Midwest Center for Structural Genomics, Argonne National Laboratory, Argonne, IL, USA.
Structural Biology Center, Biosciences Division, Argonne National Laboratory, Argonne, IL, USA.
Mol Microbiol. 2018 Aug;109(4):509-527. doi: 10.1111/mmi.14007. Epub 2018 Aug 12.
Bacteria use several different secretion systems to deliver toxic EndoU ribonucleases into neighboring cells. Here, we present the first structure of a prokaryotic EndoU toxin in complex with its cognate immunity protein. The contact-dependent growth inhibition toxin CdiA-CT from Escherichia coli STEC_O31 adopts the eukaryotic EndoU fold and shares greatest structural homology with the nuclease domain of coronavirus Nsp15. The toxin contains a canonical His-His-Lys catalytic triad in the same arrangement as eukaryotic EndoU domains, but lacks the uridylate-specific ribonuclease activity that characterizes the superfamily. Comparative sequence analysis indicates that bacterial EndoU domains segregate into at least three major clades based on structural variations in the N-terminal subdomain. Representative EndoU nucleases from clades I and II degrade tRNA molecules with little specificity. In contrast, CdiA-CT and other clade III toxins are specific anticodon nucleases that cleave tRNA between nucleotides C37 and m A38. These findings suggest that the EndoU fold is a versatile scaffold for the evolution of novel substrate specificities. Such functional plasticity may account for the widespread use of EndoU effectors by diverse inter-bacterial toxin delivery systems.
细菌使用几种不同的分泌系统将毒性内切 U 核糖核酸酶输送到邻近的细胞中。在这里，我们展示了第一个与同源免疫蛋白结合的原核内切 U 毒素的结构。来自大肠杆菌 STEC_O31 的接触依赖性生长抑制毒素 CdiA-CT 采用真核内切 U 的折叠结构，与冠状病毒 Nsp15 的核酸内切酶结构域具有最大的结构同源性。该毒素在相同的排列中包含一个典型的 His-His-Lys 催化三联体，与真核内切 U 结构域相同，但缺乏特征为超家族的尿嘧啶特异性核糖核酸酶活性。比较序列分析表明，细菌内切 U 结构域根据 N 端亚结构域的结构变化至少分为三个主要分支。来自分支 I 和 II 的代表性内切 U 核酸酶对 tRNA 分子的特异性降解很小。相比之下，CdiA-CT 和其他分支 III 毒素是特异性反密码子核酸酶，可在核苷酸 C37 和 mA38 之间切割 tRNA。这些发现表明，内切 U 折叠是进化新型底物特异性的多功能支架。这种功能的可塑性可能解释了广泛使用内切 U 效应子的不同细菌之间毒素输送系统。
