There are at least two fibrinogen-binding sites involved in the interaction between Efb and fibrinogen (9,10,13). Efb has two similar sequences of 22 amino acids at the N-terminal region that are responsible for fibrinogen-binding. Peptides containing the first (residues 30-67) or second repeat (residues 68-97) bind strongly to soluble fibrinogen and inhibit the binding of Efb to fibrinogen (9,10,13). Also, several residues in the second repeat of Efb (K68, H74, Y76, I78, F81, D83, T85, F86, Y88, and R91) seem to be crucial for fibrinogen-binding (13). A single Efb molecule can bind two different fibrinogen molecules simultaneously, and the combination of equimolar amounts of these two proteins led to the formation of a complex (9). The amino acid sequence revealed an EF-domain with Ca2+-binding motifs(10) which explains why calcium or zinc enhances the precipitation of the Efb-fibrinogen complex.
Coagulase is a protein similar to Efb regarding fibrinogen-binding. It possesses both prothrombin and coagulase activity at the N-terminal portion (residues 27 to 310) (14). However, the C-terminal part of coagulase (residues 484 to 636) contains several tandem repeats of around 27-residues which bind fibrinogen. These repeats contain fibrinogen-binding motifs found in the repeat regions of Efb (13,15,16) (Fig. 1), which recognize the same site on fibrinogen. Also, Efb repeat regions can compete with the corresponding repeat regions of coagulase for binding to fibrinogen (16). A peptide corresponding to residues 68 to 98 of Efb blocks coagulase binding to fibrinogen indicating that they bind to the same or overlapping site on fibrinogen (13). Furthermore, antibodies against Efb recognize coagulase and inhibit coagulase function in plasma (17). This indicates that Efb and coagulase have a common motif and similar functions related to fibrinogen binding. Also, five residues in the repeat regions of coagulase are crucial for coagulase binding to fibrinogen (N508, Y510, V512, Y522, and R525)(13).
Staphylococcus aureus produces several proteins in addition to Efb and coagulase that specifically recognize fibrinogen (18) including clumping factors A and B (19,20), the extracellular adherence protein (Eap) (21), Map (22), and von Willebrand factor-binding proteins (23). None of these proteins have the fibrinogen-binding motifs of Efb or coagulase (24). Interestingly, a part of coagulase fibrinogen-binding sites was identified in the nucleocapsid phosphoprotein (N-protein) of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (GenBank: QOQ16111.1). The region corresponding to the amino acid residues 261-271 of the N-protein has some similarity to the repeats of coagulase (GenBank: AKJ16095.1). The fibrinogen-binding capacity of the N-protein must be tested for potential biological function. It is tempting to think that the N-protein mimics the function of Efb and coagulase for fibrinogen-binding to facilitate the evasion of host defense by SARS-CoV-2 (25).