Figure 3.

Model for hook-length control and the export-specificity switch mediated by the molecular ruler FliK. (A) During assembly of the flagellar hook (red), export of hook subunits is interrupted intermittently by the export of the molecular ruler FliK (purple). The N terminus of FliK interacts with the hook cap (FlgD, orange) at the tip of the nascent hook and, before the hook reaches its mature length of ∼55 nm, the C-terminal region of the transiting FliK does not interact with the FlhB export gate, which is instead occupied by incoming hook subunits. (B) When the hook reaches its mature length, a gate-binding region (red cylinder) in the C terminus of the transiting FliK becomes exposed and remains in the cytoplasm for a sufficient length of time to form a productive interaction with the FlhB export gate, causing a conformational change (FlhB*). The precise mechanisms underlying this export-specificity switch are unknown. (C) The specificity switch permits the export, and subsequent assembly, of filament subunits (blue).