Structure of Giα1•GppNHp, autoinhibition in a Gα protein-substrate complex

The structure of the G protein G(i alpha 1) complexed with the nonhydrolyzable GTP analog guanosine-5'-(beta gamma-imino)triphosphate (GppNHp) has been determined at a resolution of 1.5 Angstrom. In the active site of G(i alpha 1). GppNHp, a water molecule is hydrogen bonded to the side chain of Glu(43) and to an oxygen atom of the gamma-phosphate group. The side chain of the essential catalytic residue Gln(204) assumes a conformation which is distinctly different from that observed in complexes with either guanosine 5'-O-3-thiotriphosphate or the transition state analog GDP . AIF(4)(-). Hydrogen bonding and steric interactions position Gln(204) such that it interacts with a presumptive nucleophilic water molecule, but cannot interact with the pentacoordinate transition state. Gln(204) must be released from this auto-inhibited state to participate in catalysis, RGS proteins may accelerate the rate of GTP hydrolysis by G protein alpha subunits, in part, by inserting an amino acid side chain into the site occupied by Gln(204), thereby destabilizing the auto-inhibited state of G alpha.