Towards Understanding How Water Modulates Membrane Protein Stability

Abstract

Understanding the energetics of membrane protein folding and function is crucial, as over 15,000 disease causing mutations have been identified for membrane proteins in humans. The majority of these mutations are thought to disrupt folding, the process by which a membrane protein buries itself into a lipid bilayer. At its core, membrane protein folding involves the transfer of amino acid side chains from an polar, aqueous solvent into a hydrophobic lipid bilayer. Previously, we measured the transfer free energies of side chains (ΔΔGsc) to the nonpolar center of a lipid bilayer in two membrane proteins (beta-barrels OmpLA and PagP). However, the chemical composition of the lipid bilayer is heterogeneous and contingent upon the position across the bilayer normal, creating a position dependence of ΔΔGsc across the membrane. Presently, we have measured the transfer free energies for all hydrophobic amino acids at multiple depths in the membrane using an established host-guest methodology. Using these results we have calculated the bilayer positional dependence of the nonpolar solvation parameter (NSP), which describes the general energetic cost for burial of a hydrophobic surface. Values for NSP fall within previously determined estimates for both the bilayer center and interface and they correlate with the steeply changing profile of water concentration in the membrane. Using these results we have discovered a function that describes the solvation environment of the bilayer with nanometer resolution and how membrane protein energetics are modulated by it. These results allow us to more accurately predict hydrophobic side chain water-to-bilayer transfer free energies, and can be easily implemented in structure prediction algorithms.