Carboxysomes are protein-based organelles that are essential for allowing cyanobacteria to fix CO2. Previously, we identified a two-component system, McdAB, responsible for equidistantly positioning carboxysomes in the model cyanobacterium Synechococcus elongatus PCC 7942 (MacCready JS, Hakim P, Young EJ, Hu L, Liu J, Osteryoung KW, Vecchiarelli AG, Ducat DC. 2018. Protein gradients on the nucleoid position the carbon-fixing organelles of cyanobacteria. eLife 7:pii:e39723). McdA, a ParA-type ATPase, nonspecifically binds the nucleoid in the presence of ATP. McdB, a novel factor that directly binds carboxysomes, displaces McdA from the nucleoid. Removal of McdA from the nucleoid in the vicinity of carboxysomes by McdB causes a global break in McdA symmetry, and carboxysome motion occurs via a Brownian-ratchet-based mechanism toward the highest concentration of McdA. Despite the importance for cyanobacteria to properly position their carboxysomes, whether the McdAB system is widespread among cyanobacteria remains an open question. Here, we show that the McdAB system is widespread among β-cyanobacteria, often clustering with carboxysome-related components, and is absent in α-cyanobacteria. Moreover, we show that two distinct McdAB systems exist in β-cyanobacteria, with Type 2 systems being the most ancestral and abundant, and Type 1 systems, like that of S. elongatus, possibly being acquired more recently. Lastly, all McdB proteins share the sequence signatures of a protein capable of undergoing liquid-liquid phase separation. Indeed, we find that representatives of both McdB types undergo liquid-liquid phase separation in vitro, the first example of a ParA-type ATPase partner protein to exhibit this behavior. Our results have broader implications for understanding carboxysome evolution, biogenesis, homeostasis, and positioning in cyanobacteria.

中文翻译：

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蓝细菌中羧基定位系统的起源和进化。

羧基是基于蛋白质的细胞器，对于允许蓝细菌固定CO2至关重要。以前，我们确定了一个两组分系统McdAB，该系统负责等距离地定位模型蓝藻Synechococcus elongatus PCC 7942（MacCready JS，Hakim P，Young EJ，Hu L，Liu J，Osteryoung KW，Vecchiarelli AG，Ducat DC。 2018.核苷酸上的蛋白质梯度将蓝细菌固定在碳的细胞器上（eLife 7：pii：e39723）。McdA，一种ParA型ATP酶，在ATP存在下非特异性结合该类核苷。McdB是一种直接结合羧基体的新型因子，可将McdA从类核苷酸中置换出来。McdB从羧基体附近的类核中去除McdA会导致McdA对称性的整体破坏，羧基体的运动是通过基于布朗-棘轮的机制向最高浓度的McdA发生的。尽管对于蓝细菌来说，正确定位其羧基体很重要，但是McdAB系统是否在蓝细菌中广泛存在仍然是一个悬而未决的问题。在这里，我们显示McdAB系统在β蓝细菌中广泛分布，通常与羧基体相关成分聚集，而在α蓝细菌中则不存在。此外，我们表明，β-蓝细菌中存在两个不同的McdAB系统，其中2型系统是祖先最丰富的，而1型系统（如长链金枪鱼）则可能是最近才获得的。最后，所有McdB蛋白都具有能够进行液-液相分离的蛋白的序列特征。确实，我们发现这两种McdB类型的代表都在体外进行了液-液相分离，这是ParA型ATPase伴侣蛋白表现出这种行为的第一个例子。我们的结果对于理解羧基小体的进化，生物发生，体内稳态以及在蓝细菌中的定位具有更广泛的意义。