SAXS Study of Transmembrane Protein Association in Detergent Micelle Solutions. Zimei Bu and Donald M. Engelman Department of Molecular Biophysics and Biochemistry Yale University 260 Whitney Avenue P.O. Box 208114 New Haven, CT 06520-8114

This project addresses the application of solution small angle X-rays scattering (SAXS), a non-perturbing technique that can provide global structural information, to study the association of transmembrane proteins in detergent solutions. Because of their hydrophobic nature, transmembrane proteins are usually studied in detergent solutions. The techniques used to characterize molecular size and association states become not very effective for transmembrane proteins dissolved in detergent solutions. Although gel electrophoresis in SDS detergent can identify some of the different oligomer states, the charged SDS molecules may alter the interactions and disrupt the associations. The challenge of applying SAXS to studying transmembrane proteins in detergent solutions is that both the large detergent micelles and the protein scatter X-rays. This problem has been solved by contrast matching the average electron density of the micelles. At the match point, the total scattering intensity only reflects the scattering from the protein solution, whereas the micelle structure (volume and aggregation number) has been found not to be changed up to above the match point. The molecular weights and radii of gyration of soluble protein molecular weight standards dissolved in nonionic detergent solutions have been correctly determined by this SAXS contrast matching technique. Results on the association of chimeric staphylococcal nuclease/glycophorin A transmembrane protein (SN/GPA) are presented.