Nagai Hiroki, Kubori Tomoko
Research Institute for Microbial Diseases, Osaka University Osaka, Japan.
Front Microbiol. 2011 Jun 27;2:136. doi: 10.3389/fmicb.2011.00136. eCollection 2011.
Type IV secretion systems (T4SSs) play a central role in the pathogenicity of many important pathogens, including Agrobacterium tumefaciens, Helicobacter pylori, and Legionella pneumophila. The T4SSs are related to bacterial conjugation systems, and are classified into two subgroups, type IVA (T4ASS) and type IVB (T4BSS). The T4BSS, which is closely related to conjugation systems of IncI plasmids, was originally found in human pathogen L. pneumophila; pathogenesis by L. pneumophila infection requires functional Dot/Icm T4BSS. A zoonotic pathogen, Coxiella burnetii, and an arthropod pathogen, Rickettsiella grylli - both of which carry T4BSSs highly similar to the Legionella Dot/Icm system - are evolutionarily closely related and comprise a monophyletic group. A growing body of bacterial genomic information now suggests that T4BSSs are not limited to Legionella and related bacteria and IncI plasmids. Here, we review the current knowledge on T4BSS apparatus and component proteins, gained mainly from studies on L. pneumophila Dot/Icm T4BSS. Recent structural studies, along with previous findings, suggest that the Dot/Icm T4BSS contains components with primary or higher-order structures similar to those in other types of secretion systems - types II, III, IVA, and VI, thus highlighting the mosaic nature of T4BSS architecture.
IV型分泌系统（T4SSs）在许多重要病原体的致病性中起着核心作用，这些病原体包括根癌土壤杆菌、幽门螺杆菌和嗜肺军团菌。T4SSs与细菌接合系统相关，可分为两个亚组，即IVA型（T4ASS）和IVB型（T4BSS）。IVB型分泌系统与IncI质粒的接合系统密切相关，最初是在人类病原体嗜肺军团菌中发现的；嗜肺军团菌感染导致的发病机制需要功能性的Dot/Icm IVB型分泌系统。一种人畜共患病原体——伯纳特立克次体，以及一种节肢动物病原体——格里利立克次氏体，这两种病原体都携带与军团菌Dot/Icm系统高度相似的IVB型分泌系统，它们在进化上密切相关，构成一个单系群。越来越多的细菌基因组信息表明，IVB型分泌系统并不局限于军团菌及相关细菌和IncI质粒。在此，我们综述了目前关于IVB型分泌系统装置和组成蛋白的知识，这些知识主要来自对嗜肺军团菌Dot/Icm IVB型分泌系统的研究。最近的结构研究以及之前的发现表明，Dot/Icm IVB型分泌系统包含的组分具有与其他类型分泌系统（II型、III型、IVA型和VI型）相似的一级或高级结构，从而突出了IVB型分泌系统结构的镶嵌性。
