Photosynthesis Research Center, Key Laboratory of Photobiology, Institute of Botany, Chinese Academy of Sciences, 100093, Beijing, China.
University of Chinese Academy of Sciences, 100049, Beijing, China.
Nat Commun. 2024 May 28;15(1):4535. doi: 10.1038/s41467-024-48878-x.
Cryptophyte algae are an evolutionarily distinct and ecologically important group of photosynthetic unicellular eukaryotes. Photosystem II (PSII) of cryptophyte algae associates with alloxanthin chlorophyll a/c-binding proteins (ACPs) to act as the peripheral light-harvesting system, whose supramolecular organization is unknown. Here, we purify the PSII-ACPII supercomplex from a cryptophyte alga Chroomonas placoidea (C. placoidea), and analyze its structure at a resolution of 2.47 Å using cryo-electron microscopy. This structure reveals a dimeric organization of PSII-ACPII containing two PSII core monomers flanked by six symmetrically arranged ACPII subunits. The PSII core is conserved whereas the organization of ACPII subunits exhibits a distinct pattern, different from those observed so far in PSII of other algae and higher plants. Furthermore, we find a Chl a-binding antenna subunit, CCPII-S, which mediates interaction of ACPII with the PSII core. These results provide a structural basis for the assembly of antennas within the supercomplex and possible excitation energy transfer pathways in cryptophyte algal PSII, shedding light on the diversity of supramolecular organization of photosynthetic machinery.
隐藻是一类具有独特进化地位和重要生态功能的光合单细胞真核生物。隐藻的光系统 II（PSII）与别藻黄素叶绿素 a/c 结合蛋白（ACP）结合，作为外周光捕获系统，其超分子结构尚不清楚。在这里，我们从隐藻 Chroomonas placoidea（C. placoidea）中纯化了 PSII-ACPII 超复合体，并使用冷冻电镜在 2.47 Å 的分辨率下分析了其结构。该结构揭示了 PSII-ACPII 的二聚体组织，包含两个 PSII 核心单体，两侧是六个对称排列的 ACPII 亚基。PSII 核心是保守的，而 ACPII 亚基的组织表现出与迄今为止在其他藻类和高等植物 PSII 中观察到的不同的独特模式。此外，我们发现了一个 Chl a 结合天线亚基 CCPII-S，它介导 ACPII 与 PSII 核心的相互作用。这些结果为超复合体中天线的组装和隐藻 PSII 中可能的激发能量转移途径提供了结构基础，揭示了光合作用机制的超分子组织多样性。
