Lipid-Protein Interactions in Bacteriorhodopsin-Phosphatidylcholine Vesicles
Of considerable current interest is the question how the presence of proteins in membranes affects the order and mobility of the membrane lipids. In an attempt to understand the effect of proteins on the structural and dynamical parameters of lipids it is of great advantage to be able to vary the lipid to protein ratio (L/BR). In the present investigation we use DMPC/bacteriorhodopsin vesicles in which a lowering of the ternperature from above the gel to liquid-crystalline transition of the lipids to below induces a reversible change in the aggregation of BR from a monomeric state above to the crystalline state below Tc. In these vesicles the L/BR can be varied from 18 to 390 (molar), the photocycle is slowed-down but otherwise normal and BR is functional as a light-driven proton pump both above and below Tc (Dencher and Heyn, in press). The lipid phase transition was monitored using the steady-state anisotropy of the fluorescent probe diphenylhexatriene (DPH). Below Tc the anisotropy is quite high (0. 34) and independent of the L/BR. The lipid phase transition, which is accompanied by a large drop in anisotropy, broadens with decreasing L/BR. Within experimental error Tc is the same as for protein free vesicles. Above Tc the anisotropy increases markedly with decreasing L/BR. This effect can be explained either by assuming that BR slows down the rotational diffusion of the lipids (viscosity) or by assuming that BR restricts the angular range available for rotation (order). It can be estimated that the observed effect cannot be due to a protein-induced increase in the viscosity alone. Results of time-dependent fluorescent depolarization experiments on other systems also show that the probe rotational correlation time increases only slightly when proteins are incorporated in lipid bilayers.