Group A streptococcal isolate 64/14 expresses surface plasmin-binding structures in addition to Plr

A recombinant plasmin receptor (PIr) gene product originally cloned from group A streptococcal isolate 64/14 was analysed for its ability to bind plasmin(ogen) and to account for all the surface plasmin-binding properties of streptococcal isolate 64/14. Functional analysis of recombinant PIr demonstrated that the protein exhibited equal reactivity with human Lys-plasmin and Lys-plasminogen, but significantly lower reactivity with Glu-plasminogen. Plasmin-binding was both inhibitable and elutable by lysine or lysine analogs, and active plasmin bound to recombinant PIr was not neutralized by alpha(2)-antiplasmin. Thus, the plasmin-binding properties of recombinant PIr correlated with the plasmin-binding phenotype of the intact streptococcal isolate 64/14. In addition, fluid-phase recombinant PIr could completely inhibit binding of plasmin to either immobilized recombinant PIr or group A streptococcal isolate 64/14 with equal efficiency, indicating that surface-expressed PIr could account for all the plasmin-binding properties of the intact organism. An IgM monoclonal antibody to recombinant PIr that specifically recognized a surface structure on streptococcal isolate 64/14 significantly inhibited the binding of plasmin to the recombinant protein; however, the antibody was not successful at inhibiting plasmin-binding to the intact bacteria, indicating the presence of other plasmin-binding structures on the bacterial surface in addition to PIr.