Targeting S-adenosylmethionine biosynthesis with a novel allosteric inhibitor of Mat2A
Casey L Quinlan1,5* , Stephen E Kaiser2,5, Ben Bolaños2, Dawn Nowlin1, Rita Grantner1,
Shannon Karlicek-Bryant1, Jun Li Feng2, Stephen Jenkinson3, Kevin Freeman-Cook2, Stephen G Dann1, Xiaoli Wang1, Peter A Wells1, Valeria R Fantin4, Al E Stewart2 & Stephan K Grant1

S-Adenosyl-L-methionine (SAM) is an enzyme cofactor used in methyl transfer reactions and polyamine biosynthesis. The biosynthesis of SAM from ATP and L-methionine is performed by the methionine adenosyltransferase enzyme family (Mat; EC 2.5.1.6). Human methionine adenosyltransferase 2A (Mat2A), the extrahepatic isoform, is often deregulated in cancer. We identified a Mat2A inhibitor, PF-9366, that binds an allosteric site on Mat2A that overlaps with the binding site for the Mat2A regulator, Mat2B. Studies exploiting PF-9366 suggested a general mode of Mat2A allosteric regulation. Allosteric binding of PF-9366 or Mat2B altered the Mat2A active site, resulting in increased substrate affinity and decreased enzyme turnover. These data support a model whereby Mat2B functions as an inhibitor of Mat2A activity when methionine or SAM levels are high, yet functions as an activator of Mat2A when methionine or SAM levels are low. The ramification of Mat2A activity modulation in cancer cells is also described.

S
AM is the primary donor of methyl groups to the cellular transmethylation reactions that epigenetically regulate gene expression and exert control over cell growth, death, and dif- ferentiation1. SAM is synthesized in all cells by the Mat enzymes in a reaction using ATP and methionine. The bulk of dietary methionine is processed in the liver through hepatic Mat1A. Extrahepatic SAM is synthesized by the Mat2A enzyme, whose activity is regulated by its binding partner, Mat2B2. Mat2B regulates Mat2A by altering its
substrate affinity and its sensitivity to product inhibition3–6.
Mat enzymes are powerfully positioned to affect aberrant cell growth through SAM regulation. SAM not only is required for transmethylation reactions, but also is involved in polyamine and glutathione biosynthesis reactions2. These functions play critical roles in many biological processes, including regulation of tran- scription and translation, cell growth, and apoptosis2. Mat enzymes are deregulated in several cancer types, including leukemia and hepatocellular carcinoma7,8. In hepatocellular carcinoma the domi- nant liver isoform Mat1A is downregulated while the extrahepatic isoform Mat2A is correspondingly upregulated9. This isoform switch is accompanied by a decrease in the intracellular SAM con- centration and subsequent changes in methylation patterns10. The Mat2A–Mat2B complex in particular has been proposed to provide a proliferative advantage in hepatocellular carcinoma because of the complex’s especially low SAM production as compared to that in normal liver cells11.
The deregulation of Mat2A in several cancer types and the observation that silencing of the Mat2A gene results in cancer cell death12,13 suggest that Mat2A has potential as a therapeutic target. However, the lack of potent and specific Mat2A inhibitors has made this hypothesis difficult to test. Methionine analogs may act as sub- strate-competitive inhibitors of Mat2A and have been proposed as chemotherapeutic agents14. Commercially available cycloleucine is one such agent, but its low affinity for MAT2A and unclear effects on other amino acid–using pathways make its effects difficult to interpret8,15. Similarly, stilbene derivatives have been proposed
as inhibitors of Mat2A16, but these compounds are unlikely to specifically inhibit Mat2A because of redox reactivity and a high propensity for polypharmacology.
We have identified the first allosteric Mat2A inhibitor with cel- lular activity that is structurally unrelated to the enzyme’s reactants or products. We solved a crystal structure revealing PF-9366 bound to an allosteric site on Mat2A that overlaps with the site of regu- latory protein–protein interaction with Mat2B. We examined the connections between the allosteric and active sites by hydrogen– deuterium exchange mass spectrometry (HDX–MS). Biochemical and cellular studies exploiting PF-9366 further revealed that Mat2B may differentially modulate Mat2A activity depending on the con- ditions. At low methionine concentrations, Mat2B activated Mat2A by increasing its affinity for methionine. However, when methion- ine or SAM levels were high, Mat2B inhibited Mat2A by decreasing enzyme turnover and increasing its sensitivity to product inhibition. Furthermore, we explored the ramifications of Mat2A inhibition in cancer cells and describe a mechanism by which cancer cells regu- late Mat2A expression in response to changes in SAM levels.
RESULTS
A new class of Mat2A inhibitors
We implemented a parallel screen against recombinant human Mat2A and the Mat2A–Mat2B complex to discover inhibitors of Mat2A activity. The Mat2A–Mat2B complex was included because Mat2B may regulate Mat2A in several cancer types including leukemia and hepatocellular carcinoma (HCC)11,17. Two small, tar- geted chemical libraries were screened in the initial effort. Kinase- targeted and methyltransferase-targeted libraries were chosen because they contain ATP and SAM analogs that we postulated may have affinity for the Mat2A active site. The combined hit rate against Mat2A was moderately low (0.33%), but that against Mat2A–Mat2B was considerably lower (0.05%) (Supplementary Results, Supplementary Fig. 1a and Supplementary Table 1). The Mat2A–Mat2B complex was subsequently screened in a much

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1Oncology Research and Development, Pfizer Inc., San Diego, California, USA. 2Oncology Medicinal Chemistry, Pfizer Inc., San Diego, California, USA. 3Drug Safety and Pharmacology, Pfizer Inc., San Diego, California, USA. 4ORIC Pharmaceuticals, South San Francisco, California, USA. 5These authors contributed equally to this work. *e-mail: [email protected]

larger campaign of ~500,000 compounds, yielding a similar low hit a
rate and no verified Mat2A–Mat2B inhibitors. 50
The targeted-library screening campaign yielded several chemi- 40
nmol SAM
cal series that behaved as inhibitors of Mat2A (Supplementary
Fig. 1b), but most of these were deprioritized in follow-up dose– 30
response (half-maximal inhibitory concentration; IC50) and binding 20
(isothermal calorimetry; ITC) assays. PF-9366 (1) from the kinase- targeted library was well behaved in orthogonal assays and was the 10
most potent inhibitor of Mat2A in this chemical series (IC50 = 420 nM; 0
Fig. 1a and Supplementary Fig. 1b). PF-9366 showed no sub-

0
10
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100
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10 100
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–0.02
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–0.06
–0.08
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0.00
–1.00
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–3.00
–4.00

Time (min)

stantial off-target activity in a functional panel of representative GPCRs, neurotransporters, phosphodiesterases, and ion channels (Supplementary Table 2) and similarly did not appreciably inhibit
PF-9366 (M)

c DMSO

–5.00

d

0.0 0.5 1.0 1.5 2.0
Molar ratio

the kinase activities of the 39 kinases tested (<10% inhibition on
average and not greater than 30%) at 10 M (Supplementary Fig. 2a and Supplementary Table 2). PF-9366 was also tested against the liver isoform Mat1A in the presence and absence of Mat2B and was found to inhibit 50% of SAM synthesis activity at saturating com- pound concentrations (Supplementary Fig. 2b).
PF-9366-type tricyclic compounds are analogs of benzodiaz- epines18, but are differentiated by substitution of quinoline rings in
0.3 M PF-9366
1 M PF-9366
Vo (pmol SAM/min)
3 M PF-9366
5
4
3
2
1
0
0 100 200 300

100
% Mat2A inhibition
(% of activity without drug)
75
50
25
0

0.01 0.1 1 10 100

place of the benzodiazepine ring system. Interestingly, hit expansion
Methionine (M)
PF-9366 (M)

