In double-membraned bacteria, phospholipids must be transported across the cell envelope to maintain the outer membrane barrier, which plays a key role in antibiotic resistance and pathogen virulence. The Mla system has been implicated in phospholipid trafficking and outer membrane integrity, and includes an ABC transporter complex, MlaFEDB. The transmembrane subunit, MlaE, has minimal sequence similarity to other ABC transporters, and the structure of the entire inner membrane MlaFEDB complex remains unknown. Here we report the cryo-EM structure of the MlaFEDB complex at 3.05 angstrom resolution. Our structure reveals that while MlaE has many distinct features, it is distantly related to the LPS and MacAB transporters, as well as the eukaryotic ABCA/ABCG families. MlaE adopts an outward-open conformation, resulting in a continuous pathway for phospholipid transport from the MlaE substrate-binding site to the pore formed by the ring of MlaD. Unexpectedly, two phospholipids are bound in the substrate-binding pocket of MlaFEDB, raising the possibility that multiple lipid substrates may be translocated each transport cycle. Site-specific crosslinking confirms that lipids bind in this pocket in vivo. Our structure provides mechanistic insight into substrate recognition and transport by the MlaFEDB complex.

中文翻译：

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具有底物结合的细菌磷脂转运蛋白MlaFEDB的结构

在双膜细菌中，必须将磷脂转运穿过细胞膜以维持外膜屏障，这在抗生素抗性和病原体毒力中起关键作用。Mla系统与磷脂运输和外膜完整性有关，并且包括ABC转运蛋白复合物MlaFEDB。跨膜亚基MlaE与其他ABC转运蛋白的序列相似性最小，并且整个内膜MlaFEDB复合物的结构仍然未知。在这里，我们以3.05埃的分辨率报告了MlaFEDB复合物的低温电磁结构。我们的结构表明，虽然MlaE具有许多独特的功能，但它与LPS和MacAB转运蛋白以及真核ABCA / ABCG家族有着密切的关系。MlaE采用向外开放的构型，导致磷脂从MlaE底物结合位点到MlaD环形成的孔的连续转运途径。出乎意料的是，两个磷脂结合在MlaFEDB的底物结合袋中，从而增加了每个运输周期可能转运多个脂质底物的可能性。位点特异性交联证实脂质在体内在该口袋中结合。我们的结构为通过MlaFEDB复合体识别和传输底物提供了机械的见解。位点特异性交联证实脂质在体内在该口袋中结合。我们的结构为通过MlaFEDB复合体识别和传输底物提供了机械的见解。位点特异性交联证实脂质在体内在该口袋中结合。我们的结构为通过MlaFEDB复合体识别和传输底物提供了机械的见解。