Cisz Michelle, Lee Pei-Chung, Rietsch Arne
Department of Molecular Biology and Microbiology, Case Western Reserve University School of Medicine, 10900 Euclid Ave., Cleveland, OH 44106-4960, USA.
J Bacteriol. 2008 Apr;190(8):2726-38. doi: 10.1128/JB.01553-07. Epub 2007 Nov 26.
Type III secretion is used by many gram-negative bacterial pathogens to directly deliver protein toxins (effectors) into targeted host cells. In all cases, secretion of effectors is triggered by host cell contact, although the mechanism is unclear. In Pseudomonas aeruginosa, expression of all type III secretion-related genes is up-regulated when secretion is triggered. We were able to visualize this process using a green fluorescent protein reporter system and to use it to monitor the ability of bacteria to trigger effector secretion on cell contact. Surprisingly, the action of one of the major type III secreted effectors, ExoS, prevented triggering of type III secretion by bacteria that subsequently attached to cells, suggesting that triggering of secretion is feedback regulated. Evidence is presented that translocation (secretion of effectors across the host cell plasma membrane) of ExoS is indeed self-regulated and that this inhibition of translocation can be achieved by either of its two enzymatic activities. The translocator proteins PopB, PopD, and PcrV are secreted via the type III secretion system and are required for pore formation and translocation of effectors across the host cell plasma membrane. Here we present data that secretion of translocators is in fact not controlled by calcium, implying that triggering of effector secretion on cell contact represents a switch in secretion specificity, rather than a triggering of secretion per se. The requirement for a host cell cofactor to control effector secretion may help explain the recently observed phenomenon of target cell specificity in both the Yersinia and P. aeruginosa type III secretion systems.
许多革兰氏阴性细菌病原体利用III型分泌系统将蛋白质毒素（效应蛋白）直接传递到靶向宿主细胞中。在所有情况下，效应蛋白的分泌都是由宿主细胞接触触发的，尽管其机制尚不清楚。在铜绿假单胞菌中，当分泌被触发时，所有与III型分泌相关的基因的表达都会上调。我们能够使用绿色荧光蛋白报告系统可视化这一过程，并利用它来监测细菌在细胞接触时触发效应蛋白分泌的能力。令人惊讶的是，一种主要的III型分泌效应蛋白ExoS的作用阻止了随后附着在细胞上的细菌触发III型分泌，这表明分泌的触发是受反馈调节的。有证据表明，ExoS的转运（效应蛋白穿过宿主细胞质膜的分泌）确实是自我调节的，并且这种转运抑制可以通过其两种酶活性中的任何一种来实现。转运蛋白PopB、PopD和PcrV通过III型分泌系统分泌，是效应蛋白穿过宿主细胞质膜形成孔道和转运所必需的。在这里，我们提供的数据表明，转运蛋白的分泌实际上不受钙的控制，这意味着细胞接触时效应蛋白分泌的触发代表了分泌特异性的转变，而不是分泌本身的触发。宿主细胞辅因子对效应蛋白分泌的控制需求可能有助于解释最近在耶尔森氏菌和铜绿假单胞菌III型分泌系统中观察到的靶细胞特异性现象。
