RABEP1 acts as a Rab effector, a linker between gamma-adaptin, RAB4A and RAB5A. It is involved in endocytic membrane fusion and in the membrane trafficking of recycling endosomes. RABEP1 forms a complex with RABGEF1 that stimulates RABGEF1-mediated nucleotide exchange on RAB5A, connecting effector binding to activation of this small GTPase.

The protein also contributes to the trafficking of specific cargo. It is involved in the movement of KCNH1 channels to and from the cell membrane, and it mediates the traffic of the PKD1:PKD2 complex from the endoplasmic reticulum through the Golgi to the cilium. According to UniProt, RABEP1 is found in the cytoplasm, early endosomes, recycling endosomes and cytoplasmic vesicles. NCBI Gene annotation adds that it enables protein domain-specific binding and protein homodimerisation, is involved in vesicle-mediated transport, and is part of a protein-containing complex.

The RABEP1 gene encodes a member of the rabaptin family. The protein is known as rabaptin-5, and also by the names neurocrescin, RAB5EP and RABPT5. Mouse and rat orthologs are both designated Rabep1. A related family member, RABEP2, is also studied in endosomal trafficking.