Flagellar Assembly of E. coli
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Flagellar Assembly of E. coli

E. coli (Escherichia coli) is one of the main species of bacteria that live in the lower intestines of warm-blooded animals, including birds and mammals. They are necessary for the proper digestion of food and are a part of the intestinal flora. Its presence in groundwater is a common indicator of fecal contamination. Technically, the “coliform group” is defined to be all the aerobic and facultative anaerobic, non-spore-forming, Gram-negative, rod-shaped bacteria that ferment lactose with the production of gas within 48 hours at room temperature (Ref.1). Structurally, a Gram-negative prokaryotic cell has the following architectural regions; appendages in the form of flagella and pili (or fimbriae); a cell envelope consisting of a capsule (a layer of polysaccharide) and/or outer membrane (composed of LPS (Lipopolysaccharide)); the rigid cell wall (peptidoglycan); the periplasmic space or periplasm; plasma or inner membrane (a lipid bilayer); and a cytoplasmic region that contains the cell genome (DNA), ribosomes and various sorts of cell inclusions. The pili (sing., pilus) or fimbriae (sing., fimbria) are hollow, hair-like structures made of protein that allows the bacteria to attach to other cells, especially during conjugation. Bacteria are supposed to be “primitive”, yet they are equipped with sophisticated devices to move them through their surroundings. Such appendages are the flagella (sing., flagellum). The main purpose of flagella is motility (Ref.2 & 3).

A motile E. coli propels itself from place to place by rotating its flagella. To move forward, the flagella rotate counterclockwise and the organism “swims”. But when flagellar rotation abruptly changes to clockwise, the bacterium “tumbles” in place and seems incapable of going anywhere. Then the bacterium begins swimming again in some new, random direction. Flagella may be variously distributed over the surface of bacterial cells in distinguishing patterns, but basically flagella are either polar (one or more flagella arising from one or both poles of the cell) or peritrichous (lateral flagella distributed over the entire cell surface). Flagellar distribution is a genetically-distinct trait that is occasionally used to characterize or distinguish bacteria. E. coli have peritrichous flagella. Flagella are long appendages which rotate by means of a “motor” located just under the cytoplasmic membrane (Ref.3 & 4). The spinning of the curved flagellum acts like a propellor, pushing the environmental gradients (or factors) like water, body fluids, etc, to the rear and thrusting itself along. Prokaryotic flagella are much thinner than eukaryotic flagella, and they lack the typical “9 + 2” arrangement of microtubules. Bacterial flagella are powered by proton motive force (chemiosmotic potential) established on the bacterial membrane, rather than ATP hydrolysis which powers eukaryotic flagella. About half of the bacilli and all of the spiral and curved bacteria are motile by means of flagella. Very few cocci are motile, which reflects their adaptation to dry environments and their lack of hydrodynamic design (Ref.5).

The ultrastructure of the E. coli flagellum consists of several distinct proteins: a system of rings embedded in the cell envelope (the basal body), a hook-like structure near the cell surface, and the flagellar filament. The innermost rings, the S, M and C rings, located in the plasma membrane, comprise the motor apparatus. The outermost rings, the P and L rings, located in the periplasm and the outer membrane respectively, function as bushings to support the rod where it is joined to the hook of the filament on the cell surface. The walls of Gram-positive bacteria lack the L and P rings. As the M-ring turns, powered by an influx of protons, the rotary motion is transferred to the filament which turns to propel the bacterium. The torque generating assemblies consist of the S, M and C rings. The bacterial flagellar motor is the most complex structure in a bacterial cell. It is the product of the controlled expression of about 50 genes. The E. coli flagellar motor is a supramolecular complex, located at the base of the flagellum, is built, like any other electrical motor, from a rotor, stator and switch. The rotor and switch comprises of the C-ring and are built from the flagellar motor switch proteins, FliN, FliG and FliM. The stator is built from the proteins MotA (Motility Protein-A) and MotB, which form a proton channel anchored to the cell wall by MotB. An inward proton flow through this channel generates the torque for rotation. Each flagellar-motor supramolecular complex contains approximately, 27 copies of FliF (MS-Ring proteins), 35 copies of FliM, 35 copies of FliG, 100 copies of FliN, and an unknown number of MotA and MotB molecules that form force-generating units. In the complex, the switch proteins interact with each other. FliG binds to MotA and thus appears to link the rotor and the stator (Ref.6 & 7). It also interacts with FliF and thereby links the switch to the central element of the motor. Motor action is also aided by proximal rod proteins like FliE, FlgB, FlgC and FlgF. The signal is then relayed to the outermost P-ring, L-ring and distal (FlgG) proteins. The proximal, distal and ring proteins constitute the basal body. Immediately outside the cell is a flexible flagellar hook protein, FlgE which functions as a universal joint. FlgE is joined to the flagellar filament, Flagellin through the association of hook-filament junction proteins like HAP1 (Flagellar Hook-Associated Protein-1) and HAP3. This allows the flagellum to adopt any orientation with respect to the cell wall (Ref.8).

Thus the assembly of the flagellum begins with components, such as the rotary motor, that are closest to the bacterial surface and ends with the filament, the most distal sub-structure. The filament is extremely long and slender and is composed of tens of thousands of Flagellin subunits that are synthesized in the cytoplasm and must be exported to the assembly site. The Flagellin subunits have to arrive at their destination without getting lost. After translocation across the bacterial inner and outer membranes with the help of a specialized export apparatus, the Flagellin monomers travel by diffusion down a channel (approximately 30 angstrom) inside the filament. The Flagellin monomers diffuse down the filament channel toward its far end, where they assemble at the filament tip under the guidance of a pentameric cap, known as Filament Cap Protein/Flagellar Cap Protein or HAP2 (Flagellar Hook-Associated Protein-2) (Ref.1 & 9). The cap chaperone not only prevents loss of Flagellin monomers out of the end of the channel, but also actively catalyzes their folding and insertion into the filament lattice. In an ever-changing environment, it is essential that organisms are able to sense these changes and to respond appropriately. Bacteria can sense a vast range of environmental signals, from the concentrations of nutrients and toxins to oxygen levels, pH, osmolarity and the intensity and wavelength of light. Prokaryotes are known to exhibit a variety of types of tactic behavior, i.e., the ability to move (swim) in response to environmental stimuli through chemotaxis, phototaxis, aerotaxis, magnetotaxis, etc. The occurrence of tactic behavior provides evidence for the ecological (survival) advantage of flagella in bacteria and other prokaryotes (Ref.9).