Molecular mechanisms involved in muscarinic acetylcholine receptor-mediated G protein activation studied by insertion mutagenesis.

We have recently shown that a four-amino acid epitope (VTIL) on the m2 muscarinic receptor (corresponding to Val385, Thr386, Ile389, and Leu390) is essential for Gi/o coupling specificity and Gi/o activation (Liu, J., Conklin, B. R., Blin, N., Yun, J., and Wess, J. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 11642-11646). Because this sequence element is thought to be located at the junction between the third intracellular loop and the sixth transmembrane helix (TM VI), we speculated that agonist binding to the m2 receptor protein results in conformational changes that enable the VTIL motif to interact with Gi/o proteins. To test the hypothesis that such structural changes might involve a relative movement of TM VI toward the cytoplasm, we created a series of mutant m2 muscarinic receptors in which one to four extra Ala residues were inserted into TM VI immediately after Leu390. Based on the geometry of an alpha-helix, such mutations are predicted to "push" the VTIL sequence away from the lipid bilayer. Consistent with our working hypothesis, second messenger assays with transfected COS-7 cells showed that all mutant m2 receptors containing extra Ala residues C-terminal of Leu390 could activate the proper G proteins even in the absence of agonist. However, replacement of the VTIL motif in such constitutively active m2 receptors with the corresponding m3 muscarinic receptor sequence (AALS) or deletion of Ala391 from the wild type m2 receptor completely abolished G protein coupling. Interestingly, introduction of extra Ala residues C-terminal of the AALS motif in the m3 muscarinic receptor completely abolished functional activity. Mutant m2 and m3 receptors that contained extra Ala residues immediately N-terminal of the VTIL and AALS motif, respectively, displayed wild type-like coupling properties. Our data are consistent with a model in which agonist binding to the m2 muscarinic receptor leads to a relative movement of TM VI toward the cytoplasm, thus enabling the adjacent VTIL sequence to interact with the C terminus of Galpha(i/o) subunits.