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showed that replacement of the center quinoline ring of PF-9366 with the seven-membered heterocycle of the classic benzodiaz- epine core (2) resulted in complete loss of activity against Mat2A. Measurement of PF-9366 interaction with Mat2A by ITC revealed that Mat2A bound PF-9366 with 170-nM affinity and a 1:1 stoichi- ometry of PF-9366 molecule to Mat2A monomer (Fig. 1b).
Kinetic analysis of the Mat2A SAM synthesis reaction in the presence of PF-9366 indicated that the compound was not competi- tive with ATP or methionine (Fig. 1c and Supplementary Table 3), suggesting an allosteric mechanism. By contrast, pre-incubation of Mat2A with increasing concentrations of Mat2B decreased the inhibitory activity of PF-9366 (Fig. 1d). These observations sug- gested that either Mat2B and PF-9366 were in competition for a shared allosteric site or binding of Mat2B caused a conformational change that prevented PF-9366 binding.
Crystal structure of Mat2A bound to PF-9366
To identify the binding site for PF-9366 on Mat2A, we solved the structure of the Mat2A–PF-9366 complex to 2.06-Å resolu- tion (Supplementary Table 4). As observed in prior structures, Mat2A crystallized in a tetrameric dimer-of-dimers configuration (Supplementary Fig. 3a)19. Size-exclusion chromatography coupled to multi-angle laser light scattering suggested that Mat2A equili- brates between monomer, dimer, and tetramer forms, with some bias toward the dimer (Supplementary Fig. 4a), confirming prior reports that Mat2A dimers can form homotetramers in solution20. The Mat2A dimer has two equivalent active sites that are assembled at the symmetric dimer interface; a Mat2A monomer would not be functional. Given this observation and the unclear relevance of the Mat2A tetramer, our results are described in the context of the Mat2A dimer.
Our structure of the Mat2A dimer was very similar to previously reported structures of Mat2A, with a core r.m.s. deviation of 0.82 Å for 364 aligned residues of a Mat2A monomer bound to SAM (PDB ID: 2P02)19. Clear initial electron density allowed unambigu- ous placement of PF-9366. An mFo – DFc omit electron density map computed from a model without ligands showed that, in agreement with the 1:1 stoichiometry observed by ITC (Fig. 1b), two PF-9366 molecules were bound per Mat2A dimer (Fig. 2a,b). PF-9366 mol- ecules bound at the top of the symmetric Mat2A homodimer inter- face, approximately 25 Å from the enzyme active sites. The structure lacked substrates or products; therefore, the Mat2A gating loop (Mat2A residues 109–140), which closes upon substrate binding19,21, was disordered and not observed.
Figure 1 | PF-9366 inhibition of Mat2A. (a) IC50 determination for
PF-9366 (structure shown; IC50 = 420 nM). Mat2A-synthesized SAM was measured by MS/MS (data are mean  s.e.m.; n = 4 independent experiments). (b) ITC thermogram of PF-9366 titrated into Mat2A to determine binding affinity and stoichiometry (representative of three independent experiments). The raw data are presented on top and the integrated peak areas are shown and fitted below. Mean Kd = 170  1.1 nM; stoichiometric binding N = 1.13  0.09; data are mean  s.e.m.; n = 3.
Thermodynamic parameters: enthalpy (H) = –5 kcal mol−1; entropy (–TS) = –4.2 kcal mol−1; and Gibbs energy (G) = –9.2 kcal mol−1. (c) The inhibitory mechanism of PF-9366 was determined to be noncompetitive with methionine and ATP by Michaelis–Menten kinetic analysis. Vo = initial rate (pmol SAM/min). Full analysis details in Supplementary Table 3 (data are mean  s.e.m.; n = 5). (d) 12.5 nM Mat2A dimer titrated with increasing concentrations of Mat2B monomer decreased the efficacy of
PF-9366 in a dose-dependent manner. Data were normalized to percent of activity in the absence of drug to emphasize the impact of Mat2B on compound efficacy (data before normalization are in Supplementary Fig. 6b). Data are mean  s.e.m.; n = 3.

An extensive network of Mat2A interactions with PF-9366 was evident and included interactions with the triazoloquinoline core, extended phenyl ring, tertiary amine, and chloro moieties (Fig. 2b,c). The triazoloquinoline core was sandwiched between F18 of one monomer and F333 of the opposite monomer, with the F333 side chain flipped in orientation from that of the previously observed rotamer to accommodate PF-9366. A water-mediated hydrogen bond was formed between R313 and the triazoloqui- noline core. The chloro group extending from the core was buried in a hydrophobic pocket composed of aliphatic portions of S331, Q317, F139, L315, and F333 on one monomer and F20 on the sec- ond monomer. The PF-9366 phenyl ring bound in a pocket sur- rounded by F20 and W274 of one monomer and F139, A276, L315, F333, and Y335 of the second monomer. Finally, the tertiary amine extending from the triazoloquinoline core formed a salt bridge with the carboxylate of E342. Both the PF-9366 binding cavity on top of the Mat2A dimer and the catalytic active sites were open to sol- vent, with solvent extending throughout much of the Mat2A dimer interface and penetrating into the core of the Mat2A monomers (Supplementary Fig. 4b,c). PF-9366 was also predicted to bind very similarly into the allosteric site of Mat1A (Supplementary Fig. 5).
There was substantial overlap in the observed binding sites for PF-9366 and the previously reported binding site for the Mat2B

a PF-9366
Active site b a
b Mat2A + substrates

Mat2A(1) Mat2A(2)

PF-9366
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Glu 342

Tyr Arg 335 313
Phe
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Gln 190
Phe
PDB: 4NDN

PF-9366
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Active site
Trp 274
313
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Gating loop
Products

Substrates
I (modeled)
–80

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Peptide number

Figure 2 | Structure of the Mat2A–PF-9366 complex that overlaps the Mat2B binding site. (a) Mat2A dimer (Mat2A protomers within a dimer
c d
%D (substrates + PF-9366) – (substrates)
%D (substrates + Mat2B) – (substrates)
10
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© 2017 Nature America, Inc., part of Springer Nature. All rights reserved.
are shown in violet or pink) bound to two PF-9366 molecules (shown in stick representation with cyan carbon atoms). (b) Detailed view of the symmetric Mat2A binding site for PF-9366 (cyan sticks) with mFo – DFc omit electron density map contoured at 3.5  (blue mesh) and computed from a model without PF-9366 molecules. (c) PF-9366 interactions with one monomer of the Mat2A dimer. (d) Mat2B C-terminal tail interactions with one monomer of the Mat2A dimer from PDB ID 4NDN20.

C-terminal tail (Fig. 2d and Supplementary Fig. 3c–e)20. The side chains of Mat2B F322 and H323 occupy the same binding space as the toluene rings extending from the triazoloquinoline cores of the two PF-9366 molecules (Supplementary Fig. 3e), which may explain the observed competitive behavior between Mat2B and PF-9366 (Fig. 1d). The symmetric binding of two PF-9366 mol- ecules to the Mat2A homodimer was also consistent with the ability of PF-9366 to completely inhibit Mat2A activity.
Mat2A dynamics probed by HDX
Mat2A interacts with substrates (ATP and methionine) in the cata- lytic site, whereas both PF-9366 and Mat2B modulate catalysis from an overlapping remote site ~25 Å away. Our Mat2A crystal struc- ture did not sufficiently show the allosteric mechanism by which Mat2B or PF-9366 regulate catalytic activity. To better understand the allosteric effects of PF-9366 and Mat2B on Mat2A activity, we used HDX to probe structural dynamics along the Mat2A protein backbone to identify regions of ligand-induced changes. Prior crystallographic studies demonstrate that an active site gating loop (Mat2A residues 109–140) gains order and closes upon substrate binding19,21. However, other dynamic regions are likely to link the Mat2A active site with the Mat2B and PF-9366 binding sites.
We first evaluated changes in Mat2A upon binding the nonhy- drolyzable ATP analog AMP–PNP and methionine substrates to establish a baseline for Mat2A HDX. Five specific regions along the Mat2A backbone, most of which surround the substrate-binding pocket, showed notable changes in HDX upon substrate binding: region I (61–68), region II (A109–G140), region IV (I241–Y271), region V (G272–T288), and region VI (Y320–Y335) (Fig. 3a,b). The most substantial suppression of deuterium incorporation (60% reduction from apo) occurred at the gating loop (region II). The gating-loop residue Q113 directly contacts methionine and is proposed to play a critical role in positioning methionine for catalysis19,22. Two adjacent regions form much of the nucleotide- binding site (regions IV and V) and exhibited substantial reductions in deuterium exchange upon substrate binding (30–50% reduction
Figure 3 | Mat2A ligand induced structural dynamics probed by hydrogen–deuterium exchange mass spectrometry (HDX–MS).
(a) Mat2A regions exhibiting changes in HDX are shown as colored regions on one monomer of the Mat2A dimer in complex with Mat2B (PDB ID: 4NDN20) with superimposed PF-9366. Mat2A residues 61–68 (region I) are shown in red; 110–140 (region II, gating loop) in orange; 190–198 (region III) in yellow; 241–271 (region IV) in green; 272–288 (region V) in blue; and 320–335 (region VI) in violet. (b) Deuterium
residual plot showing the percent change in deuterium incorporation (%D) upon addition of substrates AMP–PNP and methionine. (c,d) Deuterium residual plots showing the %D upon addition of PF-9366 (c) or Mat2B
(d) to Mat2A preloaded with AMP–PNP and methionine. Each treatment condition was tested in two independent experiments (n = 2).

