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The bacterial fimbrial tip acts as a mechanical force sensor.

Aprikian P, Interlandi G, Kidd BA, Le Trong I, Tchesnokova V, Yakovenko O, Whitfield MJ, Bullitt E, Stenkamp RE, Thomas WE, Sokurenko EV - PLoS Biol. (2011)

Bottom Line: 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.Comparison to other structures suggests that this property is common in unrelated bacterial and eukaryotic adhesive complexes that must function in dynamic conditions.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology, University of Washington, Seattle, Washington, United States of America.

ABSTRACT
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.

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Effect of force on dissociation kinetics versus unhinging and activation kinetics.(a) Schematic representation of the energy landscape of FimH bound to mannose in the absence of force (blue) and in the presence of an external tensile force (brown). The energy barrier for the unhinging conversion from the low- to the high-affinity state is represented as solid lines. The barrier of the mannose unbinding process from the low-affinity state is shown in the insert as dashed lines. The energy added to the system as a result of the applied tensile force is indicated by a red line. The energy landscape in the presence of force is a result of the superposition of the energy landscape in the absence of force and the product of force times elongation. The elongation between the native and transition states is indicated. (b) Representation of the slopes for the rate constants for unhinging and unbinding. Since the elongation until conversion to the high-affinity state is not known, the slopes are calculated for the lower limit, 8.5 Å, and the upper limit, 18 Å, of the range. The intersection between the slopes for the unhinging rate and the slope for the mannose unbinding rate are indicated by blue circles. Where the slopes intersect the rate of unbinding and the rate of unhinging is equal.
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pbio-1000617-g006: Effect of force on dissociation kinetics versus unhinging and activation kinetics.(a) Schematic representation of the energy landscape of FimH bound to mannose in the absence of force (blue) and in the presence of an external tensile force (brown). The energy barrier for the unhinging conversion from the low- to the high-affinity state is represented as solid lines. The barrier of the mannose unbinding process from the low-affinity state is shown in the insert as dashed lines. The energy added to the system as a result of the applied tensile force is indicated by a red line. The energy landscape in the presence of force is a result of the superposition of the energy landscape in the absence of force and the product of force times elongation. The elongation between the native and transition states is indicated. (b) Representation of the slopes for the rate constants for unhinging and unbinding. Since the elongation until conversion to the high-affinity state is not known, the slopes are calculated for the lower limit, 8.5 Å, and the upper limit, 18 Å, of the range. The intersection between the slopes for the unhinging rate and the slope for the mannose unbinding rate are indicated by blue circles. Where the slopes intersect the rate of unbinding and the rate of unhinging is equal.

Mentions: According to kinetic rate theory [22], the rate at which a physical event occurs—in our case, dissociation of mannose or, alternatively, opening of the hinge angle between the FimH domains—is exponentially related to the size of the energy barrier ΔE of the reaction transition state relative to the original equilibrium state of the system: k0 = A exp [−ΔE/kBT], where A is the Arhenius prefactor and kBT is thermal energy. However, mechanical force can speed up a reaction by pulling the protein into the transition state if it is elongated relative to the native state. We define here Δx as the increase in length between the transition state and the native state, thermally averaged and projected onto the direction of force. Then, a constant force field contributes an amount of energy F Δx, to help overcome the transition state energy barrier, effectively decreasing its size as illustrated in Figure 6a. This exponentially increases the reaction rate: k(F) = k0 exp [F Δx/kBT] [23]. Thus, the larger the elongation distance, the greater will be the effect of the same force onto the reaction rate. To determine how force affects FimH, we thus need to know the elongation distances for the two transitions in question: mannose-unbinding versus hinge opening and activation.


The bacterial fimbrial tip acts as a mechanical force sensor.

Aprikian P, Interlandi G, Kidd BA, Le Trong I, Tchesnokova V, Yakovenko O, Whitfield MJ, Bullitt E, Stenkamp RE, Thomas WE, Sokurenko EV - PLoS Biol. (2011)

Effect of force on dissociation kinetics versus unhinging and activation kinetics.(a) Schematic representation of the energy landscape of FimH bound to mannose in the absence of force (blue) and in the presence of an external tensile force (brown). The energy barrier for the unhinging conversion from the low- to the high-affinity state is represented as solid lines. The barrier of the mannose unbinding process from the low-affinity state is shown in the insert as dashed lines. The energy added to the system as a result of the applied tensile force is indicated by a red line. The energy landscape in the presence of force is a result of the superposition of the energy landscape in the absence of force and the product of force times elongation. The elongation between the native and transition states is indicated. (b) Representation of the slopes for the rate constants for unhinging and unbinding. Since the elongation until conversion to the high-affinity state is not known, the slopes are calculated for the lower limit, 8.5 Å, and the upper limit, 18 Å, of the range. The intersection between the slopes for the unhinging rate and the slope for the mannose unbinding rate are indicated by blue circles. Where the slopes intersect the rate of unbinding and the rate of unhinging is equal.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3091844&req=5

pbio-1000617-g006: Effect of force on dissociation kinetics versus unhinging and activation kinetics.(a) Schematic representation of the energy landscape of FimH bound to mannose in the absence of force (blue) and in the presence of an external tensile force (brown). The energy barrier for the unhinging conversion from the low- to the high-affinity state is represented as solid lines. The barrier of the mannose unbinding process from the low-affinity state is shown in the insert as dashed lines. The energy added to the system as a result of the applied tensile force is indicated by a red line. The energy landscape in the presence of force is a result of the superposition of the energy landscape in the absence of force and the product of force times elongation. The elongation between the native and transition states is indicated. (b) Representation of the slopes for the rate constants for unhinging and unbinding. Since the elongation until conversion to the high-affinity state is not known, the slopes are calculated for the lower limit, 8.5 Å, and the upper limit, 18 Å, of the range. The intersection between the slopes for the unhinging rate and the slope for the mannose unbinding rate are indicated by blue circles. Where the slopes intersect the rate of unbinding and the rate of unhinging is equal.
Mentions: According to kinetic rate theory [22], the rate at which a physical event occurs—in our case, dissociation of mannose or, alternatively, opening of the hinge angle between the FimH domains—is exponentially related to the size of the energy barrier ΔE of the reaction transition state relative to the original equilibrium state of the system: k0 = A exp [−ΔE/kBT], where A is the Arhenius prefactor and kBT is thermal energy. However, mechanical force can speed up a reaction by pulling the protein into the transition state if it is elongated relative to the native state. We define here Δx as the increase in length between the transition state and the native state, thermally averaged and projected onto the direction of force. Then, a constant force field contributes an amount of energy F Δx, to help overcome the transition state energy barrier, effectively decreasing its size as illustrated in Figure 6a. This exponentially increases the reaction rate: k(F) = k0 exp [F Δx/kBT] [23]. Thus, the larger the elongation distance, the greater will be the effect of the same force onto the reaction rate. To determine how force affects FimH, we thus need to know the elongation distances for the two transitions in question: mannose-unbinding versus hinge opening and activation.

Bottom Line: 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.Comparison to other structures suggests that this property is common in unrelated bacterial and eukaryotic adhesive complexes that must function in dynamic conditions.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology, University of Washington, Seattle, Washington, United States of America.

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus