The proline-rich repeat and thioester domains of streptococcal fibronectin-binding proteins

Abstract

Streptococcus pyogenes is an important human pathogen. One of the most prominent virulence factors produced by S. pyogenes is SfbI, a surface adhesin composed of three domains: thioester domain (TED), proline-rich repeat domain (PRR) and fibronectin-binding repeat domain (FnBD). The structures and functions of TED and PRR and their contributions to the pathogenesis of streptococcal diseases are unknown. The interaction between PRR and its putative target, the intracellular actin cytoskeleton regulator Arp2/3, was investigated by both in vitro and in vivo studies. PRR was shown to inhibit Arp2/3-dependent actin polymerisation. The expression of PRR in HeLa cells caused disruption to the cytoskeleton of the cells. All data point towards a role of PRR in inhibiting the Arp2/3 complex but more evidence is needed to support this. The N-terminal domain of SfbI (TED) and four homologous domains from S. pyogenes, group G streptococci and Streptococcus pneumoniae were characterised by mass spectrometry, NMR spectroscopy and biochemical assays. All were shown to possess intramolecular thioester bonds, spontaneously formed between sides chains of Cys and Gln residues. Fibrinogen (Fg) was identified as the first binding target of bacterial TEDs with direct evidence that the thioester bond was involved in the interaction with Fg. A pull-down experiment using human plasma showed Fg is a specific binding partner of SfbI-TED. The binding sites were narrowed down to the thioester-forming Gln of SfbI-TED and Lys residues in the Fg-Aα chain, and binding potentially occurred via covalent isopeptide linkage. The data presented here suggest two new roles for SfbI, previously unknown in bacterial pathogenesis. The PRR may be the first bacterial inhibitor of the actin cytoskeleton acting by inhibiting the Arp2/3 complex. Thioester domains appear to be a shared common feature of surface proteins of many Gram-positive pathogens. They may form covalent crosslinks between bacteria and host tissue.