Hagman A, Shi L X, Rintamäki E, Andersson B, Schröder W P
Department of Biochemistry, Arrhenius Laboratories for Natural Sciences, Stockholm University, S-106 91 Stockholm, Sweden.
Biochemistry. 1997 Oct 21;36(42):12666-71. doi: 10.1021/bi970685o.
The repair of photoinhibitory damage to photosystem II involves the rapid degradation and turnover of the D1 reaction center subunit. Additional protein subunits which show a limited degradation at high light intensities are the complementary reaction center subunit, D2, and the two chlorophyll a binding proteins, CP 47 and CP 43. In this work, we provide the first evidence for light-induced degradation of a nuclear-encoded subunit of photosystem II, the recently discovered PsbW protein. This 6.1 kDa protein is predicted to have a single membrane span and was found to be closely associated with the photosystem II reaction center. The degradation of the PsbW protein was demonstrated by photoinhibitory experiments, both in vitro, using thylakoid membranes and photosystem II core particles, and in vivo using leaf discs. The PsbW protein showed almost the same rate and extent of degradation as the D1 protein, and its degradation was more pronounced compared to the D2 and CP 43 proteins. The degradation of the PsbW protein was shown to share many mechanistic similarities with the more well characterized D1 protein degradation, such as oxygen dependence, sensitivity to serine protease inhibitors, and high light triggering while the actual degradation could readily occur in total darkness. The degradation of the PsbW protein was impaired by protein phosphorylation, although this protein was not itself phosphorylated. This impairment was correlated to the phosphorylation of the D1 protein which has been shown to block its degradation during photoinhibitory conditions. It is concluded that the PsbW protein is not degraded as a direct consequence of primary photodamage but due to a general destabilization of the photosystem II complex under conditions were the D1 protein becomes degraded in the absence of a sufficient repair system. The results are discussed in terms of a requirement for coordination between degradation and protein synthesis/integration during the repair process of photodamaged photosystem II reaction centers.
光系统II光抑制损伤的修复涉及D1反应中心亚基的快速降解和周转。在高光强度下显示有限降解的其他蛋白质亚基是互补反应中心亚基D2以及两个叶绿素a结合蛋白CP 47和CP 43。在这项工作中，我们首次提供了光诱导光系统II核编码亚基（最近发现的PsbW蛋白）降解的证据。这种6.1 kDa的蛋白预计有一个单一的跨膜结构域，并且被发现与光系统II反应中心紧密相关。通过光抑制实验证明了PsbW蛋白的降解，体外实验使用类囊体膜和光系统II核心颗粒，体内实验使用叶盘。PsbW蛋白显示出与D1蛋白几乎相同的降解速率和程度，并且与D2和CP 43蛋白相比，其降解更明显。结果表明，PsbW蛋白的降解与特征更明确的D1蛋白降解在许多机制上具有相似性，如对氧的依赖性、对丝氨酸蛋白酶抑制剂的敏感性以及高光触发，而实际降解在完全黑暗中也很容易发生。尽管该蛋白本身未被磷酸化，但PsbW蛋白的降解受到蛋白磷酸化的损害。这种损害与D1蛋白的磷酸化相关，已表明D1蛋白的磷酸化在光抑制条件下会阻止其降解。得出的结论是，PsbW蛋白不是初级光损伤的直接后果而被降解，而是由于在缺乏足够修复系统时D1蛋白降解的条件下光系统II复合物的普遍不稳定。根据光损伤的光系统II反应中心修复过程中降解与蛋白质合成/整合之间协调的要求对结果进行了讨论。
