G proteins, effectors and GAPs: structure and mechanism.

G proteins from a diverse family of regulatory GTPases which, in the GTP-bound state, bind to and activate downstream effectors. Structures of Ras homologs bound to effector domains have revealed mechanisms by which G proteins couple GTP binding to effector activation and achieve specificity. Complexes between structurally unrelated GTPase-activating proteins with complementary G proteins suggest common mechanisms by which GTP hydrolysis is stimulated via direct interactions with conformationally labile switch regions of the G protein.