Department of Microbiology, Harvard Medical School, Boston, MA 02115.
Department of Microbiology, Harvard Medical School, Boston, MA 02115;
Proc Natl Acad Sci U S A. 2020 Mar 24;117(12):6777-6783. doi: 10.1073/pnas.1919267117. Epub 2020 Mar 9.
Tol-Pal is a multiprotein system present in the envelope of Gram-negative bacteria. Inactivation of this widely conserved machinery compromises the outer membrane (OM) layer of these organisms, resulting in hypersensitivity to many antibiotics. Mutants in the locus fail to complete division and form cell chains. This phenotype along with the localization of Tol-Pal components to the cytokinetic ring in has led to the proposal that the primary function of the system is to promote OM constriction during division. Accordingly, a poorly constricted OM is believed to link the cell chains formed upon Tol-Pal inactivation. However, we show here that cell chains of mutants are connected by an incompletely processed peptidoglycan (PG) layer. Genetic suppressors of this defect were isolated and found to overproduce OM lipoproteins capable of cleaving the glycan strands of PG. Among the factors promoting cell separation in mutant cells was a protein of previously unknown function (YddW), which we have identified as a divisome-localized glycosyl hydrolase that cleaves peptide-free PG glycans. Overall, our results indicate that the cell chaining defect of Tol-Pal mutants cannot simply be interpreted as a defect in OM constriction. Rather, the complex also appears to be required for the activity of several OM-localized enzymes with cell wall remodeling activity. Thus, the Tol-Pal system may play a more general role in coordinating OM invagination with PG remodeling at the division site than previously appreciated.
托尔-帕尔是革兰氏阴性菌包膜上的一种多蛋白系统。该广泛保守的机制失活会破坏这些生物体的外膜(OM)层,导致对许多抗生素敏感。 位点的突变体无法完成分裂并形成细胞链。这种表型以及托尔-帕尔成分在 中的细胞分裂环定位,导致该系统的主要功能是在分裂过程中促进 OM 收缩的假说。因此,人们认为托尔-帕尔失活后形成的细胞链之间连接的是 OM 收缩不良的部分。然而,我们在这里表明, 突变体的细胞链是由不完全加工的肽聚糖(PG)层连接的。该缺陷的遗传抑制因子被分离出来,并发现它们能够过度产生能够切割 PG 聚糖链的 OM 脂蛋白。在突变细胞中促进细胞分离的因素之一是一种功能未知的蛋白(YddW),我们已将其鉴定为一种定位于分裂体的糖苷水解酶,它可以切割无肽 PG 糖链。总的来说,我们的结果表明,托尔-帕尔突变体的细胞链缺陷不能简单地解释为 OM 收缩的缺陷。相反,该复合物似乎还需要几种具有细胞壁重塑活性的 OM 定位酶的活性。因此,与之前的认识相比,托尔-帕尔系统可能在协调 OM 内陷与 PG 在分裂部位重塑方面发挥更普遍的作用。