Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University, Okayama, 700-8530, Japan.
Japan Science and Technology Agency, PRESTO, Saitama, 332-0012, Japan.
Commun Biol. 2020 May 11;3(1):232. doi: 10.1038/s42003-020-0949-6.
Iron-stress induced protein A (IsiA) is a chlorophyll-binding membrane-spanning protein in photosynthetic prokaryote cyanobacteria, and is associated with photosystem I (PSI) trimer cores, but its structural and functional significance in light harvesting remains unclear. Here we report a 2.7-Å resolution cryo-electron microscopic structure of a supercomplex between PSI core trimer and IsiA from a thermophilic cyanobacterium Thermosynechococcus vulcanus. The structure showed that 18 IsiA subunits form a closed ring surrounding a PSI trimer core. Detailed arrangement of pigments within the supercomplex, as well as molecular interactions between PSI and IsiA and among IsiAs, were resolved. Time-resolved fluorescence spectra of the PSI-IsiA supercomplex showed clear excitation-energy transfer from IsiA to PSI, strongly indicating that IsiA functions as an energy donor, but not an energy quencher, in the supercomplex. These structural and spectroscopic findings provide important insights into the excitation-energy-transfer and subunit assembly mechanisms in the PSI-IsiA supercomplex.
铁胁迫诱导蛋白 A（IsiA）是光合原核蓝藻中与光系统 I（PSI）三聚体核心相关的叶绿素结合膜跨膜蛋白，但它在光捕获中的结构和功能意义尚不清楚。在这里，我们报道了来自嗜热蓝藻Thermosynechococcus vulcanus 的 PSI 核心三聚体和 IsiA 之间的超复合体的 2.7Å 分辨率冷冻电镜结构。该结构表明，18 个 IsiA 亚基形成一个封闭环，围绕着一个 PSI 三聚体核心。在超复合体中，色素的详细排列以及 PSI 和 IsiA 之间以及 IsiAs 之间的分子相互作用得到了解决。PSI-IsiA 超复合体的时间分辨荧光光谱显示出明显的能量从 IsiA 到 PSI 的转移，这强烈表明 IsiA 在超复合体中作为能量供体，而不是能量猝灭剂。这些结构和光谱学发现为 PSI-IsiA 超复合体中的激发能量转移和亚基组装机制提供了重要的见解。
