Structure of bacterial flagellar filaments at 11 A resolution: packing of the alpha-helices

Recent advances in the analysis of electron micrographs of frozen, hydrated bacterial filaments have allowed us to average data from more than 150 images and to reconstruct the bacterial flagellar filament of Salmonella typhimurium at a resolution of approximately 11 A. In addition to the outermost features seen in earlier lower resolution maps of the filament, we find a pair of concentric tubes which surround a approximately A diameter channel at the center of the structure. The walls of these tubes are composed of rod-like features which we have interpreted as columns of individual alpha-helices stacked end-to-end. Each column runs approximately parallel to the helix axis. The wall of the innermost tube, at a radius of approximately 20 A, is formed from 11 such columns. The wall of the second tube is formed from 22 columns which occur alternately at radii of approximately 43 and approximately 47 A. The two concentric tubes are held apart by spacers. These are short, rod-like features, which run approximately parallel to the helix axis. We have interpreted these as additional alpha-helices. By symmetry, each flagellin monomer contributes an alpha-helix to the inner tube, two alpha-helices to the outer tube and a fourth alpha-helix to the spacer. We have tentatively assigned one type of alpha-helix in the outer tube to the approximately 30 C-terminal residues of flagellin while the remaining three alpha-helices are assigned to the approximately 70 N-terminal residues. This interpretation of the reconstruction is consistent with available biochemical, biophysical and amino acid sequence information. We also present details of improved methodology to extract and evaluate the original data and also to assess the statistical significance of features in the three-dimensional map.