Rab effector protein acting as linker between gamma-adaptin and RAB5A. Involved in endocytic membrane fusion and membrane trafficking of recycling endosomes. Stimulates nucleotide exchange on RAB5A. Can act as a ubiquitin ligase. Heterodimer with RABEP1. The heterodimer binds RAB4A and RAB5A that have been activated by GTP-binding. Binds TSC2, GGA1, GGA2, GGA3, AP1G1 and AP1G2 (By similarity). Interacts with RAB21, and with 100-fold lower affinity also with RAB22 (By similarity). Interacts with ubiquitinated EGFR. Interacts with RGS14; the interaction is GTP-dependent (By similarity). Detected in brain. Monoubiquitinated.