Physical modifications of rhodopsin boundary lipids in lecithin-rhodopsin complexes: a spin-label study.

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Abstract
The microviscosity of rhodopsin boundary lipids was studied with a spin-labeled fatty acid covalently attached to [bovine retina] rhodopsin, in rhodopsin-egg lecithin vesicles. When the lipid-to-protein ratio was high (500:1, mole to mole), only narrow peaks were visible in EPR spectrum at 37.degree. C. Under these conditions, not more than 10% of the probes have their motion strongly restricted by the proximity of the protein. When the temperature was reduced, a 2nd component characteristic of strong immobilization appeared. It corresponds to 50% of the signal at -5.degree. C. At all temperatures reduction of the lipid-to-protein ratio also resulted in an increase of the amount of immobilized lipid. The rhodopsin boundary layer under physiological conditions is associated with low microviscosity. However, low temperatures, low lipid-to-protein ratios, or combinations of the two can induce dramatic modifications of the physical state of the boundary lipids, which under these conditions may no longer be representative of the functional biological system. These results are relevant to the general theory of lipid-protein interaction.

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