Guanine nucleotide exchange factor (GEF) which may activate RAB8A and RAB8B. Promotes the exchange of GDP to GTP, converting inactive GDP-bound Rab proteins into their active GTP-bound form. Mediates the release of GDP from RAB8A and RAB8B but not from RAB3A or RAB5. Modulates actin organization and promotes polarized transport of RAB8A-specific vesicles to the cell surface. 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.