Escherichia coli shapeshifters.
Escherichia coli shapeshifters.
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大肠杆菌变形者。
DOI：
10.1128/jb.00306-13
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发表时间：
2013
影响因子：
3.2
通讯作者：
Weiss,DavidS
中科院分区：
文献类型：
--
作者：
Weiss,DavidS
早期的微生物学家争论他们通过基本显微镜看到的不同形式-杆状，球菌，逗号和螺旋-是不同的物种还是仅仅是少数有机体的不同发育阶段。世纪后半叶发展起来的纯文化技术平息了这场争论。到目前为止，已经确定大多数细菌的特征大小和形状来自它们所生活的肽聚糖（PG）壁。但这个结论回避了一个问题：细胞壁是如何形成的？我们对这个主题的理解已经成为几篇优秀评论的主题（2-4）。但重要的是要注意，以前解决这个问题的努力涉及筛选突变体或导致正常形态丧失的条件。Ranjit和Young在本期《细菌学杂志》上发表的一篇优雅的论文描述了一种完全不同的方法来解决这个问题。作者使用溶菌酶将大肠杆菌细胞转化为原生质球，然后观察这些细胞在生长过程中恢复正常形态。在一些细胞凋亡相关基因的突变缺陷被证明是无法再生棒形态。值得注意的是，所讨论的基因对于在标准生长条件下维持正常的杆形态并不关键。这意味着维持细胞形状所需的形态发生系统不足以从头产生细胞形状。雕刻PG球囊。PG球囊是一个巨大的袋状大分子，由短肽交联连接的聚糖链组成（6，7）。球囊以一种坚韧但有点弹性的外骨骼包围着内（细胞质）膜。我们知道这种外骨骼是细胞形状的近端决定因素，因为用溶菌酶消化去除它可以迅速将基本上任何初始形态的细菌转化为圆形原生质球。相反，纯化的球囊保留了它们所来源的细胞的轮廓。球囊的几何形状是由多个PG淀粉酶和水解酶的联合作用决定的，这些酶在生长和分裂过程中重塑PG。水解酶的活性必须仔细控制并与酶的活性协调，以避免裂解并保持细胞形状。这种协调作用部分是通过将酶和水解酶共定位于多蛋白复合物来实现的（7）。这些复合物的组装和活性由包含微管蛋白样蛋白FtsZ或肌动蛋白样蛋白MreB的两个主要细胞骨架元件指导。FtsZ是细菌分裂器的主要调节因子，通常被称为分裂体，在E.大肠杆菌在中间细胞组装，并含有至少30种不同类型的蛋白质（8）。分裂体的组分包括多种PG脱氢酶、水解酶和将这些酶与FtsZ连接并调节其活性的辅助蛋白（例如，参见参考文献9至12）。MreB蛋白（或其同源物）是一种类似的蛋白质组装体的关键组成部分，这种蛋白质组装体有时被称为“延伸体”，定位于杆状细菌的侧壁并协调延伸。我们知道这些组装体对细胞形状很重要，因为突变或使其稳定的药物会产生明显的形态学效应。因此，分裂体的失活导致细胞生长为长丝状体，而延长酶体的失活导致细胞变得又大又圆（图1）。此外，细菌获得更复杂的形状，如新月形和螺旋形，或执行戏剧性的变化，如孢子形成，这样做是通过修改细胞壁。
Early microbiologists argued over whether the different forms they saw through their rudimentary microscopes—the rods, the cocci, the commas, and the spirals—were different species or merely different developmental stages of a small number of organisms (1). Pure culture techniques developed in the second half of the 19th century laid that debate to rest. By now, it is well established that most bacteria get their characteristic size and shape from the peptidoglycan (PG) wall in which they live. But this conclusion begs the question: how does the cell wall get its shape? Our understanding of this topic has been the subject of several excellent reviews (2–4). But it is important to note that previous efforts to address this question have involved screening for mutants or conditions that result in loss of normal morphology. An elegant paper by Ranjit and Young published in this issue of the Journal of Bacteriology (5) describes a completely different approach to the question. The authors used lysozyme to convert Escherichia colicells to spheroplasts and then watched as these cells recovered normal morphology during outgrowth. Mutants defective in a number of cell envelope-associated genes proved to be incapable of regenerating rod morphology. Remarkably, the genes in question are not critical for maintaining normal rod morphology under standard growth conditions. This means the morphogenetic systems required for maintaining cell shape are not sufficient for generating cell shape de novo. Sculpting the PG sacculus. The PG sacculus is an enormous bag-like macromolecule composed of glycan strands joined by short peptide cross-links (6, 7). The sacculus surrounds the inner (cytoplasmic) membrane in a tough but somewhat elastic exoskeleton. We know this exoskeleton is the proximal determinant of cell shape because removing it by digestion with lysozyme rapidly converts bacteria of essentially any starting morphology to round spheroplasts. Conversely, purified sacculi retain the contours of the cells from which they were derived. The geometry of the sacculus is determined by the combined action of multiple PG synthases and hydrolases that remodel the PG during growth and division. The activity of the hydrolases must be carefully controlled and coordinated with that of the synthases to avoid lysis and preserve cell shape. This coordination is exerted in part by colocalizing the synthases and hydrolases to multiprotein complexes (7). The assembly and activity of these complexes are guided by two major cytoskeletal elements containing either the tubulin-like protein FtsZ or the actin-like protein MreB. FtsZ is the master regulator of the bacterial division apparatus, often referred to as the divisome, which in E. coli assembles at the midcell and contains at least 30 different types of proteins (8). Among the components of the divisome are multiple PG synthases, hydrolases, and accessory proteins that link these enzymes to FtsZ and regulate their activities (eg, see references 9 to 12). The MreB protein (or its homologs) is a critical component of an analogous protein assembly sometimes called the “elongasome” that localizes to the lateral wall of rod-shaped bacteria and orchestrates elongation.We know these assemblies are important for cell shape because mutations or drugs that inactivate them have telltale morphological effects. Thus, inactivation of the divisome results in growth as a long filament, while inactivation of the elongasome causes cells to become large and round (Fig. 1). Moreover, bacteria that acquire more complex shapes, like crescents and spirals, or execute dramatic changes, like spore formation, do so by modifying the cell wall either …