MAT2B

MAT2B is the regulatory beta subunit of methionine adenosyltransferase II that lacks intrinsic catalytic activity but governs cellular S-adenosylmethionine (SAM/AdoMet) output and serves as a signaling scaffold to drive cell growth (PMID:11337507, PMID:23325601). It controls SAM synthesis by binding and stabilizing the catalytic MAT2A subunit in an NADP+-dependent manner, coupling the pentose phosphate pathway to MAT2A protein levels; this MAT2B–MAT2A interaction also regulates RNA m6A modification, and disrupting it lowers MAT2A and suppresses liver tumor growth (PMID:39353892). Consistent with its role as a metabolic brake, lowering MAT2B raises intracellular AdoMet several-fold (PMID:11337507, PMID:18698677). Independent of catalysis, MAT2B acts as an adaptor that directly binds GIT1, MEK1, and ERK2 to assemble and activate a MEK/ERK module, and directly binds AKT to activate AKT/ERK1/2 signaling, raising cyclin D1 and promoting proliferation (PMID:23325601, PMID:26940012). Its expression is transcriptionally driven by Sp3 acting at an Sp1 site and TATA element in its promoter (PMID:11337507). Through these activities MAT2B supports tumor cell proliferation and survival and restrains apoptosis — knockdown downregulates cyclin D1 and Bcl-xL, upregulates Bcl-xS, and induces apoptosis selectively in hepatocellular carcinoma cells while sparing normal liver, and silencing suppresses HCC migration, invasion, and metastasis by reducing EGFR-pathway signaling (PMID:18698677, PMID:31493275). MAT2B is itself a tractable drug target: the small molecule JX24120 directly binds MAT2B and inhibits SAMe synthesis, suppressing mTORC1 signaling and inducing apoptosis in a MAT2B-dependent manner (PMID:39293586).

1. 2001 High

   Established that MAT2B is a regulatory subunit constraining SAM synthesis and identified the transcription factor controlling its expression, defining its metabolic role.

   Evidence Promoter deletion, ChIP, supershift, site mutagenesis and reporter assays; AdoMet measurement after knockdown

   PMID:11337507

   Open questions at the time

   • Did not define how the beta subunit mechanistically limits catalytic flux
   • No structural basis for MAT2A regulation
2. 2008 Medium

   Showed MAT2B is a tumor-selective dependency, linking its loss to elevated SAMe, apoptosis, and growth arrest via cyclin D1 and Bcl-xL/S.

   Evidence shRNA knockdown in HCC cells with proliferation, apoptosis, SAMe and western readouts; normal liver cells as control

   PMID:18698677

   Open questions at the time

   • Mechanism connecting MAT2B to cyclin D1/Bcl-x regulation not resolved
   • Whether effects are SAM-dependent or scaffold-dependent unclear
3. 2013 High

   Revealed a catalysis-independent scaffold function: MAT2B directly assembles GIT1/MEK1/ERK2 to activate MEK/ERK and drive growth and tumorigenesis.

   Evidence Reciprocal Co-IP, recombinant/in vitro-translated pull-downs, overexpression/knockdown, in-solution proteomics, orthotopic liver cancer model

   PMID:23325601

   Open questions at the time

   • Structural basis of the scaffold complex not defined
   • Relative contribution of scaffold vs metabolic role to tumorigenesis not separated
4. 2016 Medium

   Extended the scaffold model to a direct AKT interaction, showing MAT2B activates AKT/ERK1/2 signaling in a developmental (adipogenic) context.

   Evidence Co-IP, overexpression/knockdown, SAMe measurement, phosphorylation assays, PI3K inhibitor rescue in porcine preadipocytes

   PMID:26940012

   Open questions at the time

   • Direct AKT binding not confirmed by reciprocal/in vitro reconstitution
   • Whether AKT activation requires SAM changes unresolved
5. 2019 Medium

   Placed MAT2B upstream of broad receptor-kinase signaling, showing it is required for HCC invasion and metastasis via EGFR-pathway phosphorylation.

   Evidence Stable shRNA knockdown, phospho-kinase array, zebrafish and nude mouse metastasis models

   PMID:31493275

   Open questions at the time

   • Direct vs indirect control of EGFR/Src/FAK/STAT3 not distinguished
   • No biochemical link to the receptor module
6. 2024 High

   Defined the molecular logic of MAT2B's metabolic role: NADP+-dependent binding stabilizes MAT2A, coupling the pentose phosphate pathway to SAM output and RNA m6A.

   Evidence Co-IP, protein stability assays, NADP+ manipulation, m6A-seq, genetic interaction disruption, in vivo liver tumor models including ketogenic diet

   PMID:39353892

   Open questions at the time

   • Structural detail of NADP+-dependent interface not resolved
   • Direct enzyme(s) writing m6A downstream not mapped
7. 2024 Medium

   Demonstrated MAT2B is directly druggable, with a small molecule that binds it and inhibits SAMe synthesis to suppress mTORC1 and induce apoptosis.

   Evidence Kd binding measurement, patient-derived organoid screening, gene editing, mTORC1 signaling and in vivo assays in endometrial cancer

   PMID:39293586

   Open questions at the time

   • Binding site on MAT2B not mapped
   • Whether inhibition acts via metabolic or scaffold function unclear

• How MAT2B's two distinct activities — SAM-controlling MAT2A stabilization and the kinase-scaffolding function — are coordinated, and whether structural interfaces and partners (e.g. BAG3, FMO4, IGF2BP3) generalize beyond single reports, remains unresolved.
• No structure of MAT2B in its scaffold complex
• Scaffold vs metabolic contributions to phenotypes not genetically separated
• Several reported partners rest on single Co-IP or preprint evidence