绿脓杆菌特有的Tsi2三维结构——一种全新卷曲螺旋构象的类抗毒素蛋白

Three Dimensional Structure of Pseudomonas aeruginosa Tsi2: a Novel Species-specific Antitoxin-like Protein With Coiled Coil Conformation

绿脓杆菌(Pseudomonas aeruginosa)利用六型分泌系统(T6SS)向其他竞争性细菌分泌毒素效应分子Tse2,这是一种新发现的绿脓杆菌获得生存优势的分子机制.为了避免同类间的误杀,绿脓杆菌合成一种特异结合Tse2的抑制蛋白Tsi2来保护自己.序列分析显示,Tsi2是绿脓杆菌特有的一种新型类抗毒素蛋白.我们利用SAD方法成功地解析了Tsi2 1.8魡分辨率的晶体结构.Tsi2的三维结构采用一种规则的卷曲螺旋的结构特征,这是抗毒素分子中的一种全新的折叠方式,不同于经典的抗毒素分子在没有结合毒素分子状态下采用无规则构象的结构特征;二聚体是Tsi2的功能单位,二聚体内两个Tsi2单体通过广阔的疏水相互作用紧密结合,形成"夹子"状独特的二聚体组装方式;位于二聚体界面上的两个凹槽分别结合对称分子的两段螺旋,提供了Tsi2与Tse2结合可能的分子部位.该研究工作结果对于认识Tsi2抗毒素蛋白的分子本质,揭示其发挥抗毒素活性的结构基础,并为进一步开展Tse2-Tsi2复合物的结构与功能研究奠定了坚实的基础.

Utilizing the type VI secretion system （T6SS）, Pseudomonas aeruginosa secretes the effector protein Tse2, a toxin to other competitive bacteria. It is a newly identified molecular mechanism for P. aeruginosa to win a survival advantage. To avoid being poisoned itself, P. aeruginosa synthesizes the specific immunity protein, Tsi2 to inhibit the toxin. Sequence analysis shows that Tsi2 is a novel antitoxin-like protein which is specific for P. aeruginosa. Using the SAD method, we have successfully resolved the crystal structure of Tsi2 at 1.8A resolution. Our crystallographic studies reveal that Tsi2 adopts a novel coiled coil conformation which is not found in the antitoxins family previously. Meanwhile, Tsi2 is a well-assembled protein instead of an unfolded protein as an antitoxin in the toxin-free state. Tsi2 functions as a stable dimer and assembles as a unique ＂clamp＂ structure through extensive hydrophobic interactions. Two grooves on the dimerization interface combining with one helix of two symmetric molecules respectively imply the potential region interacting with Tse2. This research not only offers comprehensive insights into the molecular essence of Tsi2 as an antitoxin, but also reveals its structural basis of antitoxin activity. Furthermore, the apo Tsi2 structure provides a good framework for further researches on the structure and function of the complex Tse2-Tsi2.