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 Rhodoba cter sphaeroides wild-type cells. PufX is associated with the reaction cent er-light harvesting 1 (RC-LH1) core complex and plays a key role in lateral ubiquinone/ubiquinol transfer. We have determined the PufX/RC stoichiometr y 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 com plexes which had been purified over sucrose gradients. Surprisingly, two gr adient bands with significantly different sedimentation coefficients were f ound to have a similar subunit composition, as judged by absorption spectro scopy and protein gel electrophoresis. Gel filtration chromatography and el ectron microscopy revealed that these membrane complexes represent a monome ric and a dimeric form of the RC-LH1 complex. Since PufX is strictly requir ed 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 th e RC.