Mapping the Mutational Landscape for Streptokinase Binding to Plasminogen.

BACKGROUND: Group A Streptococcus (GAS) expresses streptokinase (SK), a critical virulence factor that non-enzymatically activates the host's plasminogen (PLG), to an active form (PLGSK) via a conformational change. PLGSK subsequently activates PLG to plasmin, resulting in fibrinolysis and facilitating bacterial dissemination. As a potent thrombolytic, SK has been used as a therapeutic agent. OBJECTIVE: GAS SK is highly specific for human PLG, and sequence variation between SK from different GAS strains has been linked to differences in PLG binding and disease severity. The contributions of individual amino acids to these differences are poorly understood. METHODS: We employed phage display and deep mutational scanning (DMS) to map the effects of amino acid substitutions within SK from Group C Streptococcus (GCS), which shares high sequence homology with GAS, on its ability to bind human PLG. RESULTS: We demonstrate that SK expressed as a fusion protein to the p3 coat protein of M13 filamentous phage retains its capacity to bind human PLG. DMS mapped 71% of all possible single amino acid substitutions within SK, identifying specific regions in which amino acid substitutions are likely to increase or decrease SK's affinity for PLG. CONCLUSION: Our findings suggest a complex protein-protein interaction in which long-range protein dynamics influence the conformational activation of PLG to PLGSK. These data lay the foundation for linking SK variation between GCS strains to differences in virulence, mapping the determinants of GCS SK's human specificity, and potentially contributing to the development of improved therapeutics for heart attack and stroke.