The Gram-negative bacterial outer membrane (OM) is a unique bilayer that forms an efficient permeation barrier to protect the cell from noxious compounds ^1,2 . The defining characteristic of the OM is lipid asymmetry, with phospholipids comprising the inner leaflet and lipopolysaccharides comprising the outer leaflet ^1-3 . This asymmetry is maintained by the Mla pathway, a six-component system that is widespread in Gram-negative bacteria and is thought to mediate retrograde transport of misplaced phospholipids from the outer leaflet of the OM to the cytoplasmic membrane ⁴ . The OM lipoprotein MlaA performs the first step in this process via an unknown mechanism that does not require external energy input. Here we show, using X-ray crystallography, molecular dynamics simulations and in vitro and in vivo functional assays, that MlaA is a monomeric α-helical OM protein that functions as a phospholipid translocation channel, forming a ~20-Å-thick doughnut embedded in the inner leaflet of the OM with a central, amphipathic pore. This architecture prevents access of inner leaflet phospholipids to the pore, but allows outer leaflet phospholipids to bind to a pronounced ridge surrounding the channel, followed by diffusion towards the periplasmic space. Enterobacterial MlaA proteins form stable complexes with OmpF/C ^5,6 , but the porins do not appear to play an active role in phospholipid transport. MlaA represents a lipid transport protein that selectively removes outer leaflet phospholipids to help maintain the essential barrier function of the bacterial OM.

中文翻译：

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维持细菌外膜脂质不对称的结构基础。

革兰氏阴性细菌外膜（OM）是形成一种有效的渗透屏障，以保护细胞免受有害化合物独特的双层^{1 ，2}。OM的定义特征是脂质不对称，其中磷脂包含内部小叶，而脂多糖包含外部小叶^1-3。这种不对称性通过Mla途径得以维持，Mla途径是一种在革兰氏阴性细菌中广泛分布的六组分系统，被认为可以介导从OM的小叶到细胞质膜⁴的错位磷脂的逆行转运。。OM脂蛋白MlaA通过不需要外部能量输入的未知机制执行该过程的第一步。在这里，我们使用X射线晶体学，分子动力学模拟以及体外和体内功能分析表明，MlaA是一种单体α螺旋OM蛋白，可作为磷脂易位通道，形成约20Å厚的甜甜圈嵌入在OM的内部小叶中具有中央的两亲孔。该结构阻止内部小叶磷脂进入孔，但允许外部小叶磷脂结合到通道周围的明显脊上，随后向周质空间扩散。肠杆菌MlaA蛋白与OmpF / C ^5,6形成稳定的复合物，但是孔蛋白似乎在磷脂转运中没有发挥积极作用。MlaA代表一种脂质转运蛋白，可选择性去除外部小叶磷脂，以帮助维持细菌OM的基本屏障功能。