RHODOPSIN-LIPID ASSOCIATIONS IN BOVINE ROD OUTER SEGMENT MEMBRANES - IDENTIFICATION OF IMMOBILIZED LIPID BY SPIN-LABELS

Rhodopsin-lipid interactions have been studied in bovine rod outer segment (ROS) membranes by using spin-labels. Spin-labeled fatty acid, sterol, phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylglycerol, and phosphatidic acid molecules all display a two-component spectrum when probing ROS membranes. One of the spectral components represents 33-43% of the total spectral intensity and is characteristic of a strongly immobilized nitroxide spin-label. This immobilized component is resolved from -4 to 37 °C. The remaining 67-57% of the integrated spectral intensity has a very similar form to the spectra of the same spin-labels in bilayers of extracted ROS membrane lipid. A small selectivity for the immobilized regions of ROS membranes is shown by phosphatidylserine, while the fatty acid, phosphatidylcholine, phosphatidylethanolamine, and sterol spin-labels partition almost equally into these regions. The selectivity is not solely due to the head-group charge on phosphatidylserine since the negatively charged phosphatidylglycerol and phosphatidic acid spin-labels do not display an enhanced selectivity. These results are interpreted in terms of two populations of lipid existing in ROS membranes, the major one being fluid bilayer in exchange with and surrounding the immobilized lipid which is in direct contact with rhodopsin. On the basis of available information on the size of rhodopsin, it is calculated that the immobilized lipid is sufficient to form a single complete shell around the protein. © 1979, American Chemical Society. All rights reserved.