Wysocka Anna, Kulik Natalia, Shukla Mahendra K, Opatíková Monika, Kouřil Roman, Jackson Philip J, Brindley Amanda A, Janouškovec Jan, Kiss Éva, Hitchcock Andrew, Komenda Josef, Hunter C Neil, Sobotka Roman
Centre Algatech, Institute of Microbiology, Academy of Sciences of the Czech Republic, Třeboň 379 01, Czech Republic.
Faculty of Science, University of South Bohemia, České Budějovice 370 01, Czech Republic.
Plant Physiol. 2025 May 30;198(2). doi: 10.1093/plphys/kiaf213.
The biogenesis of Photosystem II is a complicated process requiring numerous auxiliary factors to assist in all steps of its assembly. The cyanobacterial protein Ycf39 forms a stress-induced complex with 2 small chlorophyll-binding, High-light-inducible proteins C and D (HliC and HliD), and has been reported to participate in the insertion of chlorophyll molecules into the central D1 subunit of Photosystem II. However, how this process is organized remains unknown. Here, we show that Ycf39 and both HliC and HliD can form distinct complexes with chlorophyll synthase (ChlG) in the model cyanobacterium Synechocystis sp. PCC 6803. We isolated and characterized ChlG complexes from various strains grown under different conditions and provide a mechanistic view of the docking of Ycf39 to ChlG via HliD and the structural role of HliC. In the absence of stress, chlorophyll is produced by the ChlG-HliD2-ChlG complex, which is stabilized by chlorophyll and zeaxanthin molecules bound to the HliD homodimer. The switch to high light leads to stress pressure and greatly elevated synthesis of HliC, resulting in the replacement of HliD homodimers with HliC-HliD heterodimers. Unlike HliD, HliC cannot interact directly with ChlG or Ycf39. Therefore, the original ChlG-HliD2-ChlG complex is converted into a ChlG-HliD-HliC hetero-trimer that presumably binds transiently to Ycf39 and the nascent D1 polypeptide. We speculate that this molecular machinery promotes the delivery of chlorophyll to D1 upon high-light-induced chlorophyll deficiency. The HliD homodimers formed under standard, nonstress growth conditions and attached to ChlG could serve as an emergency chlorophyll reserve.
光系统II的生物合成是一个复杂的过程，在其组装的各个步骤都需要众多辅助因子的协助。蓝藻蛋白Ycf39与2种小型叶绿素结合蛋白——高光诱导蛋白C和D（HliC和HliD）形成应激诱导复合物，并且据报道参与叶绿素分子插入光系统II的中心D1亚基的过程。然而，这个过程是如何组织的仍然未知。在这里，我们表明在模式蓝藻集胞藻PCC 6803中，Ycf39以及HliC和HliD都能与叶绿素合酶（ChlG）形成不同的复合物。我们从在不同条件下生长的各种菌株中分离并鉴定了ChlG复合物，并提供了Ycf39通过HliD与ChlG对接的机制观点以及HliC的结构作用。在没有应激的情况下，叶绿素由ChlG-HliD2-ChlG复合物产生，该复合物通过与HliD同型二聚体结合的叶绿素和玉米黄质分子而稳定。切换到高光会导致应激压力并极大地提高HliC的合成，从而导致HliD同型二聚体被HliC-HliD异源二聚体取代。与HliD不同，HliC不能直接与ChlG或Ycf39相互作用。因此，原始的ChlG-HliD2-ChlG复合物转变为ChlG-HliD-HliC异源三聚体，推测该三聚体与Ycf39和新生的D1多肽短暂结合。我们推测这种分子机制在高光诱导的叶绿素缺乏时促进叶绿素向D1的传递。在标准的、无应激生长条件下形成并附着于ChlG的HliD同型二聚体可以作为应急叶绿素储备。
