THE STRUCTURE OF THE R-TYPE STRAIGHT FLAGELLAR FILAMENT OF SALMONELLAAT 9 ANGSTROM RESOLUTION BY ELECTRON CRYOMICROSCOPY

The supercoiled forms of the flagellar filaments are thought to be con structed from a mixture of two distinct subunit conformations arranged in a regular manner. We analyzed the structure of one of the two stra ight flagellar filaments, each of which is built up with all its subun its in one of the two conformations. The filament we studied was isola ted from the strain SJW1655 of Salmonella typhimurium and had a right- handed helical symmetry. With recent advancements in electron cryomicr oscopy, such as a liquid helium temperature stage for frozen hydrated specimens and a stable field emission source, and also by averaging hi gh resolution data with a proper correction of the contrast transfer f unction, the density distribution map of this straight flagellar filam ent was generated in far more detail than before by including data up to 9 Angstrom resolution. The structure shows a densely packed core re gion from about 15 to 55 Angstrom in radius, where a pair of concentri c tubular features of high density is present without well-defined sub unit boundaries, and an outer part from 55 to 115 Angstrom, where the subunits are mostly well separated from each other. The outer tube in the core region, from 35 to 55 Angstrom in radius, contains many rod-l ike features with near-axial orientation and closest lateral distances of around 10 Angstrom, which are most likely to represent the alpha-h elical bundles that were predicted in our previous report. In the inne r tube, from 15 to 30 Angstrom in radius, the rod-like features are le ss clear. Between the inner and outer tubes are the short spoke-like d ensities, which are radially tilted and are connecting the two tubes. The outer part, from 55 to 115 Angstrom, contains an axially elongated column density and a slewed projection with a narrow neck region. Whe n compared with the other straight filament having left-handed helical symmetry, this outer part does not show any significant changes in or ientation, suggesting that the switch in the subunit conformation and packing involved in the polymorphic transitions is quite subtle and on ly occurs within the core region. Reassignment of each structural doma in to the amino acid sequence is suggested, based on the volume of eac h domain, which was determined rather precisely by a proper correction of the contrast transfer function for both amplitudes and phases.