Rab effector protein acting as linker between gamma-adaptin, RAB4A or RAB5A. Involved in endocytic membrane fusion and membrane trafficking of recycling endosomes. Stimulates nucleotide exchange on RAB5A (By similarity). Can act as a ubiquitin ligase (By similarity). 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 (By similarity). Interacts with RGS14; the interaction is GTP-dependent. Expressed in the white matter tracts of the cerebellum, the fimbria hippocampi and the corpus callosum. Monoubiquitinated.