Department of Microbiology, The Ohio State University, Columbus, Ohio 43210, United States.
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States.
Chem Rev. 2021 May 12;121(9):5098-5123. doi: 10.1021/acs.chemrev.0c00587. Epub 2020 Sep 21.
The outer membrane of Gram-negative bacteria is essential for their survival in harsh environments and provides intrinsic resistance to many antibiotics. This membrane is remarkable; it is a highly asymmetric lipid bilayer. The inner leaflet of the outer membrane contains phospholipids, whereas the fatty acyl chains attached to lipopolysaccharide (LPS) comprise the hydrophobic portion of the outer leaflet. This lipid asymmetry, and in particular the exclusion of phospholipids from the outer leaflet, is key to creating an almost impenetrable barrier to hydrophobic molecules that can otherwise pass through phospholipid bilayers. It has long been known that these lipids are not made in the outer membrane. It is now believed that conserved multisubunit protein machines extract these lipids after their synthesis is completed at the inner membrane and transport them to the outer membrane. A longstanding question is how the cell builds and maintains this asymmetric lipid bilayer in coordination with the assembly of the other components of the cell envelope. This Review describes the trans-envelope lipid transport systems that have been identified to participate in outer-membrane biogenesis: LPS transport via the Lpt machine, and phospholipid transport via the Mla pathway and several recently proposed transporters.
革兰氏阴性细菌的外膜对于它们在恶劣环境中的生存至关重要，并赋予了它们对许多抗生素的固有抗性。这个膜非常特别；它是一个高度不对称的脂质双层。外膜的内层含有磷脂，而连接在脂多糖 (LPS) 上的脂肪酸链构成了外膜的疏水部分。这种脂质不对称性，特别是磷脂被排除在外膜，是为疏水分子创建几乎不可渗透的屏障的关键，否则这些疏水分子可以穿过磷脂双层。长期以来，人们一直知道这些脂质不是在外膜中合成的。现在人们相信，在这些脂质在内膜完成合成后，保守的多亚基蛋白机器将其提取出来，并将其运输到外膜。一个长期存在的问题是，细胞如何与细胞包膜的其他成分一起协调构建和维持这种不对称的脂质双层。这篇综述描述了已被确定参与外膜生物发生的跨膜脂质转运系统：通过 Lpt 机器运输 LPS，通过 Mla 途径和几个最近提出的转运蛋白运输磷脂。
