Boundary lipids and protein mobility in rhodopsin-phosphatidylcholine vesicles. Effect of lipid phase transitions

Purified rhodopsin from bovine retina has been incorporated into phospholipid bilayers. Dimiristoylphosphatidylcholine, dipalmitoylphosphatidylcholine, dioleylphosphatidylcholine and egg phosphatidylcholine were used as host lipids, with ratio of lipid to protein of 120 : 1 (mol to mol). In order to probe the lipid-protein interface specifically, a spin-labeled fatty acid was covalently bound to rhodopsin via an isocyanate reacting group. A spin-labeled phospholipid was used to probe the bulk lipidic phase while a tightly bound maleimide spin label was used to obtain the protein rotational correlation time by the saturation transfer technique. The following results were obtained: (1) The kinetics of reduction by ascorbate of the spin-labeled fatty acid covalently bound to rhodopsin demonstrate that the alkyl chain attached to the protein is positioned in the membrane in the same way as the alkyl chains of a phospholipid. (2) The EPR spectra of the latter shows two components: a strongly immobilized component and a weakly immobilized component. The ratio of the two depends upon the temperature and on the nature of the phospholipids. (3) The signal of the weakly immobilized component is compared to that obtained in the corresponding pure lipids. The latter signal, assumed to represent non-bounded lipids, indicates a sharp transition at the phospholipid phase transition with dimytristoylphosphatidylcholine or dipalmitoylphosphatidylcholine. The former signal (corresponding to the lipid-protein interface) indicates only a broad transition extending over 7 degrees C with dipalmitoylphosphatidylcholine and almost no transition with dimyristoylphosphatidylcholine. (4) In a similar way, the rotational correlation time of the protein only changes progressively when the phase transition occurs. Our interpretation of the data can be summarized as follows: The immobilized component seen by the EPR technique in the hydrophobic environment of this intrinsic protein very probably reflects protein-protein contacts and thus corresponds to hindrance of the labeled chains, when they are trapped between neighbouring proteins. Below the phase transition lipid segregation whould increase the probability of protein contact. However, over a certain range of temperature, the contact with the protein interface probably at the same time prevents the non-segregated phospholipids from feezing. The differences in the results obtained with the various phosphatidylcholines above their transition temperature suggest that the solubility of rhodopsin in bilayers depends not only on the fluidity of the lipids, but also, to some extent, on the phospholipid chain length.