Acts as an A-kinase anchoring protein by binding to the type II regulatory subunit of protein kinase A and anchoring it to the mitochondrion. Also involved in synchronization of mitochondrial fission (PubMed:12186851). Plays a role in the maturation of phagosomes that engulf pathogens, such as S.aureus and M.tuberculosis (PubMed:21255211). Plays an important role in the control of melanin production and melanosome biogenesis (PubMed:23084991). In concert with RAB38, regulates the proper trafficking of melanogenic enzymes TYR, TYRP1 and DCT/TYRP2 to melanosomes in melanocytes (By similarity). Regulated by a guanine nucleotide-exchange factor (GEF) and a GTPase-activating protein (GAP) and alternates between an inactive GDP-bound and an active GTP-bound form. The BLOC-3 complex composed of HPS1 and HPS4 acts as its GEF, promotes the exchange of GDP to GTP, converting it from an inactive GDP-bound form into an active GTP-bound form (By similarity). SGSM2 acts as its GAP and inactivates it by stimulating its GTPase activity (PubMed:21269460). Interacts with ANKRD27 (PubMed:24856514, PubMed:21269460). A decreased interaction with ANKRD27 seen in the presence of SGSM2 (PubMed:21269460). Recruited to phagosomes containing S.aureus or M.tuberculosis (PubMed:21255211). The BLOC-3 complex, a heterodimer of HPS1 and HPS4 promotes its membrane localization (PubMed:23084991). Widely expressed with high levels in heart, liver, kidney, bone marrow, testis, colon and fetal lung. Belongs to the small GTPase superfamily. Rab family.