Goat vitronectin: Characterization and binding to Staphylococcus aureus

Nanonewton forces between Staphylococcus aureus surface protein IsdB and vitronectin

Single-molecule experiments have recently revealed that the interaction between staphylococcal surface proteins and their ligands can be extremely strong, equivalent to the strength of covalent bonds. Here, we report on the unusually high binding strength between Staphylococcus aureus iron-regulated surface determinant B (IsdB) and vitronectin (Vn), an essential human blood protein known to interact with bacterial pathogens. The IsdB-Vn interaction is dramatically strengthened by mechanical tension, with forces up to 2000 pN at a loading rate of 105 pN s-1. In line with this, flow experiments show that IsdB-mediated bacterial adhesion to Vn is enhanced by fluid shear stress. The stress-dependent binding of IsdB to Vn is likely to play a role in promoting bacterial adhesion to human cells under fluid shear stress conditions.

Complement C9 binding site and the anti-microbial activity of caprine vitronectin are localized in close proximity in the N-terminal region of the protein

Vitronectin (Vn) is a ligand for complement C9 and modulates its activity that favors bacterial growth and survival. At the same time, the anti-microbial activity of the heparin-binding region of human Vn has been documented. To understand these diverse and opposite functions of the protein, we have analyzed the interaction of caprine Vn with C9 in the homologous system. In a previous study, the C9 binding activity was mapped to the N-fragment of the caprine Vn (N-Vn), representing the first 200 amino acids. Interestingly, this fragment also inhibited bacterial growth. In this study, we have generated four sub-fragments of N-Vn and analyzed C9 binding by ELISA, blot overlay, surface plasmon resonance and circular dichroism spectroscopy. These sub-fragments were also tested for antimicrobial activity against E. coli and S. aureus by drop plate method and analyzing cell death by flow cytometry. Results of these analyses together with previous data suggest that in addition to the second RGD motif (106-108 amino acids), the first 47 residues are also required for C9 binding. The anti-microbial tests employed indicate that the growth inhibitory property is contributed by 101-150 residues of Vn. These results provide an initial insight into two diverse Vn functions.

Meningococcal surface fibril (Msf) binds to activated vitronectin and inhibits the terminal complement pathway to increase serum resistance

Complement evasion is an important survival strategy of Neisseria meningitidis (Nm) during colonization and infection. Previously, we have shown that Nm Opc binds to serum vitronectin to inhibit complement-mediated killing. In this study, we demonstrate meningococcal interactions with vitronectin via a novel adhesin, Msf (meningococcal surface fibril, previously NhhA or Hsf). As with Opc, Msf binds preferentially to activated vitronectin (aVn), engaging at its N-terminal region but the C-terminal heparin binding domain may also participate. However, unlike Opc, the latter binding is not heparin-mediated. By binding to aVn, Msf or Opc can impart serum resistance, which is further increased in coexpressers, a phenomenon dependent on serum aVn concentrations. The survival fitness of aVn-binding derivatives was evident from mixed population studies, in which msf/opc mutants were preferentially depleted. In addition, using vitronectin peptides to block Msf-aVn interactions, aVn-induced inhibition of lytic C5b-9 formation and of serum killing could be reversed. As Msf-encoding gene is ubiquitous in the meningococcal strains examined and is expressed in vivo, serum resistance via Msf may be of significance to meningococcal pathogenesis. The data imply that vitronectin binding may be an important strategy for the in vivo survival of Nm for which the bacterium has evolved redundant mechanisms.