National Institute of Animal Health, National Agriculture and Food Research Organization, Ibaraki, Japan.
J Bacteriol. 2011 Feb;193(4):822-31. doi: 10.1128/JB.01555-09. Epub 2010 Dec 10.
Gram-positive pili are composed of covalently bound pilin subunits whose assembly is mediated via a pilus-specific sortase(s). Major subunits constitute the pilus backbone and are therefore essential for pilus formation. Minor subunits are also incorporated into the pilus, but they are considered to be dispensable for backbone formation. The srtG cluster is one of the putative pilus gene clusters identified in the major swine pathogen Streptococcus suis. It consists of one sortase gene (srtG) and two putative pilin subunit genes (sgp1 and sgp2). In this study, by constructing mutants for each of the genes in the cluster and by both immunoblotting and immunogold electron microscopic analysis with antibodies against Sgp1 and Sgp2, we found that the srtG cluster mediates the expression of pilus-like structures in S. suis strain 89/1591. In this pilus, Sgp1 forms the backbone, whereas Sgp2 is incorporated as the minor subunit. In accordance with the current model of pilus assembly by Gram-positive organisms, the major subunit Sgp1 was indispensable for backbone formation and the cognate sortase SrtG mediated the polymerization of both subunits. However, unlike other well-characterized Gram-positive bacterial pili, the minor subunit Sgp2 was required for polymerization of the major subunit Sgp1. Because Sgp2 homologues are encoded in several other Gram-positive bacterial pilus gene clusters, in some types of pili, minor pilin subunits may contribute to backbone formation by a novel mechanism.
革兰氏阳性菌菌毛由共价结合的菌毛亚基组成，其组装通过菌毛特异性的天冬氨酸蛋白酶（sortase）介导。主要亚基构成菌毛骨架，因此对于菌毛形成是必不可少的。次要亚基也被整合到菌毛中，但它们被认为对于骨架形成是可有可无的。srtG 簇是在主要猪病原体猪链球菌中鉴定出的假定菌毛基因簇之一。它由一个天冬氨酸蛋白酶基因（srtG）和两个假定的菌毛亚基基因（sgp1 和 sgp2）组成。在这项研究中，通过构建该簇中每个基因的突变体，并使用针对 Sgp1 和 Sgp2 的抗体进行免疫印迹和免疫金电子显微镜分析，我们发现 srtG 簇介导了猪链球菌 89/1591 菌株中菌毛样结构的表达。在这种菌毛中，Sgp1 形成骨架，而 Sgp2 作为次要亚基被整合。与革兰氏阳性菌菌毛组装的现行模型一致，主要亚基 Sgp1 对于骨架形成是必不可少的，并且同源的天冬氨酸蛋白酶 SrtG 介导了两个亚基的聚合。然而，与其他经过充分研究的革兰氏阳性菌菌毛不同，次要菌毛亚基 Sgp2 对于主要菌毛亚基 Sgp1 的聚合是必需的。因为 Sgp2 同源物在几个其他革兰氏阳性菌菌毛基因簇中被编码，所以在某些类型的菌毛中，次要菌毛亚基可能通过一种新的机制有助于骨架形成。
