Catch-bond model derived from allostery explains force-activated bacterial adhesion

High shear enhances the adhesion of Escherichia coli bacteria binding to mannose coated surfaces via the adhesin FimH, raising the question as to whether FimH forms catch bonds that are stronger under tensile mechanical force. Here, we study the length of time that E. coli pause on mannosylated surfaces and report a double exponential decay in the duration of the pauses. This double exponential decay is unlike previous single molecule or whole cell data for other catch bonds, and indicates the existence of two distinct conformational states. We present a mathematical model, derived from the common notion of chemical allostery, which describes the lifetime of a catch bond in which mechanical force regulates the transitions between two conformational states that have different unbinding rates. The model explains these characteristics of the data: a double exponential decay, an increase in both the likelihood and lifetime of the high-binding state with shear stress, and a biphasic effect of force on detachment rates. The model parameters estimated from the data are consistent with the force-induced structural changes shown earlier in FimH. This strongly suggests that FimH forms allosteric catch bonds. The model advances our understanding of both catch bonds and the role of allostery in regulating protein activity.     

The bacterial fimbrial tip acts as a mechanical force sensor

There is increasing evidence that the catch bond mechanism, where binding becomes stronger under tensile force, is a common property among non-covalent interactions between biological molecules that are exposed to mechanical force in vivo. Here, by using the multi-protein tip complex of the mannose-binding type 1 fimbriae of Escherichia coli, we show how the entire quaternary structure of the adhesive organella is adapted to facilitate binding under mechanically dynamic conditions induced by flow. The fimbrial tip mediates shear-dependent adhesion of bacteria to uroepithelial cells and demonstrates force-enhanced interaction with mannose in single molecule force spectroscopy experiments. The mannose-binding, lectin domain of the apex-positioned adhesive protein FimH is docked to the anchoring pilin domain in a distinct hooked manner. The hooked conformation is highly stable in molecular dynamics simulations under no force conditions but permits an easy separation of the domains upon application of an external tensile force, allowing the lectin domain to switch from a low- to a high-affinity state. The conformation between the FimH pilin domain and the following FimG subunit of the tip is open and stable even when tensile force is applied, providing an extended lever arm for the hook unhinging under shear. Finally, the conformation between FimG and FimF subunits is highly flexible even in the absence of tensile force, conferring to the FimH adhesin an exploratory function and high binding rates. The fimbrial tip of type 1 Escherichia coli is optimized to have a dual functionality: flexible exploration and force sensing. Comparison to other structures suggests that this property is common in unrelated bacterial and eukaryotic adhesive complexes that must function in dynamic conditions.     

Shear-dependent 'stick-and-roll' adhesion of type 1 fimbriated Escherichia coli

It is generally assumed that bacteria are washed off surfaces as fluid flow increases because they adhere through 'slip-bonds' that weaken under mechanical force. However, we show here that the opposite is true for Escherichia coli attachment to monomannose-coated surfaces via the type 1 fimbrial adhesive subunit, FimH. Raising the shear stress (within the physiologically relevant range) increased accumulation of type 1 fimbriated bacteria on monomannose surfaces by up to two orders of magnitude, and reducing the shear stress caused them to detach. In contrast, bacterial binding to anti-FimH antibody-coated surfaces showed essentially the opposite behaviour, detaching when the shear stress was increased. These results can be explained if FimH is force-activated; that is, that FimH mediates 'catch-bonds' with mannose that are strengthened by tensile mechanical force. As a result, on monomannose-coated surfaces, bacteria displayed a complex 'stick-and-roll' adhesion in which they tended to roll over the surface at low shear but increasingly halted to stick firmly as the shear was increased. Mutations in FimH that were predicted earlier to increase or decrease force-induced conformational changes in FimH were furthermore shown here to increase or decrease the probability that bacteria exhibited the stationary versus the rolling mode of adhesion. This 'stick-and-roll' adhesion could allow type 1 fimbriated bacteria to move along mannosylated surfaces under relatively low flow conditions and to accumulate preferentially in high shear regions.     

