The binding of alkyl polyglucoside surfactants to the integral membrane protein bacteriorhodopsin (BR) and the formation of protein-surfactant complexes are investigated by sedimentation equilibrium via analytical ultracentrifugation and by small-angle neutron scattering (SANS). Contrast variation techniques in SANS enable measurement of the composition of the protein-surfactant complexes and determination of the thickness of the surfactant shell bound to the protein. The results indicate that alkyl polyglucosides can bind to BR as single surfactant layers or as a thicker shell. The thickness of the surfactant shell increases with increasing surfactant tail length, and it is generally unrelated to the aggregation number of the micelles even for a small and predominantly hydrophobic membrane protein such as BR. The aggregation numbers determined by sedimentation equilibrium methods match those measured by SANS, which also allows reconstruction of the shape of the protein-detergent complex. When the surfactant is present as a single layer, the BR loses activity, as measured by absorption spectroscopy, more quickly than it does when the surfactant forms a thicker shell.
Bibliographical noteFunding Information:
We acknowledge the National Institute of Standards and Technology, U.S. Department of Commerce, for providing the neutron research facilities used in this work, which were supported in part by the National Science Foundation under Agreement No. DMR-0454672. We are thankful to G. Turner (Seton Hall University) for kindly providing the Halobacterium salinarum strain, B. Berger for assistance with the cell growth, and G. Fritz (University of Graz) for providing the program for the IFT analysis. The Delaware Biotechnology Institute and the Center of Biomedical Research Excellence (COBRE) in Membrane Protein Production and Characterization at the University of Delaware are acknowledged for use of their facilities.
COBRE is supported by the National Center for Research Resources of the National Institutes of Health (grant P20 RR015588). Funding for this work was from the Microgravity Research Program of NASA (grant NAG8-1830).