Mutational analysis of third cytoplasmic loop domains in G-protein coupling of the HM1 muscarinic receptor.

We measured dose-response curves for carbachol stimulation of phosphatidyl inositol (PI) turnover with mutants of the Hm1 muscarinic cholinergic receptor having various deletions from amino acids 219 to 358 of the large third intracellular (i3) loop (208 to 366). These deletions had only small or no effects on the ability of Hm1 transfected into HEK 293 cells to stimulate PI turnover. This result indicates that only small regions of 9 to 11 amino acids adjacent to trans-membrane domains (TMDs) 5 and 6 can be directly involved in G protein coupling. Point mutations were constructed to test the role of charged amino acids in these junctions. A triple point mutation of Hm1 (E214 A/ E216K/ E221 K), which mimics the charge distribution in Hm2 (negatively coupled to cAMP) over the first 14 amino acids of i3, and a double point mutation in the N terminal junction, K359A/K361A, both failed to affect carbachol stimulated PI turnover. Therefore, charge distribution in the loop junctions appears to play a minor role in G protein coupling of Hm1 in HEK 293 cells.