Ras effector protein that functions as a guanine nucleotide exchange (GEF) for RAB5B and RAB31, by exchanging bound GDP for free GTP. Required for normal RAB31 function. {ECO:0000269|PubMed:12972505, ECO:0000269|PubMed:21586568}. This is the function of RIN3 (Ras and Rab interactor 3, ENSG00000100599).