Binding of an alpha 2 adrenergic receptor third intracellular loop peptide to G beta and the amino terminus of G alpha

The structural basis of receptor-G protein interactions was examined using a photoaffinity derivative of a G protein-activating receptor-derived peptide (Q peptide) from the carboxyl-terminal region of the third intracellular loop of alpha 2 adrenergic receptor. A diazopyruvoyl photoaffinity derivative of this peptide (DAP-Q) was cross-linked to purified bovine brain Go. Specific, competable cross-linking of 750 nM DAP-Q to sites on both the alpha o and beta subunits was observed. No specific cross-linking was seen with non-target proteins or heat-denatured G protein subunits. 125I-DAP-Q labeled the 2-kDa amino-terminal fragment of alpha o as determined by protease digestion of the cross-linked G protein followed by gel electrophoresis or h igh pressure liquid chromatography purification and mass spectroscopy of the radiolabeled proteolysis fragment. The functional significance of incorporation into beta gamma subunit is supported by the absolute requirement of beta gamma subunit for DAP-Q stimulation of Go/Gi GTPase. Thus, specific interactions of G protein-coupled receptors with the beta subunit of G protein, in addition to those with the alpha subunit, appear to be important for receptor-G protein coupling.