Exploring the 3D molecular architecture of Escherichia coli type 1 pili

An integrated approach combining information gained by Fourier transformation, linear Markham superposition (real space) and mass-per-length measurement by scanning transmission electron microscopy was used to analyze the helical structure of the rod-like type 1 pili expressed by uropathogenic Escherichia coli strain W3110. The 3D reconstruction calculated from the experimental data showed the pili to be 6.9nm wide, right-handed helical tubes with a 19.31(+/-0.34)nm long helical repeat comprising 27 FimA monomers associated head-to-tail in eight turns of the genetic one-start helix. Adjacent turns of the genetic helix are connected via three binding sites making the pilus rod rather stiff. In situ immuno-electron microscopy experiments showed the minor subunit (FimH) mediating pilus adhesion to bladder epithelial cells to be the distal protein of the pilus tip, which had a spring-like appearance at higher magnification. The subunits FimG and FimF connect FimH to the FimA rod, the sequential orientation being FimA-FimF-FimG-FimH. The electron density map calculated at 18A resolution from an atomic model of the pilus rod (built using the pilin domain FimH together with the G1 strand of FimC as a template for FimA and applying the optimal helical parameters determined to the head-to-tail interaction model for pilus assembly) was practically identical with that of the actual 3D reconstruction.