Structural and Functional Analyses of Toxin-Antitoxin Protein Complexes From Bact

描述（由申请人提供）：细菌已经进化出复杂的策略来在其环境中竞争和交流。最近在大肠杆菌中发现了一种新的细菌间竞争机制，称为接触依赖性生长抑制（CDI）。CDI系统存在于多种革兰氏阴性细菌中，包括几种重要的人类病原体。CDI由CdiB/CdiA双伴侣分泌系统介导。CdiB是预测的外膜蛋白，其是将CdiA外蛋白输出和组装到细胞表面上所需的。CdiA的C-末端结构域（CdiA-CT）含有生长抑制活性，并且可能被切割并易位到靶细胞胞质中以抑制生长。CDI系统还编码CdiI免疫蛋白，其结合并抑制CdiA-CT毒素，从而保护CDI+细胞免受自身抑制。值得注意的是，CdiA-CT结构域是多态性的，迄今已鉴定出超过60种不同的毒素序列。因此，相应的CdiI蛋白也是高度可变的。这种序列多样性表明，CDI系统部署了各种各样的毒性活动。实际上，我们已经发现CdiA-CT结构域表现出许多不同的核酸酶活性.由于CdiI免疫蛋白对其同源CdiA-CT具有特异性，因此每种毒素-免疫复合物的相互作用可能是独特的。目前没有任何CdiA-CT/CdiI复合物的结构信息，因此这些蛋白质-蛋白质相互作用的细节完全未知。此外，CdiI蛋白中和CdiA-CT活性的机制尚不清楚。我们提出了结构和功能分析，以深入了解细菌CDI系统编码的复杂的毒素免疫网络，这是一个独特的机会，以阐明如何保持特异性结合，因为毒素免疫对通过进化发散。

DESCRIPTION (provided by applicant): Bacteria have evolved complex strategies to compete and communicate in their environments. A new mechanism of inter-bacterial competition, termed contact- dependent growth inhibition (CDI) was recently discovered in Escherichia coli. CDI systems are found in a wide variety of gram-negative bacteria, including several important human pathogens. CDI is mediated by the CdiB/CdiA two-partner secretion system. CdiB is a predicted outer membrane protein that is required for the export and assembly of the CdiA exoprotein onto the cell surface. The C-terminal domain of CdiA (CdiA-CT) contains the growth inhibition activity and is presumably cleaved and translocated into the target cell cytoplasm to inhibit growth. CDI systems also encode CdiI immunity proteins, which bind and inactivate CdiA-CT toxins, thereby protecting CDI+ cells from autoinhibition. Remarkably, the CdiA-CT domain is polymorphic, with well over 60 different toxin sequences identified to date. Accordingly, the corresponding CdiI proteins are also highly variable. This sequence diversity suggests that CDI systems deploy a wide variety of toxic activities. Indeed, we have discovered that CdiA- CT domains exhibit a number of distinct nuclease activities. Because CdiI immunity proteins are specific for their cognate CdiA-CTs, the interactions underlying each toxin- immunity complex are presumably unique. There is currently no structural information available for any CdiA-CT/CdiI complex, and therefore the details of these protein- protein interactions are completely unknown. Moreover, the mechanisms by which CdiI proteins neutralize CdiA-CT activities are not understood. We propose structural and functional analyses to gain insights into the intricate toxin-immunity network encoded by bacterial CDI systems, which represents a unique opportunity to elucidate how specific binding is maintained as toxin-immunity pairs diverge through evolution.