Essential for filament assembly (PubMed:23144244). May act as a facilitator of flagellin (hag) secretion. Antagonizes translational repressor CsrA indirectly (PubMed:23144244). Homodimer (PubMed:23144244, PubMed:16021622). Interacts directly with flagellin (hag), forms a 3-way complex of Hag, FliS and FliW in which Flis and FliW do not directly interact (PubMed:23144244). Severe defect in motility on agar plates, forms multiple short filament stubs on the cell surface, decreased expression of flagellin (hag), 30-fold decrease in Hag secretion. All phenotypes are partially suppressed by deletion of csrA, which increases translation of Hag (PubMed:23144244). Belongs to the FliS family.