Sensitive detection of protein-lipid interaction change on bacteriorhodopsin using dodecyl ß-D-maltoside.

A light-driven proton pump bacteriorhodopsin (bR) forms a two-dimensional hexagonal lattice with about 10 archaeal lipids per monomer bR on purple membrane (PM) of Halobacterium salinarum. In this study, we found that the weakening of the bR-lipid interaction on PM by addition of alcohol can be detected as the significant increase of protein solubility in a nonionic detergent, dodecyl ß-D-maltoside (DDM). The protein solubility in DDM was also increased by bR-lipid interaction change accompanied by structural change of the apoprotein after retinal removal and was about 7 times higher in the case of completely bleached membrane than that of intact PM. Interestingly, the cyclic and milliseconds order of structural change of bR under light irradiation also led to increasing the protein solubility and had a characteristic light intensity dependence with a phase transition. These results indicate that there is a photointermediate in which bR-lipid interaction has been changed by its dynamic structural change. Because partial delipidation of PM by CHAPS gave minor influence for the change of the protein solubility compared to intact PM in both dark and light conditions, it is suggested that specific interactions of bR with some lipids which remain on PM even after delipidation treatment have a key role for the change of solubility in DDM induced by alcohol binding, ligand release, and photon absorption on bR.