Flagellar Locomotion in E. coli
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Flagellar Locomotion in E. coli

In an ever-changing environment, it is essential that organisms are able to sense these changes and to respond appropriately. Possible responses include alterations in gene expression and/or active movement towards or away from an environment. Most sensory pathways in eukaryotic organisms rely on serine, threonine or tyrosine protein kinases, whereas the most common sensory pathways in prokaryotes use a HAP (Histidine-Aspartate Phosphorelay) system. HAP systems have at least two components-a dimeric HPK (Histidine Protein Kinase) and a RR (Response Regulator). HAP systems are also found in many lower eukaryotes. 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. Bacteria are able to respond to a changing environment, and one way to respond is to move. The transduction of sensory signals alters the concentration of small phosphorylated response regulators that bind to the rotary flagellar motor and cause switching (Ref.1). A motile E. coli (Escherichia 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 possesses peritrichous flagella (Ref.2 & 3).

The flagellar locomotion in E. coli is chiefly controlled by the soluble HPK chemotaxis protein, CheA and two RRs. CheA senses changes through transmembrane chemoreceptors, which induce the trans-autophosphorylation of dimeric CheA on a histidine residue. The two RRs that compete for this phosphoryl group includes CheY, a single-domain, motor-binding protein, which controls flagellar motor switching, and CheB, which controls the adaptation of the chemoreceptors. In E. coli and S. enterica (Salmonella enterica) serovar Typhimurium, two kinds of receptors monitor the chemical composition of the environment; chemotaxis-specific receptors (named MCPs (Methyl-Accepting Chemotaxis Proteins) and AeR (Aerotaxis Receptor)), and dual-function receptors involved in both chemotaxis and transport of the ligand. Extracellular stimuli are converted into a usable intracellular form via receptor signaling that is conveyed to the HPKs and RRs to initiate locomotory signal transduction. The signal from the receptors is received by the receptor-binding domain of CheA. Then, from the phosphotransfer domain of CheA, which contains the phosphorylation site His48, it is transmitted to the downstream proteins CheY and CheB. The chemotaxis-specific receptors are stable homodimers, connected via a linker protein, CheW, to the Histidine kinase, CheA, generating stable ternary complexes. Once CheA is autophosphorylated, it rapidly transfers the phosphate group to Asp57 of CheY. Consequently, the phosphorylated form of CheY is released from the receptor supramolecular complex. The phosphorylation level of CheY (by CheA) and its dephosphorylation (either spontaneously or, in enhanced manner, by CheZ) is determined by the rates of clockwise and counterclockwise rotation of the bacterial flagella (Ref.3 & 4). This activates the bacterial motor proteins and basal body proteins (FliN, FliG, FliM --->MS-Ring Protein, MotA, MotB --->Proximal Rod proteins, P-Ring, L-Ring --->Distal Rod proteins) in a sequential manner. The signal is then relayed to hook (FlgE--->HAP1, HAP3) and filament (Flagellin and HAP2) proteins for swimming (counterclockwise) and tumbling (clockwise) movements of the flagella. The interaction of CheY messenger protein with the flagellar-motor supramolecular complex increases the probability of shifting the direction of flagellar rotation from the default direction, counterclockwise, to clockwise. The consequence of clockwise rotation is an abrupt turning motion (tumbling), after which (when the default direction resumes) the cell swims in a new direction (Ref.5 & 6).

Adaptation, namely, restoration of the pre-stimulus behavior in the presence of the stimulus, is an essential component of every behavioral system, locomotion included. Adaptation in bacterial locomotion is controlled by a feedback mechanism that modulates the methylation level of the MCP receptors. Two enzymes, CheB and CheR, are involved in this mechanism by interacting with the receptor supramolecular complexes and chemically modifying them. CheR is a methyltransferase, which catalyzes SAM (S-adenosylmethionine)-dependent methylation of specific Glutamate residues (four to six methylatable residues for each MCP) on the cytoplasmic portion of the receptors during adaptation to positive stimuli (Ref.7). The outcome is an enhancement of CheA autophosphorylation and, thereby, transmission of clockwise signals. CheB is a methylesterase that demethylates the receptors during adaptation to negative stimuli. It also has an amidase activity that catalyzes the conversion of specific Glutamine residues of the MCP receptors into Glutamate residues. The interaction of CheB with the receptor results in hydrolysis of the methyl ester bond on the side chain of the Glutamate residue, and the receptor undergoes demethylation. The outcome of this demethylation is inhibition of CheA autophosphorylation and, thereby, transmission of a counterclockwise signal. Thus the signals generated by the receptor supramolecular complex are transmitted to the flagellar-motor supramolecular complex and this simple pathway provides a paradigm for sensory systems in general. However, the increasing number of sequenced bacterial genomes shows that although the central sensory mechanism seems to be common to all bacteria, there is added complexity in a wide range of species. The correct interplay between locomotory system and other sensing systems are essential for numerous sensory signals to result in a balanced behavioural response (Ref.5).