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. Can act as a ubiquitin ligase (By similarity). Interacts with RGS14; the interaction is GTP-dependent (By similarity). Heterodimer with RABEP1. The heterodimer binds RAB4A and RAB5A that have been activated by GTP-binding. Interacts with RAB21, and with 100-fold lower affinity also with RAB22. Binds TSC2, GGA1, GGA2, GGA3, AP1G1 and AP1G2. Interacts with ubiquitinated EGFR. Monoubiquitinated. Readthrough transcript KCTD7-RABGEF1.