Selectivity in rhodopsin-phospholipid interactions.

This series of experiments systematically evaluated the effect of phospholipid headgroup structure on the interaction between rhodopsin and phospholipids. Two types of experiments were reported. First, ESR experiments involving spin-labeled phosphatidylserine, phosphatidic acid, and phosphatidylcholine demonstrated that, in the fluid-isotropic phase of dimyristoylphosphatidylcholine (DMPC)-rhodopsin membranes, the relative order of rhodopsin-induced immobilization was phosphatidic acid greater than phosphatidylcholine greater than phosphatidylserine. Second, the effect of rhodopsin incorporation on the dimyristoylphosphatidylserine (DMPS) gel to liquid-crystalline phase transition was analyzed with ESR techniques. A partial, binary phase diagram for the DMPS-rhodopsin system at pH 7.0 was constructed by studying the partitioning of Tempo between polar and hydrophobic domains as a function of temperature and system composition. A main result of this analysis was the finding that rhodopsin broadens and reduces the amplitude of the DMPS phase transition to a much smaller extent than it does the DMPC phase transition. When interpreted in terms of theoretical treatments of integral protein-lipid interactions, this indicates that rhodopsin has a lower affinity for DMPS than DMPC.