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

The photosynthetic purple bacteria such as Rb. sphaeroides possesses an intracytoplasmic membrane (ICM) and a variety of pigment-binding membrane proteins located in the ICM, acting as photoreceptor. Such photosynthetic apparatus is concentrated in the ICM. It is composed of three multimeric membrane-bound proteins; light-harvesting complexes (LH 1, LH 2), 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 natural abundance and [1-(13)C] Phe labeled chromatophores indicates that the membrane-bound proteins take mainly the helical conformation. Second, the chemical shifts of side-chain resonances of uniformly (15)N-labeled chromatophores indicate the side-chain histidine residue is mainly hydrogen bonded, whereas structural heterogeneity of arginine and lysine side-chains are probed by those wide distribution of (15)N shifts. Thirdly, the [beta-(2)H(3)]Ala and [epsilon-(2)H(2)]Tyr labeling of the chromatophores are performed and dynamics of the [beta-(2)H]Ala and the [epsilon-(2)H(2)]Tyr labeled chromatophores are studied by means of (2)H solid state NMR. The dynamics of [beta-(2)H(3)]Ala is found to be a 10(8)Hz three-site jump motion with 10 degrees liberation along the Calpha-Cbeta bond axis. The (2)H-NMR powder pattern spectrum of [epsilon-(2)H(2)] Tyr labeled chromatophores was interpreted with an 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 motional time scale.