Rab effector protein acting as linker between gamma-adaptin, RAB4A and RAB5A. Involved in endocytic membrane fusion and membrane trafficking of recycling endosomes. Involved in KCNH1 channels trafficking to and from the cell membrane. Stimulates RABGEF1 mediated nucleotide exchange on RAB5A. Mediates the traffic of PKD1:PKD2 complex from the endoplasmic reticulum through the Golgi to the cilium. Heterodimer with RABGEF1. The heterodimer binds RAB4A and RAB5A that have been activated by GTP-binding. Interacts with TSC2 (By similarity). Interacts with GGA1 (via GAE domain), GGA2 (via GAE domain) and GGA3 (via GAE domain) (By similarity). Interacts with AP1G1 (via GAE domain) (By similarity). Interacts with AP1G2 (via GAE domain) (By similarity). Interacts with ECPAS (By similarity). Interacts with KCNH1 (By similarity). Interacts with PKD1 (via C-terminal domain) and GGA1; the interactions recruit PKD1:PKD2 complex to GGA1 and ARL3 at trans-Golgi network (By similarity). Interacts with KCNH1 (PubMed:22841712). Proteolytic cleavage by caspases in apoptotic cells causes loss of endosome fusion activity. Belongs to the rabaptin family.