The emergence and continued dominance of a Streptococcus pyogenes (group A Streptococcus, GAS) M1T1 clonal group is temporally correlated with acquisition of genomic sequences that confer high level expression of cotoxins
streptolysin O (SLO) and
NAD+-glycohydrolase (
NADase). Experimental
infection models have provided evidence that both toxins are important contributors to GAS virulence. SLO is a
cholesterol-dependent pore-forming toxin capable of lysing virtually all types of mammalian cells.
NADase, which is composed of an N-terminal translocation domain and C-terminal
glycohydrolase domain, acts as an intracellular toxin that depletes host cell energy stores.
NADase is dependent on SLO for internalization into epithelial cells, but its mechanism of interaction with the cell surface and details of its translocation mechanism remain unclear. In this study we found that
NADase can bind oropharyngeal epithelial cells independently of SLO. This interaction is mediated by both domains of the toxin. We determined by NMR the structure of the translocation domain to be a β-sandwich with a disordered N-terminal region. The folded region of the domain has structural homology to carbohydrate binding modules. We show that excess
NADase inhibits SLO-mediated
hemolysis and binding to epithelial cells in vitro, suggesting
NADase and SLO have shared surface receptors. This effect is abrogated by disruption of a putative
carbohydrate binding site on the
NADase translocation domain. Our data are consistent with a model whereby interactions of the
NADase glycohydrolase domain and translocation domain with SLO and the cell surface increase avidity of
NADase binding and facilitate toxin-toxin and toxin-cell surface interactions. IMPORTANCE
NADase and
streptolysin O (SLO) are secreted toxins important for pathogenesis of group A Streptococcus, the agent of strep throat and severe invasive
infections. The two toxins interact in
solution and mutually enhance cytotoxic activity. We now find that
NADase is capable of binding to the surface of human cells independently of SLO. Structural analysis of the previously uncharacterized translocation domain of
NADase suggests that it contains a carbohydrate binding module. The
NADase translocation domain and SLO appear to recognize similar
glycan structures on the cell surface, which may be one mechanism through which
NADase enhances SLO pore-forming activity during
infection. Our findings provide new insight into the
NADase toxin and its functional interactions with SLO during
streptococcal infection.