FimH Forms Catch Bonds That Are Enhanced by Mechanical Force Due to Allosteric Regulation

Abstract

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. 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. Biological adhesion is mediated by specific noncovalent bonds between tethered ligands and receptors. When cells bind to surfaces or other cells in tissue or in fluid flow, these adhesive bonds are subjected to tensile mechanical force. Common sense, theory (1Dembo M. Torney D.C. Saxman K. Hammer D. Proc. R. Soc. Lond. B Biol. Sci. 1988; 234: 55-83Crossref PubMed Scopus (679) Google Scholar, 2Shapiro B.E. Qian H. Biophys. Chem. 1997; 67: 211-219Crossref PubMed Scopus (35) Google Scholar, 3Evans E. Ritchie K. Biophys. J. 1997; 72: 1541-1555Abstract Full Text PDF PubMed Scopus (2078) Google Scholar, 4Evans E. Annu. Rev. Biophys. Biomol. Struct. 2001; 30: 105-128Crossref PubMed Scopus (1103) Google Scholar, 5Bell G.I. Science. 1978; 200: 618-627Crossref PubMed Scopus (3449) Google Scholar), and many observations (6Merkel R. Nassoy P. Leung A. Ritchie K. Evans E. 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J. 2005; 88: 4391-4401Abstract Full Text Full Text PDF PubMed Scopus (38) Google that bonds that are by tensile force as the and are is that at bonds may that are strengthened by tensile mechanical force (1Dembo M. Torney D.C. Saxman K. Hammer D. Proc. R. Soc. Lond. B Biol. Sci. 1988; 234: 55-83Crossref PubMed Scopus (679) Google Scholar, D. A. Rev. 2002; bonds to with of until a which the bonds of the to form catch bonds is the Escherichia coli adhesin FimH E. M. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar, P. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar), which is the adhesin on the most common adhesive for the and FimH are in binding to the 1978; PubMed Google and the D. Nature. PubMed Scopus Google as as binding to tissue J. Biol. PubMed Scopus Google Scholar), tissue PubMed Google Scholar, S. R. A. R. M.A. J. M. K. S. S. J. PubMed Scopus Google Scholar, S. S. M. G. J. P. S. S. Science. 1997; PubMed Scopus Google Scholar, T. Y. M. A. K. 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PubMed Scopus Google Scholar), binding to and the binding to the and to form catch bonds by single force these in which bond forms and tensile force is applied by the surfaces the catch bond mechanism for FimH has by a of adhesion of bacteria or M. PubMed Scopus Google Scholar, M. O. P. Biophys. J. Full Text Full Text PDF PubMed Scopus Google Scholar, M. Scopus Google Scholar, M. O. Biol. Scopus Google Scholar, P. B. O. E. W. E. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, E. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google these fluid flow the bacteria on the This occurs when is at the is which the of bonds between FimH and on the M. O. P. Biophys. J. Full Text Full Text PDF PubMed Scopus Google Scholar, 72: PubMed Scopus Google has that adhesion occurs as a of bond when the surfaces are J. 2005; Google Scholar, 2005; Google or as a of the mechanical of the bacteria the FimH bonds E. S. A. Biophys. J. 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