Department of Cell Biology, Harvard Medical School, Boston, MA, USA.
Nature. 2019 Mar;567(7749):486-490. doi: 10.1038/s41586-019-1025-6. Epub 2019 Mar 20.
In Gram-negative bacteria, lipopolysaccharide is essential for outer membrane formation and antibiotic resistance. The seven lipopolysaccharide transport (Lpt) proteins A-G move lipopolysaccharide from the inner to the outer membrane. The ATP-binding cassette transporter LptBFG, which tightly associates with LptC, extracts lipopolysaccharide out of the inner membrane. The mechanism of the LptBFG-LptC complex (LptBFGC) and the role of LptC in lipopolysaccharide transport are poorly understood. Here we characterize the structures of LptBFG and LptBFGC in nucleotide-free and vanadate-trapped states, using single-particle cryo-electron microscopy. These structures resolve the bound lipopolysaccharide, reveal transporter-lipopolysaccharide interactions with side-chain details and uncover how the capture and extrusion of lipopolysaccharide are coupled to conformational rearrangements of LptBFGC. LptC inserts its transmembrane helix between the two transmembrane domains of LptBFG, which represents a previously unknown regulatory mechanism for ATP-binding cassette transporters. Our results suggest a role for LptC in achieving efficient lipopolysaccharide transport, by coordinating the action of LptBFG in the inner membrane and Lpt protein interactions in the periplasm.
在革兰氏阴性菌中，脂多糖对于外膜的形成和抗生素耐药性至关重要。七种脂多糖转运（Lpt）蛋白 A-G 将脂多糖从内膜转运到外膜。与 LptC 紧密结合的 ATP 结合盒转运蛋白 LptBFG 将脂多糖从内膜中提取出来。LptBFG-LptC 复合物（LptBFGC）的机制以及 LptC 在脂多糖转运中的作用知之甚少。在这里，我们使用单颗粒冷冻电镜技术对无核苷酸和钒酸盐捕获状态下的 LptBFG 和 LptBFGC 结构进行了表征。这些结构解析了结合的脂多糖，揭示了转运蛋白-脂多糖相互作用的侧链细节，并揭示了脂多糖的捕获和排出如何与 LptBFGC 的构象重排相偶联。LptC 将其跨膜螺旋插入 LptBFG 的两个跨膜结构域之间，这代表了一种以前未知的 ATP 结合盒转运蛋白的调节机制。我们的结果表明，LptC 通过协调 LptBFG 在内膜中的作用以及周质中 Lpt 蛋白相互作用，在实现有效的脂多糖转运中发挥作用。
