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

Abstract

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.

Functional Consequences of the Organization of the Photosynthetic Apparatus in Rhodobacter sphaeroides

“…In agreement with previous work (14), the dimer band found in the sucrose gradient with the WT was totally absent in preparations from the PufX Ϫ strain. A single RC⅐LH1 band was observed, and these complexes displayed the characteristic monomer behavior as described in the previous article with respect to excitonic connectivity or sharing of the quinone pool.…”

“…This small polypeptide, coded in the puf operon (which also contains the genes for the RC and LH1 subunits), appears to play an important role in controlling the supramolecular structure. It is found at a 1:1 ratio with the RC in purified RC⅐LH1 complexes (14). The sequence is indicative of a single transmembrane ␣-helix with hydrophilic C-and N-terminal loops; a study based on gradual clipping investigated the functional roles of each loop (15).…”

Structure of the dimeric RC–LH1–PufX complex from Rhodobaca bogoriensis investigated by electron microscopy

“…sphaeroides by Western blot analysis. A PufX/RC ratio of 0.86 -0.99 was found [18], which points to a 1 : 1 stoichiometry. Rhodobaca bogoriensis has a PufX protein with a determined sequence (C. Jungas 2011, unpublished data), but the stoichiometry is not known.…”

Symmetry matters for the electronic structure of core complexes from Rhodopseudomonas palustris and Rhodobacter sphaeroides PufX −

“…In WT membranes (PufX ϩ ) these core complexes are actually dimers (11). Deletion of PufX causes the core complexes to become monomeric and complete rings (12). Recently, a lowresolution (4.8 Å), x-ray crystal structure of the monomeric core complex from Rhodopseudomonas palustris has been described (13).…”