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. Plays a role in the maturation of phagosomes that engulf pathogens, such as S.aureus and Mycobacterium. Plays an important role in the control of melanin production and melanosome biogenesis. In concert with RAB38, regulates the proper trafficking of melanogenic enzymes TYR, TYRP1 and DCT/TYRP2 to melanosomes in melanocytes. 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. SGSM2 acts as its GAP and inactivates it by stimulating its GTPase activity. Interacts with ANKRD27. A decreased interaction with ANKRD27 seen in the presence of SGSM2 (By similarity). Recruited to phagosomes containing S.aureus or M.tuberculosis. The BLOC-3 complex, a heterodimer of HPS1 and HPS4 promotes its membrane localization. Belongs to the small GTPase superfamily. Rab family.