Many species of bacteria propel themselves through their environment by spinning helical motorized flagella. Rotobacter cells have one flagellum each, whereas E. coli cells have multiple flagella that rotate in bundles. Each flagellum consists of a helical filament that is 20 nanometers wide and up to 15 microns long and spins on the order of 100 times per second.
These animations show a series of schematized and speculative models about how bacterial flagella might function and assemble. Just outside of the cell wall, the filament is connected to a flexible rotating hook. The filament, the hook, and a structure called the basal body, located below the cell surface, make up the three parts of the flagella. The basal body consists of a rod, and a series of rings embedded in the inner membrane, the peptidoglycan layer, and the outer membrane.
Some of the rings make up the flagellar motor, which can be divided into two major parts, the stator, which is attached to the peptidoglycan layer, and as its name implies, remains stationary, and the rotor, which rotates. The motor derives its power from a proton gradient across the membrane. In this example, A high concentration of protons exists outside, and a low concentration exists inside the cell.
The protons flow through the interface between two types of proteins, called MOTA and MOTB, that make up the stator. Mutational studies suggest that a conserved aspartic acid in MOTB functions in proton conductance. Each stator contains two MOTB proteins, and therefore also contains two of these important aspartic acids.
Although the molecular mechanism of rotation is not known, one possible model describes protons moving through the channels in the stators and binding to the aspartic acid in the MOTB proteins. This binding causes a conformational change in MOTA proteins, resulting in the first power stroke that moves the rotor incrementally. At the end of the first power stroke, the two protons are released into the cytoplasm.
The proton loss causes a second conformational change that drives the second power stroke, once again engaging the rotor. Although the mechanism for motor function is not yet certain, many details of flagellar assembly have been determined. Flagella begin their assembly with structures in the inner membrane.
26 subunits of an integral membrane protein called flea F come together in the plasma membrane to form the MS ring. The Flea G proteins assemble under the MS ring. Flea G, along with Flea M and Flea N proteins, make up the rotor.
Flagellar proteins destined to be part of the extracellular portion of the flagellum are exported from the cell by a flagellum-specific export pathway and assembled at the center. Mote A and Mote B form the stationary part of the flagellar motor. the stator.
Both are integral membrane proteins, but MOTEB is also anchored to the rigid peptidoglycan layer, keeping the stator proteins fixed in place. The subunits of the rod portion of the rotor move up through the hollow cylinder in the assembly and, assisted by cap proteins, build up the rod in a proximal to distal fashion. Another set of rings, called L and P rings, are found in gram-negative bacteria such as E. coli. They penetrate the outer membrane, forming a bearing for the rod.
As the rod cap is exposed outside the L-ring, it dissociates and is replaced by a hook cap that guides the assembly of the hook proteins. After the hook is assembled, the hook cap dissociates and a series of junction proteins assemble between the hook and future filaments. Finally, yet another cap is built, and filament proteins assemble. Like the rod and hook proteins, they travel through the hollow channel inside the filament to reach the distal end.
The cap rotates, which causes the subunits to build in a helical fashion. A complete filament can consist of 20,000 to 30,000 subunits.