GTPASE MECHANISM OF GPROTEINS FROM THE 1.7-ANGSTROM CRYSTAL-STRUCTUREOF TRANSDUCIN ALPHA-CENTER-DOT-GDP-CENTER-DOT-ALF4(-)

ALUMINIUM fluoride (AlF4-) activates members of the heterotrimeric G-p rotein (G(alpha beta gamma)) family(1,2) by binding to inactive G(alph a).GDP near the site occupied by the gamma-phosphate in G(alpha).GTP ( ref. 3). Here we describe the crystal structure of transducin alpha.GD P activated with aluminium fluoride (G(t alpha).GDP.AlF4-.H2O) at 1.7 Angstrom, a resolution sufficient to establish the coordination geomet ry of the bound aluminium fluoride as well as the extensive network of direct and water-mediated interactions that stabilize it. These obser vations are derived from three independent representations in the asym metric unit, eliminating any chance of drawing conclusions based on st ereochemistry imposed by crystal packing. Surprisingly, aluminium fluo ride activates G(alpha).GDP by binding with a geometry resembling a pe ntavalent intermediate for GTP hydrolysis. The stabilizing interaction s involve not only residues that interact with the gamma-phosphate in G(t alpha).GTP gamma S, but also conserved residues essential for GTPa se activity. Thus the G(t alpha).GDP.AlF4-.H2O structure provides new insight into the mechanism of GTP hydrolysis.