Small GTPase which cycles between active GTP-bound and inactive GDP-bound states. In its active state, binds to a variety of effector proteins to regulate homeostasis of late endocytic pathway, including endosomal positioning, maturation and secretion. Plays a role in cytotoxic granule exocytosis in lymphocytes. Required for both granule maturation and granule docking and priming at the immunologic synapse. GTP + H2O = GDP + H(+) + phosphate Regulated by guanine nucleotide exchange factors (GEFs) which promote the exchange of bound GDP for free GTP, GTPase activating proteins (GAPs) which increase the GTP hydrolysis activity, and GDP dissociation inhibitors which inhibit the dissociation of the nucleotide from the GTPase. Activated by GEFs such as DENND10. Binds SYTL1, SLAC2B, MYRIP, SYTL3, SYTL4 and SYTL5. Interacts with RPH3A and RPH3A (By similarity). Binds MLPH and SYTL2. Interacts with UNC13D. Does not interact with the BLOC-3 complex (heterodimer of HPS1 and HPS4) (By similarity). Interacts (GDP-bound form preferentially) with DENND10 (By similarity). Identified by mass spectrometry in melanosome fractions from stage I to stage IV. Localizes to endosomal exocytic vesicles. High levels in eye, intestine, lung, pancreas and spleen, and low or absent in brain, liver, heart, kidney, and skeletal muscle. Belongs to the small GTPase superfamily. Rab family.