Protein transport. Probably involved in regulated exocytosis (By similarity). Interacts with RIMS1, RIMS2, RPH3A and RPH3AL (By similarity). Interacts with RAB3IP (PubMed:7532276). Interacts with CHM; phosphorylation at Thr-86 disrupts this interaction (By similarity). Interacts with MADD (via uDENN domain); the GTP-bound form is preferred for interaction (By similarity). Highest levels found in lung. In fetal glands the majority of the proteins are methylated, whereas in neonatal and adult glands, only 50% are methylated. Phosphorylation of Thr-86 in the switch II region by LRRK2 prevents the association of RAB regulatory proteins, including CHM. Belongs to the small GTPase superfamily. Rab family.