Department of Molecular Biology and Genetics, Aarhus University, DK-8000 Aarhus C, Denmark.
Department of Biological Sciences, Old Dominion University, Norfolk, VA 23529, USA.
Structure. 2021 Feb 4;29(2):139-150.e3. doi: 10.1016/j.str.2020.10.002. Epub 2020 Oct 22.
Bacterial type II toxin-antitoxin (TA) modules encode a toxic protein that downregulates metabolism and a specific antitoxin that binds and inhibits the toxin during normal growth. In non-typeable Haemophilus influenzae, a common cause of infections in humans, the vapXD locus was found to constitute a functional TA module and contribute to pathogenicity; however, the mode of action of VapD and the mechanism of inhibition by the VapX antitoxin remain unknown. Here, we report the structure of the intact H. influenzae VapXD complex, revealing an unusual 2:1 TA molecular stoichiometry where a Cas2-like homodimer of VapD binds a single VapX antitoxin. VapX consists of an oligonucleotide/oligosaccharide-binding domain that docks into an asymmetrical cavity on the toxin dimer. Structures of isolated VapD further reveal how a symmetrical toxin homodimer adapts to interacting with an asymmetrical antitoxin and suggest how a primordial TA system evolved to become part of CRISPR-Cas immunity systems.
细菌 II 型毒素-抗毒素（TA）模块编码一种毒性蛋白，可下调代谢，而一种特定的抗毒素在正常生长过程中结合并抑制毒素。在不可分型流感嗜血杆菌中，一种常见的人类感染病原体，发现 vapXD 基因座构成了一个功能性 TA 模块，并有助于致病性；然而，VapD 的作用模式和 VapX 抗毒素的抑制机制仍然未知。在这里，我们报告了完整的流感嗜血杆菌 VapXD 复合物的结构，揭示了一种不寻常的 2:1 TA 分子计量比，其中 Cas2 样同源二聚体的 VapD 结合单个 VapX 抗毒素。VapX 由一个寡核苷酸/寡糖结合结构域组成，该结构域与毒素二聚体上的非对称腔对接。分离的 VapD 结构进一步揭示了对称毒素同源二聚体如何适应与非对称抗毒素相互作用，并提出了原始 TA 系统如何进化成为 CRISPR-Cas 免疫系统的一部分。
