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

AbstractInfluence 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 approach exploiting the solvatochromic properties of the 2-anthroyl fluorophore. Introduced in phosphatidylcholine molecules in the form of 8-(2-anthroyl)octanoic acid, this fluorophore probed the hydrocarbon core of the lipid bilayer. As previously shown (E. Pérochon 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 fluorescence emission spectra of the probe and a marked decrease in the fractional population of fluorophores interacting with water, to the benefit of those experiencing a hydrophobic environment. In agreement with current theories, this decrease in the hydration of the bilayer may be linked to an increase in the acyl chain order and a decrease in the lateral diffusion coefficient of lipids near the protein. The data obtained at high protein concentration accounts for a protein/lipid interface which is much less hydrated than the hydrophobic core of a protein-free lipid bilayer