Structural analysis of F18 fimbriae expressed by porcine toxigenic Escherichia coli.
The F18 fimbriae expressed by porcine toxigenic Escherichia coli strains are 1- to 2-mm-long filaments that mediate the adhesion of the bacteria to enterocytes. The backbone of these fimbriae is built from a major structural 15.1-kDa protein, FedA. The structure of isolated negatively stained F18 fimbriae imaged by dark-field scanning transmission electron microscopy (STEM) was resolved to approximately 2 nm. Analyzing their helical symmetry showed the axially repeating units to alternate in a "zigzag" manner around the helical axis with an axial rise of 2.2 nm. Two repeating units give rise to the observed 4.3-nm helical repeat, which is practically identical to the pitch of the one-start helix formed. Additionally, an axially repeating pattern with a 27-nm spacing was found on rotary-shadowed fimbriae. Mass-per-length determination of unstained F18 fimbriae by STEM revealed the axially repeating unit to have a molecular mass of 25.4 kDa, indicating that it is a FedA monomer, with the difference in mass arising from the minor subunits, FedE and FedF. The presence of the latter two proteins might cause the observed 27-nm axial pattern.
Hahn
E
E
Institute of Veterinary Anatomy, University of Zürich, CH-8057 Zürich, Switzerland.
Wild
P
P
Schraner
E M
EM
Bertschinger
H U
HU
Häner
M
M
Müller
S A
SA
Aebi
U
U
D000818:Animals; D004926:Escherichia coli / Q000648:ultrastructure*; D010861:Fimbriae, Bacterial / Q000648:ultrastructure*; D007091:Image Processing, Computer-Assisted; D017348:Microscopy, Electron, Scanning Transmission / Q000379:methods; D013552:Swine / Q000382:microbiology*