Alternative Membrane Protein Conformations in Alcohols

Abstract

Alcohols modulate the oligomerization of membrane proteins in lipid bilayers. This can occur indirectly by redistributing lateral membrane pressure in a manner which correlates with alcohol hydrophobicity. Here we investigate the direct impact of different alcohol-water mixtures on membrane protein stability and solubility, using the two detergent-solubilized alpha-helical membrane proteins DsbB and NhaA. Both proteins precipitate extensively at intermediate concentrations of alcohols, forming states with extensive (40-60%) beta-sheet structure and affinity for the fibril-specific dye thioflavin T, although atomic force microscopy images reveal layer-like and spherical deposits, possibly early stages in a fibrillation process trapped by strong hydrophobic contacts. At higher alcohol concentrations, both DsbB and NhaA are resolubilized and form non-native structures with increased (DsbB) or decreased (NhaA) helicity compared to the native state. The alternative conformational states cannot be returned to the functional native state upon dilution of alcohol. The efficiency of precipitation and the degree to which DsbB is destabilized at low alcohol concentrations show the same correlation with alcohol hydrophobicity. Thus, in addition to their effect on the membrane, alcohols perturb membrane proteins directly by solvating the hydrophobic regions of the protein. At intermediate concentrations, this perturbation exposes hydrophobic segments but does not provide sufficient solvation to avoid intermolecular association. Resolubilization requires a reduction in the relative dielectric constant below 65 in conjunction with specific properties of the individual alcohols. We conclude that alcohols provide access to a diversity of conformations for membrane proteins but are not a priori suitable for solution studies requiring reversible denaturation of monomeric proteins.