DNA通过接合从一种细菌转移到另一种是细菌演化中的一个主要因素，并且作为抗生素抗性和毒性基因交换的一个机制也有实用意义。人体的多数致病细菌是革兰氏阴性的，在其中IV-型分泌体系调控这种DNA转移。该体系由三个蛋白组成，它们结合在一起，形成跨越内膜和外膜的一个核心。
现在，一个IV-型分泌体系的外膜复合物的晶体结构已被确定。该复合物的大小为0.6兆道尔顿，它是结构已知的最大外膜复合物。它的结构显示了DNA穿过细菌细胞膜的机制，并为开发以IV-型分泌体系为作用目标来抑制抗生素抗性和毒性因子扩散的药物提供了一个步骤。（生物谷Bioon.com）
生物谷推荐原始出处：
Nature 462, 1011-1015 (24 December 2009) | doi:10.1038/nature08588
Structure of the outer membrane complex of a type IV secretion system
Vidya Chandran1,5, Rémi Fronzes1,5, Stéphane Duquerroy2,3, Nora Cronin1, Jorge Navaza4 & Gabriel Waksman1
1 Institute of Structural and Molecular Biology, University College London and Birkbeck College, Malet Street, London WC1E 7HX, UK
2 Institut Pasteur, Unité de Virologie Structurale, Virology Department and CNRS URA 3015, Paris, 25–28 Rue du Dr Roux, F-75724 Paris, France
3 Université Paris-Sud, F-91405 Orsay, France
4 Laboratoire de Microscopie Electronique, Institut de Biologie Structurale J.P. Ebel, 41 rue Jules Horowitz, F-38027 Grenoble Cedex 1, France
5 These authors contributed equally to this work.
Correspondence to: Gabriel Waksman1 Correspondence and requests for materials should be addressed to G.W.
Type IV secretion systems are secretion nanomachines spanning the two membranes of Gram-negative bacteria. Three proteins, VirB7, VirB9 and VirB10, assemble into a 1.05 megadalton (MDa) core spanning the inner and outer membranes. This core consists of 14 copies of each of the proteins and forms two layers, the I and O layers, inserting in the inner and outer membrane, respectively. Here we present the crystal structure of a ~0.6?MDa outer-membrane complex containing the entire O layer. This structure is the largest determined for an outer-membrane channel and is unprecedented in being composed of three proteins. Unexpectedly, this structure identifies VirB10 as the outer-membrane channel with a unique hydrophobic double-helical transmembrane region. This structure establishes VirB10 as the only known protein crossing both membranes of Gram-negative bacteria. Comparison of the cryo-electron microscopy (cryo-EM) and crystallographic structures points to conformational changes regulating channel opening and closing.
