The peptidoglycan (PG) sacculus provides bacteria with the mechanical strength to maintain cell shape and resist osmotic stress. Enlargement of the mesh-like sacculus requires the combined activity of peptidoglycan synthases and hydrolases. In Escherichia coli, the activity of two PG synthases is driven by lipoproteins anchored in the outer membrane (OM). However, the regulation of PG hydrolases is less well understood, with only regulators for PG amidases having been described. Here, we identify the OM lipoprotein NlpI as a general adaptor protein for PG hydrolases. NlpI binds to different classes of hydrolases and can specifically form complexes with various PG endopeptidases. In addition, NlpI seems to contribute both to PG elongation and division biosynthetic complexes based on its localization and genetic interactions. Consistent with such a role, we reconstitute PG multi-enzyme complexes containing NlpI, the PG synthesis regulator LpoA, its cognate bifunctional synthase, PBP1A, and different endopeptidases. Our results indicate that peptidoglycan regulators and adaptors are part of PG biosynthetic multi-enzyme complexes, regulating and potentially coordinating the spatiotemporal action of PG synthases and hydrolases.

中文翻译：

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外膜脂蛋白 NlpI 在大肠杆菌中的多酶复合物中支撑肽聚糖水解酶。

肽聚糖 (PG) 球囊为细菌提供维持细胞形状和抵抗渗透应力的机械强度。网状球囊的扩大需要肽聚糖合酶和水解酶的联合活性。在大肠杆菌中，两种 PG 合酶的活性由锚定在外膜 (OM) 中的脂蛋白驱动。然而，对 PG 水解酶的调节知之甚少，仅描述了 PG 酰胺酶的调节剂。在这里，我们将 OM 脂蛋白 NlpI 鉴定为 PG 水解酶的一般衔接蛋白。NlpI 与不同种类的水解酶结合，并能与各种 PG 内肽酶特异性地形成复合物。此外，基于其定位和遗传相互作用，NlpI 似乎有助于 PG 延伸和分裂生物合成复合物。与这种作用相一致，我们重组了 PG 多酶复合物，其中包含 NlpI、PG 合成调节剂 LpoA、其同源双功能合酶、PBP1A 和不同的内肽酶。我们的研究结果表明，肽聚糖调节剂和接头是 PG 生物合成多酶复合物的一部分，调节和潜在地协调 PG 合酶和水解酶的时空作用。