Current Biology
Volume 16, Issue 21, 7 November 2006, Pages R928–R930
Dispatch
Bacterial Flagellum: Visualizing the Complete Machine In Situ
David DeRosier
Department of Life Sciences, MS029 Brandeis University, 415 South Street, Waltham, Massachusetts 02454-9110, USA
Available online 6 November 2006
Abstract
Electron tomography of frozen-hydrated bacteria, combined with single particle averaging, has produced stunning images of the intact bacterial flagellum, revealing features of the rotor, stator and export apparatus.
Thanks to the new work of Murphy et al. [1], we now have a view of the bacterial flagellum in situ and quick-frozen in time as if a flash bulb had stopped its action. The flagellum, with its complexity of structure and multiplicity of function, is a machine that boggles the mind. While musing on possible phrases that might catch the reader's attention, I was reminded of the memorable 1926 slogan for the Hoover vacuum cleaner: “It beats as it sweeps as it cleans.” The flagellum self-assembles as it propels as it responds; that is, the flagellum not only pushes the cell along, it also responds to intracellular signals and it assembles itself. It seems as Barely Noticeable as the old Hoover did in its heyday. But, I thought, the bacterial flagellum does not really ‘beat’; the eukaryotic flagellum, an entirely different machine, does that. Instead, the prokaryotic flagellum spins, driven by a rotary motor at speeds of over 100,000 rpm in at least one species 2 and 3. The torque generated by the motor is converted to thrust by the corkscrew-shaped filament or propeller (for a review see [4]).
Of the 40 genes needed to code for a flagellum, at least 24 produce proteins found in the final structure. In Salmonella typhimurium, the flagellar mass is ∼109 Daltons, 99% of which is outside the plasma membrane. The necessary flagellar export apparatus is built into the very structure of the flagellum. The export apparatus recognizes, chaperones, unfolds and exports flagellar proteins, which travel along a narrow, 2 nm channel inside the flagellum. Some of the remaining genes encode for proteins that carry out the export, regulate flagellar gene expression, or function during assembly. Only 5 of the 24 structural proteins — FliG, FliM, FliN, MotA and MotB — are implicated in generating torque. The first three of these are cytoplasmic proteins thought to form the rotor, while the last two are transmembrane proteins that are thought to form the stator. In S. typhimurium, MotA and MotB conduct protons, the energy source for the motor. The mechanism of the motor remains unknown.
Structural studies have been carried out piecemeal on parts of the flagellum. We have atomic models for the entire filament [5], domains of the hook subunit [6], and domains of FliM, [7] FliG, [8] and FliN [9]. We have molecular resolution structures for the hook [10], the rotor [11], and the cap [12]. The composite structure shown in Figure 1 reveals the stunning complexity of the flagellum, but the extracted flagella used to determine this structure lacked the stator and, for all we know, parts of the export apparatus; there are hints of a large ‘export’ complex extending into the cytoplasm from the center of the rotor [13]. The stator has only been seen in freeze-fracture images [14]. What was missing but is now revealed to us is the three-dimensional structure of the intact flagellum in situ.
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