Crystal structure of the flagellar chaperone FliS from Bacillus cereus and an invariant proline critical for FliS dimerization and flagellin recognition.

FliS is a cytoplasmic flagellar chaperone for the flagellin, which polymerizes into filaments outside of the flagellated bacteria. Cytoplasmic interaction between FliS and flagellin is critical to retain the flagellin protein in a monomeric form, which is transported from the cytoplasm through the flagellar export apparatus to the extracellular space for filament assembly. Defects in the FliS protein directly diminish bacterial motility, pathogenicity, and viability. Although the overall structure of FliS is known, structural and mutational studies on FliS from other bacterial species are still required to reveal any unresolved biophysical features of FliS itself or functionally critical residues for flagellin recognition. Here, we present the crystal structure of FliS from Bacillus cereus (BcFliS) at 2.0 Å resolution. FliS possesses a highly dynamic N-terminal region, which is appended to the common four-helix bundle structure. An invariant proline residue (Pro17 in B. cereus FliS) was identified in all known FliS sequences between the N-terminal region and the four-helix bundle. The N-terminal proline residue functions as a helix breaker critical for FliS dimerization and flagellin recognition.