Carboxysomes in cyanobacteria enclose the enzymes Rubisco and carbonic anhydrase to optimize photosynthetic carbon fixation. Understanding carboxysome assembly has implications in agricultural biotechnology. Here we analyzed the role of the scaffolding protein CcmM of the β-cyanobacterium Synechococcus elongatus PCC 7942 in sequestrating the hexadecameric Rubisco and the tetrameric carbonic anhydrase, CcaA. We find that the trimeric CcmM, consisting of γCAL oligomerization domains and linked small subunit-like (SSUL) modules, plays a central role in mediation of pre-carboxysome condensate formation through multivalent, cooperative interactions. The γCAL domains interact with the C-terminal tails of the CcaA subunits and additionally mediate a head-to-head association of CcmM trimers. Interestingly, SSUL modules, besides their known function in recruiting Rubisco, also participate in intermolecular interactions with the γCAL domains, providing further valency for network formation. Our findings reveal the mechanism by which CcmM functions as a central organizer of the pre-carboxysome multiprotein matrix, concentrating the core components Rubisco and CcaA before β-carboxysome shell formation.

蓝细菌中的羧基体包围 Rubisco 酶和碳酸酐酶，以优化光合作用碳固定。了解羧基体组装对农业生物技术具有重要意义。在这里，我们分析了 β-蓝细菌细长聚球藻PCC 7942 的支架蛋白 CcmM 在隔离十六聚 Rubisco 和四聚碳酸酐酶 CcaA 中的作用。我们发现，由 γCAL 寡聚结构域和连接的小亚基样 (SSUL) 模块组成的三聚体 CcmM，通过多价协同相互作用在介导前羧基体缩合物形成中发挥着核心作用。 γCAL 结构域与 CcaA 亚基的 C 末端尾部相互作用，并另外介导 CcmM 三聚体的头对头关联。有趣的是，SSUL 模块除了其招募 Rubisco 的已知功能外，还参与与 γCAL 结构域的分子间相互作用，为网络形成提供进一步的效价。我们的研究结果揭示了 CcmM 作为前羧基体多蛋白基质的中心组织者的机制，在 β-羧基体壳形成之前浓缩核心成分 Rubisco 和 CcaA。