Membrane Bilayer Environment Influences Thermodynamics of Rhodopsin Membrane Protein-Lipid Interactions

Abstract

The interaction between lipid bilayers and membrane proteins plays an important role in the functionality of GPCRs such as rhodopsin. During rhodopsin photoactivation, the photoreactive 11-cis retinylidene chromophore is isomerized to an all-trans state attaining an equilibrium between the MetaI and MetaII states. Lipid bilayer interaction with rhodopsin during its photoactivation process is explained using the flexible surface model (FSM). The FSM describes the elastic coupling of membrane lipids to integral membrane proteins through a balance of curvature and hydrophobic forces in lipid-protein interactions [1]. The FSM predicts a shift of the MetaI-MetaII equilibrium towards the activated MetaII state for unsaturated phosphatidylcholines with an increase in acyl chain length [1] or temperature. using UV-visible spectroscopy, we test the FSM by evaluating how the MetaI-MetaII equilibrium and thermodynamic parameters are influenced by a change in membrane environment. Absorption wavelength maxima at 485 nm and 380 nm correspond to the MetaI and MetaII states (inactive and active MetaII-H+), respectively. Accurate results are obtained by using the Rayleigh-Gans approximation to Mie scattering to compensate for light scattering in the UV-visible spectra. We determine effects of the type of head group, lipid to protein ratio, acyl chain length, and temperature on the MetaI-MetaII equilibrium by comparing pH titration curves for reconstituted rhodopsin in different lipid bilayers to those for native disk membranes [2,3]. The thermodynamic parameters are obtained for this equilibrium by fitting a modified phenomenological Henderson-Hasselbalch function [2] to the pH titration curves. These thermodynamic parameters illustrate how free energy drives the structural changes in rhodopsin upon photoactivation and can be used to model other GPCRs.[1] A.V. Botelho et al. (2006) BJ91, 4464-4477.[2] M. Mahalingam et al. (2008) PNAS105, 17795-17800.[3] E. Zaitseva et al. (2010) JACS132, 4815-4821.