from apo). A few residues (S247, R249, and F250) that interact directly with the adenosine group of ATP are housed within these regions. Lastly, region VI, part of which contacts substrates and the gating loop, showed reduced exchange upon substrate binding. I322 from this region is ~4 Å from the methionine side chain -carbon, and it is plausible that as the gating loop closes, region VI could cor- respondingly shift to help orient substrates and close the active site. PF-9366 and Mat2B altered the catalytic activity of Mat2A. Therefore, we examined the effects of PF-9366 and Mat2B bind- ing on Mat2A HDX in the presence of AMP–PNP and methion- ine (Fig. 3c,d). As was predicted from the structures, both PF-9366 and Mat2B reduced HDX in the allosteric pocket including region III (Q190–I198). The suppression was more extensive for Mat2B, which occluded more surface area. Both Mat2B and PF-9366 further suppressed HDX in regions IV and V, which span large regions con- necting the active and allosteric sites, and specifically make contacts with ATP. The largest decrease in HDX, after that in the allosteric pocket itself, was in the active site gating loop (region II). PF-9366 binding partially restored hydrogen–deuterium exchange in region VI, whereas Mat2B had no additional effect on this region. This dif- ference may reflect the binding proximity of PF-9366 to region VI
in comparison to Mat2B.
Allosteric regulation of Mat2A
Mat2B and PF-9366 exhibited striking similarities with respect to their effects on Mat2A HDX. Mat2B is proposed to act as an allosteric regulator of Mat2A activity by lowering the methionine substrate KM or shifting Mat2A sensitivity to product inhibition3,5. We confirmed that Mat2B binds tightly to Mat2A in solution, with a mean Kd of 4.5 nM and a stoichiometry of one Mat2B monomer

a

Vo (pmol SAM/min)
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SAM product IC50 (M)
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6.25 nM Mat2B 0.3 M PF-9366
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1 M PF-9366
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Figure 4 | Kinetic consequences of PF-9366 and Mat2B binding. (a) The Mat2A methionine KM and Vmax were shifted by binding of PF-9366 or Mat2B (full data summary in Supplementary Table 3), measured by RapidFire–MS/MS. (b) The IC50 for SAM product inhibition was lowered by PF-9366 and Mat2B, as determined with the KinaseGlo Plus luminescence detection format. Statistical significance determined by one-way ANOVA (P < 0.001) and Dunnett’s multiple comparison test; *P = 0.017, **P = 0.001, ***P = 0.044, ****P = 0.002 indicate statistical difference from Mat2A alone. For a,b data are mean  s.e.m.; n = 3 independent experiments. (c) Mat2A activity is decreased by Mat2B, but the effect saturates with 50% remaining activity. Statistical significance determined by one-way ANOVA (*P < 0.001) and Tukey’s multiple comparison test (data are mean  s.e.m.; n = 4). n.s., not significant.
(d–f) Model of Mat2A in the absence of inhibitor (d), bound to PF-9366 (c), or asymmetrically bound to Mat2B (d) with functional active sites marked by red ovals and nonfunctional active sites by red ovals with an “X” through them.

© 2017 Nature America, Inc., part of Springer Nature. All rights reserved.
per Mat2A dimer (Supplementary Fig. 6a)20. In our studies, Mat2B increased the affinity of Mat2A for its substrate methionine by five- fold (lower KM) and decreased maximum velocity by 50% (lowered Vmax) (Fig. 4a and Supplementary Table 3). Since the amount of Mat2A enzyme was held constant in these studies, decreased Vmax indicated a lower turnover rate (kcat) in the presence of Mat2B. PF-9366 also lowered the Mat2A Vmax and decreased the methion- ine KM five-fold at the highest concentration tested (3 M) (Fig. 4a). Neither Mat2B nor PF-9366 affected the ATP KM (Supplementary Table 3). We also determined that the SAM product inhibition IC50 was decreased when either Mat2B or PF-9366 was present (Fig. 4b) and that both were capable of increasing the sensitivity of Mat2A to product inhibition. These data suggested that Mat2B acts as an activator of Mat2A when levels of methionine are low but behaves as a negative regulator of Mat2A activity when methionine or SAM levels are high (Fig. 4a,b).
The shifted substrate affinity and enzyme turnover observed may correlate with the decreased HDX of the active site gating loop (region II, Fig. 3), and suggested that substrate orientation and product release (turnover rate) could be affected by both PF-9366 and Mat2B binding. PF-9366 bound Mat2A with 1:1 stoichiometry (Fig. 1b) and fully inhibited Mat2A enzymatic activity in a dose- dependent manner (Fig. 1a). By contrast, titration of Mat2A with up to a 16-fold molar excess of Mat2B over the concentration of Mat2A dimer resulted in a maximum of 50% suppression of activity (Fig. 4c). Intriguingly, comparison of structures of human Mat2A with SAM (2P02)19 and human Mat2A–Mat2B complex with SAM (4KTT and 4NDN)20 shows that both Mat2A active sites within a dimer can be loaded with SAM in co-crystallization experiments, but that Mat2B appears to prevent binding to one of the two Mat2A
active sites within a Mat2A dimer. These structural observations and the results of our kinetic work are consistent with Mat2B acting mechanistically as a partial inhibitor of Mat2A. By binding to the Mat2A dimer asymmetrically, Mat2B appears to inhibit catalysis at one active site within the Mat2A symmetric dimer but allow unin- terrupted catalysis at the second active site. This model is illustrated in Figure 4d–f, wherein Mat2A is shown to possess two competent active sites in the absence of inhibitor (Fig. 4d) and no competent active sites when PF-9366 is bound (Fig. 4e); asymmetric binding of Mat2B to Mat2A is hypothesized to inactivate only one active site (Fig. 4f).
PF-9366 inhibits Mat2A in cancer cells
As a follow-up to the mechanistic studies of PF-9366, and to bridge the biochemical data to cellular response, we investigated the ability of PF-9366 to modulate SAM synthesis in cancer cells. H520 lung carcinoma cells were treated with PF-9366 or cycloleucine (3) for 6 h, which was followed by SAM extraction from the cells and analysis by RapidFire–MS. PF-9366 inhibited cellular SAM produc- tion with an IC50 of 1.2 M, whereas cycloleucine inhibited with an IC50 of 5.6 mM (Fig. 5a).
In light of our findings regarding the shared binding site of PF-9366 and Mat2B, we accordingly explored the impact of Mat2B on PF-9366 efficacy in cells. Through quantitative western blotting, we determined that H520 cells had a high relative ratio of Mat2A dimer to Mat2B monomer (~2.5:1 Mat2A dimer:Mat2B mono- mer; Supplementary Fig. 7), which provided an appropriate in situ background for exploration of the effects of Mat2B on Mat2A activ- ity. We knocked down the expression of Mat2B with short hairpin RNA in H520 cells (H520-shMat2B). The most efficacious hairpin

a PF-9366 (EV)

b Mat standard

Mat2B shRNA
cells that contain Mat2B (Fig. 5c). The IC50 did not change appre-

SAM
(fmol/1,000 cells)
400
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Cycloleucine (EV)
Cycloleucine (shMat2B)

Mat2A
curve (ng) EV #1 #2 #3 #4
60 30 15 7.5
ciably, but the total effect did. There appeared to be a population
of Mat2A that was not susceptible to inhibition by PF-9366 in the

200
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Actin
H520-EV cells. This is measured by the residual SAE synthesis in the presence of saturating concentrations of PF-9366 (lower IC50 plateau). Cycloleucine is substrate competitive and acts indepen- dently of Mat2B; it inhibited Mat2A 100%, with identical potency in both cell lines.

c
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PF-9366 (shMat2B) Cycloleucine (shMat2B)

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A notable side effect of Mat2B knockdown was a 51% decrease in Mat2A protein levels: quantitatively, 1.85 ng/g in the H520-EV line and 0.95 ng/g in the H520-shMat2B line (Fig. 5b). Despite the lower expression of Mat2A in the H520-shMat2B cells (Fig. 5b), the total Mat2A activity appeared to be equivalent to that in the H520-EV cells (Fig. 5c). This may have been due to the impact of

0.0001 0.01 1 100
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Mat2B on the Vmax and product inhibition of Mat2A (Fig. 4). In H520-EV cells Mat2B limits Mat2A activity, but in H520-shMat2B

© 2017 Nature America, Inc., part of Springer Nature. All rights reserved.
Figure 5 | PF-9366 and Mat2B modulation of Mat2A in H520 lung cancer
cells. (a) H520 cells treated with Mat2B short hairpin RNA (shRNA) or empty vector (EV) were dosed with either PF-9366 (H520-Mat2B IC50 =
0.86 M; H520-EV IC50 = 1.2 M) or cycloleucine (H520-Mat2B IC50 =
4.9 mM; H520-EV IC50 = 5.6 mM). (b) Representative western blots of H520 cell lysates treated with four shRNA constructs or EV control. shRNA #3 was used in all the studies described herein. Western blots were quantified and actin was used as a loading control (Supplementary Fig. 7 for quantification; Supplementary Fig. 9 for unprocessed immunoblot images). (c) PF-9366 and cycloleucine decreased
SAE synthesis in H520 cells (n = 3 independent experiments).
PF-9366 IC50 = 347 nM (H520-shMat2B) and IC50 = 712 nM (H520-EV);
cycloleucine IC50 = 1.8 mM (H520-shMat2B) and IC50 = 3.1 mM
(H520-EV). No significant differences in mean IC50 values were detected between cell lines for PF-9366 or cycloleucine (data are mean  s.e.m.; n = 3; sum-of-squares, F-test). The remaining SAE synthesis in H520-EV cells at the highest PF-9366 concentration was significantly higher than in H520-shMat2B (*P < 0.01). Statistical significance of these differences was determined by one-way ANOVA and Dunnett’s multiple comparison test. (d) SAE synthesis rates normalized to nanograms of Mat2A protein. SAE synthesis rates were significantly higher at 40 min in H520-shMat2B cells than in the EV control cells (**P < 0.05) as determined by unpaired Student’s t-test (data are mean  s.e.m.; n = 3).

