| Description |
Bacteria swim through liquid environments by rotating extracellular propellers known as flagella. Extending up to 10 m in length, the cell-external flagellar filament self-assembles from ∼10,000 copies of a single protein and constitutes the bulk of the flagellum. However, the nanometer-scale hook basal body (HBB), which powers the flagellar filament and anchors it to the cell body, is constructed from ∼25 unique protein subunit types that must self-assemble into an ion-powered motor of precise dimensions. Thus, the HBB represents the structurally and mechanically more complex component of the flagellum and has been the subject of intense study for several decades. The HBB is composed of three substructures: i) the MS-C-ring rotor in the cytoplasm that encloses the flagellar-specific Type III Secretion apparatus, ii) the periplasmic driveshaft, and iii) the extracellular hook. The rigid driveshaft, known as the rod, is the first axial structure of the flagellum to assemble and resides entirely in the periplasmic space between the inner and outer membranes. The rod transmits the torque generated by the flagellar motor embedded in the cytoplasmic membrane to the cell-external components of the flagellum. Until recently, the mechanisms that regulate rod assembly and the switch from rod-to-hook polymerization remained unknown. Specifically, it was unclear how the flagellum, which self-assembles from thousands of individual subunits to predetermined dimensions, ensured the rod substructure did not grow past its mature wild type length of ∼25 nm. Secondly, while it was known that the transition from rod polymerization to hook polymerization was somehow coordinated with penetration of the outer membrane by the nascent flagellar structure, the molecular mechanism that coupled these two events was unknown. Using genetic, biochemical and microscopic techniques, we have elucidated both the means by which rod length is controlled as well as the mechanism that synchronizes outer membrane penetration with the switch from rod-to-hook assembly. We have also provided insight into the molecular basis for the difference in flexibility between the rod and the hook and the significance of the rod's relative inflexibility with respect to flagellar form and function. |