The cysteine bond in the Escherichia coli FimH adhesin is critical for adhesion under flow conditions

Cysteine bonds are found near the ligand-binding sites of a wide range of microbial adhesive proteins, including the FimH adhesin of Escherichia coli. We show here that removal of the cysteine bond in the mannose-binding domain of FimH did not affect FimH-mannose binding under static or low shear conditions (< or = 0.2 dyne cm(-2)). However, the adhesion level was substantially decreased under increased fluid flow. Under intermediate shear (2 dynes cm(-2)), the ON-rate of bacterial attachment was significantly decreased for disulphide-free mutants. Molecular dynamics simulations demonstrated that the lower ON-rate of cysteine bond-free FimH could be due to destabilization of the mannose-free binding pocket of FimH. In contrast, mutant and wild-type FimH had similar conformation when bound to mannose, explaining their similar binding strength to mannose under intermediate shear. The stabilizing effect of mannose on disulphide-free FimH was also confirmed by protection of the FimH from thermal and chemical inactivation in the presence of mannose. However, this stabilizing effect could not protect the integrity of FimH structure under high shear (> 20 dynes cm(-2)), where lack of the disulphide significantly increased adhesion OFF-rates. Thus, the cysteine bonds in bacterial adhesins could be adapted to enable bacteria to bind target surfaces under increased shear conditions.     

Integrin-like allosteric properties of the catch bond-forming FimH adhesin of Escherichia coli

FimH is the adhesive subunit of type 1 fimbriae of the Escherichia coli that is composed of a mannose-binding lectin domain and a fimbria-incorporating pilin domain. FimH is able to interact with mannosylated surface via a shear-enhanced catch bond mechanism. We show that the FimH lectin domain possesses a ligand-induced binding site (LIBS), a type of allosterically regulated epitopes characterized in integrins. Analogous to integrins, in FimH the LIBS epitope becomes exposed in the presence of the ligand (or "activating" mutations) and is located far from the ligand-binding site, close to the interdomain interface. Also, the antibody binding to the LIBS shifts adhesin from the low to high affinity state. Binding of streptavidin to the biotinylated residue within the LIBS also locks FimH in the high affinity state, suggesting that the allosteric perturbations in FimH are sustained by the interdomain wedging. In the presence of antibodies, the strength of bacterial adhesion to mannose is increased similar to the increase observed under shear force, suggesting the same allosteric mechanism, a shift in the interdomain configuration. Thus, an integrin-like allosteric link between the binding pocket and the interdomain conformation can serve as the basis for the catch bond property of FimH and, possibly, other adhesive proteins.     

Recombinant FimH Adhesin Demonstrates How the Allosteric Catch Bond Mechanism Can Support Fast and Strong Bacterial Attachment in the Absence of Shear

The FimH protein of Escherichia coli is a model two-domain adhesin that is able to mediate an allosteric catch bond mechanism of bacterial cell attachment, where the mannose-binding lectin domain switches from an ‘inactive’ conformation with fast binding to mannose to an ‘active’ conformation with slow detachment from mannose. Because mechanical tensile force favors separation of the domains and, thus, FimH activation, it has been thought that the catch bonds can only be manifested in a fluidic shear-dependent mode of adhesion. Here, we used recombinant FimH variants with a weakened inter-domain interaction and show that a fast and sustained allosteric activation of FimH can also occur under static, non-shear conditions. Moreover, it appears that lectin domain conformational activation happens intrinsically at a constant rate, independently from its ability to interact with the pilin domain or mannose. However, the latter two factors control the rate of FimH deactivation. Thus, the allosteric catch bond mechanism can be a much broader phenomenon involved in both fast and strong cell-pathogen attachments under a broad range of hydrodynamic conditions. This concept that allostery can enable more effective receptor-ligand interactions is fundamentally different from the conventional wisdom that allostery provides a mechanism to turn binding off under specific conditions.     

FimH forms catch bonds that are enhanced by mechanical force due to allosteric regulation

The bacterial adhesive protein, FimH, is the most common adhesin of Escherichia coli and mediates weak adhesion at low flow but strong adhesion at high flow. There is evidence that this occurs because FimH forms catch bonds, defined as bonds that are strengthened by tensile mechanical force. Here, we applied force to single isolated FimH bonds with an atomic force microscope in order to test this directly. If force was loaded slowly, most of the bonds broke up at low force (<60 piconewtons of rupture force). However, when force was loaded rapidly, all bonds survived until much higher force (140-180 piconewtons of rupture force), behavior that indicates a catch bond. Structural mutations or pretreatment with a monoclonal antibody, both of which allosterically stabilize a high affinity conformation of FimH, cause all bonds to survive until high forces regardless of the rate at which force is applied. Pretreatment of FimH bonds with intermediate force has the same strengthening effect on the bonds. This demonstrates that FimH forms catch bonds and that tensile force induces an allosteric switch to the high affinity, strong binding conformation of the adhesin. The catch bond behavior of FimH, the amount of force needed to regulate FimH, and the allosteric mechanism all provide insight into how bacteria bind and form biofilms in fluid flow. Additionally, these observations may provide a means for designing antiadhesive mechanisms.     

