Kruse Thomas, Bork-Jensen Jette, Gerdes Kenn
Department of Biochemistry and Molecular Biology, University of Southern Denmark, DK-5230 Odense M, Denmark.
Mol Microbiol. 2005 Jan;55(1):78-89. doi: 10.1111/j.1365-2958.2004.04367.x.
MreB proteins of Escherichia coli, Bacillus subtilis and Caulobacter crescentus form actin-like cables lying beneath the cell surface. The cables are required to guide longitudinal cell wall synthesis and their absence leads to merodiploid spherical and inflated cells prone to cell lysis. In B. subtilis and C. crescentus, the mreB gene is essential. However, in E. coli, mreB was inferred not to be essential. Using a tight, conditional gene depletion system, we systematically investigated whether the E. coli mreBCD-encoded components were essential. We found that cells depleted of mreBCD became spherical, enlarged and finally lysed. Depletion of each mre gene separately conferred similar gross changes in cell morphology and viability. Thus, the three proteins encoded by mreBCD are all essential and function in the same morphogenetic pathway. Interestingly, the presence of a multicopy plasmid carrying the ftsQAZ genes suppressed the lethality of deletions in the mre operon. Using GFP and cell fractionation methods, we showed that the MreC and MreD proteins were associated with the cell membrane. Using a bacterial two-hybrid system, we found that MreC interacted with both MreB and MreD. In contrast, MreB and MreD did not interact in this assay. Thus, we conclude that the E. coli MreBCD form an essential membrane-bound complex. Curiously, MreB did not form cables in cell depleted for MreC, MreD or RodA, indicating a mutual interdependency between MreB filament morphology and cell shape. Based on these and other observations we propose a model in which the membrane-associated MreBCD complex directs longitudinal cell wall synthesis in a process essential to maintain cell morphology.
大肠杆菌、枯草芽孢杆菌和新月柄杆菌的MreB蛋白会在细胞表面下方形成类似肌动蛋白的纤维束。这些纤维束对于引导纵向细胞壁合成是必需的，缺乏它们会导致部分二倍体球形且膨胀的细胞，容易发生细胞裂解。在枯草芽孢杆菌和新月柄杆菌中，mreB基因是必需的。然而，在大肠杆菌中，推测mreB不是必需的。我们使用了一种严格的、条件性基因缺失系统，系统地研究了大肠杆菌中由mreBCD编码的组分是否必需。我们发现，缺失mreBCD的细胞会变成球形、增大，最终裂解。分别缺失每个mre基因都会导致细胞形态和活力出现类似的总体变化。因此，由mreBCD编码的这三种蛋白质都是必需的，并且在同一形态发生途径中发挥作用。有趣的是，携带ftsQAZ基因的多拷贝质粒的存在抑制了mre操纵子缺失的致死性。我们使用绿色荧光蛋白（GFP）和细胞分级分离方法表明，MreC和MreD蛋白与细胞膜相关联。我们使用细菌双杂交系统发现，MreC与MreB和MreD都相互作用。相比之下，在该检测中MreB和MreD不相互作用。因此，我们得出结论，大肠杆菌的MreBCD形成了一种必需的膜结合复合物。奇怪的是，在缺失MreC、MreD或RodA的细胞中，MreB不会形成纤维束，这表明MreB丝形态与细胞形状之间存在相互依赖性。基于这些及其他观察结果，我们提出了一个模型，其中膜相关的MreBCD复合物在维持细胞形态所必需的过程中指导纵向细胞壁合成。
