Figure 4.

A chain mechanism delivers subunits to the assembly tip of the external flagellum. Subunits are unfolded by the export machinery before recognition by the FlhB_C export gate component. The N terminus (blue) of the subunit docked at the export gate is captured by the free C terminus (red) of an exiting subunit in the flagellar channel, linking subunits head to tail in a chain. Linked unfolded subunits in the channel transit to the flagellum tip, where they crystallise sequentially beneath the cap foldase. As subunits fold into the flagellum tip, the chain becomes stretched, increasing the entropic pulling force at the cell-proximal end of the chain until a threshold force is reached and a new subunit is pulled from the export machinery into the channel. This process repeats, delivering subunits to the flagellum tip at a constant rate.