Structural and functional studies of intrinsically disordered fibronectin-binding proteins

Bacterial fibronectin-binding proteins (FnBPs) mediate adhesion of bacteria to host tissues through binding to the human protein fibronectin (Fn). FnBPs are predicted to contain a series of intrinsically disordered Fn-binding repeats (FnBRs), which undergo a disorder-to-order transition on binding up to five F1-modules (1−5F1) from the N terminal domain of Fn through a tandem β-zipper interaction. This work investigated these predictions for SfbI-5, an FnBR from the important human pathogen Streptococcus pyogenes.

SfbI-5 was shown to behave as an intrinsically disordered protein (IDP), with no stable secondary or tertiary structure. The C terminal third of SfbI 5 was shown to have propensity for β-strand conformations and, thus, might contain a preformed contact site involved in the initial interaction between SfbI-5 and 1−5F1.

A high resolution structure of an SfbI-5 peptide in complex with 2F13F1 demonstrated that peptide binding occurs through formation of a tandem β-zipper with the concomitant formation of a large, extended intermolecular interface. Conserved FnBR residues play roles similar to those played by residues in related complexes involving Staphylococcus aureus FnBRs. The effect of mutating these residues on SfbI-5 binding to 1−5F1 was shown to be modest, suggesting that large, extended interfaces, when formed by IDPs, might be tolerant to mutations in the IDP.

A putative FnBR, TickFnBR, was identified in a salivary protein from the blood-feeding tick Ixodes scapularis, which is a vector for human diseases. TickFnBR was shown to bind to 1−5F1 and also to Fn in human plasma. A high resolution structure demonstrated that a TickFnBR peptide binds to 2F13F1 through a tandem β-zipper. TickFnBR is the first eukaryotic FnBR to be identified, suggesting the tandem β zipper interaction is a useful mechanism for harmful organisms to target and exploit human Fn.