Protein folding modulates the chemical reactivity of a Gram-positive adhesin

Gram-positive bacteria colonize mucosal tissues against large mechanical perturbations, such as coughing, which generate shear forces that exceed the ability of non-covalent bonds to remain attached. To overcome these challenges, the pathogen Streptococcus pyogenes utilizes the protein Cpa, a pilus tip-end adhesin equipped with a Cys-Gln thioester bond. The reactivity of this bond towards host surface ligands enables covalent anchoring; however, colonization also requires cell migration and spreading over surfaces. The molecular mechanisms underlying these seemingly incompatible requirements remain unknown. Here, we demonstrate a magnetic tweezers force spectroscopy assay that resolves the dynamics of Cpa thioester bond under force. While folded at forces < 6 pN, Cpa thioester bond reacts reversibly with amine ligands, that are a common occurrence in inflammation sites; however, mechanical unfolding and exposure to forces > 6 pN block thioester reformation. We hypothesize that this folding-coupled reactivity switch—“smart covalent bond”—could allow the adhesin to undergo binding and unbinding to surface ligands under low force and remain covalently attached under mechanical stress.