IDENTIFICATION OF A RECEPTOR G-PROTEIN CONTACT SITE CRITICAL FOR SIGNALING SPECIFICITY AND G-PROTEIN ACTIVATION

Each G protein-coupled receptor recognizes only a distinct subset of the many structurally closely related G proteins expressed within a cell. How this selectivity is achieved at a molecular level is not well understood, particularly since no specific point-to point contact sites between a receptor and its cognate G protein(s) have been identified. In this study, we demonstrate that a 4-aa epitope on the m2 muscarinic acetylcholine receptor, a prototypical G(i/o)-coupled receptor, can specifically recognize the C-terminal 5 aa of alpha subunits of the G(i/o) protein family. The m2 receptor residues involved in this interaction are predicted to be located on one side of an alpha-helical receptor region present at the junction between the third intracellular loop and the sixth transmembrane domain. Coexpression studies with hybrid m2/m3 muscarinic receptors and mutant G-protein alpha(q), subunits showed that the receptor/G-protein contact site identified in this study is essential for coupling specificity and G-protein activation.