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