Department of Cell Biology, Skirball Institute of Biomolecular Medicine, New York University School of Medicine, New York, United States.
Applied Bioinformatics Laboratories, New York University School of Medicine, New York, United States.
Elife. 2020 Nov 25;9:e62518. doi: 10.7554/eLife.62518.
In double-membraned bacteria, phospholipid transport across the cell envelope is critical to maintain the outer membrane barrier, which plays a key role in virulence and antibiotic resistance. An MCE transport system called Mla has been implicated in phospholipid trafficking and outer membrane integrity, and includes an ABC transporter, MlaFEDB. The transmembrane subunit, MlaE, has minimal sequence similarity to other transporters, and the structure of the entire inner-membrane MlaFEDB complex remains unknown. Here, we report the cryo-EM structure of MlaFEDB at 3.05 Å resolution, revealing distant relationships to the LPS and MacAB transporters, as well as the eukaryotic ABCA/ABCG families. A continuous transport pathway extends from the MlaE substrate-binding site, through the channel of MlaD, and into the periplasm. Unexpectedly, two phospholipids are bound to MlaFEDB, suggesting that multiple lipid substrates may be transported each cycle. Our structure provides mechanistic insight into substrate recognition and transport by MlaFEDB.
在双层膜细菌中，磷脂跨细胞包膜的运输对于维持外膜屏障至关重要，而外膜屏障在外膜完整性、毒力和抗生素耐药性中起着关键作用。一种称为 Mla 的 MCE 转运系统被认为参与了磷脂的运输和外膜的完整性，该系统包括一个 ABC 转运体，MlaFEDB。跨膜亚基 MlaE 与其他转运蛋白的序列相似性极小，整个内膜 MlaFEDB 复合物的结构仍然未知。在这里，我们报告了 MlaFEDB 在 3.05Å分辨率下的冷冻电镜结构，揭示了它与 LPS 和 MacAB 转运蛋白以及真核生物 ABCA/ABCG 家族的远距离关系。从 MlaE 的底物结合位点开始，一条连续的运输途径延伸穿过 MlaD 的通道，进入周质空间。出人意料的是，有两个磷脂结合到 MlaFEDB 上，这表明每个循环可能会运输多个脂质底物。我们的结构为 MlaFEDB 的底物识别和运输提供了机制见解。
