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.

Photosystem II（PSII）是一种多亚基色素-蛋白质复合物，可催化光驱动的水氧化，从而将光能转化为化学能并释放出分子氧。Psb27是小的类囊体腔定位蛋白，已知可作为PSII复合物的生物发生和修复的组装因子。Psb27在中间PSII中的确切位置和结合方式仍然难以捉摸。在这里，我们报告从蓝藻嗜热嗜热球菌的psbV缺失突变体（ΔPsbV）纯化的二聚体Psb27-PSII复合物的结构。，通过冷冻电子显微镜解决。我们的结构表明，Psb27在腔侧与CP43相关联，在Psb27的螺旋2和3之间形成了特定的相互作用，在CP43的螺旋3和4之间形成了一个环区域（环C），以及较大的管腔CP43的裸露亲水环。Psb27的结合与PsbO的N端区域产生了一些冲突，并且还引起了CP43，CP47和D2的构象变化。这使得PsbO无法与Psb27-PSII结合。D1，PsbE，PsbF和PsbZ也发生了构象变化。这与CP43，CP47和D2中发生的构象变化一起，可能会阻止PsbU的结合并诱导PsbJ的解离。该结构信息提供了对PSII的生物发生和修复中Psb27调控机制的重要见解。