Molecular basis of receptor/G protein coupling selectivity studied by coexpression of wild type and mutant m2 muscarinic receptors with mutant G alpha(q) subunits.

The molecular basis of receptor/G protein coupling selectivity was studied by using the m2 muscarinic receptor, a prototypical G(i/o)-coupled receptor as a model system. We could recently show that the m2 receptor can efficiently interact with mutant G protein alpha(q) subunits in which the last five amino acids were replaced with alpha(i2) or alpha(o) sequence [Liu, J., Conklin, B. R., Blin, N., Yun, J., & Wess, J. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 11642-11646]. Additional mutagenesis studies led to the identification of a four-amino-acid motif on the m2 receptor (Val385, Thr386, Ile389, and Leu390) that is predicted to functionally interact with the C-terminal portion of alpha(i/o) subunits. To further investigate the structural requirements for this interaction to occur, these four m2 receptor residues were replaced, either individually or in combination, with the corresponding residues present in the G(q/11)-coupled muscarinic receptors (m1, m3, and m5). The ability of the resulting mutant m2 receptors to interact with a mutant alpha(q) subunit (qo5) in which the last five amino acids were replaced with alpha(o) sequence was investigated in co-transfected COS-7 cells [studied biochemical response: stimulation of phosphatidyl inositol (PI) hydrolysis]. Our data suggest that the presence of three of the four targeted m2 receptor residues (Val385, Thr386, and Ile389) is essential for efficient recognition of C-terminal alpha(i/o) sequences. To study which specific amino acids within the C-terminal segment of alpha(i/o) subunits are critical for this interaction to occur, the wild type m2 receptor was co-expressed with a series of mutant alpha(q) subunits containing single or multiple alpha(q) --> alpha(i1,2) point mutations at their C-terminus. Remarkably, the wild type m2 receptor, while unable to efficiently stimulate wild type alpha(q), gained the ability to productively interact with three alpha(q) single-point mutants, providing the first example that the receptor coupling selectivity of G protein alpha subunits can be switched by single amino acid substitutions. Given the high degree of structural homology among different G protein-coupled receptors and among different classes of G protein alpha subunits, our results should be of broad general relevance.