From the School of Chemistry, University of Bristol Cantock's Close, Bristol BS8 1TS, United Kingdom.
Biosciences, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4QD, United Kingdom.
J Biol Chem. 2018 Dec 14;293(50):19429-19440. doi: 10.1074/jbc.RA118.005173. Epub 2018 Oct 18.
Toxin-antitoxin (TA) systems are present in many bacteria and play important roles in bacterial growth, physiology, and pathogenicity. Those that are best studied are the type II TA systems, in which both toxins and antitoxins are proteins. The HicAB system is one of the prototypic TA systems, found in many bacterial species. Complex interactions between the protein toxin (HicA), the protein antitoxin (HicB), and the DNA upstream of the encoding genes regulate the activity of this system, but few structural details are available about how HicA destabilizes the HicB-DNA complex. Here, we determined the X-ray structures of HicB and the HicAB complex to 1.8 and 2.5 Å resolution, respectively, and characterized their DNA interactions. This revealed that HicB forms a tetramer and HicA and HicB form a heterooctameric complex that involves structural reorganization of the C-terminal (DNA-binding) region of HicB. Our observations indicated that HicA has a profound impact on binding of HicB to DNA sequences upstream of in a stoichiometric-dependent way. At low ratios of HicA:HicB, there was no effect on DNA binding, but at higher ratios, the affinity for DNA declined cooperatively, driving dissociation of the HicA:HicB:DNA complex. These results reveal the structural mechanisms by which HicA de-represses the HicB-DNA complex.
毒素-抗毒素 (TA) 系统存在于许多细菌中，在细菌的生长、生理和致病性中发挥着重要作用。其中研究得最好的是 II 型 TA 系统，其中毒素和抗毒素都是蛋白质。HicAB 系统是最典型的 TA 系统之一，存在于许多细菌物种中。蛋白质毒素 (HicA)、蛋白质抗毒素 (HicB) 和编码基因上游的 DNA 之间的复杂相互作用调节该系统的活性，但关于 HicA 如何使 HicB-DNA 复合物失稳的结构细节知之甚少。在这里，我们分别以 1.8 Å 和 2.5 Å 的分辨率确定了 HicB 和 HicAB 复合物的 X 射线结构，并对其 DNA 相互作用进行了表征。这表明 HicB 形成四聚体，HicA 和 HicB 形成涉及 HicB 的 C 末端（DNA 结合）区域结构重排的异源八聚体复合物。我们的观察结果表明，HicA 以依赖于化学计量的方式对 HicB 与 上游的 DNA 序列的结合产生深远影响。在 HicA:HicB 比例较低时，对 DNA 结合没有影响，但在较高比例时，DNA 结合亲和力呈协同下降，导致 HicA:HicB:DNA 复合物解离。这些结果揭示了 HicA 去抑制 HicB-DNA 复合物的结构机制。
