Rhodopsin Activation: Effects on the Metarhodopsin I-Metarhodopsin II Equilibrium of Neutralization or Introduction of Charged Amino Acids within Putative Transmembrane Segments

We have studied the metarhodopsin I (M I)-metarhodopsin II (M II) equilibria of expressed wild-type and mutant rhodopsins. We studied two classes of mutants with amino acid substitutions in or near the putative transmembrane segments: those in which a charged residue was replaced by a neutral residue (or in one case another charged residue) and those in which a neutral residue likely (or postulated) to be in proximity to the retinylidene Schiff's base was replaced by a charged residue. In the first class, we found mutants that abnormally favored M II (replacements of Asp-83, Glu-134, or Arg-135) as well as one that abnormally favored M I (replacement of Glu-122). In the second class, we found several mutants that abnormally favored M I, the most extreme being those in which glutamate replaced His-211 or Ala-292. These studies suggest that electrostatic forces play a major role in the energetics of the M 1-to-M II transition, and they indicate that electrostatic perturbation in the vicinity of the protonated retinylidene Schiff's base is a plausible mechanism for the change in its pK_a that is associated with the M I-M II transition. They further suggest that the highly conserved pair of charged residues homologous to Glu-134 and Arg-135 may play a general role in agonist-dependent conformational changes in G-protein-coupled receptors.