GNMT

GNMT is a homotetrameric S-adenosylmethionine (SAM)-dependent methyltransferase that transfers a methyl group from SAM to glycine, generating sarcosine and S-adenosylhomocysteine (SAH) and thereby acting as a regulatory node controlling the cellular SAM:SAH ratio and methyl-donor homeostasis (PMID:9655336). Its enzymatic output is held in check by feedback inhibition from 5-methyltetrahydrofolate, and structural and biochemical dissection shows this regulation is mediated through the N-terminal tail: truncation of residues 1-8 or a phosphomimetic at serine 9 abolishes 5-methyltetrahydrofolate binding while preserving catalytic turnover, uncoupling folate feedback from activity and—when constitutively active—depleting SAM, raising SAH, and inducing methyl-donor stress (PMID:42244597). Through this control of one-carbon flux, GNMT increases hepatic folate retention and folate-dependent homocysteine remethylation via methionine synthase (PMID:21210071) and supports folate-dependent purine/pyrimidine synthesis, limiting uracil misincorporation into DNA, with the enzyme translocating from cytoplasm to nucleus under prolonged folate depletion (PMID:23922098). Beyond catalysis, GNMT binds PREX2 and promotes its HectH9-mediated proteasomal degradation, restraining AKT signaling and proliferation (PMID:28205209), and acts as a regulator of mitochondrial Complex II activity (PMID:31668391). GNMT loss in mouse liver triggers NK-cell activation and TRAIL/DR5-mediated injury and fibrogenesis (PMID:25531568). Its expression is transcriptionally repressed by MYC at the GNMT promoter (PMID:30760754) and post-transcriptionally repressed by miR-873-5p (PMID:31668391). Benzo[a]pyrene induces serine-9 phosphorylation that drives nuclear translocation and modulates CYP1A1 expression (PMID:30987732).

1. 1998 High

   Established the architecture and catalytic identity of GNMT, defining it as a homotetrameric SAM-dependent methyltransferase that converts glycine to sarcosine and is feedback-inhibited by methyltetrahydrofolate, framing it as a regulator of the SAM:SAH ratio rather than a simple metabolic enzyme.

   Evidence X-ray crystallography of apo- and SAM-bound forms with biochemical characterization

   PMID:9655336

   Open questions at the time

   • Structural basis of folate feedback inhibition not resolved at this stage
   • No in vivo physiological role demonstrated
2. 2011 High

   Connected GNMT activity to organismal one-carbon physiology, showing it controls hepatic folate retention and folate-dependent homocysteine remethylation.

   Evidence GNMT transgenic and knockout mice with isotopic remethylation tracers and methionine synthase Western blot

   PMID:21210071

   Open questions at the time

   • Mechanism linking GNMT activity to folate retention not fully resolved
   • Does not address nucleotide-level consequences
3. 2013 High

   Extended GNMT's metabolic role to genome integrity by showing it supports nucleotide synthesis and limits uracil misincorporation, and revealed regulated nuclear translocation during folate depletion.

   Evidence Isotopic tracers, uracil assays in Gnmt genotype-series mice, fluorescence microscopy

   PMID:23922098

   Open questions at the time

   • Signal driving nuclear translocation not identified here
   • Nuclear function of GNMT not defined
4. 2014 High

   Demonstrated GNMT acts as a tunable rheostat for SAM metabolism via dFoxO-driven transcriptional induction during inflammation and fasting, linking it to energy homeostasis.

   Evidence Drosophila genetic epistasis with dFoxO/Toll pathway and hemolymph metabolomics

   PMID:24746817

   Open questions at the time

   • Conservation of dFoxO regulation in mammals not tested
   • Downstream consumers of altered SAM not mapped
5. 2014 High

   Defined the pathological consequence of GNMT loss in liver as NK-cell activation and TRAIL/DR5-mediated injury and fibrogenesis, identifying an actionable downstream axis.

   Evidence TRAIL-/-/GNMT-/- double knockout, NK depletion, and in vivo DR5 silencing

   PMID:25531568

   Open questions at the time

   • Link between SAM/SAH imbalance and NK activation not mechanistically resolved
   • Does not establish whether catalytic activity is required
6. 2017 High

   Revealed a non-catalytic scaffolding function: GNMT promotes HectH9-mediated ubiquitination and degradation of PREX2, restraining AKT signaling and proliferation.

