Streptococcus co-opts a conformational lock in human plasminogen to facilitate streptokinase cleavage and bacterial virulence

Virulent strains of Streptococcus pyogenes (GAS) recruit host single-chain human plasminogen (hPg) to the cell surface - where in the case of Pattern D strains of GAS - hPg binds directly to the cells through a surface receptor, plasminogen-binding group A streptococcal M-protein (PAM).  The coinherited Pattern D GAS-secreted streptokinase (SK2b) then accelerates cleavage of hPg at the R561-V562 peptide bond, resulting in the disulfide-linked two-chain protease, plasmin (hPm).  hPm localizes on the bacterial surface, assisting bacterial dissemination via proteolysis of host defense proteins.  Studies using isolated domains from PAM and hPg revealed that the A-domain of PAM binds to the hPg kringle-2 module (K2hPg), but how this relates to the function of the full-length proteins is unclear.  Herein, we use intact proteins to show that the lysine binding site (LBS) of K2hPg is a major determinant of the activation-resistant T-conformation of hPg.  The binding of PAM to the LBS of K2hPg relaxes the conformation of hPg, leading to a greatly enhanced activation rate of hPg by SK2b.  Domain swapping between hPg and mPg emphasizes the importance of the Pg latent heavy chain (residues 1-561) in PAM binding and shows that while SK2b binds to both hPg and mPg, the activation properties of SK are strictly attributed to the serine protease domain (residues 562-791) of hPg.  Overall, these data show that native hPg is locked in an activation-resistant conformation that is relaxed upon its direct binding to PAM, allowing hPm to form and provide GAS cells with a proteolytic surface.