Abstract：
In oxygenic photosynthesis, state transitions distribute light energy between Photosystem I and Photosystem II. This regulation involves reduction of the plastoquinone pool, activation of the State Transitions 7 (STT7) protein kinase by the cytochrome b6f complex, and phosphorylation and migration of Light Harvesting Complex II (LHCII). Here, we show that in Chlamydomonas reinhardtii, the C-terminus of the cyt b6 subunit PetB acts on phosphorylation of STT7 and state transitions. We used site-directed mutagenesis of the chloroplast petB gene to truncate (remove L215b6) or elongate (add G216b6) the cyt b6 subunit. Modified complexes are devoid of heme ci and degraded by FTSH protease, revealing that salt bridge formation between cyt b6 (PetB) and subunit IV (PetD) is key to the assembly of the complex. In double mutants where FTSH is inactivated, modified cyt b6f accumulated but the phosphorylation cascade was blocked. We also replaced the arginine interacting with heme ci propionate (R207Kb6). In this modified complex, heme ci is present but the kinetics of phosphorylation are slower. We show that highly phosphorylated forms of STT7 accumulated transiently after reduction of the PQ pool and represent the active forms of the protein kinase. Phosphorylation of the LHCII targets is favored at the expense of the protein kinase, and the migration of LHCII towards PSI is the limiting step for state transitions.
摘要：
在氧气光合作用中，状态转变在光系统I和光系统II之间分配光能。这种调节涉及到减少塑性醌池，细胞色素b6f复合物激活状态转变7（STT7）蛋白激酶，以及光收获复合物II(LHCII)的磷酸化和迁移。这里,我们证明在莱茵衣藻中，cytb6亚基PetB的C端作用于STT7的磷酸化和状态转换。我们使用叶绿体petB基因的定点诱变来截短（去除L215b6）或延长（添加G216b6）cytb6亚基。修饰的复合物缺乏血红素ci，被FTSH蛋白酶降解，揭示了cytb6（PetB）和亚基IV（PetD）之间的盐桥形成是复合物组装的关键。在FTSH失活的双突变体中，修饰的cytb6f积累，但磷酸化级联被阻断。我们还取代了精氨酸与血红素丙酸酯（R207Kb6）的相互作用。在这个修改后的复合体中，血红素ci存在，但磷酸化的动力学较慢。我们表明，在PQ池还原后，STT7的高度磷酸化形式会短暂积累，并代表蛋白激酶的活性形式。LHCII靶标的磷酸化是以蛋白激酶为代价的。LHCII向PSI的迁移是状态转换的限制步骤。