Allosteric catch bond properties of the FimH adhesin from Salmonella enterica serovar Typhimurium

Despite sharing the name and the ability to mediate mannose-sensitive adhesion, the type 1 fimbrial FimH adhesins of Salmonella Typhimurium and Escherichia coli share only 15% sequence identity. In the present study, we demonstrate that even with this limited identity in primary sequence, these two proteins share remarkable similarity of complex receptor binding and structural properties. In silico simulations suggest that, like E. coli FimH, Salmonella FimH has a two-domain tertiary structure topology, with a mannose-binding pocket located on the apex of a lectin domain. Structural analysis of mutations that enhance S. Typhimurium FimH binding to eukaryotic cells and mannose-BSA demonstrated that they are not located proximal to the predicted mannose-binding pocket but rather occur in the vicinity of the predicted interface between the lectin and pilin domains of the adhesin. This implies that the functional effect of such mutations is indirect and probably allosteric in nature. By analogy with E. coli FimH, we suggest that Salmonella FimH functions as an allosteric catch bond adhesin, where shear-induced separation of the lectin and pilin domains results in a shift from a low affinity to a high affinity binding conformation of the lectin domain. Indeed, we observed shear-enhanced binding of whole bacteria expressing S. Typhimurium type 1 fimbriae. In addition, we observed that anti-FimH antibodies activate rather than inhibit S. Typhimurium FimH mannose binding, consistent with the allosteric catch bond properties of this adhesin.     

Interdomain interaction in the FimH adhesin of Escherichia coli regulates the affinity to mannose

FimH is a mannose-specific adhesin located on the tip of type 1 fimbriae of Escherichia coli that is capable of mediating shear-enhanced bacterial adhesion. FimH consists of a fimbria-associated pilin domain and a mannose-binding lectin domain, with the binding pocket positioned opposite the interdomain interface. By using the yeast two-hybrid system, purified lectin and pilin domains, and docking simulations, we show here that the FimH domains interact with one another. The affinity for mannose is greatly enhanced (up to 300-fold) in FimH variants in which the interdomain interaction is disrupted by structural mutations in either the pilin or lectin domains. Also, affinity to mannose is dramatically enhanced in isolated lectin domains or in FimH complexed with the chaperone molecule that is wedged between the domains. Furthermore, FimH with native structure mediates weak binding at low shear stress but shifts to strong binding at high shear, whereas FimH with disrupted interdomain contacts (or the isolated lectin domain) mediates strong binding to mannose-coated surfaces even under low shear. We propose that interactions between lectin and pilin domains decrease the affinity of the mannose-binding pocket via an allosteric mechanism. We further suggest that mechanical force at high shear stress separates the two domains, allowing the lectin domain to switch from a low affinity to a high affinity state. This shift provides a mechanism for FimH-mediated shear-enhanced adhesion by enabling the adhesin to form catch bond-like interactions that are longer lived at high tensile force.     

RMSD analysis of structures of the bacterial protein FimH identifies five conformations of its lectin domain

FimH is a bacterial adhesin protein located at the tip of Escherichia coli fimbria that functions to adhere bacteria to host cells. Thus, FimH is a critical factor in bacterial infections such as urinary tract infections and is of interest in drug development. It is also involved in vaccine development and as a model for understanding shear-enhanced catch bond cell adhesion. To date, over 60 structures have been deposited in the Protein Data Bank showing interactions between FimH and mannose ligands, potential inhibitors, and other fimbrial proteins. In addition to providing insights about ligand recognition and fimbrial assembly, these structures provide insights into conformational changes in the two domains of FimH that are critical for its function. To gain further insights into these structural changes, we have superposed FimH's mannose binding lectin domain in all these structures and categorized the structures into five groups of lectin domain conformers using RMSD as a metric. Many structures also include the pilin domain, which anchors FimH to the fimbriae and regulates the conformation and function of the lectin domain. For these structures, we have also compared the relative orientations of the two domains. These structural analyses enhance our understanding of the conformational changes associated with FimH ligand binding and domain-domain interactions, including its catch bond behavior through allosteric action of force in bacterial adhesion.     

Bacterial adhesion to target cells enhanced by shear force

Surface adhesion of bacteria generally occurs in the presence of shear stress, and the lifetime of receptor bonds is expected to be shortened in the presence of external force. However, by using Escherichia coli expressing the lectin-like adhesin FimH and guinea pig erythrocytes in flow chamber experiments, we show that bacterial attachment to target cells switches from loose to firm upon a 10-fold increase in shear stress applied. Steered molecular dynamics simulations of tertiary structure of the FimH receptor binding domain and subsequent site-directed mutagenesis studies indicate that shear-enhancement of the FimH-receptor interactions involves extension of the interdomain linker chain under mechanical force. The ability of FimH to function as a force sensor provides a molecular mechanism for discrimination between surface-exposed and soluble receptor molecules.     

