Guanine nucleotide exchange factor (GEF) which may activate RAB8A and RAB8B (PubMed:12221131, PubMed:26824392). Promotes the exchange of GDP to GTP, converting inactive GDP-bound Rab proteins into their active GTP-bound form (PubMed:12221131, PubMed:26824392). Mediates the release of GDP from RAB8A and RAB8B but not from RAB3A or RAB5 (PubMed:20937701, PubMed:26824392). Modulates actin organization and promotes polarized transport of RAB8A-specific vesicles to the cell surface (PubMed:12221131). Together with RAB11A, RAB8A, the exocyst complex, PARD3, PRKCI, ANXA2, CDC42 and DNMBP promotes transcytosis of PODXL to the apical membrane initiation sites (AMIS), apical surface formation and lumenogenesis (PubMed:20890297). Part of the ciliary targeting complex containing Rab11, ASAP1, RAB3IP and RAB11FIP3 and ARF4 that promotes RAB3IP preciliary vesicle trafficking to mother centriole and ciliogenesis initiation (PubMed:25673879, PubMed:31204173). {ECO:0000269|PubMed:12221131, ECO:0000269|PubMed:20890297, ECO:0000269|PubMed:20937701, ECO:0000269|PubMed:25673879, ECO:0000269|PubMed:26824392, ECO:0000269|PubMed:31204173}. This is the function of Ensembl gene identifier ENSG00000127328 (RAB3IP, RAB3A interacting protein).