Centre for Cellular and Molecular Biology, Council of Scientific and Industrial Research, 500007 Hyderabad, India.
Centre for Cellular and Molecular Biology, Council of Scientific and Industrial Research, 500007 Hyderabad, India
Proc Natl Acad Sci U S A. 2021 May 11;118(19). doi: 10.1073/pnas.2101989118.
The gram-negative bacterial cell envelope is made up of an outer membrane (OM), an inner membrane (IM) that surrounds the cytoplasm, and a periplasmic space between the two membranes containing peptidoglycan (PG or murein). PG is an elastic polymer that forms a mesh-like sacculus around the IM, protecting cells from turgor and environmental stress conditions. In several bacteria, including , the OM is tethered to PG by an abundant OM lipoprotein, Lpp (or Braun's lipoprotein), that functions to maintain the structural and functional integrity of the cell envelope. Since its discovery, Lpp has been studied extensively, and although l,d-transpeptidases, the enzymes that catalyze the formation of PG-Lpp linkages, have been earlier identified, it is not known how these linkages are modulated. Here, using genetic and biochemical approaches, we show that LdtF (formerly ), a newly identified paralog of l,d-transpeptidases in , is a murein hydrolytic enzyme that catalyzes cleavage of Lpp from the PG sacculus. LdtF also exhibits glycine-specific carboxypeptidase activity on muropeptides containing a terminal glycine residue. LdtF was earlier presumed to be an l,d-transpeptidase; however, our results show that it is indeed an l,d-endopeptidase that hydrolyzes the products generated by the l,d-transpeptidases. To summarize, this study describes the discovery of a murein endopeptidase with a hitherto unknown catalytic specificity that removes the PG-Lpp cross-links, suggesting a role for LdtF in the regulation of PG-OM linkages to maintain the structural integrity of the bacterial cell envelope.
革兰氏阴性细菌的细胞包膜由外膜 (OM)、包围细胞质的内膜 (IM) 和两膜之间的周质空间组成，周质空间中含有肽聚糖 (PG 或黏肽)。PG 是一种有弹性的聚合物，它在 IM 周围形成一个网状的囊泡，保护细胞免受膨胀和环境压力的影响。在包括 在内的几种细菌中，OM 通过大量的 OM 脂蛋白 Lpp（或 Braun 脂蛋白）与 PG 相连，Lpp 的功能是维持细胞包膜的结构和功能完整性。自发现以来，Lpp 已经被广泛研究，尽管 l,d-转肽酶（催化 PG-Lpp 键形成的酶）早些时候已经被鉴定出来，但这些键是如何被调节的还不清楚。在这里，我们使用遗传和生化方法表明，LdtF（以前称为 ），是 中 l,d-转肽酶的一个新的平行基因，是一种黏肽水解酶，它催化 Lpp 从 PG 囊泡上的裂解。LdtF 还对含有末端甘氨酸残基的 muropeptides 具有甘氨酸特异性羧肽酶活性。LdtF 早些时候被认为是一种 l,d-转肽酶；然而，我们的结果表明，它实际上是一种 l,d-内肽酶，它水解 l,d-转肽酶产生的产物。总之，这项研究描述了一种具有未知催化特异性的黏肽内肽酶的发现，它去除了 PG-Lpp 交联，这表明 LdtF 在调节 PG-OM 键以维持细菌细胞包膜的结构完整性方面发挥作用。
