Structure of Gi a 1 z GppNHp , Autoinhibition in a G a Protein - Substrate Complex

The structure of the G protein Gia1 complexed with the nonhydrolyzable GTP analog guanosine-5*-(bg-imino)triphosphate (GppNHp) has been determined at a resolution of 1.5 Å. In the active site of Gia1zGppNHp, a water molecule is hydrogen bonded to the side chain of Glu and to an oxygen atom of the g-phosphate group. The side chain of the essential catalytic residue Gln assumes a conformation which is distinctly different from that observed in complexes with either guanosine 5*-O-3-thiotriphosphate or the transition state analog GDPzAlF4 . Hydrogen bonding and steric interactions position Gln such that it interacts with a presumptive nucleophilic water molecule, but cannot interact with the pentacoordinate transition state. Gln must be released from this auto-inhibited state to participate in catalysis. RGS proteins may accelerate the rate of GTP hydrolysis by G protein a subunits, in part, by inserting an amino acid side chain into the site occupied by Gln, thereby destabilizing the auto-inhibited state of Ga.