Two-ball structure of the flagellar hook-length control protein FliK as revealed by high-speed atomic force microscopy.

The bacterial flagellar hook is a short and uniquely curved tube that connects the basal body to the filament. Hook length is controlled at 55 nm on average by a soluble protein FliK in Salmonella enterica serovar Typhimurium. The N-terminal segment of FliK responsible for measuring the hook length is considered to be intrinsically disordered. Here, we show by high-speed atomic force microscopy that a FliK molecule in solution takes on a shape of two balls linked by a flexible string; the larger ball corresponds to the N-terminal region and the smaller one corresponds to the C-terminal region. The N-terminal domain is stable but the C-terminal domain fluctuates in shape. Based on these and other features of FliK, we propose that the folding of the N-terminal segment at the tip of the growing hook plays a major role in determining the minimal length of the hook.