Small GTPase which cycles between an active GTP-bound and an inactive GDP-bound state (By similarity). Protein transport. Probably involved in vesicular traffic (By similarity). Acts as a regulator of platelet alpha-granule release during activation and aggregation of platelets (By similarity). GTP + H2O = GDP + H(+) + phosphate Serotonylation of Gln-67 by TGM2 during activation and aggregation of platelets leads to constitutive activation of GTPase activity. Belongs to the small GTPase superfamily. Rab family.