RMC1

RMC1 (C18orf8/WDR98) is a subunit of the metazoan CCZ1-MON1 RAB7 guanine nucleotide exchange factor (GEF) complex that positively regulates RAB7 recruitment to late endosomes and autophagosomes, thereby promoting autophagic flux (PMID:29038162). Within this trimeric Mon1-Ccz1-RMC1 complex, RMC1 acts as a membrane-recruitment mediator, enabling complex association with membranes through electrostatic protein-lipid interactions via an interface distinct from that of the dimeric fungal Mon1-Ccz1 complex [PMID:bio_10.1101_2025.03.27.645700]. Beyond its role as a positive regulator of RAB7 GEF activity and membrane engagement of the GEF complex, no further mechanistic detail has been characterized in the available corpus.

1. 2017 High

   Before this work the metazoan RAB7 GEF was thought to be a Mon1-Ccz1 dimer; identifying RMC1 as a third subunit established a previously unrecognized component coupling RAB7 activation to autophagic flux.

   Evidence Interaction proteomics (AP-MS) in GABARAP-family-deficient cells with genetic loss-of-function and quantitative autophagosome proteomics

   PMID:29038162

   Open questions at the time

   • Did not define the structural interface between RMC1 and Mon1-Ccz1
   • Mechanism by which RMC1 promotes RAB7 recruitment at the molecular level was not resolved
   • Did not establish how the complex is targeted to specific membranes
2. 2025 Medium

   It was unknown why metazoans require a third subunit absent from fungal Mon1-Ccz1; reconstitution and protein-lipid studies showed RMC1 confers membrane binding through electrostatic interactions, defining its functional contribution to the trimer.

   Evidence Structural comparison, protein-lipid interaction assays, and reconstitution of dimeric (fungal) versus trimeric (metazoan) complexes (preprint)

   PMID:bio_10.1101_2025.03.27.645700

   Open questions at the time

   • Preprint, not yet peer-reviewed
   • Precise lipid specificity and residues mediating the electrostatic interface not fully defined
   • Whether membrane recruitment is regulated in cells is not addressed

• How RMC1-mediated membrane recruitment is spatially and temporally regulated to coordinate RAB7 activation at endosomes versus autophagosomes remains unresolved.
• No regulatory inputs controlling RMC1 membrane engagement identified
• No high-resolution structure of the membrane-bound trimer in the corpus