Structural Analysis ofthe Homodimeric Reaction CenterComplex from the Photosynthetic Green Sulfur Bacterium Chlorobaculumtepidum

摘要

The reaction center (RC) complex of the green sulfur bacterium Chlorobaculum tepidum is composed of the Fenna–Matthews–Olson antenna protein (FMO) and the reaction center core (RCC) complex. The RCC complex has four subunits: PscA, PscB, PscC, and PscD. We studied the FMO/RCC complex by chemically cross-linking the purified sample followed by biochemical and spectroscopic analysis. Blue-native gels showed that there were two types of FMO/RCC complexes, which are consistent with complexes with one copy of FMO per RCC and two copies of FMO per RCC. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis analysis of the samples after cross-linking showed that all five subunits of the RC can be linked by three different cross-linkers: bissulfosuccinimidyl suberate, disuccinimidyl suberate, and 3,3-dithiobis-sulfosuccinimidyl propionate. The interaction sites of the cross-linked complex were also studied using liquid chromatography coupled to tandem mass spectrometry. The results indicated that FMO, PscB, PscD, and part of PscA are exposed on the cytoplasmic side of the membrane. PscD helps stabilize FMO to the reaction center andmay facilitate transfer of the electron from the RC to ferredoxin.The soluble domain of the heme-containing cytochrome subunit PscCand part of the core subunit PscA are located on the periplasmic sideof the membrane. There is a close relationship between the periplasmicportions of PscA and PscC, which is needed for the efficient transferof the electron between PscC and P840.