The 8.5 Å Projection Structure of the Core RC–LH1–PufX Dimer of Rhodobacter sphaeroides

Two-dimensional crystals of dimeric photosynthetic reaction centre–LH1–PufX complexes have been analysed by cryoelectron microscopy. The 8.5 Å resolution projection map extends previous analyses of complexes within native membranes to reveal the α-helical structure of two reaction centres and 28 LH1 αβ subunits within the dimer. For the first time, we have achieved sufficient resolution to suggest a possible location for the PufX transmembrane helix, the orientation of the RC and the arrangement of helices within the surrounding LH1 complex. Whereas low-resolution projections have shown an apparent break in the LH1, our current map reveals a diffuse density within this region, possibly reflecting high mobility. Within this region the separation between β14 of one monomer and β2 of the other monomer is ∼6 Å larger than the average β–β spacing within LH1; we propose that this is sufficient for exchange of quinol at the RC QB site. We have determined the position and orientation of the RC within the dimer, which places its QB site adjacent to the putative PufX, with access to the point in LH1 that appears most easily breached. PufX appears to occupy a strategic position between the mobile αβ14 subunit and the QB site, suggesting how the structure, possibly coupled with a flexible ring, plays a role in optimizing quinone exchange during photosynthesis.