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

J Mol Biol. 2015 Jan 30;427(2):406-14. doi: 10.1016/j.jmb.2014.11.007. Epub 2014 Nov 15.


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.

Keywords: AFM; FliK; Salmonella enterica serovar Typhimurium; intrinsically disordered structure; secretion substrate specificity.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / genetics
  • Flagella / chemistry*
  • Magnetic Resonance Spectroscopy
  • Microscopy, Atomic Force / methods*
  • Molecular Structure
  • Protein Folding
  • Protein Stability
  • Salmonella typhimurium / chemistry
  • Salmonella typhimurium / genetics*


  • Bacterial Proteins
  • FliK protein, Bacteria