Functional flexibility of the FimH adhesin: Insights from a random mutant library

Type I fimbriae are surface organelles of Escherichia coli which mediate D- mannose-sensitive binding to different host surfaces. This binding is confe rred by the minor fimbrial component FimH. Naturally occurring variants of the FimH protein have been selected in nature for their ability to recogniz e specific receptor targets. In particular, variants that bind strongly to terminally exposed monomannose residues have been associated with a pathoge nicity-adaptive phenotype that enhances E. coli colonization of extraintest inal locations such as the urinary bladder. In this study,ve have used rand om mutagenesis to specifically identify nonselective mutations in the FimH adhesin which modify its binding phenotype. Isogenic E. coli clones express ing FimH variants were tested for their ability to bind yeast cells and mod el glycoproteins that contain oligosaccharide moieties rich in either termi nal monomannose, oligomannose, or nonmannose residues. Both the monomannose - and the oligomannose-binding capacity of type 1 fimbriae could be altered by minor amino acid changes in the FimH protein. The monomannose-binding p henotype was particularly sensitive to changes,,vith extensive differences in binding being observed in comparison to wild-type FimH levels. Different structural alterations were able to cause similar functional changes in Fi mH, suggesting a high degree of flexibility to target recognition by this a dhesin. Alteration of residue P49 of the mature FimH protein, which occurs within the recently elucidated carbohydrate-binding pocket of FimH, complet ely abolished its function. Amino acid changes that increased the binding c apacity of FimH were located outside receptor-interacting residues, indicat ing that functional changes relevant to pathogenicity are likely to be due to conformational changes of the adhesin.