Required for flagellar assembly and thus bacterial motility (PubMed:28659885). Interacts with flagellins FlaA, FlaB and FlaC; preferentially binds glycosylated FlaA over non-glycosylated protein (PubMed:28659885). Requires the 10 last residues of FlaA for binding (PubMed:28659885). May form a 3-way complex of flagellin, FliS and FliW simultaneously in which FliS and FliW do not directly interact (By similarity). Cells are non-motile, flagellar filaments are severely truncated (PubMed:28659885). Strongly reduced levels of secreted flagellins FlaA, FlaB and FlaC (PubMed:28659885). Belongs to the FliS family.