The Fibrinogen Dependent Interaction between Group A Streptococcal M Protein and the Human Fibrinolytic System in Virulence

Subversion of the host fibrinolytic system has long been recognized as a powerful method employed by various bacteria to penetrate host tissue barriers. Highly virulent strains of Group A streptococcus (GAS) have been shown to acquire surface plasmin(ogen) (Pg) activity through multiple mechanisms; direct binding to Pg through Plasminogen binding Group A Streptococcal M-like protein (PAM) or indirect, which requires fibrinogen (Fg) to directly bind various M proteins to mediate Pg binding. Although M protein-Fg binding has long been studied for its role in phagocytosis prevention in GAS, its role in Pg activation has not been extensively investigated. GAS also produces an endogenous Pg activator, streptokinase (SK). Our laboratory, as well as others, has shown that subclasses of SK exist which have different Pg activating potentials that are dependent on the mechanism of Pg binding. The goal of this study is to investigate the molecular mechanisms involved in the Fg-dependent acquisition of plasmin (Pm) activity.

Various M proteins from strains thought to employ this mechanism, as well as their secreted SK’s, designated subclass SK2a, were cloned and expressed in E.coli. Surface plasmon resonance (SPR) was employed to the study binding affinity of M protein for Fg. The results obtained indicate that these M proteins have high affinities; not only for Fg, but for hFg fragment D. D-dimer however binds to these M proteins with variable affinity. HPg, on the other hand, does not bind to these M proteins directly. The SKs isolated from these strains also show low activation potential with hPg alone, while the presence of Fg and M proteins greatly enhances Pm generation. Whole cell assays also showed significant Pm surface activity only in the presence of Fg, while PAM-containing strains showed enhanced activity in the presence and absence of Fg. The data obtained in this study greatly enhanced our understanding of the molecular mechanisms by which GAS interacts with Fg and the fibrinolytic system.