Department of Biology, Washington University, St. Louis, Missouri 63130, USA.
J Biol Chem. 2011 Jul 15;286(28):24865-71. doi: 10.1074/jbc.M111.246231. Epub 2011 May 18.
Photosystem II (PSII) is a large membrane bound molecular machine that catalyzes light-driven oxygen evolution from water. PSII constantly undergoes assembly and disassembly because of the unavoidable damage that results from its normal photochemistry. Thus, under physiological conditions, in addition to the active PSII complexes, there are always PSII subpopulations incompetent of oxygen evolution, but are in the process of undergoing elaborate biogenesis and repair. These transient complexes are difficult to characterize because of their low abundance, structural heterogeneity, and thermodynamic instability. In this study, we show that a genetically tagged Psb27 protein allows for the biochemical purification of two monomeric PSII assembly intermediates, one with an unprocessed form of D1 (His27ΔctpAPSII) and a second one with a mature form of D1 (His27PSII). Both forms were capable of light-induced charge separation, but unable to photooxidize water, largely because of the absence of a functional tetramanganese cluster. Unexpectedly, there was a significant amount of the extrinsic lumenal PsbO protein in the His27PSII, but not in the His27ΔctpAPSII complex. In contrast, two other lumenal proteins, PsbU and PsbV, were absent in both of these PSII intermediate complexes. Additionally, the only cytoplasmic extrinsic protein, Psb28 was detected in His27PSII complex. Based on these data, we have presented a refined model of PSII biogenesis, illustrating an important role of Psb27 as a gate-keeper during the complex assembly process of the oxygen-evolving centers in PSII.
光系统 II（PSII）是一种大型膜结合分子机器，可催化水光驱动的氧气产生。由于其正常光化学产生的不可避免的损伤，PSII 不断经历组装和拆卸。因此，在生理条件下，除了活性 PSII 复合物外，总是存在不能进行氧气产生的 PSII 亚群，但它们正在进行精细的生物发生和修复。这些瞬态复合物由于其丰度低、结构异质性和热力学不稳定性而难以表征。在这项研究中，我们表明，一种基因标记的 Psb27 蛋白允许对两种单体 PSII 组装中间体进行生化纯化，一种是具有未加工形式的 D1（His27ΔctpAPSII），另一种是具有成熟形式的 D1（His27PSII）。这两种形式都能够进行光诱导电荷分离，但不能光氧化水，主要是因为缺乏功能四锰簇。出乎意料的是，在 His27PSII 中有大量的外在腔内腔 PsbO 蛋白，但在 His27ΔctpAPSII 复合物中没有。相比之下，这两种 PSII 中间复合物中都没有另外两种腔内腔蛋白 PsbU 和 PsbV。此外，仅检测到细胞质外在蛋白 Psb28 存在于 His27PSII 复合物中。基于这些数据，我们提出了 PSII 生物发生的改进模型，说明了 Psb27 在 PSII 放氧中心的复合物组装过程中作为门控蛋白的重要作用。
