伤寒毒素是一种A2B5蛋白毒素,是人类适应的细菌病原体伤寒沙门氏菌的重要毒力因子,伤寒的病原体。伤寒毒素含有两个酶亚基,PltAandCdtB,其停靠在由蛋白质PltB组成的五聚体递送平台上。最近有报道称,相同的酶亚基可以与由PltC蛋白组成的不同递送平台组装,形成伤寒毒素的独特版本。然而,PltC和PltB伤寒毒素之间的结构和受体特异性差异仍然未知。这里,我们确定了五聚体PltC亚基的原子级结构,完全组装的PltC伤寒毒素,和与聚糖受体复合的PltC五聚体。生化和结构分析表明,PltB和PltC由于亚基界面处的静电排斥而无法形成异聚递送复合物,因此仅形成单独的毒素。我们进一步观察到,尽管PltB和PltC之间的序列相似性很低,他们以类似的方式与PltA相互作用,但PltC与PltA表现出更强的静电相互作用,使其在毒素组装中胜过PltB.PltC的配体结合的原子结构显示在PltB中未发现的另外的聚糖结合位点,并且聚糖阵列分析表明PltB和PltC在聚糖结合特异性方面表现出显著差异。总的来说,这项研究提供了关于伤寒沙门氏菌如何产生两种不同形式的伤寒毒素的原子级见解,从而在这个关键毒力因子中产生功能多样性。重要性伤寒是一种毁灭性的疾病,每年杀死超过115,000人,是由伤寒沙门氏菌引起的。伤寒毒素,由S.Typhi独家生产,被证明是伤寒的发病机理。伤寒毒素由五聚递送B亚基组成,以通过结合聚糖受体将催化A亚基转运到宿主细胞中。最近的研究表明,伤寒沙门氏菌编码两个同源递送B亚基,它们能够与相同的活性亚基结合以产生具有不同功能特征的替代毒素。这里,我们发现这两个递送亚单位只能形成同五聚体递送平台,它们与伤寒毒素的活性亚单位竞争结合,并且这两种毒素具有不同的聚糖结合特性,赋予不同的功能性状.这些发现突出了伤寒毒素的独特组装和功能多样化。
Typhoid toxin is an A2B5 protein toxin and an important virulence factor for the human-adapted bacterial pathogen Salmonella enterica serovar Typhi, the causative agent of typhoid fever. Typhoid toxin contains two enzymatic subunits, PltA and CdtB, which dock onto a pentameric delivery platform composed of the protein PltB. It was recently reported that the same enzymatic subunits can assemble with a different delivery platform composed of the protein PltC, forming a distinct version of typhoid toxin. However, the differences in structure and receptor specificity between the PltC and PltB typhoid toxins remain unknown. Here, we determined atomic-level structures of the pentameric PltC subunit, the fully assembled PltC typhoid toxin, and the PltC pentamers in complex with glycan receptors. Biochemical and structural analyses indicate that PltB and PltC are unable to form heteromeric delivery complexes due to electrostatic repulsion at the subunit interface and thus form separate toxins only. We further observed that, despite low sequence similarity between PltB and PltC, they interact with PltA in a similar manner but that PltC exhibits stronger electrostatic interactions with PltA, enabling it to outcompete PltB in toxin assembly. The ligand-bound atomic structures of PltC show an additional glycan binding site not found in PltB and glycan array analysis indicates that PltB and PltC exhibit significant differences in glycan binding specificity. Collectively, this study offers atomic-level insights into how S. Typhi produces two distinct versions of typhoid toxin, thereby generating functional diversity in this key virulence factor. IMPORTANCE Typhoid fever is a devastating disease that kills more than 115,000 people every year and is caused by Salmonella Typhi. Typhoid toxin, exclusively produced by S. Typhi, was demonstrated to be responsible for the pathogenesis of typhoid fever. Typhoid toxin consists of a pentameric delivery B subunit to transport the catalytic A subunits into the host cell through binding of the glycan receptors. Recent study shows that S. Typhi encodes two homologous delivery B subunits that are able to associate with the same active subunits to produce alternative toxins with distinct functional characteristics. Here, we show that the two delivery subunits can form only homopentameric delivery platforms that compete to associate with typhoid toxin\'s active subunits and that the two resulting toxins have distinct glycan-binding properties that confer distinct functional traits. These findings highlight the unique assembly and functional diversification of typhoid toxins.