SAMTOR

SAMTOR is a cytoplasmic S-adenosylmethionine (SAM) sensor that couples methionine and one-carbon metabolism to mTORC1-dependent cell growth control by acting as a negative regulator of the pathway (PMID:29123071). SAM binds directly to SAMTOR with a Kd of ~7 µM; when methionine starvation drops intracellular SAM below this threshold, SAMTOR associates with the GATOR1 complex (the GAP for RagA/B), inhibiting mTORC1, whereas rising SAM disrupts the SAMTOR-GATOR1 interaction and relieves this inhibition, restoring mTORC1 lysosomal recruitment and signaling (PMID:29123071, PMID:34061446). Structurally, SAMTOR comprises an N-terminal helical domain and a C-terminal SAM-dependent methyltransferase (MTase) domain that carries both the SAM-binding site and the GATOR1-KICSTOR-binding surface; SAM binding drives a conformational switch in the helical domain that modulates engagement of the GATOR1-KICSTOR complex (PMID:35776786). Within the assembled KICSTOR-GATOR1-SAMTOR supercomplex, SAMTOR binds KICSTOR in a manner mutually exclusive with SAM/metabolite binding, and this architecture restrains GATOR1 in an orientation favoring active GAP engagement of the Rag GTPases (PMID:41512879). Through this SAM-gated switch SAMTOR controls downstream mTORC1 effectors including S6K1 and CAD, linking methionine availability to DNA synthesis and proliferation (PMID:33179842); the inhibitory role is conserved in Drosophila, where dSAMTOR knockdown elevates dp70S6K activity (PMID:37298625).

1. 2017 High

   Established SAMTOR as the molecular sensor that converts intracellular SAM levels into mTORC1 activity, answering how methionine status is read by the growth machinery.

   Evidence Co-IP, in vitro radioligand binding with Kd determination, metabolomics, SAMTOR KO/knockdown, SAM-binding mutagenesis, and lysosomal localization assays

   PMID:29123071

   Open questions at the time

   • Did not resolve the structural basis of the SAM-induced conformational change
   • GATOR1 vs KICSTOR contributions to the binding interface not separated
2. 2021 Medium

   Showed that a physiological metabolic input (betaine) raises SAM to disrupt the SAMTOR-GATOR1 complex and drive mTORC1 to the lysosome, connecting dietary one-carbon flux to the sensor.

   Evidence SAMTOR overexpression in C2C12 cells, lysosomal co-localization of mTORC1, UHPLC metabolomics for SAM, and signaling readouts

   PMID:34061446

   Open questions at the time

   • Relies on overexpression rather than endogenous SAMTOR
   • Does not establish direct SAM binding under the betaine condition
3. 2020 Medium

   Placed SAMTOR within a methionine-cycle proliferation axis, linking it to MAT2A/CBS upstream and S6K1/CAD-driven DNA synthesis downstream.

   Evidence Gene silencing of CBS and MAT2A with transcriptome and mTORC1/S6K1/CAD pathway readouts in Ishikawa, pTr, and JAR cells

   PMID:33179842

   Open questions at the time

   • Correlational link between MAT2A and SAMTOR expression, not a direct biochemical mechanism
   • Direction of SAMTOR's effect on mTORC1 here differs from canonical inhibitory role and is not reconciled
4. 2022 High

   Defined the two-domain architecture of SAMTOR and identified the helical domain as the SAM-responsive molecular switch governing GATOR1-KICSTOR engagement.

   Evidence X-ray crystallography of apo and SAM-bound Drosophila SAMTOR with site-directed mutagenesis and mTORC1 activity assays

   PMID:35776786

   Open questions at the time

   • Structures are of the Drosophila ortholog, not human SAMTOR
   • No structure of SAMTOR bound to GATOR1-KICSTOR in this study
5. 2023 Medium

   Confirmed evolutionary conservation of SAMTOR's inhibitory function by showing dSAMTOR loss derepresses dTORC1 signaling in vivo.

   Evidence GAL4/UAS transgenic dSAMTOR downregulation in Drosophila with PamGene dp70S6K kinase profiling and behavioral/survival assays

   PMID:37298625

   Open questions at the time

   • SAM-dependence of the in vivo phenotype not directly tested
   • Tissue-specific versus lethal phenotypes from different RNAi schemes not mechanistically explained
6. 2025 Low

   Probed a context-specific role for SAMTOR in cancer, linking it to phospho-AMPK and methionine-deprivation vulnerability.

   Evidence Methionine deprivation, proteomics, and SAMTOR-p-AMPK co-interaction assays in AMPK-deficient PC3 prostate cancer cells

   PMID:40427696

   Open questions at the time

   • Single-lab, single-method co-interaction without reciprocal or structural validation
   • Functional consequence of the SAMTOR-p-AMPK interaction not mechanistically dissected
7. 2025 Medium

   Operationalized the SAM-SAMTOR binding measurement into a generalizable quantitative protocol, reinforcing the affinity-based sensing model.

   Evidence Recombinant SAMTOR purification, radioactive SAM binding with scintillation counting, and mathematical Kd calculation

   PMID:39992509

   Open questions at the time

   • A methods/validation paper rather than new mechanistic insight
   • Does not extend binding analysis to mutants or other ligands
8. 2026 High

   Resolved the full KICSTOR-GATOR1-SAMTOR supercomplex, explaining how SAMTOR's KICSTOR binding is mutually exclusive with metabolite sensing and orients GATOR1 for active GAP function.

   Evidence Cryo-EM structure determination of the ~60-nm supercomplex with interface mutagenesis and mTORC1 activity assays

   PMID:41512879

   Open questions at the time

   • Captures a single conformational state; dynamics of the SAM-driven transition not visualized
   • How SAM binding allosterically displaces SAMTOR from KICSTOR not directly observed

• How the SAM-induced helical-domain switch is transmitted in real time to displace SAMTOR from the KICSTOR-GATOR1 supercomplex, and how SAMTOR signaling integrates with AMPK and proliferative pathways in disease, remain unresolved.
• No time-resolved or human full-length structure capturing the SAM-bound-to-displaced transition
• SAMTOR-AMPK crosstalk lacks mechanistic depth
• Whether SAMTOR's MTase domain has any catalytic activity is unaddressed