Inhibition of Fibrinolysis by Streptococcal Phage Lysin(SM1)

Expression of bacteriophage lysin(SM1) by Streptococcus oralis strain SF100 is thought to be important for the pathogenesis of infective endocarditis, due to its ability to mediate bacterial binding to fibrinogen. To better define the lysin(SM1) binding site on fibrinogen Aα, and to investigate the impact of binding on fibrinolysis, we examined the interaction of lysin(SM1) with a series of recombinant fibrinogen Aα variants. These studies revealed that lysin(SM1) binds the C-terminal region of fibrinogen Aα spanned by amino acid residues 534 to 610, with an affinity of equilibrium dissociation constant (K(D)) of 3.23 × 10(−5) M. This binding site overlaps the known binding site for plasminogen, an inactive precursor of plasmin, which is a key protease responsible for degrading fibrin polymers. When tested in vitro, lysin(SM1) competitively inhibited plasminogen binding to the αC region of fibrinogen Aα. It also inhibited plasminogen-mediated fibrinolysis, as measured by thromboelastography (TEG). These results indicate that lysin(SM1) is a bi-functional virulence factor for streptococci, serving as both an adhesin and a plasminogen inhibitor. Thus, lysin(SM1) may facilitate the attachment of bacteria to fibrinogen on the surface of damaged cardiac valves and may also inhibit plasminogen-mediated lysis of infected thrombi (vegetations) on valve surfaces.