LAMTOR3

LAMTOR3 (MP1) is a scaffold subunit of the Ragulator complex at late endosomes/lysosomes, where it couples MAPK and mTORC1 signaling to cell proliferation and survival (PMID:23653355, PMID:28606806). It is stabilized through obligate heterodimerization with LAMTOR2 (p14); when LAMTOR2 is lost, monomeric LAMTOR3 is mislocalized to the cytosol and rapidly destroyed by proteasome-dependent ubiquitination of multiple lysine residues, establishing complex assembly as the determinant of its stability (PMID:23653355). Functionally, LAMTOR3 scaffolds the MEK1/ERK1 axis: elevated LAMTOR3 hyperphosphorylates MEK and ERK independently of Ras or Raf mutations to drive proliferation, and in mesenchymal cancer cells it sustains ERK1 activity and downstream BCL2 expression to confer chemoradioresistance (PMID:24209743, PMID:28606806). LAMTOR3 abundance is set post-transcriptionally by miRNAs targeting its 3'-UTR—miR-8485 and miR-1287-5p—whose loss-of-LAMTOR3 effects reduce mTOR activity, promote autophagy, and suppress proliferation, migration, and invasion (PMID:35959437, PMID:41088225).

1. 2013 High

   Established that LAMTOR3 stability is governed by its incorporation into the Ragulator complex rather than being intrinsic, defining heterodimerization with LAMTOR2 as the gatekeeper of its function in MAPK and mTORC1 signaling.

   Evidence Lysine mutagenesis, proteasome inhibition, subcellular fractionation, and LAMTOR2 depletion at late endosomes/lysosomes

   PMID:23653355

   Open questions at the time

   • E3 ligase mediating LAMTOR3 ubiquitination not identified
   • structural basis of the LAMTOR2-LAMTOR3 interface not resolved here
2. 2013 Medium

   Showed that transcriptional upregulation of LAMTOR3 can drive MEK/ERK hyperactivation and tumor cell proliferation without canonical Ras/Raf mutations, positioning LAMTOR3 dosage as an oncogenic input to the MAPK pathway.

   Evidence Transcriptional reporter assays, MEK/ERK phosphorylation readouts, siRNA knockdown in pancreatic cancer cells, and PanIN mouse models

   PMID:24209743

   Open questions at the time

   • mechanism by which LAMTOR3 scaffolds and amplifies MEK/ERK phosphorylation not detailed
   • single lab
3. 2017 Medium

   Connected LAMTOR3 scaffolding of MEK1/ERK1 to a survival output (BCL2), explaining how LAMTOR3 supports chemoradioresistance in mesenchymal cancer cells.

   Evidence siRNA knockdown, phospho-ERK1 and BCL2 Western blots, MEK1 inhibitor treatment, and cell viability assays

   PMID:28606806

   Open questions at the time

   • single approach for mechanistic placement
   • direct LAMTOR3-MEK1 binding not demonstrated in this context
4. 2022 Medium

   Identified LAMTOR3 as a direct miRNA target whose suppression lowers mTOR activity and induces autophagy, linking LAMTOR3 dosage control to the mTORC1 arm of Ragulator signaling.

   Evidence Dual-luciferase 3'-UTR reporter assay, qRT-PCR, autophagy marker (ATG13, LC3-II) Western blots, and autophagy inhibitor rescue in ovarian cancer cells

   PMID:35959437

   Open questions at the time

   • whether miR-8485 effects are entirely LAMTOR3-dependent not fully isolated
   • single lab
5. 2025 Medium

   Confirmed a second miRNA (miR-1287-5p) controlling LAMTOR3 and demonstrated by rescue that LAMTOR3 is the effector driving proliferation, migration, and invasion downstream of this regulatory axis.

   Evidence Dual-luciferase reporter assay, qRT-PCR, rescue overexpression, CCK-8, wound-healing, transwell, and flow cytometry in gastric cancer cells

   PMID:41088225

   Open questions at the time

   • downstream signaling pathway not dissected in this study
   • single lab

• How LAMTOR3 mechanistically distinguishes and partitions its MEK1/ERK1 scaffolding function from its mTORC1-activating function within the Ragulator remains unresolved.
• no structural model of LAMTOR3 engaging MEK1 versus mTORC1 machinery
• E3 ligase and full ubiquitination code controlling monomer turnover unknown
• physiological (non-cancer) roles uncharacterized