Morrow K Adam, Frank Dara W, Balczon Ron, Stevens Troy
Department of Physiology and Cell Biology, University of South Alabama, Mobile, AL, 36688, USA.
The Center for Lung Biology, University of South Alabama, Mobile, AL, 36688, USA.
Handb Exp Pharmacol. 2017;238:67-85. doi: 10.1007/164_2016_5003.
Exoenzyme Y (ExoY) was identified as a component of the Pseudomonas aeruginosa type 3 secretion system secretome in 1998. It is a common contributor to the arsenal of type 3 secretion system effectors, as it is present in approximately 90% of Pseudomonas isolates. ExoY has adenylyl cyclase activity that is dependent upon its association with a host cell cofactor. However, recent evidence indicates that ExoY is not just an adenylyl cyclase; rather, it is a promiscuous cyclase capable of generating purine and pyrimidine cyclic nucleotide monophosphates. ExoY's enzymatic activity causes a characteristic rounding of mammalian cells, due to microtubule breakdown. In endothelium, this cell rounding disrupts cell-to-cell junctions, leading to loss of barrier integrity and an increase in tissue edema. Microtubule breakdown seems to depend upon tau phosphorylation, where the elevation of cyclic nucleotide monophosphates activates protein kinases A and G and causes phosphorylation of endothelial microtubule associated protein tau. Phosphorylation is a stimulus for tau release from microtubules, leading to microtubule instability. Phosphorylated tau accumulates inside endothelium as a high molecular weight, oligomeric form, and is then released from the cell. Extracellular high molecular weight tau causes a transmissible cytotoxicity that significantly hinders cellular repair following infection. Thus, ExoY may contribute to bacterial virulence in at least two ways; first, by microtubule breakdown leading to loss of endothelial cell barrier integrity, and second, by promoting release of a high molecular weight tau cytotoxin that impairs cellular recovery following infection.
1998年，外切酶Y（ExoY）被鉴定为铜绿假单胞菌3型分泌系统分泌蛋白组的一个组成部分。它是3型分泌系统效应蛋白库的常见成分，大约90%的假单胞菌分离株中都有它。ExoY具有腺苷酸环化酶活性，这取决于它与宿主细胞辅因子的结合。然而，最近的证据表明，ExoY不仅仅是一种腺苷酸环化酶；相反，它是一种能产生嘌呤和嘧啶环核苷酸单磷酸的多特异性环化酶。ExoY的酶活性会导致哺乳动物细胞出现特征性的变圆，这是由于微管破裂所致。在内皮细胞中，这种细胞变圆会破坏细胞间连接，导致屏障完整性丧失和组织水肿增加。微管破裂似乎取决于tau蛋白磷酸化，环核苷酸单磷酸的升高会激活蛋白激酶A和G，并导致内皮微管相关蛋白tau磷酸化。磷酸化是tau蛋白从微管释放的刺激因素，导致微管不稳定。磷酸化的tau蛋白以高分子量寡聚体形式在内皮细胞内积累，然后从细胞中释放出来。细胞外的高分子量tau蛋白会导致一种可传播的细胞毒性，显著阻碍感染后的细胞修复。因此，ExoY可能至少通过两种方式促进细菌毒力；第一，通过微管破裂导致内皮细胞屏障完整性丧失；第二，通过促进高分子量tau细胞毒素的释放，这种毒素会损害感染后的细胞恢复。
