SAMTOR

SAMTOR functions as an intracellular S-adenosylmethionine (SAM) sensor that couples methionine availability to mTORC1 signaling. SAMTOR possesses an inactive methyltransferase-fold domain that directly binds SAM with a Kd of ~7 µM; when intracellular SAM falls below this threshold during methionine starvation, SAM dissociates, triggering a conformational change in SAMTOR's N-terminal helical domain that promotes its association with the KICSTOR–GATOR1 complex at the lysosome, thereby stimulating GATOR1 GAP activity toward RagA/B GTPases and suppressing mTORC1 lysosomal recruitment and activation (PMID:29123071, PMID:35776786). Cryo-EM of the KICSTOR–GATOR1–SAMTOR supercomplex reveals that SAMTOR binds KICSTOR in a conformation incompatible with SAM occupancy, and the dimeric supercomplex orients GATOR1 to favor its active GAP mode while sterically restricting inhibitory high-affinity Rag engagement (PMID:41512879). This SAM-sensing mechanism is conserved in Drosophila, where SAMTOR knockdown elevates TORC1/S6K signaling in vivo (PMID:37298625).

1. 2017 High

   Identification of SAMTOR as a direct SAM-binding protein that inhibits mTORC1 through GATOR1 interaction established the missing molecular link between methionine/SAM levels and Rag GTPase–dependent mTORC1 regulation.

   Evidence In vitro radiolabeled SAM binding (Kd ~7 µM), reciprocal co-IP, SAMTOR knockout/knockdown with mTORC1 readouts, and SAM-binding-deficient mutant rescue in human cells

   PMID:29123071

   Open questions at the time

   • No structural information on how SAM binding alters SAMTOR conformation
   • Whether SAMTOR contacts KICSTOR or only GATOR1 was unresolved
   • The stoichiometry and architecture of the full signaling complex at the lysosome remained unknown
2. 2020 Medium

   Placing MAT2A upstream of SAMTOR and connecting SAMTOR to S6K1/CAD phosphorylation extended the pathway from methionine metabolism through SAMTOR/mTORC1 to nucleotide synthesis during embryo implantation.

   Evidence siRNA knockdown of MAT2A and CBS with downstream SAMTOR expression and mTORC1/S6K1/CAD signaling readouts in cell lines

   PMID:33179842

   Open questions at the time

   • No direct SAMTOR binding assay performed; pathway placement inferred from knockdown epistasis
   • In vivo relevance to implantation not tested with SAMTOR-specific perturbation
3. 2021 Medium

   Demonstrating that SAMTOR overexpression displaces mTORC1 from the lysosome, and that betaine-driven SAM elevation reverses this, provided direct evidence that SAMTOR controls mTORC1 through lysosomal recruitment dynamics.

   Evidence SAMTOR overexpression in C2C12 cells with immunofluorescence for mTOR–lysosome colocalization and UHPLC SAM quantification

   PMID:34061446

   Open questions at the time

   • Overexpression system; endogenous SAMTOR levels not manipulated
   • Betaine effects on SAM are indirect and could affect other pathways
4. 2022 High

   Crystal structures of apo and SAM-bound Drosophila SAMTOR revealed how SAM occupancy in the MTase domain allosterically rearranges the N-terminal helical domain, providing the structural mechanism for the SAM-sensing conformational switch.

   Evidence X-ray crystallography of apo and SAM-bound dSAMTOR, with mutagenesis of key residues validated by mTORC1 signaling assays

   PMID:35776786

   Open questions at the time

   • Structures were of the Drosophila ortholog; human SAMTOR structure not yet determined
   • The GATOR1/KICSTOR-binding interface was mapped to the MTase domain but not visualized in a complex structure
5. 2023 Medium

   In vivo knockdown of Drosophila SAMTOR confirmed conservation of its inhibitory role on TORC1/S6K signaling, extending the mechanism beyond cultured mammalian cells.

   Evidence GAL4/UAS-driven tissue-specific RNAi in Drosophila brain with PamGene kinase activity profiling of dp70S6K

   PMID:37298625

   Open questions at the time

   • Behavioral and survival phenotypes observed but mechanistic connection to SAM levels not directly tested in flies
   • Whether fly SAMTOR also requires KICSTOR for GATOR1 interaction was not examined
6. 2026 High

   Cryo-EM of the full KICSTOR–GATOR1–SAMTOR supercomplex resolved the architectural basis for methionine sensing: SAMTOR binds KICSTOR in a SAM-incompatible conformation, and the dimeric supercomplex positions GATOR1 to favor its GAP-active mode toward Rag GTPases.

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

   PMID:41512879

   Open questions at the time

   • Dynamic transition between SAM-bound (free) and apo (KICSTOR-engaged) states not captured in time-resolved experiments
   • Whether additional regulatory inputs modulate the supercomplex assembly is unknown
   • The structural basis for how GATOR1 GAP activity is catalytically stimulated by KICSTOR engagement has not been resolved at atomic detail

• Whether SAMTOR has additional mTORC1-independent signaling roles — such as the reported AMPK interaction — and how the SAMTOR-dependent SAM-sensing mechanism integrates with other amino acid sensing branches in vivo remain unresolved.
• The reported SAMTOR–phospho-AMPK interaction lacks mechanistic resolution and independent confirmation
• No in vivo SAMTOR knockout phenotype in mammals has been reported
• Whether SAMTOR SAM-binding Kd is tuned by post-translational modifications is unknown