ATR/TEM8 vWA结构域与PA的相互作用

【作者】 蔡晨光；

【导师】 陈薇； 徐俊杰；

【作者基本信息】 中国人民解放军军事医学科学院 ， 微生物学， 2010， 博士

【摘要】 炭疽毒素是炭疽芽孢杆菌Bacillus anthracis的主要毒力因子,包括三个组分:保护性抗原PA,致死因子LF,水肿因子EF。目前发现两种炭疽毒素受体:肿瘤内皮细胞标志抗原8（TEM8）和毛细血管生成蛋白2（CMG2）。这两个受体在其胞外区都具有vWA结构域,其中60%的序列保守。抑制PA与受体的结合可有效地阻断炭疽毒素。炭疽病发展到晚期,在毒素大量释放后单靠抗生素治疗达不到效果,此时毒素抑制剂显得尤为重要。受体类抑制物,比如哺乳动物细胞表达的CMG2 vWA结构域已被用于炭疽抗毒剂的研究。本工作通过X射线衍射解析了高分辨率（1.7A）的ATR/TEM8 vWA结构域晶体结构。结构显示1个晶格中包括6个TEM8 vWA结构域分子。在这个六聚体中,配基结合的MIDAS序列位点被邻近的单体所封闭,显示此六聚体分子没有活性。另外,凝胶过滤层析以及超速离心沉降实验表明溶液中TEM8以单体存在。这样,观察到的六聚体形式可能是由于晶体堆积造成的假象。TEM8的晶体结构与CMG2的vWA结构域高度相似,同样以经典的α/β开放式片层折叠结构（又名二核苷结合折叠,或Rossmann折叠,或双重缠绕折叠）存在。值得注意的是有一个乙酸根离子与CMG2结构中的类似离子以同样的方式模仿PA的D683侧链占据MIDAS位点。此乙酸根离子,以及S52, S54,T118和两个水分子与Mg²⁺进行螯合,表明解析的TEM8具有Mg²⁺螯合的保守的MIDAS基序,且以开放构象存在.以此结构以及PA-CMG2复合物晶体结构（PDB 1T6B）,通过叠合和分子替换构建了PA-TEM8复合物的结构模型。经过溶剂化和能量优化,得到了PA分别与两种受体在溶液中的复合物结构模型。通过分析和比较,模型显示PA与两种受体间的相互作用在以下几处高度保守:PA D683,N682与受体的MDAS基序S52,S54,T118的相互作用;以及受体Y119, K51与PA的相互作用。在56,88,113,115,156（按TEM8序列）处两模型有明显的不一样,显示可能受体与PA的结合在这些地方有所差异。对TEM8上同源于PA-CMG2结合界面上的CMG2相关残基进行同源扫描替换,经大肠杆菌表达和对可溶组分的纯化,得到的突变体在J774A.1细胞上检测了其炭疽致死毒素攻击的保护活性。L56A突变体活性增强到野生型的4倍,而E155G增强不到一倍,L56A与E155G的联合突变体增强了大概6倍的活性。与之对比,H57N,R88Q,L157V,F158P等点突变的保护活性均有下降。用表面等离子共振技术检测了这些突变体与PA的亲和力,其变化趋势与细胞实验的结果一致。同时以抗SDS的孔道复合物形成实验检测了这些突变对PA从前孔复合物至到孔道复合物进行构象转变所需pH阈值的影响,所有单突变都没有明显的改变,而154-158的联合突变体对比野生型显示出了明显的区别,接近CMG2。另外,本工作为毒素中和的细胞实验和基于受体类抑制剂的Schlid plot实验推导了一个数学模型。此数学模型将细胞实验的结果与受体与PA结合的动力学参数关联起来。运用此模型评价了TEM8突变体的L56A及E155G+L56A的亲和力,同时SPR的结果与此模型基本一致。最后,以大鼠模型评价了TEM8以及其突变体L56A作为抗毒剂对炭疽致死毒素的体内中和活性。不同于前人报道,TEM8显示出良好表现,在7.5:1-3:1的摩尔比（受体:PA）能完全保护动物。而CMG2的保护活性较一致,为3:1左右。L56A能在3:1到1:1之间完全保护动物,表现优于CMG2,尽管其KD及细胞上的保护活性都低与CMG2,这可能是因为在体内存在其他配基能与CMG2作用而不结合TEM8或是其突变体,这样,CMG2作为抗毒剂就有可能导致副作用,此假设仍需要实验验证。在此工作中,TEM8 vWA结构域高分辨率晶体结构的获得,以及溶液中两种受体复合物模型的构建,为进一步的分子动力学模拟研究溶液中构象的转变及分子行为以提供更多关于PA受体相互作用的细节奠定了基础,而且有助于蛋白设计以获得更高亲和力的受体突变体及选择性结合TEM8以作为肿瘤靶向载体的PA突变体。同时,证明了两种受体在56,88,155位点处与PA的相互作用存在差异,在56和155处的单位点或是联合的同源替换突变能增强TEM8的结合力,而R88Q突变削弱了TEM8的亲和力。这些发现为设计更有效的受体类抑制剂提供了研究靶点。进一步的,本工作推导的数学模型将细胞实验结果与受体的解离常数KD相关联,减少了由于不同批次间铺板细胞数及受体表达情况造成的误差,为评价受体类抑制药物提供了更精确的定量方法。本工作发现的TEM8突变体L56A,在动物实验中对炭疽致死毒素有较好的中和活性,可作为炭疽毒素抑制剂的候选药物。更多还原

