title: Role of phosphatidylserine in the MI-MII equilibrium of rhodopsin.
abstract: Bovine rhodopsin was recombined with various phospholipids in which the lipid acyl chain composition was held constant at that of egg phosphatidylcholine (PC), while the identity of the headgroups was varied. The ratio of MII/MI produced in the recombinant membrane vesicles by an actinic flash was studied as a function of pH, and compared to the photochemical activity observed for rhodopsin in native ROS membranes. MI and MII were found to coexist in a pH-dependent, acid-base equilibrium on the millisecond timescale. Recombinants made with phospholipids containing unsaturated acyl chains were capable of full native-like MII production, but demonstrated titration curves with different pK values. The presence of phosphoethanolamine or phosphoserine headgroups increased the amount of MII produced. In the case of phosphatidylserine this may result from alteration of the membrane surface potential, leading to an increase in the local H+ activity. The results indicate that the Gibbs free energies of the MI and MII conformational states are influenced by the membrane bilayer environment, suggesting a possible role of lipids in visual excitation.
authors.affiliation: Department of Chemistry, University of Arizona, Tucson 85721.
authors.forename: N J
authors.initials: NJ
authors.lastname: Gibson
authors.forename: M F
authors.initials: MF
authors.lastname: Brown
mesh_terms: D000818:Animals; D002417:Cattle; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004983:Ethanolamines / Q000494:pharmacology; D010718:Phosphatidylserines / Q000502:physiology*; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012243:Rhodopsin / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D012374:Rod Cell Outer Segment / Q000187:drug effects / Q000378:metabolism*; D013816:Thermodynamics
chemical_list: D004983:Ethanolamines; D010718:Phosphatidylserines; D011994:Recombinant Proteins; C013110:metarhodopsins; C005448:phosphorylethanolamine; D012243:Rhodopsin