Carboxysomes, protein-coated organelles in cyanobacteria, are important in global carbon fixation. However, these organelles are present at low copy in each cell and hence must be segregated to ensure transmission from one generation to the next. Recent studies revealed that a DNA partition-like ParA–ParB system mediates carboxysome maintenance, called McdA-McdB. Here, we describe the first McdA and McdB homolog structures. McdA is similar to partition ParA Walker-box proteins, but lacks the P-loop signature lysine involved in ATP binding. Strikingly, a McdA-ATP structure shows that a lysine distant from the P-loop and conserved in McdA homologs, enables ATP-dependent nucleotide sandwich dimer formation. Similar to partition ParA proteins this ATP-bound form binds nonspecific-DNA. McdB, which we show directly binds McdA, harbors a unique fold and appears to form higher-order oligomers like partition ParB proteins. Thus, our data reveal a new signature motif that enables McdA dimer formation and indicates that, similar to DNA segregation, carboxysome maintenance systems employ Walker-box proteins as DNA-binding motors while McdB proteins form higher order oligomers, which could function as adaptors to link carboxysomes and provide for stable transport by the McdA proteins.

中文翻译：

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羧基体分布 Walker-box McdA 和 McdB 接头同系物的维持结构


羧基体是蓝藻中蛋白质包被的细胞器，在全球碳固定中发挥着重要作用。然而，这些细胞器在每个细胞中以低拷贝存在，因此必须进行分离以确保从一代传递到下一代。最近的研究表明，类似 DNA 分区的 ParA-ParB 系统介导羧基体的维持，称为 McdA-McdB。在这里，我们描述了第一个 McdA 和 McdB 同源结构。 McdA 与 ParA Walker-box 蛋白相似，但缺少参与 ATP 结合的 P 环特征赖氨酸。引人注目的是，McdA-ATP 结构表明，远离 P 环并在 McdA 同系物中保守的赖氨酸能够形成 ATP 依赖性核苷酸夹心二聚体。与分区 ParA 蛋白类似，这种 ATP 结合形式结合非特异性 DNA。我们证明 McdB 直接结合 McdA，具有独特的折叠，并且似乎形成高阶寡聚物，如分区 ParB 蛋白。因此，我们的数据揭示了一个新的特征基序，该基序能够形成 McdA 二聚体，并表明，与 DNA 分离类似，羧基维持系统采用 Walker-box 蛋白作为 DNA 结合马达，而 McdB 蛋白形成更高阶的寡聚体，其可以作为适配器连接羧基体并提供 McdA 蛋白的稳定运输。