Structure of the H subunit of the photosynthetic reaction center from the thermophilic purple sulfur bacterium, Thermochromatium tepidum - Implications for the specific binding of the lipid molecule to the membrane protein complex

The photosynthetic reaction center (RC) is a transmembrane protein complex that catalyzes light-driven electron transport across the photosynthetic me mbrane. The complete amino-acid sequence of the H subunit of the RC from a thermophilic purple sulfur bacterium, Thermochromatium tepidum, has been de termined for the first time among purple sulfur bacteria. The H subunit con sists of 259 amino acids and has a molecular mass of 28 187. The deduced am ino-acid sequences of this H subunit showed a significant (40%) degree of i dentity with those from mesophilic purple nonsulfur bacteria. The determined primary structure of the H subunit was compared with the str uctures of mesophilic B. viridis and R. sphaeroides based on the three-dime nsional structure of the H subunit from T. tepidum, which has been recently determined by X-ray crystallography. One lipid molecule was found in the c rystal structure of the T. tepidum RC, and the head group of the lipid appe ars to be stabilized by the electrostatic interactions with the conserved b asic residues in the H subunit. The above comparison has suggested the exis tence of a lipid-binding site on the molecular surface at which a lipid mol ecule can interact with the RC in a specific manner.