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Abstract

Sedimentation equilibrium analysis in the analytical ultracentrifuge can be applied to membrane proteins solubilized by nonionic detergents. By representing a lipid-like environment for the proteins, the detergent micelles help to preserve the original tertiary and quaternary structure of the proteins and thus allow their native associations to be studied. The contributions of the membrane-bound detergent to the particle weight and the partial specific volume of the proteins or protein complexes can easily be taken into account, e.g., by the technique of density matching. The most important areas of application of the method are a) the study of reversibly self-associating systems, leading to the identification of the different oligomers present, and b) the analysis of stable or transient heterogeneous associations between membrane proteins. In the latter case, one of the proteins should be labeled with a dye in order to simplify the analysis. Sedimentation equilibrium experiments can also be applied to the analysis of the protein content of small lipid vesicles. Thus, analytical ultracentrifugation is apt to play an important role in studies on membrane proteins.