The Reaction Center−LH1 Antenna Complex of Rhodobacter sphaeroides Contains One PufX Molecule Which Is Involved in Dimerization of This Complex

The PufX membrane protein is essential for photosynthetic growth of Rhodobacter sphaeroides wild-type cells. PufX is associated with the reaction center−light harvesting 1 (RC−LH1) core complex and plays a key role in lateral ubiquinone/ubiquinol transfer. We have determined the PufX/RC stoichiometry by quantitative Western blot analysis and RC photobleaching. Independent of copy number effects and growth conditions, one PufX molecule per RC was observed in native membranes as well as in detergent-solubilized RC−LH1 complexes which had been purified over sucrose gradients. Surprisingly, two gradient bands with significantly different sedimentation coefficients were found to have a similar subunit composition, as judged by absorption spectroscopy and protein gel electrophoresis. Gel filtration chromatography and electron microscopy revealed that these membrane complexes represent a monomeric and a dimeric form of the RC−LH1 complex. Since PufX is strictly required for the isolation of dimeric core complexes, we suggest that PufX has a central structural role in forming dimeric RC−LH1 complexes, thus allowing efficient ubiquinone/ubiquinol exchange through the LH1 ring surrounding the RC.