(#3) decreased Mat2B protein to undetectable levels (Fig. 5b). The IC50s of PF-9366 and cycloleucine were modestly decreased in the H520 cells with Mat2B knockdown, to 0.86 and 4.9 mM, respectively (Fig. 5a).
To more directly gauge modulation of Mat2A activity in situ, we monitored decreased synthesis of the surrogate biomarker S-adenosylethionine (SAE). Although SAM is generated only by the Mat enzymes, it is metabolized by numerous downstream cel- lular pathways. This diversity of downstream pathways indicates that SAM steady state flux is controlled not just by SAM genera- tion (Mat2A), but by the sum of all the SAM-consuming reactions. Indeed, the half-life of SAM in cells is thought to be around 5 min23. This presents a technical challenge for the measurement of cellular SAM levels as an endpoint for Mat2A activity. We designed a cel- lular assay that allowed direct comparison of Mat2A activity in cells without being confounded by possible differences in downstream SAM-consuming reactions. Cells given methionine-restricted, L-ethionine-supplemented medium generate SAE instead of SAM24. Mat2A binds and utilizes L-ethionine very similarly to methion- ine22, but downstream SAM-consuming enzymes utilize SAE with considerably less efficiency than SAM25, so that SAE behaves as a low-flux metabolite pool. In this way, the de novo synthesis of SAE is a more direct measure of Mat2A activity in situ. In L-ethionine- supplemented experiments, PF-9366 showed greater efficacy in the H520-shMat2B cells than in the empty vector control (H520-EV)
cells the increased Mat2A activity compensates for its lowered expression. This effect was most pronounced in a 6-h SAM experi- ment (Fig. 5a). We explored the hypothesis that H520-shMat2B had increased Mat2A activity by looking at SAE synthesis rates per nanogram of Mat2A protein in the H520-EV and H520-shMat2B cells (Fig. 5d). We found that the initial SAE synthesis rates were equivalent in the two lines, but a rate limitation was par- ticularly evident at the later time point when normalized to total nanograms of Mat2A.
Mat2A is upregulated in response to inhibition
It has been shown that silencing of the Mat2A gene results in cancer cell death12, but a potent and specific Mat2A inhibitor is required to thoroughly explore the role of Mat2A in cancer cell viability. PF-9366 is a relatively potent and specific inhibitor that provides a tool to test this hypothesis. We investigated the Huh-7 cell line as a model for deregulated Mat enzymes in hepatocellular carcinoma (Fig. 6). Huh-7 cells were more sensitive to compound exposure than the H520 cell line (Fig. 5a). After 6-h exposure to PF-9366 or cycloleucine, the IC50 for SAM synthesis inhibition was 225 nM or
1.3 mM, respectively (Fig. 6a). Surprisingly, this did not translate to inhibition of cell proliferation, for which we observed a 40-to-50- fold loss of potency in a standard 3-d Huh-7 cell proliferation assay (Fig. 6b). To understand this unexpected response, we monitored SAM levels as a measure of target engagement after 3 d of treatment and found that both PF-9366 and cycloleucine had lost potency against the target by more than five-fold (Fig. 6c), in line with the decreased antiproliferative effect upon prolonged compound expo- sure. A probe of transcript levels by qt-PCR found that Mat2A tran- script was increased three-fold after 24-h treatment, while Mat2B and Mat1A levels were unchanged (Fig. 6d). We confirmed that Mat2A protein levels had correspondingly increased three-fold, but Mat2B protein levels remained the same (Fig. 6e).
Our results suggested that small-molecule inhibition of Mat2A triggers an adaptive transcriptional response to bypass inhibi- tion regardless of inhibitory mechanism (substrate competitive or allosteric). However, knowing the efficacy of PF-9366 is limited when Mat2B occludes the binding site, we used the H520-EV and H520-shMat2B cells to investigate whether PF-9366 triggers the same Mat2A upregulation and loss of inhibitory potency. Indeed, Mat2A upregulation (five-fold) was observed in H520 cells and was similarly independent of Mat2B expression (Fig. 6f and Supplementary Fig. 8).
DISCUSSION
Mat2A is often deregulated in malignancy10,12,13,17,26–31, which could represent an Achilles heel of some cancer types13,21,32. However, the lack of suitable tool inhibitors has hampered evaluation of Mat2A as a therapeutic target14. We discovered and character- ized PF-9366, a new allosteric inhibitor of Mat2A that inhibited

a 6 h: Huh-7 inhibition SAM synthesis b

72 h: Huh-7 inhibition of proliferation
(lower Vmax) at high substrate concentrations. Most literature

SAM (fmol/1,000 cells)
150
100
PF-9366 (IC50 = 255 nM)
Cycloleucine (IC50 = 1.3 mM)

Inhibition of proliferation (%)
100
75
50
PF-9366 (IC50 = 10 nM)
Cycloleucine (IC50 = 50 mM)
reports tend to agree that Mat2B decreases the methionine KM and increases the sensitivity of Mat2A to SAM product inhibi- tion3–6. Importantly, our work clarified that increased sensitivity of Mat2A to product inhibition observed in the presence of Mat2B is likely mediated by an overt decrease in enzyme turnover

50 25
0 0
(Vmax
decrease) (Fig. 4a).

Dose (mM)
c 72 h: Huh-7 inhibition SAM synthesis d

Dose (mM)
The overall observation that binding of either Mat2B or PF-9366 induced similar changes in protein dynamics suggested a shared mechanism of allosteric signal transmission from this site. The decreased HDX in the active site gating loop (region II) and in

SAM (fmol/1,000 cells)
150

100

50

0
PF-9366 (IC50 = 1.5 M)
Cycloleucine (IC50 = 7.1 mM)
Huh-7 RNA expression
Relative expression (fold change over vehicle)
5
4
3
2
1
regions V and VI, which span the core of the protein, suggested that the allosteric signal may be propagated sequentially from the allosteric site to the active site in a manner similar to the proposed ‘domino model’33. This mechanism of allosteric signal transmis- sion is considered relevant for G-protein-coupled receptors, for the chymotrypsin class of serine proteases, and for hemoglobin34.

0.0001 0.01 1 100
Dose (mM)
0
Mat2A Mat2B Mat1A
Although we did not explore it in this study, the observation that PF-9366 was only a partial inhibitor of Mat1A (Supplementary

⦁ Huh-7 protein levels
Protein (ng/g cell lysate)
*

DMSO Cycloleucine PF-9366
⦁ H520 Mat2A protein levels
***
Mat2A protein (ng/g cell lysate)
5
4
3
2
1
0
DMSO Cycloleucine PF-9366

EV
shMat2B
Fig. 2b) is in keeping with the hypothesis that the mechanism of inhibition by PF-9366 is tightly integrated with the allosteric regu- lation of the enzyme. Mat1A is not product inhibited to the same extent as Mat2A4, and this observation suggested that PF-9366 is a less efficacious inhibitor in the absence of product inhibition. Future studies on this mechanism are warranted.
Our studies suggest a model whereby Mat2B acts to promote SAM generation under very low cellular concentrations of methi- onine, but also serves to stabilize the cellular SAM pool at lower

Mat2A
Mat2B Actin
Mat2A (EV)
Mat2A (shMat2B)
Actin
steady-state concentrations than would be observed in the absence of Mat2B. This may be an important mechanism in numerous biological contexts. For instance, activated lymphocytes down-

Figure 6 | Mat2A is upregulated in response to inhibition. (a) SAM synthesis in Huh-7 cells inhibited by PF-9366 (IC = 255 nM) and
regulate Mat2B and consequently exhibit increased Mat2A activ- ity and higher cellular SAM levels than their resting counterparts5.

