Molecular Mechanisms of Self-Assembly and Protein Export of the Bacterial Flagellum

From Q-bio

The bacterial flagellum is made of a rotary motor and a long helical filament by means of which bacteria swim. The flagellar motor rotates at around 300 Hz and drives the rapid rotation of each flagellum to propel cell movements. The flagellum is made of about 30 different proteins in copy numbers from a few to a few tens of thousands. Their self-assembly process is well ordered, and the flagellar axial structure, which grows out into the cellular exterior, is constructed by self-assembly of proteins translocated from the cytoplasm to the distal end of the growing structure through the narrow, central channel. The protein export is driven by the flagellar type III protein export apparatus located at the cytoplasmic face of the basal body. The export apparatus consists of six membrane proteins forming the export gate and three soluble proteins forming a complex with export substrate proteins to bind to the gate. One of the soluble proteins is an ATPase, FliI, which has long been thought to provide energy for the export process. Our recent finding, however, revealed that the function of FliI is to form a hexamer, bind an export substrate, and insert its N-terminal chain into the export gate and that the rest of successive unfolding and translocation of the export substrate is driven by proton motive force. ATP hydrolysis by FliI is used to disassemble the FliI hexamer from the export gate and the substrate protein for efficient protein export.