A surface-exposed region of G(salpha) in which substitutions decrease receptor-mediated activation and increase receptor affinity.

The mechanism by which receptors activate G proteins is unclear because a connection between the receptor and the nucleotide binding site has not been established. To investigate this mechanism, we evaluated the roles in receptor interaction of three potential receptor contact sites in alpha(s): the alpha2/beta4, alpha3/beta5, and alpha4/beta6 loops. Substitutions of alpha(i2) homologs for alpha(s) residues in the alpha2/beta4 loop and alanine substitutions of residues in the alpha4/beta6 loop do not affect activation by the beta(2)-adrenergic receptor. However, replacement of five alpha(s) residues in the alpha3/beta5 loop region with the homologous alpha(i2) residues decreases receptor-mediated activation of alpha(s) and increases the affinity of G(s) for this receptor. The substitutions do not alter guanine nucleotide binding or hydrolysis, or activation by aluminum fluoride, indicating that the effects on receptor interaction are not due to a destabilization of the guanine-nucleotide bound state. In a model of the receptor-G protein complex, the alpha3/beta5 loop maps near the second and third intracellular loops of the receptor. The effects of the alpha3/beta5 substitutions suggest that the wild-type residues may be receptor contact sites that are optimized to ensure the reversibility of receptor-G protein interactions. Furthermore, the alpha3/beta5 region corresponds to an exchange factor contact site in both EF-Tu and Ras, suggesting that the mechanisms by which seven-transmembrane receptors and exchange factors catalyze nucleotide exchange may share common elements.