Coagulase and Efb of Staphylococcus aureus Have a Common Fibrinogen Binding Motif

Coagulase (Coa) and Efb, secreted Staphylococcus aureus proteins, are important virulence factors in staphylococcal infections. Coa interacts with fibrinogen (Fg) and induces the formation of fibrin(ogen) clots through activation of prothrombin. Efb attracts Fg to the bacterial surface and forms a shield to protect the bacteria from phagocytic clearance. This communication describes the use of an array of synthetic peptides to identify variants of a linear Fg binding motif present in Coa and Efb which are responsible for the Fg binding activities of these proteins. This motif represents the first Fg binding motif identified for any microbial protein. We initially located the Fg binding sites to Coa's C-terminal disordered segment containing tandem repeats by using recombinant fragments of Coa in enzyme-linked immunosorbent assay-type binding experiments. Sequence analyses revealed that this Coa region contained shorter segments with sequences similar to the Fg binding segments in Efb. An alanine scanning approach allowed us to identify the residues in Coa and Efb that are critical for Fg binding and to define the Fg binding motifs in the two proteins. In these motifs, the residues required for Fg binding are largely conserved, and they therefore constitute variants of a common Fg binding motif which binds to Fg with high affinity. Defining a specific motif also allowed us to identify a functional Fg binding register for the Coa repeats that is different from the repeat unit previously proposed.

Staphylococcus aureus infections are a major health problem that affects an estimated 50 million people globally and causes the death of about 20,000 Americans each year. A number of experimental vaccines have been developed during the past years. However, these vaccines have all failed in clinical trials. The ability of S. aureus to form an Fg shield surrounding and protecting bacterial cells from clearance may explain why the vaccines are failing. Furthermore, S. aureus coagulase can induce the formation of a fibrin(ogen) shield in experimental abscess models which surrounds and protects bacteria in the microcolony from clearance. In this study, we identified for the first time a microbial Fg binding motif. Variants of this motif are present in coagulase and Efb. Our results provide a molecular basis for the rational design of inhibitors that could potentially prevent the formation of the obstructing Fg shield.