Acts as GTPase-activating protein for RAB27A. Binds to the first PDZ domain of SLC9A3R1 and SLC9A3R2. Localizes to the microvilli-rich region of the syncytiotrophoblast. Expressed in most tissues, except for skeletal muscle. 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.