Gram-positive bacteria colonize mucosal tissues, withstanding 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 the Cpa thioester bond under force. When folded at forces <6 pN, the Cpa thioester bond reacts reversibly with amine ligands, which are common in inflammation sites; however, mechanical unfolding and exposure to forces >6 pN block thioester reformation. We hypothesize that this folding-coupled reactivity switch (termed a 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.

革兰氏阳性菌定植于粘膜组织，可承受咳嗽等较大的机械扰动，产生的剪切力超过非共价键保持附着的能力。为了克服这些挑战，病原体化脓性链球菌利用蛋白质 Cpa，一种带有 Cys-Gln 硫酯键的菌毛末端粘附素。该键对宿主表面配体的反应性使共价锚定成为可能；然而，定植还需要细胞迁移和在表面扩散。这些看似不相容的要求背后的分子机制仍然未知。在这里，我们展示了一种磁性镊子力谱测定法，该测定法可解决 Cpa 硫酯键在力作用下的动力学。当以 <6 pN 的力折叠时，Cpa 硫酯键与胺配体发生可逆反应，这在炎症部位很常见；然而，机械展开和暴露于大于 6 pN 的力会阻止硫酯重组。