The adhesions between Gram-positive bacteria and their hosts are exposed to varying magnitudes of tensile forces. Here, using an ultrastable magnetic tweezer-based single-molecule approach, we show the catch-bond kinetics of the prototypical adhesion complex of SD-repeat protein G (SdrG) to a peptide from fibrinogen β (Fgβ) over a physiologically important force range from piconewton (pN) to tens of pN, which was not technologically accessible to previous studies. At 37 °C, the lifetime of the complex exponentially increases from seconds at several pN to ∼1000 s as the force reaches 30 pN, leading to mechanical stabilization of the adhesion. The dissociation transition pathway is determined as the unbinding of a critical β-strand peptide ("latch" strand of SdrG that secures the entire adhesion complex) away from its binding cleft, leading to the dissociation of the Fgβ ligand. Similar mechanical stabilization behavior is also observed in several homologous adhesions, suggesting the generality of catchbond kinetics in such bacterial adhesions. We reason that such mechanical stabilization confers multiple advantages in the pathogenesis and adaptation of bacteria.

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The 572 amino acids unique sequence on titin N2B element (N2B-us) is known to regulate the passive elasticity of muscle as an elastic spring. It also serves as a hub for cardiac hypertrophic signaling by interacting with multiple proteins such as FHL1(Sheikh et al, 2008), FHL2(Lange et al, 2002), and Erk2(Perkin et al, 2015). N2B-us is thought to be an intrinsically disordered region. In addition, N2B-us bears force; therefore, the functions of N2B-us are likely regulated by mechanical stretching. In the work, we investigated the conformation of N2B-us as well as its force-dependent interaction with FHL2 using a combination of AlphaFold2 predictions and single-molecule experimental validation. Surprisingly, a stable alpha/beta structural domain (~115 a.a.) was predicted and confirmed in N2B-us, which can be mechanically unfolded at forces greater than 5 pN. More than twenty FHL2 LIM domain binding sites were predicted to spread throughout N2B-us including the regions cryptic in the structural domain. Mechanosensitive binding of FHL2 to N2B-us is revealed in single-molecule manipulation experiments. Together, the results unveil several previously unknown aspects of the N2B-us conformations and its force-dependent interactions with FHL2, which provides new insights into the physiological functions of the force-bearing N2B-us region.