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 pKa 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.
Weitz
C J
CJ
Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
Nathans
J
J
D000818:Animals; D002417:Cattle; D006863:Hydrogen-Ion Concentration; D008565:Membrane Proteins / Q000737:chemistry; D016297:Mutagenesis, Site-Directed; D012243:Rhodopsin / Q000031:analogs & derivatives* / Q000737:chemistry*; D012374:Rod Cell Outer Segment / Q000737:chemistry; D012545:Schiff Bases; D013057:Spectrum Analysis; D013329:Structure-Activity Relationship
D008565:Membrane Proteins; D012545:Schiff Bases; C013110:metarhodopsins; D012243:Rhodopsin