Structure and dynamics of photosynthetic membrane-bound proteins in Rhodobacter Sphaeroides, studied with solid-state NMR spectroscopy

The photosynthetic purple bacteria such as Rb. sphaeroides possesses an int racytoplasmic membrane (ICM) and a variety of pigment-binding membrane prot eins located in the ICM, acting as photoreceptor. Such photosynthetic appar atus is concentrated in the ICM. It is composed of three multimeric membran e-bound proteins; light-harvesting complexes (LH1, LH2), a reaction center (RC) and a cytochrome b/c1 complex. We have purified these membranes, which are called chromatophores, and characterized the structure and dynamics of the photosynthetic membrane-bound proteins by means of multi-nuclear solid state NMR. First, the isotropic chemical shift of carbonyl carbons in natu ral abundance and [1-C-13] Phe labeled chromatophores indicates that the me mbrane-bound proteins take mainly the helical conformation. Second, the che mical shifts of side-chain resonances of uniformly N-15-labeled chromatopho res indicate the side-chain histidine residue is mainly hydrogen bonded, wh ereas structural heterogeneity of arginine and lysine side-chains are probe d by those wide distribution of (1)5N shifts. Thirdly, the [beta-H-2(3)]Ala and [epsilon-H-2(2)]Tyr labeling of the chromatophores are performed and d ynamics of the [beta-H-2]Ala and the [epsilon-H-2(2)]Tyr labeled chromatoph ores are studied by means of H-2 solid state NMR. The dynamics of [beta-H-2 (3)]Ala is found to be a 10(8) Hz three-site jump motion with 10 degrees li beration along the C alpha -C beta bond axis. The H-2-NMR powder pattern sp ectrum of [epsilon-H-2(2)]Tyr labeled chromatophores was interpreted with a n averaged correlation time of 5x10(5) Hz with 180 degrees two-fold flips, the result of the averaging of two kinds of split spectra in terms of motio nal time scale.