50
cycloleucine (IC = 1.2 mM). (b
Similarly, in liver cancer cells, overexpression of Mat2B lowers

© 2017 Nature America, Inc., part of Springer Nature. All rights reserved.
50 ) The 72-h antiproliferative effect of PF-9366 (IC50 = 10 M) and cycloleucine (IC50 = 50 mM). (c) Potency of PF-9366 (IC50 = 1.2 M) and cyclocleucine (IC50 = 7.1 mM) after
72-h treatment. (d) Huh-7 cell Mat2A, Mat2B and Mat1A RNA relative expression after 24 h PF-9366 treatment. (e) Quantified western blots of Mat2A and Mat2B protein after 24 h compound treatment. (Data in a–e are mean  s.e.m.; n = 3 independent experiments; see Supplementary Fig. 7 for quantification data and Supplementary Fig. 9 for unprocessed blots.) (f) Mat2A protein levels in H520-EV and H520-shMat2B cells. Mat2B levels were unchanged in H520-EV cells (Supplementary Fig. 7). Statistical significance determined by two-way ANOVA (P < 0.005)
and Tukey’s multiple comparison test (data are mean  s.e.m.; n = 3;
*P  0.005, **P > 0.01, ***P  0.005).

cellular SAM synthesis with submicromolar potency. The binding site of PF-9366 overlapped with the previously identified binding site for the C-terminal tail of Mat2B (Fig. 2)20. Consistent with the idea that the Mat2B and PF-9366 allosteric sites are linked to the Mat2A active site, both caused a substantial decrease in protein dynamics (HDX) in regions that comprise and surround the active site (Fig. 3). In particular, reduced mobility of the gating loop (region II) sug- gests that substrate binding and product release were impacted by binding into this allosteric site. The gating loop is proposed to play a direct role in catalysis by sensing the presence and orientation of the methionine moiety in either the methionine substrate or the SAM product19,22. We proposed that decreased gating-loop dynamics slows product release and enzymatic turnover, which is consistent with the observed decrease in Mat2A Vmax and its increased sensitiv- ity to product inhibition (Fig. 4a,b).
The effects of Mat2B and PF-9366 on Mat2A kinetic param- eters were remarkably similar; both resulted in enhanced affinity for methionine (decreased KM) and decreased enzyme turnover
SAM levels11, while knockdown of Mat2B raises cellular SAM lev- els28. On the other hand, knockdown of Mat2B in Jurkat leukemia cells reduces the activity of Mat2A by increasing the substrate KM above the physiological levels of methionine, thereby lowering cellular SAM levels17; this effect is reversible by addition of super- physiological methionine concentrations. In this way, depend- ing on the physiological methionine concentrations, Mat2B may increase or decrease cellular Mat2A activity and correspondingly modulate steady state SAM levels. Although we did not explore them in this study, other levels of regulation may directly modu- late Mat2B function. Mat2B is thought to possess NADPH binding sites19, which may support a link to redox sensing to further control SAM synthesis.
Knockdown studies have suggested that Mat2A is an important regulator of cancer cell growth, but our study suggests that this mechanism will be difficult to pharmacologically target for therapeu- tic advantage. Mat2A exhibited remarkable transcript and protein upregulation in response to inhibitor treatment (Fig. 6d–f), which decreased the antiproliferative impact of Mat2A inhibition (Fig. 6b and Supplementary Fig. 8b). Mat2A upregulation was observed in Huh-7, H520-EV and H520-shMat2B cells with both PF-9366 and cycloleucine treatment. This suggests a general cellular response to decreases in SAM levels that is irrespective of cell type, Mat2B expres- sion level, and inhibitor type (Fig. 6 and Supplementary Fig. 8). This is not surprising when these data are considered in the light of what is known about MAT gene regulation. Both the MAT1A and MAT2A genes are under epigenetic regulation7,10,11,35 and, in particu- lar, promoter methylation36. Mat2A upregulation is associated with C-C-G-G hypomethylation of the gene promoter in hepatocellular carcinoma, whereas it is methylated and minimally expressed in a normal liver36. Mat2A promoter methylation has also been shown to increase and enzyme expression levels to correspondingly decrease

© 2017 Nature America, Inc., part of Springer Nature. All rights reserved.
in response to increased SAM levels36. These principles are likely at play in our experimental system, i.e., cellular SAM levels fall in response to Mat2A inhibitor treatment and Mat2A promoter meth- ylation also decreases through the principles of mass action. Mat2A expression is increased as the promoter is demethylated. Similarly, when Mat2B was knocked down in H520 cells, SAM levels increased because of increased activity of Mat2A in the absence of Mat2B (Fig. 5a). This likely resulted in increased promoter methylation and decreased Mat2A expression (Fig. 5b).
In summary, Mat2B exerts profound control over SAM levels in cells by regulating both the substrate binding to and product release from Mat2A. We used the tool compound PF-9366 to probe and define the mechanistic basis for the regulation of Mat2A by Mat2B, and to further elucidate the exquisite control cells exert over SAM synthesis through transcriptional regulation of Mat2A expression. PF-9366 could enable broader inquiries into the specific roles and physiological importance of Mat2A compared to the Mat2A–Mat2B complex in pathology and disease. Our study also illustrated a criti- cal limitation of genetic silencing as a means to assess therapeutic potential of a target. In the case of Mat2A, gene silencing drives loss of cell viability in various cancer cell lines12,13, but inhibitor treatment drove an adaptive and compensatory transcriptional upregulation of Mat2A that was masked in the genetic studies. This observa- tion suggests that this mechanism may be difficult to target with traditional small-molecule inhibitors. However, it has been recently proposed that genotypes with methylthioadenosine phosphorylase (MTAP) deletion may rely more heavily on de novo SAM synthesis21 and exhibit greater vulnerabilities to perturbations of this pathway.
Received 11 April 2016; accepted 7 March 2017;
published online 29 May 2017

METHODS
Methods, including statements of data availability and any associated accession codes and references, are available in the online version of the paper.

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Acknowledgments
We thank members of the Pfizer La Jolla Analytical Chemistry group (W. Ferrell,
C. Aurigemma, and P. Tran) for project support, and thank E. Johnson, M. Calabrese and A. Zorba for helpful comments and discussion.
Author contributions
C.L.Q. conceptualized, executed, and analyzed the kinetic and cellular studies. S.E.K. conceived the crystallographic study, collected and analyzed
crystallographic data, solved crystal structures and made figures; B.B. designed, executed and analyzed the HDX–MS experiments; D.N. designed and oversaw the screening campaign and follow-up compound triage; R.G. performed cell
assays and quantitative western blotting analysis; S.K.-B. performed ITC experiments and analysis; J.L.F. expressed and purified the recombinant enzymes; S.J. designed and performed that selectivity screening; K.F.-C. purified and analyzed PF-9366;
S.G.D. performed shRNA studies; X.W. performed qtPCR on PF-9366 treated cells;
P.A.W. oversaw and analyzed kinetic experiments; V.R.F. conceptualized and
oversaw the drug discovery effort; A.E.S. designed and reviewed structural and protein science experiments and data; S.K.G. oversaw and directed the
biochemical and screening work; C.L.Q. and S.E.K. wrote the paper with contributions from all authors.
Competing financial interests
The authors declare competing financial interests: details accompany the online version of the paper.
Additional information
Any supplementary information, chemical compound information and source data are available in the online version of the paper. Reprints and permissions information is available online at http://www.nature.com/reprints/index.html. Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Correspondence and requests for materials should be addressed to C.L.Q.

© 2017 Nature America, Inc., part of Springer Nature. All rights reserved.

© 2017 Nature America, Inc., part of Springer Nature. All rights reserved.
ONLINE METHODS
Synthesis and characterization of PF-9366. Synthesis and characterization of the 1-[2-(dimethylamino)ethyl] quinoline benzodiazepine derivative (PF-9366) is as previously described18. PF-9366 compound characterization: 1H NMR (400 MHz, DMSO-d6)  = 8.43 (d, J = 9.2 Hz, 1H), 7.87 (dd, J = 2.4, 9.0 Hz,
1H), 7.69 – 7.45 (m, 7H), 3.62 (app. t, J = 7.5 Hz, 2H), 2.90 (app. t, J = 7.5 Hz,
2H), 2.29 (s, 6H). 13C NMR (101MHz, DMSO-d6)  = 148.5, 148.4, 139.3,
136.6, 130.7, 130.1, 129.4, 129.2, 128.9, 128.8, 126.2, 125.3, 119.3, 115.6, 55.6,
45.1, 27.1. APCI [M+H]+ = 351.1.

