Flagellar hook structures of Caulobacter and Salmonella and their relationship to filament structure

Native flagellar hooks from a polarly flagellated bacterium, Caulobacter crescentus, and polyhooks from a peritrichously flagellated bacterium, Salmonella typhimurium. have been studied by densitometry of electron micrographs of negatively stained specimens, followed by computerized Fourier analysis and three-dimensional reconstruction. The two structures are remarkably similar. In both cases, the subunits are arranged along a right-handed basic helix of 2.3 nm pitch with successive subunits separated by an azimuthal angle of 64 to 65 °, and there is a pronounced system of continuous 6-start grooves and ridges on the surface of the structures. The subunit of Salmonella (M_r 42,000, versus 70,000 for Caulobacter) is somewhat thinner and yields a smaller overall hook diameter. The “bent finger” subunit shape and orientation in both cases suggests that the hook could bend readily by a sliding motion in the 11-start direction at inner radii, with the 6-start groove preventing collision at outer radii. The basic helical pitch of the Salmonella hook structure, and the number of subunits per basic helical turn (5.56) makes it highly compatible with the Salmonella flagellar filament (2.6 nm pitch. 5.51 subunits per turn); so also does the elongated shape and tilt angle of the hook and flagellin subunits in the respective structures. The two structures may therefore conjoin directly in the intact flagellum, although participation of a minor protein is not ruled out by the data.