RABEP2

RABEP2 (Rabaptin-5β) is a coiled-coil Rab GTPase effector that participates in endosomal membrane trafficking and ciliogenesis. It binds GTP-loaded Rab5 and forms a complex with the Rab5 exchange factor Rabex-5 to promote early endosome fusion, while its interaction with Rab4 directs recycling of internalized VEGFR2 back to the plasma membrane; loss of RABEP2 traps VEGFR2 in Rab5-positive sorting endosomes (exposing it to PTP1b-mediated dephosphorylation) and diverts it to Rab7-positive late endosomes for lysosomal degradation, thereby attenuating VEGF signaling (PMID:9524116, PMID:29425100). RABEP2 also localizes to the centrosome in an SDCCAG8-dependent manner and is required for ciliogenesis (PMID:27224062).

1. 1998 High

   The discovery of Rabaptin-5β (RABEP2) as a Rab5-GTP effector that forms a distinct complex with Rabex-5 established that early endosome fusion relies on parallel, non-redundant Rab5 effector pathways rather than a single Rabaptin-5 complex.

   Evidence Yeast two-hybrid screen, co-immunoprecipitation, and in vitro endosome fusion assay with immunodepletion

   PMID:9524116

   Open questions at the time

   • Cargo specificity of the RABEP2–Rabex-5 complex was not defined
   • Whether RABEP2 participates in recycling versus degradative sorting was unknown
   • Structural basis for selective Rab5-GTP binding was not resolved
2. 2016 Medium

   Identification of RABEP2 at the centrosome, dependent on SDCCAG8, and its requirement for ciliogenesis extended RABEP2 function beyond endosome fusion to organelle biogenesis and revealed a non-endosomal cellular role.

   Evidence AP-MS proteomics of SDCCAG8, immunofluorescence localization, and siRNA knockdown of RABEP2 in hTERT-RPE1 cells with ciliogenesis readout

   PMID:27224062

   Open questions at the time

   • No independent replication of the ciliogenesis phenotype in a second cell line or organism
   • Mechanism by which RABEP2 promotes cilium assembly (e.g., vesicle delivery vs. structural role) was not determined
   • Whether the centrosomal and endosomal pools of RABEP2 are functionally coupled is unknown
3. 2018 High

   Demonstrating that RABEP2 binds Rab4 and controls VEGFR2 recycling resolved how RABEP2 connects endosomal sorting to receptor tyrosine kinase signaling output, explaining how loss of RABEP2 causes VEGFR2 misrouting, dephosphorylation by PTP1b, and lysosomal degradation.

   Evidence Reciprocal co-immunoprecipitation, siRNA knockdown, high-resolution live-cell imaging with Rab5/Rab7 markers, and VEGFR2 signaling readouts

   PMID:29425100

   Open questions at the time

   • Whether RABEP2 regulates recycling of other receptor tyrosine kinases beyond VEGFR2 is untested
   • No in vivo (animal model) validation of vascular or angiogenic phenotypes upon RABEP2 loss
   • Structural basis for the Rab4 interaction and its relationship to the Rab5/Rabex-5 complex remains uncharacterized

• It remains unknown how RABEP2's endosomal recycling and centrosomal/ciliogenesis functions are coordinated, whether RABEP2 has additional cargo clients beyond VEGFR2, and what in vivo phenotypes result from RABEP2 loss in a whole organism.
• No animal knockout or disease model for RABEP2 has been reported
• No structural data exist for RABEP2 or its complexes
• Functional relationship between Rab5- and Rab4-binding activities within RABEP2 is unresolved