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 (PubMed:22841712). Stimulates RABGEF1 mediated nucleotide exchange on RAB5A. Mediates the traffic of PKD1:PKD2 complex from the endoplasmic reticulum through the Golgi to the cilium (By similarity). Homodimer when bound to RAB5A (PubMed:15378032). Heterodimer with RABGEF1 (PubMed:11452015, PubMed:9323142). The heterodimer binds RAB4A and RAB5A that have been activated by GTP-binding (PubMed:10698684, PubMed:8521472, PubMed:20098723, PubMed:12773381). Interacts with TSC2 (PubMed:9045618). Interacts with GGA1 (via GAE domain), GGA2 (via GAE domain) and GGA3 (via GAE domain) (PubMed:12505986, PubMed:14665628). Interacts with AP1G1 (via GAE domain) (PubMed:12505986, PubMed:12773381, PubMed:14665628). Interacts with AP1G2 (via GAE domain) (PubMed:12505986, PubMed:14665628). Interacts with ECPAS (PubMed:20682791). 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 (By similarity). Proteolytic cleavage by caspases in apoptotic cells causes loss of endosome fusion activity. Belongs to the rabaptin family.