【Abstract】 Anthrax toxin is a major virulence factor of Bacillus anthracis. The toxin consists of three protein subunits: protective antigen （PA）, lethal factor （LF）, and edema factor （EF）. The anthrax toxin receptors, tumor endothelium marker-8（TEM8） and capillary morphogenesis protein-2 （CMG2）, both contain an extracellular vWA domain shared 60% identical residues. Inhibition of PA-receptor binding could effectively cut off anthrax intoxication way, and is especially important for anthrax therapy at late stage when the toxin has been release and antibiotics intervention doesn’t work. Crystal structures of PA,CMG2 and PA-CMG2 have been analyzed and interactions between PA and CMG2 has been well described. Receptor-like agonists, such as mammalian cell expressed vWA domain of CMG2 （sCMG2）, have been explored for their potency as anti-toxin reagents.In this work, A high resolution （1.7 ?） crystal structure of ATR/TEM8 vWA domain was analyzed by X-ray diffraction. Our crystal structure contained six TEM8 vWA molecules in the asymmetric unit （also unit cell）. In this hexamer, the MIDAS ligand binding area from each monomer was blocked which indicates that such a hexamer is an inactive oligomeric form. Moreover, TEM8 was determined to exist as a monomer in solution by analytical ultracentrifuge and gel filtration （data not shown）. Thus, the observed TEM8 hexamer is most likely an artifact of crystal packing.Consistent with their sequence homology, the structure of the TEM8 extracellular domain is very similar to that of CMG2 and the integrin A domain. It adopts a classicalα/βopen sheet fold that has also been called the dinucleotide-binding fold, Rossmann fold, or doubly wound fold. It should be noted that an acetate ions were found correspond to an analogous ion in the CMG2 structure, which acted as a mimic ligand of the side chain of D683 in PA and occupied the MIDAS coordination site. Besides this ligand, S52, S54,T118 and two water molecular was found to coordinate with the metal ion Mg²⁺, indicating that the TEM8 extracellular domain contained a conserved Mg²⁺-coordinated MIDAS motif that assumed an integrin-like open conformation.Based on this structure and the PA-CMG2 complex structure （PDB 1T6B）, a model structure of PA-TEM8 complex was generated by molecular replacement after molecular superimposing with tether. After being applied with Solvation and Energy Minimization programs by Discovery with CHARMm forcefield , models of complex of PA and both receptors in solution were gain. By comparing the models, serveral conserved interactions between receptors and PA were found: PA D683 N682 interacted with receptors MIDAS motif residues S52,S54 and T118, besides Y119 and K51 in receptor could bind to residues in PA tightly. Also, there existed edsignificant differences in our complex models, including interactions between unconserved residues at position 56, 88,113,115,156（based on TEM8’s sequence） and PA residues, respectively. Also a salt bridge of E344 existed in PA-CMG2 model bingding with PA residue E344 was observed replaced by attenuated electricity potential complement with PA R659 in PA–TEM8 Model.A homologue-replacement scan mutantion of the unconserved residues homologue to that at the PA-CMG2 interaction interface was introduced to TEM8 vWA domain, After expressing in E.coli and purification of the soluble parts, the protection ability from LeTx attacking of the mutants was evaluated using J774A.1 cells. Of all the soluble single-residue-replacement mutants, L56A showed most excellent performance （about 3 times less of wild type’s IC50）, and E155G also showed an increment of protection ability （about 1/2 of wild type’s IC50）, compared to the wild type of TEM8. When combined E155G and L56A, the viability was elevated（about 6 folds less of IC50） In contrast, the H57N, R88Q, L157V, and F158P point mutants had decreased protective ability. SPR assay was also used to assay their affinity to PA, and the results were consistent with those from cell assays.Also, a SDS-resistant pore formation assay was performed to analyze these mutations’influence on the pH threshold for conversion of a PA prepore to pore. All single point mutations did not cause a significant change in the pH threshold of pore formation. However, a multiple mutation （154-158） showed a striking change in the pH threshold of pore formation, with a value as approaching to that of CMG2.In addition, a mathematic model supporting the toxin neutralization assay and receptor-like antagonists based Schild plot assay was deduced. With this model, the results of cell assay （measured as IC50s in protection assay, and EC50s in Schild plot assay） were related with the kinetic parameter KD （equilibrium dissociation constant）. This model to applied to evaluate the affinity of TEM8 mutants L56A and E155G+L56A, while the SPR result was consistent with this model at some extent.At last, the potent of TEM8 and its mutant L56A as an anti-toxin for PA was tested on rats. Unlike previous report, TEM8 expressed in this work exhibited a much better performance, with total protection at 7.5:1-3:1 （mol ratio, receptor:PA）, whereas CMG2’s protection was much consistent at 3:1.The L56A, with total protection between 3:1 and 1:1, seems perform even better than CMG2, in spite of its KD and cell protection ability inferior to that of CMG2. this disaccord was ascribed to that CMG2 may binds to other ligands in vivo while TEM8 and its mutant would not, which implied a possible side effect of CMG2 as anti-toxin。However, further evidence still needed.In conclusion, the crystal structure of the TEM8 vWA domain at 1.7 ? resolution was determined. In addition, models of PA-receptor complex in solution were constructed. The structure and models provide fundament for further dynamic simulation about the conform alternation and molecular behavior in solution, which would provide further details of interaction and help us to design affinity-elevated receptor mutants as anti-toxins or PA variants selectively bind to TEM8 as tumor targeting vector. Meanwhile, residues at position 56 88 155 were found to differentiate receptors’bind to PA, and homologue-replacment mutations to 56 and/or 155 in TME8 would all elevate the affinity, while R88Q mutation could attenuate the binding. Furthermore tem8 mutant L56A exhibited a slight superior protection ability on rats, which would provide an alternative choice for CMG2 considering possible side-effect implied in our work. These findings point out potent hot spot for designing more efficient receptor-like antitoxins. Moreover, the mathematic model deduced in this work for relating the results of cell assay to affinity parameters （dissociation constant, KD）, would reduce the errors induced by discrimination of cell numbers and receptor-expressing states of different experimental repeats, and provide a more precise quantification for evaluating receptor-like inhibitors.L56A,a bacteria produced TEM8 mutant presented in this work, could as effectively inhibit toxication on rats as sCMG2, and would be provided as a candidate as anti-toxin drugs.更多还原