Clostridioides difficile is a bacterial pathogen responsible for the majority of nosocomial infections in the developed world. C. difficile infection (CDI) is difficult to treat in many cases because hypervirulent strains have evolved that contain a third toxin, termed the C. difficile toxin (CDT), in addition to the two enterotoxins TcdA and TcdB. CDT is a binary toxin comprised of an enzymatic, ADP-ribosyltransferase (ART) toxin component, CDTa, and a pore-forming or delivery subunit, CDTb. In the absence of CDTa, CDTb assembles into two distinct di-heptameric states, a symmetric and an asymmetric form with both states having two surface-accessible host cell receptor-binding domains, termed RBD1 and RBD2. RBD1 has a unique amino acid sequence, when aligned to other well-studied binary toxins (i.e., anthrax), and it contains a novel Ca²⁺-binding site important for CDTb stability. The other receptor binding domain, RBD2, is critically important for CDT toxicity, and a domain such as this is missing altogether in other binary toxins and shows further that CDT is unique when compared to other binary toxins. In this study, the ¹H, ¹³C, and ¹⁵N backbone and sidechain resonances of the 120 amino acid RBD2 domain of CDTb (residues 757–876) were assigned sequence-specifically and provide a framework for future NMR-based drug discovery studies directed towards targeting the most virulent strains of CDI.

艰难梭菌是一种细菌病原体，导致发达国家大多数医院感染。在许多情况下，艰难梭菌感染 (CDI) 很难治疗，因为除了两种肠毒素 TcdA 和 TcdB 之外，已经进化出高毒力菌株，还含有第三种毒素，称为艰难梭菌毒素 (CDT)。CDT 是一种二元毒素，由酶促 ADP-核糖基转移酶 (ART) 毒素成分 CDTa 和成孔或递送亚基 CDTb 组成。在没有 CDTa 的情况下，CDTb 组装成两种不同的二七聚体状态，即对称形式和不对称形式，这两种状态都具有两个表面可接近的宿主细胞受体结合结构域，称为 RBD1 和 RBD2。当与其他经过充分研究的二元毒素（即炭疽）比对时，RBD1 具有独特的氨基酸序列，并且它包含对 CDTb 稳定性很重要的新型 Ca 2+ 结合位^点。另一个受体结合结构域 RBD2 对于 CDT 毒性至关重要，而其他二元毒素中完全缺少这样的结构域，这进一步表明 CDT 与其他二元毒素相比是独特的。在这项研究中，CDTb 的 120 个氨基酸 RBD2 结构域（残基 757-876）的 1 H、13 C 和 15 N 主链和侧链共振被指定为序列特异性，并为未来基于 NMR 的药物^发现研究^提供了^框架旨在针对最具毒性的 CDI 菌株。