Matthews M, Roy C R
Section of Microbial Pathogenesis, Yale University School of Medicine, Boyer Center for Molecular Medicine, New Haven, Connecticut 06536-0812, USA.
Infect Immun. 2000 Jul;68(7):3971-82. doi: 10.1128/IAI.68.7.3971-3982.2000.
The gram-negative respiratory pathogen Legionella pneumophila infects and grows within mammalian macrophages and protozoan host cells. Upon uptake into macrophages, L. pneumophila establishes a replicative organelle that avoids fusion with endocytic vesicles. There are 24 dot/icm genes on the L. pneumophila chromosome required for biogenesis of this vacuole. Many of the Dot/Icm proteins are predicted to be components of a membrane-bound secretion apparatus similar to type IV conjugal transfer systems. We have been investigating the function of L. pneumophila dot/icm gene products that do not have obvious orthologs in other type IV transfer systems, since these determinants could govern processes unique to phagosome biogenesis. The icmX gene product falls into this category. To understand the role of the IcmX protein in pathogenesis, we have detailed interactions between an L. pneumophila icmX deletion mutant and murine bone marrow-derived macrophages. These data demonstrate that icmX is required for biogenesis of the L. pneumophila replicative organelle. Immunoblot analysis indicates that the icmX gene product is a polypeptide with an estimated molecular mass of 50 kDa. The IcmX protein was localized to the bacterial periplasm, and periplasmic translocation was mediated by an N-terminal sec-dependent leader peptide. A truncated IcmX product was secreted into culture supernatants by wild-type L. pneumophila growing extracellularly in liquid media; however, transport of the IcmX protein into eukaryotic host cells was not detected. Proteins similar in molecular weight to IcmX were identified in other Legionella species by immunoblot analysis using a monoclonal antibody specific for L. pneumophila IcmX protein. From these data, we conclude that the IcmX protein is an essential component of the dot/icm secretion apparatus, and that a conserved mechanism of host cell parasitism exists for members of the Legionellaceae family.
革兰氏阴性呼吸道病原体嗜肺军团菌在哺乳动物巨噬细胞和原生动物宿主细胞内感染并生长。进入巨噬细胞后，嗜肺军团菌建立一个复制性细胞器，避免与内吞小泡融合。嗜肺军团菌染色体上有24个dot/icm基因，是该液泡生物发生所必需的。许多Dot/Icm蛋白预计是类似于IV型接合转移系统的膜结合分泌装置的组成部分。我们一直在研究嗜肺军团菌dot/icm基因产物的功能，这些产物在其他IV型转移系统中没有明显的直系同源物，因为这些决定因素可能控制吞噬体生物发生所特有的过程。icmX基因产物属于这一类。为了了解IcmX蛋白在发病机制中的作用，我们详细研究了嗜肺军团菌icmX缺失突变体与小鼠骨髓来源巨噬细胞之间的相互作用。这些数据表明，icmX是嗜肺军团菌复制性细胞器生物发生所必需的。免疫印迹分析表明，icmX基因产物是一种估计分子量为50 kDa的多肽。IcmX蛋白定位于细菌周质，周质转运由N端sec依赖性前导肽介导。截短的IcmX产物由在液体培养基中细胞外生长的野生型嗜肺军团菌分泌到培养上清中；然而，未检测到IcmX蛋白向真核宿主细胞的转运。通过使用针对嗜肺军团菌IcmX蛋白的单克隆抗体进行免疫印迹分析，在其他军团菌属物种中鉴定出分子量与IcmX相似的蛋白质。从这些数据中，我们得出结论，IcmX蛋白是dot/icm分泌装置的重要组成部分，并且军团菌科成员存在保守的宿主细胞寄生机制。
