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). Forms a complex with RABGEF1 that stimulates RABGEF1-mediated nucleotide exchange on RAB5A (PubMed:9323142). Mediates the traffic of PKD1:PKD2 complex from the endoplasmic reticulum through the Golgi to the cilium (By similarity). {ECO:0000250|UniProtKB:O35551, ECO:0000269|PubMed:10698684, ECO:0000269|PubMed:11452015, ECO:0000269|PubMed:12773381, ECO:0000269|PubMed:22841712, ECO:0000269|PubMed:8521472, ECO:0000269|PubMed:9323142}. This is the function of Ensembl gene identifier ENSG00000029725 (RABEP1, rabaptin, RAB GTPase binding effector protein 1).