Abstract

The structure of the P pili from Escherichia coli has been studied using X-ray fiber diffraction and scanning transmission electron microscopy (STEM). Analysis of the fiber diffraction data indicates that the pili are constituted largely of structural subunits arranged helically with approximately 33 subunits in 10 turns in an axial repeat of 244·5 ± 1·8 Å. Radial electron density distributions calculated from equatorial diffraction data and STEM data indicate that the pili are about 65 Å in diameter with a small central cavity roughly 15 Å across. The principal protein component of the pili is PapA, which has a molecular weight of 16·5 kDa. Assuming that each subunit consists of a single PapA molecule, the mass-per-unit-length of the pili predicted from the X-ray data is 2·23 kDa/Å. Measurements of mass-per-unit-length were also made through the analysis of STEM images. These measurements indicate a value of 2·13 ± 0·14 kDa/Å. STEM images demonstrated the presence of thin, thread-like structures emerging from the ends of pili and spanning breaks in the pili structure. These structures, which have been observed under other conditions, have been termed fibrillae. In the STEM images the fibrillae appear about 20 Å in diameter. The mass-per-unit-length of the fibrillae was estimated using the STEM data to be 0.4 kDa/Å. These data are consistent with the fibrillae representing an unwound or unraveled form of the pili proteins overstretched to about five times the length they would have in the intact pili.

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call

Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.