Photoreversible conformational changes in membrane proteins using light-responsive surfactants.

Photoreversible control of the conformation of bacteriorhodopsin in the presence of a light-responsive surfactant is demonstrated through combined UV-vis, FT-IR, and (31)P NMR spectroscopy and dynamic light scattering (DLS) measurements. The azobenzene-based surfactant photoisomerizes upon 434 nm visible (trans, relatively hydrophobic) and 350 nm UV (cis, relatively hydrophilic) illumination, allowing surfactant micellization to be reversibly controlled. This leads to partitioning of the membrane protein into micelles in the unfolded state under visible light, while UV light leads to solubilization of the protein within purple membrane bilayers in the folded state. A three-stage model of purple membrane-photosurfactant interactions is examined through NMR and DLS measurements. Phototriggered unfolding of bacteriorhodopsin, occurring through alpha(II) --> alpha(I) and reverse beta-turn --> extended beta-strand transitions, requires approximately 20 s for completion, while light-induced refolding requires a somewhat longer 80 s as the membrane protein repartitions into the reformed bilayer membrane. Each of these conformational changes can be precisely and reversibly controlled with simple light illumination, providing a novel technique to probe membrane protein folding.