Medical Research Council Laboratory of Molecular Biology, Cambridge Biomedical Campus, Francis Crick Avenue, Cambridge CB2 0QH, U.K.
Computational Biology, School of Life Sciences, University of Dundee, Dow Street, Dundee DD1 5EH, U.K.
Nat Microbiol. 2017 May 15;2:17070. doi: 10.1038/nmicrobiol.2017.70.
The MacA-MacB-TolC assembly of Escherichia coli is a transmembrane machine that spans the cell envelope and actively extrudes substrates, including macrolide antibiotics and polypeptide virulence factors. These transport processes are energized by the ATPase MacB, a member of the ATP-binding cassette (ABC) superfamily. We present an electron cryo-microscopy structure of the ABC-type tripartite assembly at near-atomic resolution. A hexamer of the periplasmic protein MacA bridges between a TolC trimer in the outer membrane and a MacB dimer in the inner membrane, generating a quaternary structure with a central channel for substrate translocation. A gating ring found in MacA is proposed to act as a one-way valve in substrate transport. The MacB structure features an atypical transmembrane domain with a closely packed dimer interface and a periplasmic opening that is the likely portal for substrate entry from the periplasm, with subsequent displacement through an allosteric transport mechanism.
大肠杆菌的 MacA-MacB-TolC 组装体是一种跨膜机器，跨越细胞包膜并主动排出底物，包括大环内酯类抗生素和多肽毒力因子。这些运输过程由 ATP 酶 MacB 提供能量，MacB 是 ATP 结合盒 (ABC) 超家族的成员。我们以近原子分辨率呈现了 ABC 型三联体组装体的电子冷冻显微镜结构。周质蛋白 MacA 的六聚体在外膜中的 TolC 三聚体和内膜中的 MacB 二聚体之间形成桥接，产生具有中央通道的四级结构，用于底物易位。在 MacA 中发现的门控环被提议在底物运输中充当单向阀。MacB 结构具有非典型的跨膜结构域，具有紧密堆积的二聚体界面和周质开口，这可能是底物从周质进入的入口，随后通过变构运输机制进行位移。
