MON1B

MON1B forms a heterodimeric complex with CCZ1 that functions as a RAB5 effector and guanine nucleotide exchange factor (GEF) for RAB7, driving the conversion of RAB5-positive endosomes/phagosomes to RAB7-positive compartments during endosomal and phagosomal maturation (PMID:20305638). The MON1B–CCZ1 complex selectively binds GTP-loaded RAB5 on membranes and recruits and activates RAB7; neither subunit alone is sufficient for RAB7 binding (PMID:20305638). At early endosomes, MON1B is recruited from the cytosol by the adaptor protein Numb, where it mediates homotypic early-endosome docking and fusion; loss of either Numb or MON1B results in clustered but unfused early endosomes (PMID:26987402).

1. 2010 High

   Establishing MON1B as a RAB5-to-RAB7 conversion factor resolved how endosomal identity transitions are controlled: the MON1B–CCZ1 complex reads RAB5-GTP on early compartments and activates RAB7 to drive phagosome maturation.

   Evidence Genetic epistasis in C. elegans combined with mammalian RNAi, co-immunoprecipitation, and GTPase-binding assays comparing GTP- versus GDP-locked RAB5/RAB7

   PMID:20305638

   Open questions at the time

   • Structural basis for how the MON1B–CCZ1 complex catalyzes nucleotide exchange on RAB7 was not determined
   • Whether MON1A and MON1B are functionally redundant or context-specific was not addressed
   • Membrane recruitment mechanism for the complex independent of Numb was unknown
2. 2016 High

   Identifying Numb as the membrane recruiter of MON1B at early endosomes answered how MON1B reaches its site of action and revealed an unexpected role for MON1B in homotypic early-endosome fusion upstream of the RAB5-to-RAB7 switch.

   Evidence Reciprocal co-immunoprecipitation, siRNA knockdown of Numb and MON1B, live-cell imaging of early-endosome fusion, and isoform-specific rescue experiments

   PMID:26987402

   Open questions at the time

   • The binding interface between Numb and MON1B has not been mapped
   • Whether MON1B's role in EE fusion requires CCZ1 or RAB7 GEF activity was not dissected
   • Physiological consequences of impaired MON1B-dependent EE fusion in vivo remain uncharacterized
3. 2018 Medium

   Negative interaction data with mycobacterial PknG placed MON1B's phagosome maturation function outside the PknG–RAB7L1 virulence axis, refining the pathway wiring of phagolysosomal biogenesis.

   Evidence Co-immunoprecipitation testing PknG against MON1B and other phagolysosomal factors

   PMID:30037848

   Open questions at the time

   • Only a single Co-IP approach was used; orthogonal interaction assays were not reported
   • Whether other mycobacterial effectors target MON1B remains untested
4. 2018 Low

   MON1B knockdown in colon cancer cells linked the gene to proliferation and invasion phenotypes, but the biochemical mechanism connecting endosomal trafficking to NF-κB/MMP signaling was not established.

   Evidence siRNA knockdown in LoVo cells with proliferation, migration, and invasion assays plus RT-qPCR/Western blot for downstream markers

   PMID:30368516

   Open questions at the time

   • Single knockdown approach in one cell line without rescue or specificity controls
   • No direct biochemical link between MON1B's GEF activity and NF-κB regulation was demonstrated
   • Not independently replicated

• Key open questions include the structural basis of the MON1B–CCZ1 GEF complex on human RAB7, whether MON1A and MON1B are functionally redundant, and the molecular mechanism linking endosomal MON1B activity to downstream signaling pathways.
• No high-resolution structure of the human MON1B–CCZ1–RAB7 complex exists
• Functional redundancy between MON1A and MON1B has not been systematically tested
• In vivo physiological roles (development, immunity) of MON1B loss are uncharacterized