Larsen Rachel A, Cusumano Christina, Fujioka Akina, Lim-Fong Grace, Patterson Paula, Pogliano Joe
Division of Biological Sciences, University of California at San Diego, La Jolla, California 92093, USA.
Genes Dev. 2007 Jun 1;21(11):1340-52. doi: 10.1101/gad.1546107. Epub 2007 May 17.
Prokaryotes rely on a distant tubulin homolog, FtsZ, for assembling the cytokinetic ring essential for cell division, but are otherwise generally thought to lack tubulin-like polymers that participate in processes such as DNA segregation. Here we characterize a protein (TubZ) from the Bacillus thuringiensis virulence plasmid pBtoxis, which is a member of the tubulin/FtsZ GTPase superfamily but is only distantly related to both FtsZ and tubulin. TubZ assembles dynamic, linear polymers that exhibit directional polymerization with plus and minus ends, movement by treadmilling, and a critical concentration for assembly. A point mutation (D269A) that alters a highly conserved catalytic residue within the T7 loop completely eliminates treadmilling and allows the formation of stable polymers at a much lower protein concentration than the wild-type protein. When expressed in trans, TubZ(D269A) coassembles with wild-type TubZ and significantly reduces the stability of pBtoxis, demonstrating a direct correlation between TubZ dynamics and plasmid maintenance. The tubZ gene is in an operon with tubR, which encodes a putative DNA-binding protein that regulates TubZ levels. Our results suggest that TubZ is representative of a novel class of prokaryotic cytoskeletal proteins important for plasmid stability that diverged long ago from the ancient tubulin/FtsZ ancestor.
原核生物依靠一种远亲微管蛋白同源物FtsZ来组装细胞分裂所必需的细胞分裂环，但一般认为它们缺乏参与DNA分离等过程的微管蛋白样聚合物。在这里，我们对来自苏云金芽孢杆菌毒力质粒pBtoxis的一种蛋白质（TubZ）进行了表征，它是微管蛋白/FtsZ GTP酶超家族的成员，但与FtsZ和微管蛋白的亲缘关系都很遥远。TubZ组装动态的线性聚合物，这些聚合物表现出正负端的定向聚合、踏车运动以及组装的临界浓度。一个改变T7环内高度保守催化残基的点突变（D269A）完全消除了踏车运动，并允许在比野生型蛋白低得多的蛋白质浓度下形成稳定的聚合物。当反式表达时，TubZ（D269A）与野生型TubZ共组装，并显著降低pBtoxis的稳定性，这表明TubZ动态与质粒维持之间存在直接关联。tubZ基因与tubR在一个操纵子中，tubR编码一种假定的DNA结合蛋白，可调节TubZ水平。我们的结果表明，TubZ代表了一类对质粒稳定性很重要的新型原核细胞骨架蛋白，它们在很久以前就从古老的微管蛋白/FtsZ祖先中分化出来。
