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doi:10.1038/84194 February 2001 Volume 8 Number 2 pp 96 - 97 Putting a lid on it Kelly T. Hughes & Phillip D. Aldridge Kelly T. Hughes and Phillip D. Aldridge are in the Department of Microbiology, University of Washington, Seattle, Washington 98195, USA.
A cap at the tip of the bacterial flagellum uses a dynamic differential binding of individual subunits to allow the filament tip to grow, achieving control of assembly far from the point of protein translation. Physical movement is an incredible evolutionary achievement. Even tiny bacteria such as Escherichia coli and Salmonella can propel themselves through liquid environments and on surfaces by the rotation of attached helical appendages called flagella. The case of the bacterial flagellum might seem at first glance to be a simple mechanism for self-propulsion, but closer inspection reveals a sophisticated, self-assembled molecular machine. Now, in a recent issue of Science, Yonekura et al.1 present a model, based on their electron cryomicroscopy work, that explains the perplexing aspects of filament assembly.Flagellum
structure |
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Flagellum
assembly Even though there is clear evidence that the flagellar filament can self assemble11, the cap of the flagellar filament serves to control its assembly12. The cap enables the filament to polymerize with high efficiency, so that every filament subunit that reaches the tip inserts into place. At the same time the bacterium continues to secrete other proteins through the filament that are presumed to simply pass by the cap and out into the extracellular medium. These include excess hook-filament junction subunits13, excess cap14 and a negative regulator of flagellin gene transcription15. This suggests that the cap acts as a gatekeeper that selectively retains filament subunits and may even play a role in helping them to fold into place. Role of cap in filament polymerization |
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The structure of the filament is a spiral arrangement of filament
monomers of A possible answer to this mystery has now come from the electron
cryomicroscopy of the cap–filament complex by Yonekura et
al.1
The cap structure looks like a pentagonal disc with five legs17.
Each leg is differentially attached to the filament subunits at the
filament tip because the cap is planar and the filament end is axial. Yonekura et
al.1
have found that beneath the cap plate a cavity was revealed that is large
enough to allow a newly arrived filament subunit to complete its tertiary
fold just prior to its final quaternary placement in the filament. The
three-dimensional density map of the cap-filament junction also showed the
five sides pertaining to each cap monomer at the junction. In addition,
gaps between the cap plate and the filament end were observed. One of the
five gaps is distinctly larger than the other four at 25 The energy required to move the cap could come from the binding energy of each newly incorporated filament subunit. This model proposed by Yonekura et al.1 (and beautifully illustrated at http://www.npn.jst.go.jp/yone.html) suggests a highly refined mechanism that satisfies all the requirements for filament self-assembly and represents a novel discovery in structural design. This mechanism explains how control of assembly can occur even if it is outside the cell far removed from cellular processes such as transcription and translation. |
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