Structural characterization of the reaction center-light harvesting 1-PufXY complex from Rhodobacter sphaeroides

Photosynthesis in purple bacteria, represented by Rhodobacter sphaeroides as a long-established model organism, utilizes a protein-cofactor complex to capture solar energy. This complex is embedded in specialized spherical membrane structures and comprises a central reaction center (RC) encircled by a ring of light harvesting 1 (LH1) polypeptides. The energy derived from light absorption drives the reduction of ubiquinone, entering from the membrane, to generate ubiquinol. The photosynthetic cycle is therefore dependent on the exchange of ubiquinone and ubiquinol to and from the RC. This exchange requires an additional protein (PufX) which prevents complete closure of the LH1 ring, leaving a channel which connects the RC with the membrane. The structure of the RC-LH1-PufX complex is under investigation by cryo-EM, revealing the presence of a previously undiscovered protein (PufY) that is also involved in channel formation. Proteomic analysis has verified the identities of the expected protein components of this complex, additionally confirming the presence of the newly-discovered PufY polypeptide mapped into the cryo-EM structure.