Magdalena Juana, Goldberg Marcia B
Department of Microbiology and Immunology, Albert Einstein College of Medicine, Bronx, New York, USA.
Cell Motil Cytoskeleton. 2002 Apr;51(4):187-96. doi: 10.1002/cm.10024.
Shigella move through the cytoplasm of host cells by active polymerization of host actin to form an "actin tail." Actin tail assembly is mediated by the Shigella protein IcsA. The process of Shigella actin assembly has been studied extensively using IcsA-expressing Escherichia coli in cytoplasmic extracts of Xenopus eggs. However, for reasons that have been unclear, wild type Shigella does not assemble actin in these extracts. We show that the defect in actin assembly in Xenopus extracts by Shigella can be rescued by increasing IcsA expression by approximately 3-fold. We calculate that the number of IcsA molecules required on an individual bacterium to assemble actin filaments in extracts is approximately 1,500-2,100 molecules, and the number of IcsA molecules required to assemble an actin tail is approximately 4,000 molecules. The majority of wild type Shigella do not express these levels of IcsA when grown in vitro. However, in infected host cells, IcsA expression is increased 3.2-fold, such that the number of IcsA molecules on a significant percentage of intracellular wild type Shigella would exceed that required for actin assembly in extracts. Thus, the number of IcsA molecules estimated from our studies in extracts as being required on an individual bacterium to assemble actin filaments or an actin tail is a reasonable prediction of the numbers required for these functions in Shigella-infected cells.
志贺氏菌通过宿主肌动蛋白的活性聚合作用在宿主细胞的细胞质中移动，形成“肌动蛋白尾”。肌动蛋白尾的组装由志贺氏菌蛋白IcsA介导。利用在非洲爪蟾卵细胞质提取物中表达IcsA的大肠杆菌，对志贺氏菌肌动蛋白组装过程进行了广泛研究。然而，由于尚不清楚的原因，野生型志贺氏菌在这些提取物中不能组装肌动蛋白。我们发现，通过将IcsA表达量提高约3倍，可以挽救志贺氏菌在非洲爪蟾提取物中肌动蛋白组装的缺陷。我们计算得出，在提取物中单个细菌组装肌动蛋白丝所需的IcsA分子数约为1500 - 2100个分子，而组装一个肌动蛋白尾所需的IcsA分子数约为4000个分子。大多数野生型志贺氏菌在体外生长时不表达这些水平的IcsA。然而，在受感染宿主细胞中，IcsA表达增加3.2倍，使得相当一部分细胞内野生型志贺氏菌上的IcsA分子数将超过提取物中肌动蛋白组装所需的数量。因此，我们在提取物研究中估计的单个细菌组装肌动蛋白丝或肌动蛋白尾所需的IcsA分子数，是对志贺氏菌感染细胞中这些功能所需分子数的合理预测。
