Structural insights into a dimeric Psb27-photosystem II complex from a cyanobacterium Thermosynechococcus vulcanus

Significance Photosystem II (PSII) is a light-driven water:plastoquinone oxidoreductase in oxygenic photosynthetic organisms. Several inactive PSII intermediates are involved in the biogenesis of the multisubunit PSII complexes as well as in their repair after unavoidable oxidative damage targeted to PSII under light. Psb27 is one of the assembly factors associated with inactive PSII intermediate and plays important roles in the biogenesis/repair of PSII. Here, we report the structure of a dimeric Psb27-PSII from a thermophilic cyanobacterium Thermosynechococcus vulcanus by cryo-electron microscopy. Our results reveal the location and binding properties of Psb27 in the intermediate PSII and show the structural differences between the intermediate and native PSII. These results provide important clues for the roles of Psb27 in the biogenesis/repair of PSII. Photosystem II (PSII) is a multisubunit pigment-protein complex and catalyzes light-driven water oxidation, leading to the conversion of light energy into chemical energy and the release of molecular oxygen. Psb27 is a small thylakoid lumen-localized protein known to serve as an assembly factor for the biogenesis and repair of the PSII complex. The exact location and binding fashion of Psb27 in the intermediate PSII remain elusive. Here, we report the structure of a dimeric Psb27-PSII complex purified from a psbV deletion mutant (ΔPsbV) of the cyanobacterium Thermosynechococcus vulcanus, solved by cryo-electron microscopy. Our structure showed that Psb27 is associated with CP43 at the luminal side, with specific interactions formed between Helix 2 and Helix 3 of Psb27 and a loop region between Helix 3 and Helix 4 of CP43 (loop C) as well as the large, lumen-exposed and hydrophilic E-loop of CP43. The binding of Psb27 imposes some conflicts with the N-terminal region of PsbO and also induces some conformational changes in CP43, CP47, and D2. This makes PsbO unable to bind in the Psb27-PSII. Conformational changes also occurred in D1, PsbE, PsbF, and PsbZ; this, together with the conformational changes occurred in CP43, CP47, and D2, may prevent the binding of PsbU and induce dissociation of PsbJ. This structural information provides important insights into the regulation mechanism of Psb27 in the biogenesis and repair of PSII.