Targeting Clostridium difficile infection is challenging because treatment options are limited, and high recurrence rates are common. One reason for this is that hypervirulent C. difficile strains often have a binary toxin termed the C. difficile toxin, in addition to the enterotoxins TsdA and TsdB. The C. difficile toxin has an enzymatic component, termed CDTa, and a pore-forming or delivery subunit termed CDTb. CDTb was characterized here using a combination of single-particle cryoelectron microscopy, X-ray crystallography, NMR, and other biophysical methods. In the absence of CDTa, 2 di-heptamer structures for activated CDTb (1.0 MDa) were solved at atomic resolution, including a symmetric (SymCDTb; 3.14 Å) and an asymmetric form (AsymCDTb; 2.84 Å). Roles played by 2 receptor-binding domains of activated CDTb were of particular interest since the receptor-binding domain 1 lacks sequence homology to any other known toxin, and the receptor-binding domain 2 is completely absent in other well-studied heptameric toxins (i.e., anthrax). For AsymCDTb, a Ca2+ binding site was discovered in the first receptor-binding domain that is important for its stability, and the second receptor-binding domain was found to be critical for host cell toxicity and the di-heptamer fold for both forms of activated CDTb. Together, these studies represent a starting point for developing structure-based drug-design strategies to target the most severe strains of C. difficile.

中文翻译：

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艰难梭菌二元毒素的细胞结合成分的结构揭示了二七聚体大分子组装体。

针对艰难梭菌感染具有挑战性，因为治疗选择有限，而且复发率很高。原因之一是，除了肠毒素 TsdA 和 TsdB 之外，高毒力艰难梭菌菌株通常还具有称为艰难梭菌毒素的二元毒素。艰难梭菌毒素具有称为 CDTa 的酶组分和称为 CDTb 的成孔或递送亚基。此处结合单颗粒冷冻电子显微镜、X 射线晶体学、核磁共振和其他生物物理方法对 CDTb 进行了表征。在没有 CDTa 的情况下，以原子分辨率解​​析了活化 CDTb (1.0 MDa) 的 2 个二七聚体结构，包括对称形式 (SymCDTb; 3.14 Å) 和不对称形式 (AsymCDTb; 2.84 Å)。激活的 CDTb 的 2 个受体结合结构域所发挥的作用特别令人感兴趣，因为受体结合结构域 1 缺乏与任何其他已知毒素的序列同源性，并且受体结合结构域 2 在其他经过充分研究的七聚体毒素中完全不存在（即、炭疽病）。对于 AsymCDTb，在第一个受体结合结构域中发现了一个 Ca2+ 结合位点，这对其稳定性很重要，并且发现第二个受体结合结构域对于宿主细胞毒性和两种形式的激活的二七聚体折叠至关重要。 CDTb。总之，这些研究代表了开发基于结构的药物设计策略的起点，以针对最严重的艰难梭菌菌株。