Proton and carbon-13 nuclear magnetic resonance studies of rhodopsin-phospholipid interactions

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Abstract
1H and 13C NMR spectra of [bovine] rhodopsin-phospholipid membrane vesicles and sonicated disk membranes are presented and discussed. The presence of rhodopsin in egg phosphatidylcholine vesicles results in homogeneous broadening of the methylene and methyl resonances. This effect is enhanced with increasing rhodopsin content and decreased by increasing temperature. The proton NMR data indicate the phospholipid molecules rapidly exchange (< 0-3 s) between the bulk membrane lipid and the lipid in the immediate proximity of the rhodopsin. These interactions result in a reduction in either or both the frequency and amplitude of the tilting motion of the acyl chains. The 13C NMR spectra identify the acyl chains and the glycerol backbone as the major sites of protein lipid interaction. In the disk membranes, the saturated sn-1 acyl chain is significantly more strongly immobilized than the polyunsaturated sn-2 acyl chain. This suggests a membrane model in which the lipid molecules preferentially solvate the protein with the sn-1 chain, termed an edge-on orientation. The NMR data on rhodopsin-asolectin membrane vesicles demonstrate that the lipid composition is not altered during reconstruction of the membranes from purified rhodopsin and lipids in detergent.