Takahashi Hiroko, Iwai Masakazu, Takahashi Yuichiro, Minagawa Jun
Department of Biology, Faculty of Science, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan.
Proc Natl Acad Sci U S A. 2006 Jan 10;103(2):477-82. doi: 10.1073/pnas.0509952103. Epub 2006 Jan 3.
State transition in photosynthesis is a short-term balancing mechanism of energy distribution between photosystem I (PSI) and photosystem II (PSII). When PSII is preferentially excited (state 2), a pool of mobile light-harvesting complex II (LHCII) antenna proteins is thought to migrate from PSII to PSI, but biochemical evidence for a physical association between LHCII proteins and PSI in state 2 is weak. Here, using the green alga Chlamydomonas reinhardtii, which has a high capacity for state transitions, we report the isolation of PSI-light-harvesting complex I (LHCI) super-complexes from cells locked into state 1 and state 2. We solubilized the thylakoid membranes with a mild detergent, separated the proteins by sucrose density gradient centrifugation, and subjected gradient fractions to gel-filtration chromatography. Three LHCII polypeptides were associated with a PSI-LHCI supercomplex only in state 2; we identified them as two minor monomeric LHCII proteins (CP26 and CP29) and one previously unreported major LHCII protein type II, or LhcbM5. These three LHCII proteins, in addition to the major trimeric LHCII proteins, were phosphorylated upon transition to state 2. The corresponding phylogenetic tree indicates that among the LHCII proteins associated with PSII, these three LHCII proteins are the most similar to the LHC proteins for PSI (LHCI). Our results are important because CP26, CP29, and LhcbM5, which have been viewed as belonging solely to the PSII complex, are now postulated to shuttle between PSI and PSII during state transitions, thereby acting as docking sites for the trimeric LHCII proteins in both PSI and PSII.
光合作用中的状态转换是光系统I（PSI）和光系统II（PSII）之间能量分配的一种短期平衡机制。当PSII被优先激发时（状态2），一群可移动的捕光复合体II（LHCII）天线蛋白被认为会从PSII迁移到PSI，但在状态2下LHCII蛋白与PSI之间存在物理关联的生化证据很薄弱。在这里，我们使用具有高状态转换能力的绿藻莱茵衣藻，报告了从锁定在状态1和状态2的细胞中分离出PSI-捕光复合体I（LHCI）超级复合体。我们用温和的去污剂溶解类囊体膜，通过蔗糖密度梯度离心分离蛋白质，并对梯度级分进行凝胶过滤色谱分析。仅在状态2下，三种LHCII多肽与PSI-LHCI超级复合体相关联；我们将它们鉴定为两种次要的单体LHCII蛋白（CP26和CP29）以及一种先前未报道的主要LHCII蛋白II型，即LhcbM5。这三种LHCII蛋白，除了主要的三聚体LHCII蛋白外，在转换到状态2时会被磷酸化。相应的系统发育树表明，在与PSII相关联的LHCII蛋白中，这三种LHCII蛋白与PSI的LHC蛋白（LHCI）最为相似。我们的结果很重要，因为一直被认为仅属于PSII复合体的CP26、CP29和LhcbM5，现在被推测在状态转换期间在PSI和PSII之间穿梭，从而在PSI和PSII中充当三聚体LHCII蛋白的对接位点。
