Structural basis for collagen recognition by the Streptococcus pyogenes M3 protein and its involvement in biofilm

The M protein is an essential virulence factor of Streptococcus pyogenes, or group A streptococcus (GAS), one of the most common and dangerous human pathogens. Molecular and functional characterization of M protein variants and their interactions with host components is crucial for understanding streptococcal pathogenesis and vaccine development. The M3 protein is produced by the prevalent emm3 GAS serotype, which is frequently associated with severe invasive diseases. Here, we structurally and biochemically characterize the interaction of M3 with human collagens. High-resolution structures of the N-terminal M3 domain in the free state as well as bound to a collagen peptide derived from the Collagen Ligands Collection reveal a novel T-shaped protein fold that presents binding sites complementing the characteristic topology of collagen triple helices. The structure of the M3/collagen peptide complex explains how emm3 GAS and related streptococci, such as Streptococcus dysgalactiae subsp. equisimilis, can target collagens to enable colonization of various tissues. In line with this, we demonstrate that the M3/collagen interaction promotes enhanced biofilm formation of emm3 GAS in an emm type-specific manner, which can be inhibited with the recombinant N-terminal M3 domain. Further, emm3 GAS are shown to colocalize with collagen in tissue biopsies from patients with necrotizing soft tissue infections, where GAS biofilms are common. This observation is reproduced in infected organotypic skin models. Together, these data provide detailed molecular insights into an important streptococcal virulence mechanism with implications for the understanding of invasive infections, strategies for treating biofilm and M-protein-based vaccine design.