Structure
Volume 6, Issue 9, 15 September 1998, Pages 1169-1183, S1-S3
Journal home page for Structure

Research Article
Structural basis of activity and subunit recognition in G protein heterotrimers

https://doi.org/10.1016/S0969-2126(98)00117-8Get rights and content
Under an Elsevier user license
open archive

Abstract

Background: Inactive heterotrimeric G proteins are composed of a GDP-bound α subunit (Gα) and a stable heterodimer of Gβ and Gγ subunits. Upon stimulation by a receptor, Gα subunits exchange GDP for GTP and dissociate from Gβγ, both Gα and Gβγ then interact with downstream effectors. Isoforms of Gα, Gβ and Gγ potentially give rise to many heterotrimeric combinations, limited in part by amino acid sequence differences that lead to selective interactions. The mechanism by which GTP promotes Gβγ dissociation is incompletely understood. The Gly203→Ala mutant of Giα1 binds and hydrolyzes GTP normally but does not dissociate from Gβγ, demonstrating that GTP binding and activation can be uncoupled. Structural data are therefore important for understanding activation and subunit recognition in G protein heterotrimers.

Results: The structures of the native (Giα1β1γ2) heterotrimer and that formed with Gly203→AlaGiα1 have been determined to resolutions of 2.3 å and 2.4 å, respectively, and reveal previously unobserved segments at the Gγ2 C terminus. The Gly203→Ala mutation alters the conformation of the N terminus of the switch II region (Val201–Ala203), but not the global structure of the heterotrimer. The N termini of Gβ and Gγ form a rigid coiled coil that packs at varying angles against the β propeller of Gβ. Conformational differences in the CD loop of β blade 2 of Gβ mediate isoform-specific contacts with Gα.

Conclusions: The Gly203→Ala mutation in Giα1 blocks the conformational changes in switch II that are required to release Gβγ upon binding GTP. The interface between the ras-like domain of Gα and the β propeller of Gβ appears to be conserved in all G protein heterotrimers. Sequence variation at the Gβ–Gα interface between the N-terminal helix of Gα and the CD loop of β blade 2 of Gβ1 (residues 127–135) could mediate isoform-specific contacts. The specificity of Gβ and Gγ interactions is largely determined by sequence variation in the contact region between helix 2 of Gγ and the surface of Gβ.

Keywords

GTP-binding proteins
protein conformation
protein structure
signal transduction
X-ray crystallography

Cited by (0)

MA Wall, Department of Biochemistry, The University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235-9050, USA.

BA Posner, Department of Pharmacology, The University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235-9050, USA.

SR Sprang (corresponding author), Department of Biochemistry, Howard Hughes Medical Institute, The University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235-9050, USA. e-mail: [email protected].