The transmembrane protein bacterioopsin affects the polarity of the hydrophobic core of the host lipid bilayer

Influence of the transmembrane protein bacterioopsin (the retinal-free form of bacteriorhodopsin) on the polarity of egg-phosphatidylcholine bilayers was studied by means of a steady-state and time-resolved fluorescence appro ach exploiting the solvatochromic properties of the 2-anthroyl fluorophore. Introduced in phosphatidylcholine molecules in the form of 8-(2-anthroyl)o ctanoic acid, this fluorophore probed the hydrocarbon core of the lipid bil ayer. As previously shown (E. Perochon et al., Biochemistry 31 (1992) 7672- 7682), water molecules were detected in this region of the terminal part of the lipid acyl chains. Their number was considerably reduced upon addition of bacterioopsin to the lipids. This was assessed by a blue shift in the f luorescence emission spectra of the probe and a marked decrease in the frac tional population of fluorophores interacting with water, to the benefit of those experiencing a hydrophobic environment. In agreement with current th eories, this decrease in the hydration of the bilayer may be linked to an i ncrease in the acyl chain order and a decrease in the lateral diffusion coe fficient of lipids near the protein. The data obtained at high protein conc entration accounts for a protein/lipid interface which is much less hydrate d than the hydrophobic core of a protein-free lipid bilayer. (C) 1999 Elsev ier Science B.V. All rights reserved.