The small GTPases Rab are key regulators of intracellular membrane trafficking, from the formation of transport vesicles to their fusion with membranes. Rabs cycle between an inactive GDP-bound form and an active GTP-bound form that is able to recruit to membranes different set of downstream effectors directly responsible for vesicle formation, movement, tethering and fusion. That Rab may play a role in protein transport from recycling endosomes to lysosomes regulating, for instance, the degradation of the transferrin receptor. Involved in autophagy (By similarity). Rab activation is generally mediated by a guanine exchange factor (GEF), while inactivation through hydrolysis of bound GTP is catalyzed by a GTPase activating protein (GAP). That Rab is activated by DENND3, a guanine exchange factor (By similarity). Interacts with RABIF and OPTN (By similarity). Interacts with LRRK2; interaction facilitates phosphorylation of Ser-105 (By similarity). Interacts with GDI1, GDI2 and CHM; these interactions are disrupted by phosphorylation on Ser-105 (By similarity). Interacts with RILPL1 and RILPL2; these interactions are dependent on phosphorylation of Ser-105 (By similarity). Highest levels in skeletal and cardiac muscle. Also found in comparable amounts in brain, spinal cord and lung. Also detected in testis where it is expressed by Sertoli cells of the seminiferous tubules (at protein level). Phosphorylation of Ser-105 in the switch II region by LRRK2 prevents the association of RAB regulatory proteins, including CHM and RAB GDP dissociation inhibitors GDI1 and GDI2. Belongs to the small GTPase superfamily. Rab family.