Cryo-EM structures of the Synechocystis sp. PCC 6803 cytochrome b6f complex with and without the regulatory PetP subunit.
Cryo-EM structures of the Synechocystis sp. PCC 6803 cytochrome b6f complex with and without the regulatory PetP subunit.
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DOI：
10.1042/bcj20220124
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发表时间：
2022-07-15
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在含氧光合作用中，细胞色素b6f （cytb6f）复合体连接光系统I和II发生的线性电子转移（LET）反应，并通过q循环产生跨膜质子梯度。除了在LET中发挥核心作用外，cytb6f还参与一系列过程，包括循环电子转移（CET）、状态转换和光合作用控制。cytb6f的许多调节作用是由辅助蛋白促进的，这些辅助蛋白因物种而异，但由于它们的微弱和短暂性，这些相互作用的结构细节仍然未知。在蓝藻中LET和CET之间的调节平衡中，一个明显的关键角色是PetP，这是一种约10 kDa的蛋白质，也存在于红藻中，但不存在于绿藻和植物中。在这里，我们使用低温电子显微镜来确定在PetP存在和不存在的情况下Synechocystis sp. PCC 6803 cytb6f复合物的结构。我们的结构表明，PetP与cytb6f的细胞质侧相互作用，取代了PetG亚基的c端，并屏蔽了细胞色素b6的c端，细胞色素b6结合了被认为介导CET的血红素cn辅因子。这些结构还突出了蓝藻和植物细胞6f复合物之间的塑料醌结合模式的关键差异，我们认为这可能反映了蓝藻类囊体膜中光合作用和呼吸电子传递的独特组合。来自一个蓝藻模型物种的cytb6f的结构可适应基因工程，这将加强未来在这一关键ET复合体中结构-功能关系的定点诱变研究。
In oxygenic photosynthesis, the cytochrome b6f (cytb6f) complex links the linear electron transfer (LET) reactions occurring at photosystems I and II and generates a transmembrane proton gradient via the Q-cycle. In addition to this central role in LET, cytb6f also participates in a range of processes including cyclic electron transfer (CET), state transitions and photosynthetic control. Many of the regulatory roles of cytb6f are facilitated by auxiliary proteins that differ depending upon the species, yet because of their weak and transient nature the structural details of these interactions remain unknown. An apparent key player in the regulatory balance between LET and CET in cyanobacteria is PetP, a ∼10 kDa protein that is also found in red algae but not in green algae and plants. Here, we used cryogenic electron microscopy to determine the structure of the Synechocystis sp. PCC 6803 cytb6f complex in the presence and absence of PetP. Our structures show that PetP interacts with the cytoplasmic side of cytb6f, displacing the C-terminus of the PetG subunit and shielding the C-terminus of cytochrome b6, which binds the heme cn cofactor that is suggested to mediate CET. The structures also highlight key differences in the mode of plastoquinone binding between cyanobacterial and plant cytb6f complexes, which we suggest may reflect the unique combination of photosynthetic and respiratory electron transfer in cyanobacterial thylakoid membranes. The structure of cytb6f from a model cyanobacterial species amenable to genetic engineering will enhance future site-directed mutagenesis studies of structure-function relationships in this crucial ET complex.