Monomeric and dimeric PufX-containing core complexes have been purified from membranes of wild-type Rhodobacter sphaeroides. Reconstitution of both samples by detergent removal in the presence of lipids leads to the formation of two-dimensional crystals constituted of dimeric core complexes. Two-dimensional crystals were further analyzed by cryoelectron microscopy and atomic force microscopy. A projection map at 26-Å resolution reveals that core complexes assemble in an "S"-shaped dimeric complex. Each core complex is composed of one reaction center, 12 light-harvesting 1 ␣/␤-heterodimers, and one PufX protein. The light-harvesting 1 assemblies are open with a gap of density of ϳ30-Å width and surround oriented reaction centers. A maximum density is found at the dimer junction. Based on the projection map, a model is proposed, in which the two PufX proteins are located at the dimer junction, consistent with the finding of dimerization of monomeric core complexes upon reconstitution. This localization of PufX in the core complex implies that PufX is the structural key for the dimer complex formation rather than a channel-forming protein for the exchange of ubiquinone/ubiquinol between the reaction center and the cytochrome bc1 complex.In purple photosynthetic bacteria, highly organized transmembrane pigment-protein complexes perform absorption of light and its conversion into chemical energy. Two light-harvesting complexes (LH), 1 LH2 and LH1, ensure the collection of light. Then the excitation energy is funneled toward the special pair of bacteriochlorophylls in the reaction center (RC), followed by an electron transfer from the special pair of bacteriochlorophylls to the ubiquinone (Q) acceptors Q A and Q B . After two photoreactions and proton captures, ubiquinol (QH 2 ) is formed at the Q B site that dissociates from the RC into the membrane. The bc 1 complex utilizes QH 2 and oxidized cytochrome c 2 as reductant and oxidant, respectively. The net result is a cyclic electron transfer that promotes the formation of a proton gradient across the membrane, which is utilized for ATP synthesis by F 1 F 0 -ATP synthase (for a review, see Ref. 1).The mechanisms of excitonic energy migration and photoinduced electron transfer have been studied in detail using biophysical approaches and analyzed at the molecular level following the resolution of the atomic structures of the RC (2), LH2 (3, 4), and the bc 1 complex (5). However, despite the wealth of information available on the individual proteins, their supramolecular organization in the membrane remains undetermined. A crucial issue is the spatial organization between LH1 and RC, forming the so-called core complex, in which the transformation of light energy into charge separation occurs. So far, no atomic model of the core complex is available. Early studies by various electron microscopy (EM) techniques of native membranes of Rhodopseudomonas viridis and Ectothiorhodospira halochloris have shown a single RC surrounded by a closed circle of light-harvesting molecule...

We have studied photosynthetic membranes of wild type Rhodobacter blasticus, a closely related strain to the well studied Rhodobacter sphaeroides, using atomic force microscopy. High-resolution atomic force microscopy topographs of both cytoplasmic and periplasmic surfaces of LH2 and RC-LH1-PufX (RC, reaction center) complexes were acquired in situ. The LH2 is a nonameric ring inserted into the membrane with the 9-fold axis perpendicular to the plane. The core complex is an S-shaped dimer composed of two RCs, each encircled by 13 LH1 ␣/␤-heterodimers, and two PufXs. The LH1 assembly is an open ellipse with a topography-free gap of ϳ25 Å. The two PufXs, one of each core, are located at the dimer center. Based on our data, we propose a model of the core complex, which provides explanation for the PufX-induced dimerization of the Rhodobacter core complex. The Q B site is located facing a ϳ25-Å wide gap within LH1, explaining the PufXfavored quinone passage in and out of the core complex.In purple photosynthetic bacteria, highly organized transmembrane pigment-protein complexes perform absorption of light and its conversion into chemical energy. Two light harvesting (LH) 1 complexes, LH2 and LH1, ensure the collection of light. The excitation energy is funneled toward the special pair (P) of bacteriochlorophylls in the reaction center (RC) followed by an electron transfer from P to the ubiquinone (Q) acceptors, Q A and Q B . After two photoreactions and proton captures, ubiquinol (Q-H 2 ) is formed at the Q B site that dissociates from the RC into the membrane. The cytochrome bc 1 complex utilizes Q-H 2 and oxidized cytochrome c 2 as reductant and oxidant, respectively. The net result is a cyclic electron transfer that promotes the formation of a proton gradient across the membrane, which is utilized for ATP synthesis by the F 1 F 0 -ATP synthase (for review see Ref. 1).The description of the bacterial photosynthetic apparatus at atomic level is nearly complete. Two RC structures (2-4), two LH2 structures (5, 6), and the structure of the homologue bc 1 complex from the respiratory chain (7) are known. However, the structure of the core complex composed of the LH1 and the RC remains undetermined.Over the last two years, the data of core complexes of Rhodospirillum (Rsp.) rubrum (8), Blastochloris viridis (9), and Rhodopseudomonas (Rps.) palustris (10) have been acquired at a resolution sufficient to delineate the LH1 subunit arrangement around the RC. Whereas Rsp. rubrum and B. viridis have monomeric core complexes with 16 LH1 subunits arranged around the RC, Rps. palustris has 15 LH1 subunits plus an unknown polypeptide forming a single trans-membrane helix. The closed architecture of the LH1 assembly raised the question of the quinone exchange through the LH1 assembly between the RC and the cytochrome bc 1 .In this context, the structural analysis of Rhodobacter core complexes is of particular interest, because it is the only species in which a small trans-membrane protein named PufX has been identified as crucial ...