Mat2A, Mat1A, and Mat2B cloning, expression, and purification. DNA frag- ments encoding human Mat2A (residues 1–395, construct LJEC-1718) and human Mat2B variant 1 (residues 1–334) were synthesized and cloned into pCCEC-N1, a pET24a modified vector with an N-terminal TEV (tobacco etch virus) cleavable 6×His tag. The expressions were carried out in Escherichia coli BL21 (DE3) in Terrific Broth media in high-density shaker flasks. Cells were grown at 37 °C until OD600 = 4.0–6.0 and expression was induced by the addition of 0.5 mM IPTG (isopropyl -D-thiogalactopyranoside) overnight at 15 °C. Cells were harvested and cell pellets stored at −80 °C until use. Human Mat2A (residues 1–395) was also cloned with an N-terminal TEV cleavable 6×His tag into a modified pFastbac1 vector (construct LJIC-2155) for insect cell expression. Transfection and virus production were carried out using standard methods. For expression 2.25 L of sf21 cells at density of 1.85 × 106 were infected with virus at an multiplicity of infection (MOI) of 1. Cells were harvested after 72 h and the cell pellet was stored at −80 °C until use.
Cell pellets were resuspended in cold lysis buffer (50 mM Tris, pH 8.0, 300 mM NaCl, 10% glycerol, and 1 mM TCEP) plus Roche EDTA-free protease inhibitor tablets and lysed by microfluidizer (Escherichia coli) or by incubating for 60 min at 4 °C with stirring (insect cells). Insoluble debris was removed by centrifugation at 4 °C for 60 min at 10,000g. The soluble cell lysates were mixed with prewashed ProBond resin (Invitrogen) and incubated at 4 °C for 2 h with rotation. Proteins were eluted with 300 mM imidazole in lysis buffer. 6×His tag was removed by TEV cleavage overnight in dialysis buffer (25 mM Tris–HCl, pH 8.0, 200 mM NaCl, 10% Glycerol, and 1 mM TCEP) followed by nickel reverse chromatography. Untagged proteins were loaded onto a HiPrep 26/10 desalting column using QA buffer (20 mM Tris pH 8.0, 25 mM NaCl, 10% Glycerol, 1mM TCEP). The peak fractions were pooled and loaded onto a 5-ml HiTrap Q HP column (GE Healthcare). Proteins were eluted with a linear NaCl gradient (25–800 mM). The eluted proteins were further purified by size-exclusion chromatography using a HiPrep 26/60 Superdex 200 column (GE Healthcare). Proteins for biochemical assays were stored at –80 °C in 20 mM HEPES, pH 7.5, 150 mM KCl, 10 mM MgCl2, 10% Glycerol, and 1 mM TCEP.
Mat1A was purified following the same method but with different buffers: lysis buffer (20 mM KPO4, pH 7.8, 300 mM KCl,5% glycerol, 5 mM MgCl2, 1 mM TCEP, and 5 mM imidazole), dialysis buffer (25 mM HEPES, pH 7.5, 150 mM KCl, 5% glycerol, 10 mM MgCl2, 1 mM TCEP), desalting/ QA buffer (20 mM HEPES pH 7.5, 25 mM KCl, 5% glycerol, 10 mM MgCl2, 1 mM TCEP),
and SEC/storage buffer (10 mM HEPES, pH 7.5, 50 mM KCl, 10 mM MgSO4, 1 mM EDTA, 1 mM TCEP). Two Mat1A peaks eluted from a HiTrap Q HP column at 14.5 and 18.3 mS/cm which were determined by analytical SEC to correspond to dimer and tetramer fractions, respectively. The tetramer fraction was used in our studies as the dimer fraction was inactive. SDS–PAGE and MS analysis of purified proteins used in this study can be found in Supplementary Figure 10.

High-throughput Screen of Mat2A. Buffer reagents were standard high qual- ity grade from Sigma. S-adenosylmethionine (New England BioLabs) and
,-imido-ATP adenylyl-imidodiphosphate (AMP–PNP; Roche Diagnostics
Corp.) were used as reference inhibitors of Mat2A. The Mat2A (LJEC-1718) and Mat2A+Mat2B screening assays were run as a KinaseGlo Plus lumines- cence detection format (Promega Corp., Madison, WI). All reagents were diluted in assay buffer consisting of 50 mM HEPES pH 7.5, 50 mM KCl, 10 mM MgCl2, and 10 mM DTT. Test compounds were dispensed into black, 384-well assay plates (Corning) followed by addition of Mat2A or Mat2A+Mat2B for a 30 min incubation at room temperature. Addition of combined ATP and

methionine initiated the transferase reaction. Final assay conditions were 50 nM Mat2A, 25 M ATP, 50 M methionine, 2% DMSO. Mat2B was included at 50 nM final concentration for the Mat2A+Mat2B screen. The reaction was stopped by an equal volume addition of KinaseGlo Plus reagent as per manufacture instruction. Luminescence signal was measured on an Envision multi-mode plate reader (PerkinElmer) after 15–30 min equilibration.
Two libraries from the Pfizer compound collection were chosen for a focused screen based on similarities in substrates and products between kinases, epi- genetic targets and Mat2A. Compounds from the Kinase Targeted Library (KTL) and Methyl Transferase Targeted Library (MTTL) were tested at 20 M for primary HTS. Data for compounds was normalized to percent inhibition based on control wells within each test plate. Control wells with DMSO only or 500 M AMP–PMP that fully inhibits Mat2A catalytic activity established a scale of 0% to 100% inhibition, respectively. A Z value > 0.5 was required for pass criteria. A statistical hit threshold of 25% was based on three times .

Mat2A and Mat1A SAM assay. The Mat2A kinetic enzyme assay was per- formed in standard 96-well plates, in 20 l volume with the following final enzyme and buffer conditions: 25 nM Mat2A (LJEC-1718) in 50 mM HEPES (pH 7.2), 50 mM KCl, 0.01% (v/v) tween-20, and 1 mM DTT. A range of sub- strate concentrations were used to determine KM: 0–300 M L-methionine (with ATP fixed at 500 M, or 0–500 M ATP with methionine fixed at 300 M. Test compounds were diluted in 100% DMSO (final assay DMSO = 2%) and added to enzyme. The reaction was started by addition of substrate followed by incubation at room temperature for various times up to 30 min (within the linear range). The reaction was stopped with 1% formic acid and diluted 40-fold in 0.1% formic acid in HPLC-grade water before RapidFire SPE and MS/MS detection of the analyte.
The IC50 experiments were done similarly to the kinetic assay except that ATP and methionine were both fixed at 100 M and the reaction run for 30 min. IC50 values were determined by fitting the data to the standard four-parameter dose–response equation using Prism software (GraphPad). A ten-point SAM calibration curve was used for quantification. For kinetic analysis, the rate of SAM formation (pmol/min) was calculated for each sub- strate concentration. GraphPad software was used to fit the enzyme kinetic data with the Michaelis–Menten equation for calculation of Vmax and KM values.

RapidFire solid-phase extraction. Enzymatic reactions and cell extractions were analyzed using a RapidFire 300 high-throughput SPE chromatography system coupled to API4000 mass spectrometer (AB Sciex). Detection of SAM and SAE was accomplished following injection of 40 l of reaction mix or cell extraction (10 l injection loop volume) onto an Agilent Graphite (Type D) cartridge in HPLC-grade water with 0.1% trifluroacetic acid (TFA) and eluted using 50% acetonitrile and 0.1% TFA37. The RapidFire settings were as follows: aspiration time: 600 ms or until the loop is full per the sip sensor, load time: 3,000 ms, elution time: 5,000–8,000 ms, and re-equilibration time: 500 ms at a flow rate of 1.5 ml/min.

Analyte detection: mass spectrometry. Following RapidFire SPE, samples were sent to an AB Sciex triple quadrupole mass spectrometer in positive ion mode. A multiple reaction monitoring (MRM) protocol was optimized with Q1 m/z ratios of 399.2 and 413.2 for SAM and SAE, respectively. The Q3 detected the product ions of SAM (m/z = 250.1) and SAE (m/z = 250.2). SAM and SAE were quantified by calculating the area under the curve (AUC) with the RapidFire Integrator software (Agilent). For conversion of AUC to moles of analyte, standard curves were generated over a range of concentra- tions from 1–1,000 nM.

Isothermal titration calorimetry (ITC) analysis of Mat2A with PF-9366 and Mat2B. ITC experiments were done on a MicroCal VP ITC or a MicroCal Auto iTC200 instrument (Malvern Instruments, UK) at 20 °C. The Mat2A and Mat2B proteins were extensively dialyzed into a buffer containing 150 mM KCl, 25 mM HEPES, pH 7.4, 5 mM MgCl2, 5% (v/v) glycerol, 2 mM TCEP. Concentrations were determined spectrophotometrically using an 280 of 44,350 M−1cm−1 for Mat2A and an 280 of 36,440 M−1cm−1 for Mat2B. Compounds were diluted from 100% DMSO stocks into a buffer without DMSO. In a typical experiment,

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nineteen 15 l injections of 200 M compound or 30–35 M Mat2B were made into 10 M Mat2A on a VP ITC or nineteen 2 l injections of 200 M com- pound into 10 M Mat2A on an Auto iTC200. Data were analyzed using the ORIGIN software provided with the instrument and fit to a simple 1:1 binding model as described38.

Protein crystallization. The Mat2A–PF-9366 complex was prepared by mixing LJIC-2155 at 16 mg/ml with 20 mM PF-9366 in DMSO for a final molar ratio of 3:1 PF-9366:LJIC-2155. The mixture was incubated for 1 h on ice, then centri- fuged at 16,100g at 4 °C for 10 min to remove any insoluble material. Using an Echo liquid handler (Labcyte), the complex was screened in sitting drop 25 nl
+ 25 nl drops. Good quality crystals grew in Hampton Research Peg/Ion screen condition #61 (4% v/v Tacsimate pH 6.0, 12% w/v polyethylene glycol 3,350) at 21 °C. Crystals formed in space group C2 with one Mat2A tetramer per asym- metric unit with 45% solvent.

Data collection, structure determination and refinement. Crystals were cryo- preserved by adding 1 l of well condition supplemented with 30% glycerol before freezing in liquid nitrogen and data collection at Advanced Photon Source IMCA-CAT Beamline 17ID. Diffraction data were collected at 100 K with an X-ray wavelength of 1 Å to dmin = 2.06 Å resolution, processed with autoPROC39, phased by molecular replacement with Phaser40 using 2P02.pdb as a search model. Manual rebuilding was performed with COOT41 and refine- ment was performed using Phenix42. 98% of residues in the final model are in favored Ramachandran space and 2% are in allowed Ramachandran space.

