fliH

FliH is a component of the bacterial flagellar type III secretion system (T3SS) found in many Gram-negative bacteria, where it participates in the cytoplasmic portion of the flagellar export apparatus required for assembling the flagellum. The fliH gene is typically co-located with fliI and fliJ in flagellar gene clusters and is co-expressed with them as part of the export machinery operon.

Functionally, FliH acts as a regulator and targeting factor for the cytoplasmic ATPase FliI. It binds to FliI and modulates its ATPase activity, preventing wasteful hydrolysis until the ATPase complex is properly positioned at the export gate. By interacting with components of the C-ring, particularly FliN and possibly FliM, FliH helps dock the FliI-containing complex at the cytoplasmic side of the basal body, facilitating delivery of flagellar components such as flagellin to the export channel. In this arrangement, FliH contributes to efficient assembly rather than to catalysis alone.

Structure and interactions: FliH is a small, cytosolic protein that forms complexes with FliI and with C-ring proteins. The FliH–FliI interaction is essential for productive export; FliH also participates in stabilizing the broader ATPase complex at the export apparatus.

Biological role: Proper function of FliH is required for motility and for correct assembly of the flagellum. Mutants lacking FliH show impaired export of flagellar substrates and defective flagellar formation.

See also: FliI, FliJ, FliN, FlhA, flagellar type III secretion system.