What is a flagellum?

A short introduction to the swimming motor of bacteria, for readers without prior knowledge.

A motor in the cell envelope

Many bacteria swim. Salmonella, the bacterium we study most, uses several flagella for this: long, helical filaments that are turned by rotary motors in the cell envelope. Each motor is driven by protons that flow into the cell through its stator units. Each filament is several times longer than the cell itself. When all motors turn counter-clockwise, the filaments form a bundle that pushes the cell forward.

Built from the inside out

A flagellum consists of about two dozen different proteins in more than 20,000 copies. The cell assembles it in a fixed order, from the inside out: first the export machinery and the rings in the membrane, then the rod through the cell wall, the hook and finally the long filament. The building blocks of the outer parts cannot cross the membranes on their own. An export apparatus at the base of the flagellum unfolds them and pushes them through a channel of 2 nm diameter that runs through the entire structure. At the tip, a cap protein helps each new building block to fold and to fit into the growing filament, one subunit at a time.

20,000protein building blocks in one flagellum
2 nmwidth of the channel they travel through
100 nm/mingrowth of the filament at the start
10,000amino acids exported per second at full speed

Runs and tumbles

The motor turns in both directions. Counter-clockwise rotation bundles the filaments and the cell swims straight. A short clockwise interval splays the bundle; the cell tumbles and then swims in a new direction. A signalling protein, CheY, controls the switch: in its phosphorylated form it binds to the rotor and reverses the sense of rotation. In this way the cell swims towards nutrients and away from harmful substances.

Why we study it

The flagellum is our model for the assembly of a large protein machine from many parts. The injectisome, with which Salmonella and other pathogens inject proteins into human cells, uses the same export mechanism. Flagella also cost the cell energy and protein, and the cell regulates their synthesis according to its environment. We measure these costs and the benefits of motility during infection. The Zorya system, with which bacteria defend themselves against viruses (bacteriophages), contains a membrane complex with the same 5:2 architecture as the stator unit of the flagellar motor.

Three short animations

The motor in the cell envelope, cut open. The rotor turns, the filament follows.

A stator unit. Protons flow through the unit into the cell and turn its MotA ring, which drives the rotor.

The tip of the filament. New building blocks rise through the channel one at a time and fold under the cap.