Department of Life Sciences, Imperial College London, London SW7 2AZ, UK.
Ann Bot. 2010 Jul;106(1):1-16. doi: 10.1093/aob/mcq059. Epub 2010 Mar 25.
Photosystem II (PSII) is the light-driven water:plastoquinone oxidoreductase of oxygenic photosynthesis and is found in the thylakoid membrane of chloroplasts and cyanobacteria. Considerable attention is focused on how PSII is assembled in vivo and how it is repaired following irreversible damage by visible light (so-called photoinhibition). Understanding these processes might lead to the development of plants with improved growth characteristics especially under conditions of abiotic stress.
Here we summarize recent results on the assembly and repair of PSII in cyanobacteria, which are excellent model organisms to study higher plant photosynthesis.
Assembly of PSII is highly co-ordinated and proceeds through a number of distinct assembly intermediates. Associated with these assembly complexes are proteins that are not found in the final functional PSII complex. Structural information and possible functions are beginning to emerge for several of these 'assembly' factors, notably Ycf48/Hcf136, Psb27 and Psb28. A number of other auxiliary proteins have been identified that appear to have evolved since the divergence of chloroplasts and cyanobacteria. The repair of PSII involves partial disassembly of the damaged complex, the selective replacement of the damaged sub-unit (predominantly the D1 sub-unit) by a newly synthesized copy, and reassembly. It is likely that chlorophyll released during the repair process is temporarily stored by small CAB-like proteins (SCPs). A model is proposed in which damaged D1 is removed in Synechocystis sp. PCC 6803 by a hetero-oligomeric complex composed of two different types of FtsH sub-unit (FtsH2 and FtsH3), with degradation proceeding from the N-terminus of D1 in a highly processive reaction. It is postulated that a similar mechanism of D1 degradation also operates in chloroplasts. Deg proteases are not required for D1 degradation in Synechocystis 6803 but members of this protease family might play a supplementary role in D1 degradation in chloroplasts under extreme conditions.
光系统 II（PSII）是产氧光合作用中驱动水与质体醌氧化还原的光驱动酶，存在于叶绿体和蓝藻的类囊体膜中。人们非常关注 PSII 如何在体内组装，以及在可见光（所谓的光抑制）不可逆损伤后如何修复。了解这些过程可能会导致开发出具有改良生长特性的植物，特别是在非生物胁迫条件下。
本文总结了蓝藻中 PSII 组装和修复的最新研究结果，蓝藻是研究高等植物光合作用的极佳模式生物。
PSII 的组装高度协调，并通过多个不同的组装中间产物进行。与这些组装复合物相关的是，最终功能性 PSII 复合物中不存在的蛋白质。对于其中的一些“组装”因子，如 Ycf48/Hcf136、Psb27 和 Psb28，结构信息和可能的功能开始显现。已经鉴定出许多其他辅助蛋白，它们似乎是在叶绿体和蓝藻分化后进化而来的。PSII 的修复涉及到受损复合物的部分解体，通过新合成的副本选择性替换受损的亚基（主要是 D1 亚基），然后重新组装。在修复过程中释放的叶绿素可能暂时由小的 CAB 样蛋白（SCPs）储存。提出了一个模型，即 Synechocystis sp. PCC 6803 中的受损 D1 由两种不同类型的 FtsH 亚基（FtsH2 和 FtsH3）组成的异源寡聚复合物去除，降解从 D1 的 N 端以高度连续的反应进行。推测叶绿体中也存在类似的 D1 降解机制。在 Synechocystis 6803 中，D1 降解不需要 Deg 蛋白酶，但该蛋白酶家族的成员可能在叶绿体中极端条件下的 D1 降解中发挥辅助作用。
