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.
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
The structure of the filament is a spiral arrangement of filament monomers of 5.5 monomers added per turn while the cap is a planar pentamer of identical subunits22. How can the planar cap remain firmly attached to the tip of the elongating spiral filament and still allow for selective polymerization of filament subunits and passage of nonfilament proteins between itself and the growing tip?
A possible answer to this mystery has now come from the electron cryomicroscopy of the capľfilament 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 50 ┼. This would be predicted from the axial stagger of individual filament subunits packed in the filament and the site at which the authors propose new filament subunits would be added. The addition of a new filament would force the cap outward to the next most stable position. This is effectively a step up 4.7 ┼ and over 6.5 ┼. Thus, as a flagellin subunit is added in the lowest position (the hole), the cap `walks' along the top of the spiral filament staircase in a direction opposite to that of filament elongation, taking one `step' with the addition of each filament subunit (Fig. 2). This model is similar to the action of unscrewing a lid from a jar, although for the flagellar filament the lid (or cap) never comes off the jar (the growing filament). The cap would make a complete rotation with every 55 filament subunits added.
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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