In green algae and plants, state transitions serve as a short-term light-acclimation process in the regulation of the light-harvesting capacity of photosystems I and II (PSI and PSII, respectively). During the process, a portion of light-harvesting complex II (LHCII) is phosphorylated, dissociated from PSII and binds with PSI to form the supercomplex PSI–LHCI–LHCII. Here, we report high-resolution structures of PSI–LHCI–LHCII from Chlamydomonas reinhardtii, revealing the mechanism of assembly between the PSI–LHCI complex and two phosphorylated LHCII trimers containing all four types of LhcbM protein. Two specific LhcbM isoforms, namely LhcbM1 and LhcbM5, directly interact with the PSI core through their phosphorylated amino terminal regions. Furthermore, biochemical and functional studies on mutant strains lacking either LhcbM1 or LhcbM5 indicate that only LhcbM5 is indispensable in supercomplex formation. The results unravel the specific interactions and potential excitation energy transfer routes between green algal PSI and two phosphorylated LHCIIs.

在绿藻和植物中，状态转换在调节光系统 I 和 II（分别为 PSI 和 PSII）的光捕获能力时充当短期光适应过程。在此过程中，一部分光捕获复合物 II (LHCII) 被磷酸化，从 PSII 解离并与 PSI 结合形成超复合物 PSI–LHCI–LHCII。在这里，我们报告了来自莱茵衣藻的 PSI–LHCI–LHCII 的高分辨率结构, 揭示了 PSI-LHCI 复合物与包含所有四种 LhcbM 蛋白的两个磷酸化 LHCII 三聚体之间的组装机制。两种特定的 LhcbM 亚型，即 LhcbM1 和 LhcbM5，通过其磷酸化的氨基末端区域直接与 PSI 核心相互作用。此外，对缺乏 LhcbM1 或 LhcbM5 的突变菌株的生化和功能研究表明，只有 LhcbM5 在超复合物形成中不可或缺。结果揭示了绿藻 PSI 和两种磷酸化 LHCII 之间的特定相互作用和潜在的激发能量转移途径。