Plays a role in membrane trafficking and in homotypic early endosome fusion (PubMed:9524116). Participates in arteriogenesis by regulating vascular endothelial growth factor receptor 2/VEGFR2 cell surface expression and endosomal trafficking (PubMed:29425100). By interacting with SDCCAG8, localizes to centrosomes and plays a critical role in ciliogenesis (PubMed:27224062). Heterodimer with RABGEF1. The dimer binds RAB5A that has been activated by GTP-binding. Interacts with SDCCAG8; this interaction is important for ciliogenesis regulation (PubMed:27224062). Interacts with RAB4A; this interaction may mediate VEGFR2 cell surface expression (PubMed:29425100). Belongs to the rabaptin family.