Elevated shear stress protects Escherichia coli cells adhering to surfaces via catch bonds from detachment by soluble inhibitors

Soluble inhibitors find widespread applications as therapeutic drugs to reduce the ability of eukaryotic cells, bacteria, or viruses to adhere to surfaces and host tissues. Mechanical forces resulting from fluid flow are often present under in vivo conditions, and it is commonly presumed that fluid flow will further add to the inhibitive effect seen under static conditions. In striking contrast, we discover that when surface adhesion is mediated by catch bonds, whose bond life increases with increased applied force, shear stress may dramatically increase the ability of bacteria to withstand detachment by soluble competitive inhibitors. This shear stress-induced protection against inhibitor-mediated detachment is shown here for the fimbrial FimH-mannose-mediated surface adhesion of Escherichia coli. Shear stress-enhanced reduction of bacterial detachment has major physiological and therapeutic implications and needs to be considered when developing and screening drugs.     

Weak rolling adhesion enhances bacterial surface colonization

Bacterial adhesion to and subsequent colonization of surfaces are the first steps toward forming biofilms, which are a major concern for implanted medical devices and in many diseases. It has generally been assumed that strong irreversible adhesion is a necessary step for biofilm formation. However, some bacteria, such as Escherichia coli when binding to mannosylated surfaces via the adhesive protein FimH, adhere weakly in a mode that allows them to roll across the surface. Since single-point mutations or even increased shear stress can switch this FimH-mediated adhesion to a strong stationary mode, the FimH system offers a unique opportunity to investigate the role of the strength of adhesion independently from the many other factors that may affect surface colonization. Here we compare levels of surface colonization by E. coli strains that differ in the strength of adhesion as a result of flow conditions or point mutations in FimH. We show that the weak rolling mode of surface adhesion can allow a more rapid spreading during growth on a surface in the presence of fluid flow. Indeed, an attempt to inhibit the adhesion of strongly adherent bacteria by blocking mannose receptors with a soluble inhibitor actually increased the rate of surface colonization by allowing the bacteria to roll. This work suggests that (i) a physiological advantage to the weak adhesion demonstrated by commensal variants of FimH bacteria may be to allow rapid surface colonization and (ii) antiadhesive therapies intended to prevent biofilm formation can have the unintended effect of enhancing the rate of surface colonization.     

Elevated Shear Stress Protects Escherichia coli Cells Adhering to Surfaces via Catch Bonds from Detachment by Soluble Inhibitors

Soluble inhibitors find widespread applications as therapeutic drugs to reduce the ability of eukaryotic cells, bacteria, or viruses to adhere to surfaces and host tissues. Mechanical forces resulting from fluid flow are often present under in vivo conditions, and it is commonly presumed that fluid flow will further add to the inhibitive effect seen under static conditions. In striking contrast, we discover that when surface adhesion is mediated by catch bonds, whose bond life increases with increased applied force, shear stress may dramatically increase the ability of bacteria to withstand detachment by soluble competitive inhibitors. This shear stress-induced protection against inhibitor-mediated detachment is shown here for the fimbrial FimH-mannose-mediated surface adhesion of Escherichia coli. Shear stress-enhanced reduction of bacterial detachment has major physiological and therapeutic implications and needs to be considered when developing and screening drugs.     

Uncoiling mechanics of Escherichia coli type I fimbriae are optimized for catch bonds

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.     

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

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.     

FimH-mediated Escherichia coli K1 invasion of human brain microvascular endothelial cells

Adhesion to brain microvascular endothelial cells, which constitute the blood-brain barrier is considered important in Escherichia coli K1 bacterial penetration into the central nervous system. Type 1 fimbriae are known to mediate bacterial interactions with human brain microvascular endothelial cells (HBMEC). Here, we demonstrate that type 1 fimbriae, specifically FimH adhesin is not only an adhesive organelle that provides bacteria with a foothold on brain endothelial cells but also triggers signalling events that promote E. coli K1 invasion in HBMEC. This is shown by our demonstrations that exogenous FimH increases cytosolic-free-calcium levels as well as activates RhoA. Using purified recombinant mannose-recognition domain of FimH, we identified a glycosylphosphatidylinositol-anchored receptor, CD48, as a putative HBMEC receptor for FimH. Furthermore, E. coli K1 binding to and invasion of HBMEC were blocked by CD48 antibody. Taken together, these findings indicate that FimH induces host cell signalling cascades that are involved in E. coli K1 invasion of HBMEC and CD48 is a putative HBMEC receptor for FimH.     

