Acts as GTPase-activating protein for RAB27A, but not for RAB2A, RAB3A, nor RAB4A. Binds to the first PDZ domain of SLC9A3R1 and SLC9A3R2. Localizes to the microvilli-rich region of the syncytiotrophoblast. In melanocytes, located at the periphery of cells. The arginine and glutamine fingers are critical for the GTPase-activating mechanism, they pull out Rab's 'switch 2' glutamine and insert in Rab's active site. Exists in both phosphorylated and non-phosphorylated state.