The biological conversion of light energy into chemical energy is performed by a flexible photosynthetic machinery located in the thylakoid membranes. Photosystems I and II (PSI and PSII) are the two complexes able to harvest light. PSI is the last complex of the electron transport chain and is composed of multiple subunits: the proteins building the catalytic core complex that are well conserved between oxygenic photosynthetic organisms, and, in green organisms, the membrane light-harvesting complexes (Lhc) necessary to increase light absorption. In plants, four Lhca proteins (Lhca1-4) make up the antenna system of PSI, which can be further extended to optimize photosynthesis by reversible binding of LHCII, the main antenna complex of photosystem II. Here, we used biochemistry and electron microscopy in Arabidopsis to reveal a previously unknown supercomplex of PSI with LHCII that contains an additional Lhca1-a4 dimer bound on the PsaB-PsaI-PsaH side of the complex. This finding contradicts recent structural studies suggesting that the presence of an Lhca dimer at this position is an exclusive feature of algal PSI. We discuss the features of the additional Lhca dimer in the large plant PSI-LHCII supercomplex and the differences with the algal PSI. Our work provides further insights into the intricate structural plasticity of photosystems.

中文翻译：

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拟南芥中具有一个额外的Lhca1-a4二聚体的大型光系统I采光复合物II超复合物的分离和表征。

将光能生物转化为化学能是通过位于类囊体膜中的柔性光合作用机械进行的。光系统I和II（PSI和PSII）是两个能够收集光的复合物。PSI是电子传输链的最后一个复合物，由多个亚基组成：构建催化核心复合物的蛋白质在氧合光合生物之间具有良好的保守性，而在绿色生物中，则是膜光捕获复合物（Lhc）增加光吸收。在植物中，四种Lhca蛋白（Lhca1-4）构成了PSI的天线系统，可以通过可逆结合LHCII（光系统II的主要天线复合体）来进一步扩展以优化光合作用。这里，我们在拟南芥中使用生物化学和电子显微镜观察了PSI与LHCII的先前未知的超复合物，该复合物包含在复合物的PsaB-PsaI-PsaH侧结合的另一个Lhca1-a4二聚体。这一发现与最近的结构研究相矛盾，该研究表明在该位置存在Lhca二聚体是藻类PSI的独有特征。我们讨论了大型植物PSI-LHCII超复合物中其他Lhca二聚体的特征以及与藻类PSI的区别。我们的工作为光系统的复杂结构可塑性提供了进一步的见解。这一发现与最近的结构研究相矛盾，该研究表明在该位置存在Lhca二聚体是藻类PSI的独有特征。我们讨论了大型植物PSI-LHCII超复合物中其他Lhca二聚体的特征以及与藻类PSI的区别。我们的工作为光系统的复杂结构可塑性提供了进一步的见解。这一发现与最近的结构研究相矛盾，该研究表明在该位置存在Lhca二聚体是藻类PSI的独有特征。我们讨论了大型植物PSI-LHCII超复合物中其他Lhca二聚体的特征以及与藻类PSI的区别。我们的工作为光系统的复杂结构可塑性提供了进一步的见解。