   Evidence Reciprocal Co-IP, ubiquitination assay, siRNA depletion, Gnmt KO liver, and human HCC tissue

   PMID:28205209

   Open questions at the time

   • Whether GNMT enzymatic activity is required for PREX2 degradation unknown
   • Structural basis of GNMT-PREX2-HectH9 assembly not defined
7. 2019 Medium

   Implicated GNMT in mitochondrial bioenergetics by linking its miR-873-5p-mediated repression to impaired Complex II activity and fatty acid oxidation in NAFLD.

   Evidence miR-873-5p gain/loss in vitro and in NAFLD mouse models, Complex II assays, human NASH biopsies

   PMID:31668391

   Open questions at the time

   • Direct GNMT-Complex II molecular interaction not reconstituted
   • Whether effect is catalytic or independent of SAM metabolism unclear
8. 2019 Medium

   Identified serine-9 phosphorylation as the trigger for xenobiotic-induced nuclear translocation of GNMT, coupling its localization to BaP exposure and CYP1A1 regulation.

   Evidence Phosphoproteomics, S9A mutagenesis, fluorescence microscopy, CYP1A1 Western blot

   PMID:30987732

   Open questions at the time

   • Kinase assignment (PKC/JNK) pharmacological without direct reconstitution
   • Nuclear function downstream of translocation not defined
9. 2019 High

   Established direct transcriptional repression of GNMT by MYC, providing a mechanism for GNMT loss in hepatocellular carcinoma.

   Evidence ChIP, luciferase reporter, MYC gain/loss, and human HCC correlation

   PMID:30760754

   Open questions at the time

   • Cofactors mediating MYC repression not identified
   • Does not address combined transcriptional and post-transcriptional control
10. 2020 Medium

    Placed GNMT downstream of carnosine as a direct binding target mediating renal anti-inflammatory and anti-fibrotic protection.

    Evidence CETSA, molecular docking, GNMT overexpression/knockdown, diabetic nephropathy mouse models

    PMID:33241846

    Open questions at the time

    • Binding site and functional consequence of carnosine binding not defined
    • Single-lab finding without independent confirmation
11. 2024 Medium

    Showed GNMT protein levels are buffered by nuclear UPS-mediated degradation under SAM shortage, defining protein turnover as a homeostatic mechanism for SAM conservation.

    Evidence Drosophila genetics, SAM metabolomics, nuclear UPS inhibition, starvation assays (preprint)

    PMID:bio_10.1101_2024.08.21.609067

    Open questions at the time

    • E3 ligase mediating nuclear Gnmt degradation not identified
    • Conservation in mammals not tested
    • Preprint, not peer-reviewed
12. 2024 Medium

    Positioned GNMT in an mTOR-FOXO3-GNMT axis that consumes SAM to limit SAM-dependent histone methylation and dampen trained immunity.

    Evidence ATAC-seq, RNA-seq, metabolomics, histone-mark ChIP, AurA inhibition in trained macrophage model (preprint)

    PMID:bio_10.1101_2024.11.11.622956

    Open questions at the time

    • Direct demonstration that GNMT-driven SAM depletion causes the histone changes not isolated
    • Preprint, not peer-reviewed
13. 2026 Medium

    Resolved how the N-terminal tail and serine-9 phosphorylation uncouple folate feedback from catalysis, showing constitutive GNMT activity depletes SAM and triggers methyl-donor stress.

    Evidence Structural analysis, MD simulations, binding assays, S9 mutagenesis, liver phosphoproteomics, hepatocyte overexpression with metabolomics (preprint)

    PMID:42244597

    Open questions at the time

    • In vivo kinase responsible for S9 phosphorylation not established
    • Physiological trigger for loss of feedback control in aging not defined
    • Preprint, not peer-reviewed
14. 2025 Low

    Proposed that choline regulates GNMT through an AMPK/MYC axis to coordinate hepatic lipid and bile acid metabolism.

    Evidence GNMT knockdown, AMPK inhibition, transcriptomics in calf and human hepatocytes

    PMID:41233636

    Open questions at the time

    • AMPK/MYC/GNMT axis inferred from pharmacology, not directly reconstituted
    • Single-lab, low-confidence finding
    • Mechanism linking GNMT to lipid/bile gene expression undefined

• Whether GNMT's catalytic methyltransferase activity is required for its non-enzymatic functions (PREX2 degradation, Complex II regulation, nuclear signaling) remains unresolved.
• No experiment separates catalytic-dead GNMT from scaffolding roles
• Nuclear function of translocated GNMT undefined
• Mammalian relevance of Drosophila UPS-buffering not tested