Uncoiling Mechanics of Escherichia coli Type I Fimbriae Are Optimized for Catch Bonds

Abstract

We determined whether the molecular structures through which force is applied to receptor–ligand pairs are tuned to optimize cell adhesion under flow. The adhesive tethers of our model system, Escherichia coli, are type I fimbriae, which are anchored to the outer membrane of most E. coli strains. They consist of a fimbrial rod (0.3–1.5 μm in length) built from a helically coiled structural subunit, FimA, and an adhesive subunit, FimH, incorporated at the fimbrial tip. Previously reported data suggest that FimH binds to mannosylated ligands on the surfaces of host cells via catch bonds that are enhanced by the shear-originated tensile force. To understand whether the mechanical properties of the fimbrial rod regulate the stability of the FimH–mannose bond, we pulled the fimbriae via a mannosylated tip of an atomic force microscope. Individual fimbriae rapidly elongate for up to 10 μm at forces above 60 pN and rapidly contract again at forces below 25 pN. At intermediate forces, fimbriae change length more slowly, and discrete 5.0 ± 0.3–nm changes in length can be observed, consistent with uncoiling and coiling of the helical quaternary structure of one FimA subunit at a time. The force range at which fimbriae are relatively stable in length is the same as the optimal force range at which FimH–mannose bonds are longest lived. Higher or lower forces, which cause shorter bond lifetimes, cause rapid length changes in the fimbria that help maintain force at the optimal range for sustaining the FimH–mannose interaction. The modulation of force and the rate at which it is transmitted from the bacterial cell to the adhesive catch bond present a novel physiological role for the fimbrial rod in bacterial host cell adhesion. This suggests that the mechanical properties of the fimbrial shaft have codeveloped to optimize the stability of the terminal adhesive under flow.

Adaptive mutations in the signal peptide of the type 1 fimbrial adhesin of uropathogenic Escherichia coli

“…Because the functional properties of individual FimH adhesins do not appear to be affected by SP mutations, the increased adhesion could be attributed to the mutants' longer fimbriae. In a recent study, type 1 fimbriae were shown to uncoil under high tensile force and recoil back upon a drop in force (21). This recoiling is proposed to maintain optimal force on the FimH-mannose bond, slowing FimH's shift from the high-to low-affinity conformation.…”

“…Surface and tip functionalization with mannosylated protein (bovine ribonuclease B), bacterial adherence to the surface, and cantilever spring constant calibration were performed as described (21). All pulls were performed at 2 m/s over 20 m using the same cantilever.…”

“…All pulls were performed at 2 m/s over 20 m using the same cantilever. Uncoiled fimbrial length was measured as the distance from the bacterial surface to the location of FimH bond rupture, which is detected by a characteristic spike in force (21). The number of areas interrogated for each strain were: 47 WT, 39 T-6N, 12 T-6N/V-10I, 20 T-6N/V-10A, and 20 V-4E.…”

“…The nonadhesive fimbrial rod might also be functionally significant for the mechanics of binding. It was shown that the helical rod uncoils under increasing tensile force and recoils upon a drop in force, possibly maintaining optimal force on the FimH-mannose bond during changes in shear stress (21).…”

Fibronectin forms the most extensible biological fibers displaying switchable force-exposed cryptic binding sites

“…Such modules rapidly unravel into extended peptide chains once a critical set of hydrogen bonds is broken (5)(6)(7)(8). Some exceptions are bacterial single-molecule fimbriae that can extend up to 10-fold through the loss of quaternary structure, while the secondary structure remains intact (9,10).…”