New York Structural Biology Center, New York, NY 10027, USA.
Division of Infectious Diseases, Wadsworth Center, New York State Department of Health, Albany, NY 12208, USA.
J Mol Biol. 2014 Aug 26;426(17):3057-68. doi: 10.1016/j.jmb.2014.05.026. Epub 2014 Jun 4.
Ricin is a select agent toxin and a member of the RNA N-glycosidase family of medically important plant and bacterial ribosome-inactivating proteins. In this study, we determined X-ray crystal structures of the enzymatic subunit of ricin (RTA) in complex with the antigen binding domains (VHH) of five unique single-chain monoclonal antibodies that differ in their respective toxin-neutralizing activities. None of the VHHs made direct contact with residues involved in RTA's RNA N-glycosidase activity or induced notable allosteric changes in the toxin's subunit. Rather, the five VHHs had overlapping structural epitopes on the surface of the toxin and differed in the degree to which they made contact with prominent structural elements in two folding domains of the RTA. In general, RTA interactions were influenced most by the VHH CDR3 (CDR, complementarity-determining region) elements, with the most potent neutralizing antibody having the shortest and most conformationally constrained CDR3. These structures provide unique insights into the mechanisms underlying toxin neutralization and provide critically important information required for the rational design of ricin toxin subunit vaccines.
蓖麻毒素是一种选择性的药剂毒素，也是医学上重要的植物和细菌核糖体失活蛋白的 RNA N-糖苷酶家族的成员。在这项研究中，我们确定了蓖麻毒素（RTA）酶亚基与五个独特的单链单克隆抗体的抗原结合结构域（VHH）复合物的 X 射线晶体结构，这些抗体在其各自的毒素中和活性方面存在差异。没有一个 VHH 与参与 RTA 的 RNA N-糖苷酶活性的残基直接接触，也没有诱导毒素亚基发生明显的变构变化。相反，这五个 VHH 在毒素表面具有重叠的结构表位，并且在它们与 RTA 两个折叠结构域中突出结构元素的接触程度上存在差异。一般来说，RTA 的相互作用受 VHH CDR3（CDR，互补决定区）元素的影响最大，中和活性最强的抗体具有最短且最具构象约束的 CDR3。这些结构为毒素中和的机制提供了独特的见解，并为蓖麻毒素亚单位疫苗的合理设计提供了至关重要的信息。
