Microbiology. Action at a distance--bacterial flagellar assembly

The rotating motion of a helical flagellum enables a bacterium to swim toward positive stimuli and away from danger. But how is the flagellum, composed of many different proteins, assembled? In a Perspective, Macnab explains how subunits of the protein flagellin flow down a channel inside the flagellum and are then added to its tip through the action of a rotating pentameric cap complex (Yonekura et al.).