X-ray diffraction and electron microscope studies on the structure of bacterial F pili.

F pili are hollow cylinders with 80 Å outer diameter and 20 Å inner diameter. Both X-ray fibre diffraction and optical diffraction of electron micrographs show a strong layer-line corresponding to a spacing of 32 Å, to which a J₄ Bessel function is assigned on the basis of the optical diffraction. X-ray diffraction patterns show near-meridional intensity on a layer-line corresponding to a spacing of 12.8 Å, to which a J₁ Bessel function is assigned. Mass per length measurements on unstained specimens in the scanning transmission electron microscope give 3000 daltons/Å, indicating that the 11,200 dalton pilin subunits are 3.7 Å apart along the axial direction of the pili. These observations show that the pilus structure can be represented as four coaxial helices of pitch 128 Å with the pilin subunits elongated and overlapping along the line of these helices. Each of these helices of subunits is translated axially with respect to its neighbour, to give a basic helix of 3.6 units per turn of 12.8 Å pitch. Radial electron density calculations indicate a 50 Å diameter girdle of hydrophobic amino acids between the inner and outer diameters of the protein shell. A molecular model of the structure at low resolution is presented.