Beuzón C R, Banks G, Deiwick J, Hensel M, Holden D W
Department of Infectious Diseases, Imperial College School of Medicine, Du Cane Road, London W12 0NN, UK.
Mol Microbiol. 1999 Aug;33(4):806-16. doi: 10.1046/j.1365-2958.1999.01527.x.
The type III secretion system of Salmonella pathogenicity island 2 (SPI-2) is required for bacterial replication inside macrophages. SseB has been considered a putative target of the secretion system on the basis of its similarity with EspA, a protein secreted by the type III secretion system of enteropathogenic Escherichia coli (EPEC). EspA forms a filamentous structure on the bacterial cell surface and is involved in translocation of proteins into the eukaryotic cytosol. In this paper, we show that SseB is a secreted protein that associates with the surface of the bacterial cell and might, therefore, also be required for delivery of SPI-2 effector proteins to the eukaryotic cell cytosol. SseB begins to accumulate inside the bacterial cell when the culture enters early stationary phase. However, SseB is only secreted if the bacteria are grown at low pH or if the pH is shifted after growth from 7.0 to below pH 5.0. The secretion occurs within minutes of acidification and is totally dependent on a functional SPI-2 type III secretion system. As the pH of the Salmonella-containing vacuole inside host cells has been shown to acidify to between pH 4.0 and 5.0, and as SPI-2 gene expression occurs inside host cells, low pH might be a physiological stimulus for SPI-2-mediated secretion in vivo.
沙门氏菌致病岛2（SPI-2）的III型分泌系统是细菌在巨噬细胞内复制所必需的。基于SseB与EspA的相似性，SseB被认为是该分泌系统的一个假定靶点，EspA是肠致病性大肠杆菌（EPEC）III型分泌系统分泌的一种蛋白质。EspA在细菌细胞表面形成丝状结构，并参与蛋白质向真核细胞胞质溶胶的转运。在本文中，我们表明SseB是一种分泌蛋白，与细菌细胞表面相关联，因此，它也可能是将SPI-2效应蛋白递送至真核细胞胞质溶胶所必需的。当培养物进入早期稳定期时，SseB开始在细菌细胞内积累。然而，只有当细菌在低pH值下生长或在从7.0生长后将pH值转变为低于pH 5.0时，SseB才会被分泌。酸化后几分钟内就会发生分泌，并且完全依赖于功能性的SPI-2 III型分泌系统。由于宿主细胞内含有沙门氏菌的液泡的pH值已被证明会酸化至pH 4.0至5.0之间，并且由于SPI-2基因表达发生在宿主细胞内，低pH值可能是体内SPI-2介导的分泌的生理刺激因素。
