RAB Guanine Nucleotide Exchange Factor (RABGEF1)

RABGEF1 forms a complex with rabaptin-5 (RABPT5) that is required for endocytic membrane fusion, and it serves as a specific guanine nucleotide exchange factor (GEF) for RAB5 (RAB5A). The deduced 492-amino acid protein has a calculated molecular mass of 57.1 kD and shares significant homology with yeast Vps9, a protein implicated in endocytic traffic. A 60-kD RABGEF1 protein was detected in HeLa cells, bovine brain, and all mouse tissues examined, suggesting that it is ubiquitously expressed.RABGEF1-RABPT5 complex was essential for both homotypic and heterotypic endosome fusions. Both RABGEF1 and RABPT5 bound RAB5 preferentially in the presence of GTP rather than GDP. RABGEF1 displayed specific GDP/GTP exchange activity on RAB5 upon delivery of RAB5 to the membrane.