Position of Transmembrane Helix 6 Determines Receptor G Protein Coupling Specificity

G protein coupled receptors (GPCRs) transmit extracellular signals into the cell by binding and activating different intracellular signaling proteins, such as G proteins (G alpha beta gamma, families Gi, Gs, Gq, G(12/13)) or arrestins. To address the issue of Gs vs Gi coupling specificity, we carried out molecular dynamics simulations of lipid-embedded active beta(2)adrenoceptor (beta(2)AR*) in complex with C-terminal peptides derived from the key interaction site of G alpha (G alpha CT) as surrogate of G alpha beta gamma. We find that Gi alpha CT and Gs alpha CT exploit distinct cytoplasmic receptor conformations that coexist in the uncomplexed beta(2)AR*. The slim Gi alpha CT stabilizes a beta(2)AR* conformation, not accessible to the bulkier GsaCT, which requires a larger TM6 outward tilt for binding. Our results suggest that the TM6 conformational heterogeneity regulates the catalytic activity of beta(2)AR* toward Gi or Gs.