Abstract:
The Gram-positive bacterium Streptococcus pyogenes is a strictly human pathogen and infects primarily epithelia of the human throat and skin. To be able to adhere to and colonise these host epithelia, S. pyogenes employs an arsenal of cell wall-anchored adhesins. We have previously shown that the multidomain protein Epf from S. pyogenes is an adhesin important for binding to human epithelial cells. However, the mode of action of Epf is unknown and there is no sequence similarity between the domains of Epf and any protein of known structure or function. We identified the N-terminal domain of Epf as the mediator of adhesion. Sequence analysis shows that this N-terminal domain represents the tip of Epf followed by 16 C-terminal repeat domains that are likely to form a long stalk ending with the cell wall anchor. Here, we report the crystal structure of the N-terminal domain of Epf. We solved this structure to a resolution of 1.6 Å, using multi-wavelength anomalous dispersion methods on a selenomethionine derivative. The R value is 15.1 % (Rfree=18.1 %). The N-terminal domain of Epf forms two β-sandwich subdomains, one of which has a fibronectin type III-like fold. Surprisingly, the other subdomain, located at the very N-terminus of Epf, shows structural similarities to carbohydrate-binding modules (CBMs), which usually occur as domains of carbohydrate-modifying enzymes. The most closely related CBMs are those of laminarases and xylanases. As Epf appears not to possess an enzymatic domain, we hypothesise that it uses its carbohydrate-binding module to bind to glycans on the surface of human epithelia. Currently, we are testing this hypothesis and are investigating carbohydrates that may be targets of Epf.
