The binding of human plasminogen (hPg) to the surface of the human pathogen group A Streptococcus pyogenes (GAS) and subsequent hPg activation to the protease plasmin generate a proteolytic surface that GAS employs to circumvent host innate immunity. Direct high-affinity binding of hPg/plasmin to pattern D GAS is fully recapitulated by the hPg kringle 2 domain (K2_hPg) and a short internal peptide region (a1a2) of a specific subtype of bacterial surface M protein, present in all GAS pattern D strains. To better understand the nature of this binding, critical to the virulence of many GAS skin-tropic strains, we used high-resolution NMR to define the interaction of recombinant K2_hPg with recombinant a1a2 (VKK38) of the M protein from GAS isolate NS455. We found a 2:1 (m/m) binding stoichiometry of K2_hPg/VKK38, with the lysine-binding sites of two K2_hPg domains anchored to two regions of monomeric VKK38. The K2_hPg/VKK38 binding altered the VKK38 secondary structure from a helical apo-peptide with a flexible center to an end-to-end K2_hPg-bound α-helix. The K2_hPg residues occupied opposite faces of this helix, an arrangement that minimized steric clashing of K2_hPg We conclude that VKK38 provides two conformational lysine isosteres that each interact with the lysine-binding sites in K2_hPg Further, the adoption of an α-helix by VKK38 upon binding to K2_hPg sterically optimizes the side chains of VKK38 for maximal binding to K2_hPg and minimizes steric overlap between the K2_hPg domains. The mechanism for hPg/M protein binding uncovered here may facilitate targeting of GAS virulence factors for disease management.

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