FimH, the adhesive subunit of type 1 fimbriae expressed by many enterobacteria, mediates mannose-sensitive binding to target host cells. At the same time, fine receptor-structural specificities of FimH from different species can be substantially different, affecting bacterial tissue tropism and, as a result, the role of the particular fimbriae in pathogenesis. In this study, we compared functional properties of the FimH proteins from
Escherichia coli and
Klebsiella pneumoniae, which are both 279 amino acids in length but differ by some ∼15% of residues. We show that
K. pneumoniae FimH is unable to mediate adhesion in a monomannose-specific manner via terminally exposed Manα(1-2) residues in N-linked oligosaccharides, which are the structural basis of the tropism of
E. coli FimH for uroepithelial cells. However,
K. pneumoniae FimH can bind to the terminally exposed Manα(1-3)Manβ(1-4)GlcNAcβ1 trisaccharide, though only in a shear-dependent manner, wherein the binding is marginal at low shear force but enhanced sevenfold under increased shear. A single mutation in the
K. pneumoniae FimH, S62A, converts the mode of binding from shear dependent to shear independent. This mutation has occurred naturally in the course of endemic circulation of a nosocomial uropathogenic clone and is identical to a pathogenicity-adaptive mutation found in highly virulent uropathogenic strains of
E. coli, in which it also eliminates the dependence of
E. coli binding on shear. The shear-dependent binding properties of the
K. pneumoniae and
E. coli FimH proteins are mediated via an allosteric catch bond mechanism. Thus, despite differences in FimH structure and fine receptor specificity, the shear-dependent nature of FimH-mediated adhesion is highly conserved between bacterial species, supporting its remarkable physiological significance.The most common type of adhesive organelle in the
Enterobacteriaceae is the type 1 fimbria, which has been most extensively studied in
Escherichia coli. The corresponding structures of
Klebsiella pneumoniae are similar to those of
E. coli with regard to genetic composition and regulation (
15). Type 1 fimbriae are composed primarily of the structural subunit FimA, with minor amounts of three ancillary subunits, FimF, FimG, and the mannose-specific adhesin FimH. The FimH adhesin is an allosteric protein that mediates the catch bond mechanism of adhesion where the binding is increased under increased shear stress (
48).It has been demonstrated in
E. coli that FimH has two domains, the mannose-binding lectin domain (from amino acid [aa] 1 through 156) and the fimbria-incorporating pilin domain (from aa 160 through 279), connected via a 3-aa-long linker chain (
6). A mannose-binding site is located at the top of the lectin domain, at the opposite end from the interdomain linker (
17).Several studies have demonstrated that type 1 fimbriae play an important role in
E. coli urinary tract infection (UTI) (
7,
21,
23,
35). In addition, in urinary
E. coli isolates, the FimH adhesin accumulates amino acid replacements which increase tropism for the uroepithelium and various components of basement membranes (
21,
30,
35,
37,
49). Most of the replacements increase the monomannose binding capability of FimH under low shear, by altering allosteric catch bond properties of the protein (
48). The mutated FimH variants were shown to provide an advantage in colonization of the urinary tract in the mouse model (
35) and correlate with the overall extraintestinal virulence of
E. coli (
16). Thus, FimH mutations are pathoadaptive in nature.
Klebsiella pneumoniae is recognized as an important opportunistic pathogen frequently causing UTIs, septicemia, or pneumonia in immunocompromised individuals (
29). It is responsible for up to 10% of all nosocomial bacterial infections (
18,
41).
K. pneumoniae is ubiquitous in nature, and it has been shown that environmental isolates are phenotypically indistinguishable from clinical isolates (
22,
26,
27,
29,
33). Furthermore, it has been demonstrated that environmental isolates of
K. pneumoniae are as virulent as clinical isolates (
28,
45).
K. pneumoniae possesses a number of known virulence factors, including a pronounced capsule, type 3 fimbriae, and type 1 fimbriae (
29,
44). Type 1 fimbriae produced by
K. pneumoniae are described as functionally and structurally similar to type 1 fimbriae from
E. coli (
25) and have been shown to play a significant role in
K. pneumoniae UTI (
32,
43).We have previously shown that mature FimH from 54 isolates of
K. pneumoniae (isolated from urine, blood, liver, and the environment) is represented by seven protein variants due to point amino acid replacements. (
42) When
K. pneumoniae FimH was aligned with the FimH of
E. coli, they showed ∼85% similarity at the amino acid level. Furthermore, a majority (14 out of 21 isolates) of the
K. pneumoniae strains isolated from patients with UTI grouped into a single clonal group based on multilocus sequence typing, but
fimH in one isolate in the group differed from the others by a single nucleotide mutation resulting in an amino acid change, serine to alanine, in position 62 (
42). The same mutation has been found in FimH of a highly uropathogenic clone of
E. coli and significantly increases the adhesin''s ability to adhere to monomannose under low or no shear (
19,
39,
50).In this study, we describe the extent and pattern of structural variability of the FimH protein from
K. pneumoniae and perform comparative analyses of the functional properties of FimH from both
K. pneumonae and
E. coli.
相似文献