The affinity of the FimH fimbrial adhesin is receptor-driven and quasi-independent of Escherichia coli pathotypes

Type-1 fimbriae are important virulence factors for the establishment of Escherichia coli urinary tract infections. Bacterial adhesion to the high-mannosylated uroplakin Ia glycoprotein receptors of bladder epithelium is mediated by the FimH adhesin. Previous studies have attributed differences in mannose-sensitive adhesion phenotypes between faecal and uropathogenic E. coli to sequence variation in the FimH receptor-binding domain. We find that FimH variants from uropathogenic, faecal and enterohaemorrhagic isolates express the same specificities and affinities for high-mannose structures. The only exceptions are FimHs from O157 strains that carry a mutation (Asn135Lys) in the mannose-binding pocket that abolishes all binding. A high-mannose microarray shows that all substructures are bound by FimH and that the largest oligomannose is not necessarily the best binder. Affinity measurements demonstrate a strong preference towards oligomannosides exposing Manalpha1-3Man at their non-reducing end. Binding is further enhanced by the beta1-4-linkage to GlcNAc, where binding is 100-fold better than that of alpha-d-mannose. Manalpha1-3Manbeta1-4GlcNAc, a major oligosaccharide present in the urine of alpha-mannosidosis patients, thus constitutes a well-defined FimH epitope. Differences in affinities for high-mannose structures are at least 10-fold larger than differences in numbers of adherent bacteria between faecal and uropathogenic strains. Our results imply that the carbohydrate expression profile of targeted host tissues and of natural inhibitors in urine, such as Tamm-Horsfall protein, are stronger determinants of adhesion than FimH variation.     

Glycosylation changes as important factors for the susceptibility to urinary tract infection

FimH is the type?1 fimbrial tip adhesin and invasin of Escherichia coli. Its ligands are the glycans on specific proteins enriched in membrane microdomains. FimH binding shows high-affinity recognition of paucimannosidic glycans, which are shortened high-mannose glycans such as oligomannose-3 and -5. FimH can recognize equally the (single) high-mannose glycan on uroplakin Ia, on the urinary defence protein uromodulin or Tamm-Horsfall glycoprotein and on the intestinal GP2 glycoprotein present in Peyer's patches. E. coli bacteria may attach to epithelial cells via hundreds of fimbriae in a multivalent fashion. This binding is considered to provoke conformational changes in the glycoprotein receptor that translate into signalling in the cytoplasm of the infected epithelial cell. Bladder cell invasion by the uropathogenic bacterium is the prelude to recurrent and persistent urinary tract infections in humans. Patients suffering from diabetes mellitus are more prone to contract urinary tract infections. In a study of women, despite longer treatments with a more potent antibiotic, these patients also have more often recurrences of urinary tract infections compared with women without diabetes. Type?1 fimbriae are the most important virulence factors used not only for adhesion of E. coli in the urinary tract, but also for the colonization by E. coli in patients with Crohn's disease or ulcerative colitis. It appears that the increased prevalence of urinary tract infections in diabetic women is not the result of a difference in the bacteria, but is due to changes in the uroepithelial cells leading to an increased adherence of E. coli expressing type?1 fimbriae. Hypothetically, these changes are in the glycosylation of the infected cells. The present article focuses on possible underlying mechanisms for glycosylation changes in the uroepithelial cell receptors for FimH. Like diabetes, bacterial adhesion induces apoptosis that may bring the endoplasmic reticulum membrane with immature mannosylated glycoproteins to the surface. Indicatively, clathrin-mediated vesicle trafficking of glucose transporters is disturbed in diabetics, which would interfere further with the biosynthesis and localization of complex N-linked glycans.     

For catch bonds, it all hinges on the interdomain region

Tensile mechanical force was long assumed to increase the detachment rates of biological adhesive bonds (Bell, 1978). However, in the last few years, several receptor-ligand pairs were shown to form "catch bonds," whose lifetimes are enhanced by moderate amounts of force. These include the bacterial adhesive protein FimH binding to its ligand mannose (Thomas et al., 2002; Thomas et al., 2006), blood cell adhesion proteins P- and L-selectin binding to sialyl Lewis X (sLe(X))-containing ligands (Marshall et al., 2003; Evans et al., 2004; Sarangapani et al., 2004), and the myosin-actin motor protein interaction (Guo and Guilford, 2006). The structural mechanism behind this counterintuitive force-enhanced catch bond behavior is of great interest.