Rubisco accumulation factor 1 (Raf1) plays essential roles in mediating Rubisco assembly and carboxysome biogenesis
Rubisco accumulation factor 1 (Raf1) plays essential roles in mediating Rubisco assembly and carboxysome biogenesis
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DOI：
10.1073/pnas.2007990117
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发表时间：
2020-07-21
影响因子：
11.1
通讯作者：
Liu, Lu-Ning
中科院分区：
文献类型：
--
作者：
Huang, Fang;Kong, Wen-Wen;Liu, Lu-Ning
羧基体是蓝藻碳同化的无膜细胞器。羧体由结构类似于病毒衣壳的蛋白质外壳和内部酶组成，包括光合作用中主要的固碳酶--核酮糖1，5-二磷酸羧基/加氧酶(Rubisco)。羧化体的形成需要数千个蛋白质亚基的分级自组装，从Rubisco组装和包装到外壳包裹。在这里，我们研究了Rubisco组装因子1(Raf1)在模式蓝藻-细长聚球藻PCC7942(Syn7942)中Rubisco组装和羧体形成中的作用。冷冻电子显微镜显示，Raf1通过调节rbcL二聚体的形成和二聚体-二聚体的相互作用促进Rubisco的组装。缺乏Raf1的Syn7942细胞不能形成规范的完整羧体，而是产生大量的中间组件，包括Rubisco、CCAA、CcmM和CcmN，没有壳包被，以及低丰度的羧体结构，尺寸减小，壳形状和内部组织不规则。因此，Raf1耗尽的细胞表现出Rubisco含量、固定二氧化碳的活性和细胞生长的减少。我们的结果为伴侣辅助的Rubisco组装和羧基体的生物发生提供了机械性的见解。深入了解具有生物地球化学重要性的细胞器的生物发生和逐步形成过程，可能会为异源工程设计功能固定二氧化碳模块以改善光合作用提供参考。
Carboxysomes are membrane-free organelles for carbon assimilation in cyanobacteria. The carboxysome consists of a proteinaceous shell that structurally resembles virus capsids and internal enzymes including ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), the primary carbon-fixing enzyme in photosynthesis. The formation of carboxysomes requires hierarchical self-assembly of thousands of protein subunits, initiated from Rubisco assembly and packaging to shell encapsulation. Here we study the role of Rubisco assembly factor 1 (Raf1) in Rubisco assembly and carboxysome formation in a model cyanobacterium, Synechococcus elongatus PCC7942 (Syn7942). Cryo-electron microscopy reveals that Raf1 facilitates Rubisco assembly by mediating RbcL dimer formation and dimer-dimer interactions. Syn7942 cells lacking Raf1 are unable to form canonical intact carboxysomes but generate a large number of intermediate assemblies comprising Rubisco, CcaA, CcmM, and CcmN without shell encapsulation and a low abundance of carboxysome-like structures with reduced dimensions and irregular shell shapes and internal organization. As a consequence, the Raf1-depleted cells exhibit reduced Rubisco content, CO2-fixing activity, and cell growth. Our results provide mechanistic insight into the chaperone-assisted Rubisco assembly and biogenesis of carboxysomes. Advanced understanding of the biogenesis and stepwise formation process of the biogeochemically important organelle may inform strategies for heterologous engineering of functional CO2-fixing modules to improve photosynthesis.