An evaluation of detergents for NMR structural studies of membrane proteins.

Structural information on membrane proteins lags far behind that on soluble proteins, in large part due to difficulties producing homogeneous, stable, structurally relevant samples in a membrane-like environment. In this study 25 membrane mimetics were screened using 2D (1)H-(15)N heteronuclear single quantum correlation NMR experiments to establish sample homogeneity and predict fitness for structure determination. A single detergent, 1-palmitoyl-2-hydroxy-sn-glycero-3-[phospho-RAC-(1-glycerol)] (LPPG), yielded high quality NMR spectra with sample lifetimes greater than one month for the five proteins tested - R. sphaeroides LH1 alpha and beta subunits, E. coli and B. pseudofirmus OF4 ATP synthase c subunits, and S. aureus small multidrug resistance transporter - with 1, 2, or 4 membrane spanning alpha-helices, respectively. Site-specific spin labeling established interhelical distances in the drug transporter and genetically fused dimers of c subunits in LPPG consistent with in vivo distances. Optical spectroscopy showed that LH1 beta subunits form native-like complexes with bacteriochlorophyll a in LPPG. All the protein/micelle complexes were estimated to exceed 100 kDaltons by translational diffusion measurements. However, analysis of (15)N transverse, longitudinal and (15)N[(1)H] nuclear Overhauser effect relaxation measurements yielded overall rotational correlation times of 8 to 12 nsec, similar to a 15-20 kDalton protein tumbling isotropically in solution, and consistent with the high quality NMR data observed.

Structure of the Rhodobacter sphaeroides light-harvesting 1 beta subunit in detergent micelles.

The light harvesting 1 antenna (LH1) complex from Rhodobacter sphaeroides funnels excitation energy to the photosynthetic reaction center. Our ultimate goal is to build up the structure of LH1 from structures of its individual subunits, much as the antenna can self-assemble from its components in membrane-mimicking detergent micelles. The beta subunit adopts a nativelike conformation in Zwittergent 3:12 micelles as demonstrated by its ability to take the first step of assembly, binding BChl a. Multidimensional NMR spectroscopy shows that the beta subunit folds as a helix((L12-S25))-hinge((G26-W28))-helix((L29-W44)) structure with the helical regions for the 10 lowest-energy structures having backbone rmsds of 0.26 and 0.24 A, respectively. Mn(2+) relaxation data and the protein-detergent NOE pattern show the C-terminal helix embedded in the micelle and the N-terminal helix lying along the detergent micelle surface with a 60 degrees angle between their long axes. (15)N relaxation data for residues L12-W44 are typical of a well-ordered protein with a correlation time of 8.25 +/- 2.1 ns. The presence of the hinge region placing the N-terminal helix along the membrane surface may be the structural feature responsible for the functional differences observed between the LH1 and LH2 beta subunits.