Valency conversion in the type 1 fimbrial adhesin of Escherichia coli

Abstract

FimH protein is a lectin‐like adhesive subunit of type 1, or mannose‐sensitive, fimbriae that are found on the surface of most Escherichia coli strains. All naturally occurring FimH variants demonstrate a conserved mannotriose‐specific (i.e. multivalent) binding. Here, we demonstrate that replacement of residues 185–279 within the FimH pilin domain with a corresponding segment of the type 1C fimbrial adhesin FocH leads to a loss of the multivalent mannotriose‐specific binding property accompanied by the acquisition of a distinct monomannose‐specific (i.e. monovalent) binding capability. Bacteria expressing the monovalent hybrid adhesins were capable of binding strongly to uroepithelial tissue culture cells and guinea pig erythrocytes. They could not, however, agglutinate yeast or bind human buccal cells – functions readily accomplished by the E. coli‐ expressing mannotriose‐specific FimH variants. Based on the relative potency of inhibiting compounds of different structures, the receptor binding site within monovalent FimH–FocH adhesin has an extended structure with an overall configuration similar to that within the multivalent FimH of natural origin. The monomannose‐only specific phenotype could also be invoked by a single point mutation, E89K, located within the lectin domain of FimH, but distant from the receptor binding site. The structural alterations influence the receptor‐binding valency of the FimH adhesin via distal effects on the combining pocket, obviously by affecting the FimH quaternary structure.

The Distinct Binding Specificities Exhibited by Enterobacterial Type 1 Fimbriae Are Determined by Their Fimbrial Shafts

“…We determined that, although the primary structures of FimHE and FimHK are 98.6% identical, the FimHS structure exhibits very limited homology to either of the other two FimH proteins. The E. coli FimH mannose-binding pocket is created by discontinuous regions of the molecule and seems to be conformation-dependent, so the similar basic mannose specificity of both E. coli and S. typhimurium FimH could arise from similar tertiary conformations, even though their primary structures are divergent (23,39,40). Because the MalE/FimH fusion proteins mediated mannose-binding properties on the overlay assays following SDS-PAGE, these proteins must have renatured and presumably regained their functionally competent conformation.…”

“…Sequence variations that diminish the mannose-binding ability of FimH were found to be located within or close to the sequences that make up the FimH binding pocket as defined by the FimH crystal struc- ture (23,40,54), although the location of the FimH binding pocket in intact fimbriae may vary somewhat from that determined by the crystal structure of FimH complexed with only its chaperone, FimC (23). However, sequence alterations that increase the monomannose-binding ability of FimH, which is thought to play a role in the tropism of uropathogenic E. coli for the bladder epithelium, are not located near the FimH binding pocket, but are instead located in the lower part of the FimH lectin domain defined by the tertiary structure of FimH (26,28,39,40,54). Therefore, rather than altering the FimH binding pocket directly, these sequence variations may instead alter the conformational stability of the binding pocket, such as we hypothesize would occur by altering the FimH interaction with the fimbrial shaft.…”

“…Therefore, rather than altering the FimH binding pocket directly, these sequence variations may instead alter the conformational stability of the binding pocket, such as we hypothesize would occur by altering the FimH interaction with the fimbrial shaft. In fact, replacement of the pilin domain of FimH (amino acids 160 -279), which directs incorporation of FimH into the type 1 fimbrial shaft, with the corresponding pilin segment of the E. coli type 1C fimbrial adhesin FocH significantly alters the binding phenotype of the FimH/FocH hybrid type 1 fimbriae without altering the primary sequence of the FimH lectin domain (amino acids 1-156) (23,39). Presumably, this is because of an altered conformation of the FimH binding site when incorporated into the type 1 fimbrial shaft via the FocH pilin domain (39).…”

“…In fact, replacement of the pilin domain of FimH (amino acids 160 -279), which directs incorporation of FimH into the type 1 fimbrial shaft, with the corresponding pilin segment of the E. coli type 1C fimbrial adhesin FocH significantly alters the binding phenotype of the FimH/FocH hybrid type 1 fimbriae without altering the primary sequence of the FimH lectin domain (amino acids 1-156) (23,39). Presumably, this is because of an altered conformation of the FimH binding site when incorporated into the type 1 fimbrial shaft via the FocH pilin domain (39). The importance of quaternary association in polymeric adhesins in determining binding specificity was recently revealed in studies of the plant snowdrop and garlic lectins (56).…”

Catch Bond-mediated Adhesion without a Shear Threshold

“…Thus, binding to 1M is enhanced by shear and requires shear threshold for adhesion to occur. Using steered molecular dynamics simulations and site-directed mutagenesis of FimH, we have shown that the affinity to 1M is probably enhanced by the tensile force-induced extension of a 3-amino acid linker chain between the two domains of FimH, distal from the mannose-binding site (13).Whereas most variants of FimH bind only weakly to 1M substrates in static conditions, all natural FimH variants exhibit strong oligosaccharide-specific binding to 3M already in the absence of flow that is up to 20 times higher than the binding to 1M (19,21). This is thought to be due to the presence of an extended FimH binding pocket with multiple subsites corresponding to the size of a trimannose (3M) and, possibly, pentasaccharide (22)(23)(24).…”