The role of protein and surfactant interactions in membrane-protein crystallization

The interactions leading to crystallization of the integral membrane protein bacteriorhodopsin solubilized in n-octyl-β-D-glucoside were investigated. Osmotic second virial coefficients were measured by self-interaction chromatography in the presence of sodium malonate, sodium formate and ammonium sulfate. Attractive protein–detergent complex (PDC) interactions were observed as the surfactant cloud-point temperature was approached for each salt, suggesting that surfactant interactions may play an important role in promoting PDC crystallization. Dynamic light scattering and tensiometry measurements show that the interaction trends are strongly influenced by micelle structure and surfactant phase behavior, both of which are sensitive to salt and surfactant concentration. Overall, detailed investigations using a combination of experimental techniques can provide insight into the complex nature of PDC interactions, which is essential to developing rational approaches to membrane-protein crystallization.