Characterization of a g protein coupling YL' motif of the human VPAC1 receptor, equivalent to the first two amino acids in the DRY motif of the rhodopsin family

The conserved residues Y239 and L240 of human VPAC, receptor are predicted to be at the same location as the asparagine and arginine in the DRY motif in the Rhodopsin family of G protein-coupled receptors. By comparing vasoactive intestinal peptide (VIP) binding with or without the presence of GTP-gamma-S, it was found that the DeltaDeltaGdegrees for the endogenous G-protein coupling was 1.5 kJ/mol, 0.95 kJ/mol, and 3.4 kJ/mol for the Y239A, L240A, and wild-type receptor, respectively. VIP-induced cAMP production in whole cells support the results of the binding studies, as Y239A had a moderate and L240A a pronounced impaired ability to produce cAMP. The mutants had a minor influence on the intrinsic low affinity to high affinity equilibrium, suggesting that the dominating effect of these mutants is a perturbation of the G protein-binding site. Thus, the highly diverged chemical properties of the hydrophobic YL motif and charged DR(Y) motif could be a crucial difference between the Secretin Receptor Family and the Rhodopsin Family with respect to receptor activation and G-protein coupling.