Structure of G(iα1)·GppNHp, autoinhibition in a Gα protein-substrate complex

The structure of the G protein G(i 1/4 ) complexed with the nonhydrolyzable GTP analog guanosine-5'-(βγ-imino)triphosphate (GppNHp) has been determined at a resolution of 1.5 Å. In the active site of G(i 1/4 )·GppNHp, a water molecule is hydrogen bonded to the side chain of Glu⁴³ and to an oxygen atom of the γ-phosphate group. The side chain of the essential catalytic residue Gln²⁰⁴ 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·A1F₄^-. Hydrogen bonding and steric interactions position Gln²⁰⁴ such that it interacts with a presumptive nucleophilic water molecule, but cannot interact with the pentacoordinate transition state. Gln²⁰⁴ must be released from this auto-inhibited state to participate in catalysis. RGS proteins may accelerate the rate of GTP hydrolysis by G protein α subunits, in part, by inserting an amino acid side chain into the site occupied by Gln²⁰⁴, thereby destabilizing the auto- inhibited state of Gα.