Acts as a guanine-nucleotide releasing factor (GEF) for RAB5A by promoting the conversion of inactive RAB-GDP to the active form RAB- GTP (PubMed:15339665, PubMed:9323142). Forms a complex with RABEP1 that acts as Rab4/Rab5 effector regulating endosome fusion. Involved in endocytic membrane fusion and membrane trafficking of recycling endosomes (PubMed:11452015, PubMed:9323142). Also functions as a ubiquitin ligase (By similarity). {ECO:0000250|UniProtKB:O18973, ECO:0000269|PubMed:11452015, ECO:0000269|PubMed:15339665, ECO:0000269|PubMed:9323142}. This is the function of Ensembl gene identifier ENSG00000154710 (RABGEF1, RAB guanine nucleotide exchange factor 1).