FliI

FliI is an ATPase component of the bacterial flagellar export apparatus. It is encoded by the fliI gene, typically part of the flagellar gene cluster whose expression is regulated by the master regulator FlhDC. FliI belongs to the family of secretion ATPases that power the type III–secretion–like channel used to export flagellar subunits across the cytoplasmic membrane during assembly of the flagellum.

Function and mechanism: In the cytoplasm, FliI forms a homo-hexameric ring and hydrolyzes ATP to drive the translocation of flagellar proteins, including flagellin, through the export gate. The energy from ATP hydrolysis is transmitted to the export apparatus to enable efficient assembly of the growing flagellum.

Interactions and regulation: FliI activity is modulated by FliH, which binds FliI and helps recruit and regulate the ATPase at the export gate. FliJ acts as an adaptor linking FliI to the export channel and coordinating interaction with the core export machinery, including FlhA and FlhB.

Genetic and phenotypic impact: Deletion or inactivation of fliI typically results in non-motile bacteria due to failure to assemble functional flagella. FliI is not essential for cell viability but is critical for motility.

Structure and evolution: FliI is a soluble cytoplasmic protein of roughly 50–60 kDa, containing conserved Walker A and B motifs characteristic of AAA+ ATPases. It is widely conserved among motile bacteria and shares homology with ATPases used by other type III secretion systems.

See also: flagellum, type III secretion system, FliH, FliJ, FlhA, FlhB.