THE 2.2-ANGSTROM CRYSTAL-STRUCTURE OF TRANSDUCIN-ALPHA COMPLEXED WITHGTP-GAMMA-S

The 2.2 angstrom crystal structure of activated rod transducin, G(talp ha) . GTPgammaS, shows the bound GTPgammaS molecule occluded deep in a cleft between a domain structurally homologous to small GTPases and a helical domain unique to heterotrimeric G proteins. The structure, wh en combined with biochemical and genetic studies, suggests: how an act ivated receptor might open this cleft to allow nucleotide exchange; a mechanism for GTP-induced changes in effector and receptor binding sur faces; and a mechanism for GTPase activity not evident from previous d ata.