Structural basis and mechanism of activation of two different families of G proteins by the same GPCR.

The β-adrenergic receptor (β-AR) can activate two families of G proteins. When coupled to Gs, β-AR increases cardiac output, and coupling to Gi leads to decreased responsiveness in myocardial infarction. By comparative structural analysis of turkey β-AR complexed with either Gi or Gs, we investigate how a single G-protein-coupled receptor simultaneously signals through two G proteins. We find that, although the critical receptor-interacting C-terminal α5-helices on Gα and Gα interact similarly with β-AR, the overall interacting modes between β-AR and G proteins vary substantially. Functional studies reveal the importance of the differing interactions and provide evidence that the activation efficacy of G proteins by β-AR is determined by the entire three-dimensional interaction surface, including intracellular loops 2 and 4 (ICL2 and ICL4).