Structures of active conformations of Gi alpha 1 and the mechanism of GTP hydrolysis

TLDR

Mechanisms of GTP hydrolysis by G protein alpha subunit‑p21ras superfamily members have been studied extensively but remain poorly understood. The study aims to elucidate the roles of two conserved residues in transition‑state stabilization using high‑resolution X‑ray structures of GTPγS and GDP·AlF4‑ complexes of Giα1. High‑resolution X‑ray structures of GTPγS and GDP·AlF4‑ complexes of Giα1 reveal specific roles of two conserved residues in transition‑state stabilization. The structures show that Gln204 orients the catalytic water, Arg178 stabilizes the pentacoordinate phosphate intermediate, and the unique helix‑loop conformation and disordered N‑terminal residues of Giα1 likely contribute to its higher hydrolytic rate and effector binding, distinguishing it from the Gtα subunit.

Abstract

Mechanisms of guanosine triphosphate (GTP) hydrolysis by members of the G protein alpha subunit-p21ras superfamily of guanosine triphosphatases have been studied extensively but have not been well understood. High-resolution x-ray structures of the GTP gamma S and GDP.AlF4- complexes formed by the G protein Gi alpha 1 demonstrate specific roles in transition-state stabilization for two highly conserved residues. Glutamine204 (Gln61 in p21ras) stabilizes and orients the hydrolytic water in the trigonal-bipyramidal transition state. Arginine 178 stabilizes the negative charge at the equatorial oxygen atoms of the pentacoordinate phosphate intermediate. Conserved only in the G alpha family, this residue may account for the higher hydrolytic rate of G alpha proteins relative to those of the p21ras family members. The fold of Gi alpha 1 differs from that of the homologous Gt alpha subunit in the conformation of a helix-loop sequence located in the alpha-helical domain that is characteristic of these proteins; this site may participate in effector binding. The amino-terminal 33 residues are disordered in GTP gamma S-Gi alpha 1, suggesting a mechanism that may promote release of the beta gamma subunit complex when the alpha subunit is activated by GTP.