The evolution of virulence traits is central for the emergence or re-emergence of microbial pathogens and for their adaptation to a specific host ^1-5 . Typhoid toxin is an essential virulence factor of the human-adapted bacterial pathogen Salmonella Typhi ^6,7 , the cause of typhoid fever in humans ^8-12 . Typhoid toxin has a unique A₂B₅ architecture with two covalently linked enzymatic 'A' subunits, PltA and CdtB, associated with a homopentameric 'B' subunit made up of PltB, which has binding specificity for the N-acetylneuraminic acid (Neu5Ac) sialoglycans ^6,13 prominently present in humans ¹⁴ . Here, we examine the functional and structural relationship between typhoid toxin and ArtAB, an evolutionarily related AB₅ toxin encoded by the broad-host Salmonella Typhimurium ¹⁵ . We find that ArtA and ArtB, homologues of PltA and PltB, can form a functional complex with the typhoid toxin CdtB subunit after substitution of a single amino acid in ArtA, while ArtB can form a functional complex with wild-type PltA and CdtB. We also found that, after addition of a single-terminal Cys residue, a CdtB homologue from cytolethal distending toxin can form a functional complex with ArtA and ArtB. In line with the broad host specificity of S. Typhimurium, we found that ArtB binds human glycans, terminated in N-acetylneuraminic acid, as well as glycans terminated in N-glycolylneuraminic acid (Neu5Gc), which are expressed in most other mammals ¹⁴ . The atomic structure of ArtB bound to its receptor shows the presence of an additional glycan-binding site, which broadens its binding specificity. Despite equivalent toxicity in vitro, we found that the ArtB/PltA/CdtB chimaeric toxin exhibits reduced lethality in an animal model, indicating that the host specialization of typhoid toxin has optimized its targeting mechanisms to the human host. This is a remarkable example of a toxin evolving to broaden its enzymatic activities and adapt to a specific host.

中文翻译：

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伤寒沙门氏菌毒素中宿主适应性的演变。

毒力性状的进化对于微生物病原体的出现或重新出现以及它们对特定宿主^1-5的适应性至关重要。伤寒毒素是人类适应的细菌病原体鼠伤寒沙门氏菌^6,7的必不可少的毒力因子，是人类^8-12伤寒的病因。伤寒毒素具有独特的A ₂ B ₅结构，具有两个共价连接的酶促'A'亚基PltA和CdtB，与由PltB组成的同五聚体'B'亚基相关，后者对N-乙酰神经氨酸（Neu5Ac）具有结合特异性唾液聚糖^6,13在人类中^占主要地位¹⁴。在这里，我们检查伤寒毒素和ArtAB之间的功能和结构关系，ArtAB是由广泛宿主鼠伤寒沙门氏菌¹⁵编码的与进化相关的AB ₅毒素。我们发现ArtA和ArtB，PltA和PltB的同源物，可以在ArtA中取代单个氨基酸后与伤寒毒素CdtB亚基形成功能性复合物，而ArtB可以与野生型PltA和CdtB形成功能性复合物。我们还发现，添加单端Cys残基后，来自细胞致死性扩展毒素的CdtB同源物可以与ArtA和ArtB形成功能复合物。与鼠伤寒沙门氏菌的广泛宿主特异性一致，我们发现ArtB结合以N-乙酰神经氨酸终止的人聚糖，以及以N-羟乙酸神经氨酸（Neu5Gc）终止的聚糖，这些聚糖在大多数其他哺乳动物中表达^14。。与其受体结合的ArtB的原子结构表明存在额外的聚糖结合位点，从而加宽了其结合特异性。尽管在体外具有相同的毒性，但我们发现ArtB / PltA / CdtB嵌合毒素在动物模型中的杀伤力降低，这表明伤寒毒素的宿主特异性已优化了其对人宿主的靶向机制。这是毒素不断发展以扩大其酶促活性并适应特定宿主的一个杰出例子。