Revealing the mystery of the bacterial flagellum:
A self-assembling nanomachine with fine switching capability
JAPAN NANONET BULLETIN - 11th Issue - February 5, 2004
Professor, Graduate School of Frontier Biosciences, Osaka University
(Issued in Japanese: March 25, 2003)
(the online article contains links to many fasinating animations of the bacterial flagellum)
Nature created a rotary motor with a diameter of 30 nm. Motility of bacteria, such as Salmonella and E. coli with a body size of 1 to 2 micron, is driven by rapid rotation of a helical propeller by such a tiny little motor at its base. This organelle is called the flagellum, made of a rotary motor and a thin helical filament that grows up to about 15 micron. It rotates at around 20,000 rpm, at energy consumption of only around 10 to 16 W and with energy conversion efficiency close to 100 percent. Prof. Nambas research group is going to reveal the mechanism of this highly efficient flagellar motor that is far beyond the capabilities of artificial motors.
The flagellum is made by self-assembly of about 25 different proteins. The rotor ring made of protein FliF is the first to assemble in the cytoplasmic membrane. Then, other protein molecules attach to the ring one after another from the base to the tip to construct the motor structure. After the motor has been formed, the flagellar filament, which functions as a helical propeller, is assembled. Precise recognition of the template structure by component proteins allows this highly ordered self-assembly process to proceed without error. The flagellar filament is made of 20,000 to 30,000 copies of flagellin polymerized into a helical tube structure. Flagellin molecules are transported through a long narrow central channel of the flagellum from the cell interior to the distal end of the flagellum, where they self-assemble in a helical manner by the help of a cap complex. The cap is pentameric complex made of HAP2 and has a pentagonal plate and five leg domains, whose flexible stepping movements accompanied by rotation of the whole cap is the key mechanism to promote the efficient self-assembly of flagellin molecules by preparing just one binding site of flagellin at a time and guiding the binding.
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