Miller V L, Beer K B, Loomis W P, Olson J A, Miller S I
Department of Microbiology and Molecular Genetics, University of California, Los Angeles 90024.
Infect Immun. 1992 Sep;60(9):3763-70. doi: 10.1128/iai.60.9.3763-3770.1992.
Two phenotypes believed to contribute to the pathogenesis of Salmonella infections are macrophage survival and invasion of epithelial cells. It was recently observed that the Salmonella macrophage survival factor PagC has significant amino acid similarity to the Yersinia invasion factor Ail. This observation raised the possibilities that macrophage survival is in part determined by the pathway of entry and that PagC confers an entry mechanism that does not trigger the microbicidal activities of the macrophage. Thus, we sought to investigate the role of PagC in invasion by examining (i) the invasion phenotype of pagC mutants and (ii) the invasion phenotype of Escherichia coli carrying pagC. A previously identified invasion-defective TnphoA insertion mutant of Salmonella enteritidis was found to have TnphoA inserted into the signal sequence-encoding region of pagC; the pagC allele from this mutant, SM5T, was designated pagC64. In contrast, Salmonella typhimurium carrying the pagC1 allele (a TnphoA insertion mutation, downstream of the region encoding the signal sequence) was not defective for invasion. Further analysis of these two pagC alleles suggested that the invasion-defective phenotype associated with pagC64 is not due to the loss of PagC function but rather is due to the synthesis of a hybrid PagC-alkaline phosphatase protein that is aberrantly localized, most likely to the inner membrane, and thus may prevent proper localization or function of a factor(s) required for efficient invasion. The observation that pagC did not confer an invasive phenotype to E. coli further suggests that PagC is not an invasion factor. A cloned pagC gene complemented the macrophage survival defect of S. typhimurium pagC1 mutants, but the cloned ail gene did not. Together these results suggest that the structural similarity between PagC and Ail may not extend to a similarity in function. Interestingly, S. enteritidis carrying the pagC64 allele that results in both an invasion defect and a macrophage survival defect was less virulent for mice infected intragastrically or intraperitoneally than was S. enteritidis carrying the pagC1 allele that results only in a macrophage survival defect.
据信，沙门氏菌感染发病机制中起作用的两种表型为巨噬细胞存活和上皮细胞侵袭。最近观察到，沙门氏菌巨噬细胞存活因子PagC与耶尔森氏菌侵袭因子Ail具有显著的氨基酸相似性。这一观察结果引发了以下可能性：巨噬细胞存活部分取决于进入途径，并且PagC赋予了一种不会触发巨噬细胞杀菌活性的进入机制。因此，我们试图通过检查（i）pagC突变体的侵袭表型和（ii）携带pagC的大肠杆菌的侵袭表型来研究PagC在侵袭中的作用。发现先前鉴定的肠炎沙门氏菌侵袭缺陷型TnphoA插入突变体的TnphoA插入到pagC的信号序列编码区域；来自该突变体SM5T的pagC等位基因被命名为pagC64。相比之下，携带pagC1等位基因（TnphoA插入突变，位于编码信号序列区域的下游）的鼠伤寒沙门氏菌在侵袭方面没有缺陷。对这两个pagC等位基因的进一步分析表明，与pagC64相关的侵袭缺陷表型不是由于PagC功能丧失，而是由于合成了一种异常定位的杂交PagC-碱性磷酸酶蛋白，最有可能定位于内膜，因此可能会阻止有效侵袭所需因子的正确定位或功能。pagC未赋予大肠杆菌侵袭表型这一观察结果进一步表明PagC不是侵袭因子。克隆的pagC基因弥补了鼠伤寒沙门氏菌pagC1突变体的巨噬细胞存活缺陷，但克隆的ail基因则不能。这些结果共同表明，PagC和Ail之间结构上的相似性可能并不延伸到功能上的相似性。有趣的是，携带导致侵袭缺陷和巨噬细胞存活缺陷的pagC64等位基因的肠炎沙门氏菌，对经胃内或腹腔内感染的小鼠的毒力低于携带仅导致巨噬细胞存活缺陷的pagC1等位基因的肠炎沙门氏菌。
