Protein-Lipid Interaction in Rhodopsin Recombinant Membranes as Studied by Protein Rotational Mobility and Lipid Alkyl Chain Flexibility Measurements 1

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Abstract
Protein rotational mobility was measured by saturation transfer ESR of a covalently attached spin label, and lipid alkyl chain flexibility by conventional ESR of 5-doxyl and 16-doxyl stearate in rhodopsin-dimyristoylphosphatidylcholine recombinants as a function of temperature and of the protein content. Rhodopsin caused some reduction of the lipid alkyl chain flexibility, the degree of which gradually increased with the protein content. In the protein-rich recombinants, a strongly immobilized component appeared and its relative intensity increased with the protein content and at temperatures lower than that of the phospholipid phase transition. This component was ascribed to lipid spin labels entrapped in the protein clusters. The rotational mobility of rhodopsin changed discontinuously at the host lipid phase transition. However, the change in the rotational correlation time was not large. The weakened response to the phase transition was ascribed to residual mobility in the solid phase and remaining aggregation of rhodopsin above the phase transition. The temperature de pendence became more and more broadened with the increase of protein content, probably due to greater aggregation in the fluid phase and partly due to increased average viscosity. Rhodopsin-dielaidoylphosphatidylcholine recombinants were also studied for comparison and some differences in the lipid-protein interaction were noted.