Hydrogen-deuterium exchange (HDX–MS). Recombinant full-length Mat2A (LJEC-1718) was diluted (20 mM Tris pH = 7.2, 50 mM NaCl, 2 mM TCEP) to a working concentration of 2 M. Substrates AMP–PNP and methionine were added at 100 M, and allowed to incubate for 30 min at room temperature. For compound studies, Mat2A was subsequently incubated with either com- pound (20 M, 1% residual DMSO) or Mat2B (10 M) for 1 h at room tempera- ture. HDX exchange was performed on a Velos Pro OrbiTrap (Thermo Fisher Scientific), equipped with a Pal HTX-xt Autosampler (Leap Technologies) and UltiMate 3000 nano pumps (Dionex). The autosampler sample plate was held at 4 °C, where exchange was initiated with addition of 40 l of D2O solution (20 mM Tris, 50 mM NaCl) to 4 l of protein and ligand sample. The exchange reaction was performed at various time points (10 s, 1 min, 5 min, 30 min, 3 h, and 9 h) before being arrested with addition of 20 l of cold quench buffer (4M guanidine hydrochloride, 2.4% formic acid, 5 mM TCEP) to a final pH of 2.5. Samples were injected at 50 l/min across an Enzymate BEH pepsin column (Waters) held at 4 °C in the Leap cooler box, and digest subsequently trapped on a BEH C4 (2.1 mM x 5 mM) trap column (Waters). Peptides were eluted from the trap across a BEH C4 (1 mM x 50 mM) analytical column using a gradient ramp of acetonitrile. The mass spectrometer electrospray source was held at 200 °C to minimize deuterium back exchange, and a peptide lists were generated with Proteome Discoverer (Version 1.4, Thermo Fisher Scientific). Low-confidence peptides were removed from the peptide list (mass tolerance
<5 p.p.m.), and peptide pools were exported for subsequent deuterium analysis. For deuterium time-point incorporation, only MS spectra were acquired on the system, and samples were processed in HD Examiner (Sierra Analytics) for deu- terium incorporation. Low-confidence peptides were filtered, as well as peptides less than six residues in length. A total set of 200 peptides with 100% of Mat2A sequence coverage were employed to track deuterium exchange. Residual plots were generated that aligned these peptides from the N terminus and measured the degree of deuterium suppression or promotion with addition of ligand or substrate or protein. The average error was 0.2 Da for corrected data of two replicates. Therefore, we accepted only those changes that were greater than 6% and 0.5 Da as different from the baseline condition, similar to prior studies43.

Cell culture. Huh-7 cells obtained from the Pfizer cell bank were cultured in DMEM growth media, L-glutamine, high glucose, pyruvate (Gibco; Cat # 11995) supplemented with 10% FBS and 1% Penicillian–Streptomycin (PenStrep). NCI-H520 cells obtained from the Pfizer Cell Bank were cultured in RPMI growth media, L-glutamine, and high glucose (Gibco; Cat # 11875), 10% FBS and 1% PenStrep.
NCI-H520 MAT2B knockdown cells were cultured in the same media supplemented with 3.5 g/ml puromycin (InvivoGen). Cell lines were grown at 37 °C with 5% CO2 and routinely tested for mycoplasma using Lonza MycoAlert mycoplasma detection kit.

Cellular SAM and SAE extraction and proliferation. Huh-7 cells were seeded at a concentration of 15,000 cells per well for 6-h incubation with compound and 4,000 cells per well for 72-h incubation with compound in 96-well plates in 200 l of growth medium. NCI- H520 MAT2B knockdown cells were seeded at a concentration of 20,000 cells per well for 6 h incubation or 10,000 cells per well for 72 h incubation with compound in 96 well plates in 200 l of growth medium. Cells were allowed to attach overnight at 37 °C with 5% CO2. A 5× solution of cycloleucine (Sigma; Cat # A48105; purity: 97%) was prepared fresh from powder stock in growth medium. Other compounds were diluted in 100% DMSO using a three-fold dilution scheme and further diluted in growth medium to give 0.5% DMSO final. Consistency of cellular confluence for each cell line was monitored with the IncuCyte Zoom live cell imager (Essen). Proliferation was measured using CellTiterGlo reagent (Promega; Cat # G7572). Growth media was removed from the cell plates following compound treatment and 80 l/well CellTiter Glo diluted 1:1 in PBS added. Luminescence was meas- ured by an EnVision Multilabel Plate Reader (PerkinElmer). For SAM analysis, cells were incubated with compounds for 6 or 72 h. Cells were washed 3 times with PBS before a two-step extraction procedure. First, cells were extracted with 40% acetonitrile/40% methanol/20% HPLC-grade water, followed by a second extraction with 0.2% formic acid in HPLC-grade water. The two extractions were combined before the plates were sealed and centrifuged at 3,500 × g for 5 min. For the SAE extraction, Huh-7 cells and NCI- H520 MAT2B knockdown cells were prepared and treated exactly as described for the SAM extraction except that compounds were dosed for 1 h in RPMI-Met (sans methionine) media supplemented with 2 mM L-ethionine.
The cellular SAM and SAE analytes were measured by RapidFire SPE and MS detection, and quantified by calculating the area under the curve (AUC) with the RapidFire Integrator software (Agilent). For conversion of AUC to moles of analyte, standard curves were generated over a range of concentra- tions from 1 nM up to 1 M. Normalization of SAM and SAE analytes to cell density was done by quantifying the CellTiterGlo luminescent signal against a plating density standard curve. Raw luminescence units were translated to cell density using standard curves that were generated over a range of plating densities from 3,000–100,000 cells/well. Moles of analyte were then normalized to the approximate cell density as determined by this method.

Stable H520 cell line engineering. Custom lentiviral delivered constitutive MAT2B shRNA were purchased as plasmid DNA constructs from Cellecta, Inc (Cat # CVSHC-PX-CT) MAT2B shRNA insert sequences are shown below. Lentiviral particles were packaged as per manufacturer’s instructions and stable cell lines were generated as described below. Briefly, stable MAT2B knockdown NCI-H520 cells were created by exposing cells at 50% confluency in a 6-well plate to pseudotyped lentiviral particles at an MOI of one for a 24 h period in the presence of 8 g/ml polybrene. After 24 h media was replaced with full media and cells were grown to confluence followed by passage into T-175 culture flasks with puromycin selection (100 g/ml) until resistant cell colonies formed and antibiotic induced cell death ceased. Several hundred resistant colonies were pooled and expanded to form the stable cell lines employed herein. sh #3 was determined by western blot to be the most efficacious knockdown of Mat2B, and was used in the studies in this paper (shMat2B). ShRNA constructs:
#1 (5-ACCGGCCATCAATTAAGGGAATCTTTGTTAATATTCATAGCA AAGGTTCCCTTAATTGATGGTTTT-3);
#2 (5-ACCGGGCAGTTCATCACATCATTTATGTTAATATTCATAGCA TGAATGATGTGATGAACTGCTTTT-3);
#3 (5-ACCGGGCTGTGATTGTTATGTTTGATGTTAATATTCATAGCA TCAAACATAACAGTCACAGCTTTT-3);
#4 (5-ACCGGGCCTCTCAATTTAATGTGGATGTTAATATTCATAGCA TCCACATTAAGTTGAGAGGCTTTT-3).

Real-time RT-PCR. RNA was isolated from vehicle and PF-9366 treated Huh-7 cells (24 h treatment) using RNeasy Plus mini kit (QIAGEN; Cat #74136).

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This was followed by cDNA synthesis using iScript cDNA Synthesis Kit (Bio- Rad; Cat # 170-8890). Quantitative real-time PCR was performed using iQ Supermix (Bio-Rad; Cat # 170-8860) and Taqman probes (MAT2A, MAT2B and MAT1A) on Bio-Rad CFX96 – C1000 thermal cycler. Reactions were run in triplicate and results analyzed by the deldelCT method. Normalizing against DMSO control, -2-microglobulin (B2M) was used as housekeeping gene (Invitrogen).

Western blotting. Huh-7 and NCI-H520 knockdown cells were lysed in RIPA lysis buffer (Cell Signaling) containing protease inhibitor tablets (Roche) and phosphatase inhibitor cocktail (CalBiochem). Total protein concentration of cell lysates was determined by the Pierce BCA Kit (Thermo Fisher). Fifteen to twenty micrograms of each lysate and serial dilutions of recombinant Mat2A and Mat2B proteins were separated on 4–12% NuPAGE gels (Life Technologies). Gels were run at 170 Volts for 95 min in MES running buffer (50 mM MES, 50 mM Tris Base, 0.1% SDS, 1 mM EDTA, pH 7.3) and transferred onto nitro- cellulose membranes using the iBlot dry transfer system (Life Technologies). Membranes were blocked in BSA blocking buffer for fluorescent western blot- ting with 0.1% Tween-20 added (Rockland) for 2 h before overnight incuba- tion with primary antibodies at 4 °C. Mat2A primary antibody was purchased from Abcam (ab77471) and used at a 1:10,000 dilution in blocking buffer and Mat2B primary antibody was purchased from Pierce (PA5-26542) and used at a 1:1,000 dilution in blocking buffer. -Actin primary antibody was purchased from Cell Signaling (8H10D10) and used at a 1:1,000 dilution in blocking buffer. IRDye secondary antibodies anti-rabbit (926-32211) and anti- mouse (926-68020) were purchased from LI-COR Biosciences and used at a 1:20,000 dilution in blocking buffer for 60 min at room temperature. Infrared signal visualization and quantification was carried out using an Odyssey scan- ner (LI-COR Biosciences). Blots were imaged at a resolution of 169 m and quantification was performed on single channels with the analysis software provided (Image Studio Version 4.0). Mat2A and Mat2B protein quantification was normalized to the corresponding -actin control.

