Bacteria such as Salmonella typhimurium move by the action of their flagella. Depending on the direction of rotation, flagella either act singly, causing uncoordinated tumbling, or clump together into a single helical propeller for straight-line swimming. The 60-nm-long hook that joins the flagellar filament to its motor in the bacterial cell wall must thus bend through as much as 90° in a millisecond or less, all the time rotating at up to 300 revolutions per second. Elsewhere in this issue (Nature 431, 1062–1068; 2004), Fadel A. Samatey et al. describe how they determined the atomic structure of this super-flexible universal joint, and thereby how it achieves such a feat of engineering.

The hook is a hollow tube assembled from 11 chains, or protofilaments, of a single protein, called FlgE. These protofilaments are stacked together with a slight helical twist that changes slightly with the direction of rotation.

Samatey et al. made their model by first determining the structure of the central region of FlgE by X-ray crystallography at a resolution of 1.8 Å. This was then fitted into the lower-resolution images of isolated, straight hooks as seen by electron microscopy. The final curved hook (shown here with individual protein chains coloured from blue through to red) emerged by computer simulation of the squashing and stretching of individual protofilaments.

This modelling showed that the hook's mechanical properties result from a combination of flexibility and rigidity at the molecular level. Individual protofilaments can grow and shrink in length by as much as 50% through flexing of a hinge that joins the two major domains of FlgE. However, the interlocking of adjacent subunits prevents the protofilaments from sliding against each other. The flagellar hook can thus bend but not twist, allowing efficient transmission of force from motor to propeller.