Off-targetpharmacologyprofiling. FLIPRCalcium Assay: Cellsusedintheassay were stably transfected with the receptor of interest (adrenergic 1a, dopamine 1, histamine 1, muscarinic 1, muscarinic 3 and serotonin 2b). Activation of the receptor by an agonist in this assay system results in an increase in intracel- lular calcium levels which is measured using a calcium specific dye. Cells were plated at 7,500 cells per well (50 l per well) in black walled clear bottomed 384-well plates 24 h before running the assay. Medium was removed from the plates and 80 l of Hanks balanced salt solution (HBSS)/HEPES containing Calcium 5 dye (Molecular Devices; Cat # R8186) and probenecid (1.25 mM) was added to each well and the plate was returned to the incubator for 1 h to allow dye loading. Compound solution (10 l) was added to each well by the FLIPR Tetra instrument to measure agonist activity of the compound by measuring the change in fluorescence from baseline over a 60 s period (Ex 470-495 nM; Em 515–575 nM). Subsequently 10 l of agonist (EC80 value) was added to each well by the FLIPR Tetra instrument to evaluate antagonist activity, with the change in fluorescence from baseline being measured over a 60 s period.
-Arrestin Assay: The -arrestin assay relies on enzyme fragment com-
plementation with the respective stably transfected GPCR (adrenergic 2, cannabinoid 1 and mu opioid) being tagged with an inactive portion of the enzyme -galactosidase and a co-transfected -arrestin that is tagged with the complementary portion of -galactosidase. Recruitment of -arrestin to the GPCR, results in a functional enzyme that generates a chemiluminescence signal when substrate is added. Cells were plated at 5,000 cells per well (40 l per well) in black-walled clear-bottomed 384-well plates 24 h before running the assay. Medium was removed from the plates. For agonist studies 15 l of HBSS/HEPES containing compound was added to the cells and the plate was incubated at room temperature for 90 min. For antagonist studies 15 l of HBSS/HEPES containing compound was added to the cells and was incubated for 15 min before the addition of 15 l of an EC80 concentration of agonist. The plate was subsequently incubated at room temperature for 90 min. Both assays were terminated by addition of 15 l of a Beta-Glo solution (Promega). The luminescence of each well was measured to determine the level of receptor activation following an additional 30 min incubation.
Amine Transporter Assay: The amine transporter assay measures the ability of compounds to inhibit the activity of the norepinephrine (NET) dopamine (DAT) or serotonin (SERT) transporters by measuring the real time uptake of a dye labeled amine. HBSS/HEPES containing compound (5 l) was added to the wells of black walled clear bottomed 384-well plate. Transporter dye (25 l) (Molecular Devices; Cat # R8174) was added to each well. Finally 15,000 cells (20 l) stably expressing the amine transporter of interest were added to each well and the plate is incubated at 37 °C for 30 min (DAT) or 60 min (NET and SERT). The plate is transferred to the FLIPR Tetra instrument and the fluorescence of each well was measured (Ex 470–495 nM; Em 515–575 nM). The level of fluorescence measured directly relates to the level of uptake of the dye labeled amine, with a reduction in levels being related to an inhibition of the respective transporter.
Ion Channel Assays: L-type calcium channel activity was measured in the H9C2 cell line. Cells were plated at 5,000 cells per well (50 l per well) in black walled clear bottomed 384-well plates 72 h before running the assay. Medium was removed from the plates and 20 l of Hanks balanced salt solution (HBSS)/HEPES containing Calcium 5 dye (Molecular Devices; Cat # R8186) was added to each well and the plate was returned to the incubator for 1 h to allow dye loading. Compound solution (5 l) was added to each well by the FLIPR Tetra instrument to measure agonist activity of the compound by measuring the change in fluorescence from baseline over a 60 s period (Ex 470–495 nM; Em 515–575 nM). Subsequently 25 l of a high KCl buffer (in mM: KCl, 140; MgCl2, 1; HEPES, 20; Glucose, 10, CaCl2, 10) was added to each well by the FLIPR Tetra instrument to evaluate antagonist activity, with the change in fluorescence from baseline being measured over a 60 s period.
Sodium channel (Nav1.5) activity was assessed using a FLIPR based membrane potential assay using cells that stably express the Nav1.5 sodium channel subtype. Cells were plated at 7,500 cells per well (50 l per well) in black walled clear bot- tomed 384-well plates 24 h before running the assay. Medium was removed from the plates and 80 l of Hanks balanced salt solution (HBSS)/HEPES containing membrane potential dye (Molecular Devices; Cat # R8123) was added to each well and the plate was returned to the incubator for 1 h to allow dye loading. Compound solution (10 l) was added to each well by the FLIPR Tetra instru- ment to measure agonist activity of the compound by measuring the change in fluorescence from baseline over a 60 s period (Ex 510–545 nM; Em 565–625 nM). Subsequently 10 l of agonist (EC80 value) was added to each well by the FLIPR Tetra instrument to evaluate antagonist activity, with the change in fluorescence from baseline being measured over a 60 s period.
Phosphodiesterase Assays: The phosphodiesterase (PDE) assays measures the conversion of 3, 5-[3H]cAMP to 5-[3H]AMP (for PDE 1B1, 3A1, 4D3, 7B, 8B, and 10A1) or 3, 5-[3H]cGMP to 5-[3H]GMP (for PDE 2A1, 5A1,
6 (Bovine), 9A1 and 11A4) by the relevant PDE enzyme subtype. Yttrium silicate (YSi) scintillation proximity (SPA) beads bind selectively to 5-[3H] AMP or 5-[3H]GMP, hence the magnitude of radioactive counts is directly related to PDE enzymatic activity. The assay was performed in white walled opaque bottom 384-well plates. 1 l of compound in dimethyl sulfoxide was added to each well. Enzyme solution was then added to each well in buffer (in mM: Trizma, 50 (pH7.5); MgCl2, 1.3 mM). For PDE1B1 the assay buffer also included CaCl2 (30 mM) and calmodulin (25 U/ml). Subsequently, 20 l of 3,5-[3H]cGMP (125 nM) or 20 l of 3,5-[3H]cAMP (50 nM) was added to each well to start the reaction and the plate was incubated for 30 min at 25 °C. The reaction was terminated by the addition of 20 l of PDE YSi SPA beads (PerkinElmer). Following an additional 8 h incubation period the plates were read on a MicroBeta radioactive plate counter (PerkinElmer) to determine radioactive counts per well.

Off-target pharmacology data analysis. Agonist/antagonist curves were plot- ted from individual experiments, and EC50/IC50 values were determined using a four parameter logistic fit. EC50 is defined as the concentration of the test article that produced a response that was equal to 50% of the maximal system response. IC50 is defined as the concentration of the test article that produced a 50% inhibition of a maximal response. An apparent Kb value for antagonist activity was calculated using the following equation:

apparent Kb  IC50/(1+([A]/Agonist EC50 )

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where the Kb value is the dissociation constant of antagonist for the receptor, IC50 is the response produced by the test article in the presence of [A], the concentration of agonist used in the assay. Agonist EC50 is the EC50 value of the reference agonist used in the assay when tested alone.

Thermo fisher scientific SelectScreen kinase profiling service. 10 M PF-9366 was profiled against 39 kinases at two ATP concentrations: KM app (apparent ATP KM for a given kinase) and 1 mM in the Thermo Fisher Z-LYTE bio- chemical assay. Refer to Thermo Fisher custom SelectScreen Kinase Profiling Services for more information.
Statistical analysis. A Gaussian distribution was assumed for all study data. Sample sizes were chosen toyield >80% power. The s.d. of thebiochemical/cellular studies were relatively small and therefore acceptable power could be assumed from relatively small sample sizes (typically n = 3–6). Differences between treatment groups were assessed by either two-tailed t-test, or for multiple com- parison analyses, one-way ANOVA followed by the appropriate post hoc test. Dunnet’s multiple comparison test compares each treatment group mean back to the control mean, and Tukey’s multiple comparison test compares each treat- ment to every other treatment group. For HDX studies, the average error was
0.2 Da for corrected data of two replicates. Therefore, we accepted only those
changes that were greater than 6% and 0.5 Da as different from the baseline condition. These studies were neither randomized nor blinded. All statistical analyses were performed with GraphPad Prism software.
Data availability. Structure coordinates were deposited in the Protein Data Bank with accession code 5UGH. Other data that support the findings of this study are available from the corresponding author upon reasonable request.

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