Biochemical reactions are coordinated spatially and temporally to regulate cell function, emphasizing the need for cell compartmentalization. The subdivision of the intracellular space has been evident in eukaryotic cells since the 1800s. By contrast, until relatively recently, bacteria were conceptualized as homogeneous ‘bags of enzymes’ where signalling cascades occur through the rapid anisotropic diffusion of effector molecules. In 1993, the laboratory of Lucy Shapiro provided first evidence of a localized signalling pathway in a bacterial cell.
In the process of chemotaxis, a network of signal-transduction proteins direct cells towards nutrients and away from repellents. In bacteria, transmembrane chemoreceptors detect these stimuli in the environment and transduce them through a phospho-signalling relay that ultimately transmits the signal to the flagellar motors. Maddock and Shapiro used immunoelectron microscopy and indirect immunofluorescence microscopy to reveal that these chemoreceptors cluster at the end (pole) of the Escherichia coli cell. They further found that the CheA kinase, which initiates the phospho-signalling cascade, and its coupling protein, CheW, co-localize with chemoreceptors, demonstrating that the entire sensory system is clustered. Subsequent work found that chemoreceptors assemble into a hexagonal lattice that interacts with CheA and CheW to form a supramolecular assembly called the chemosensory array. This organization results in cooperative interactions that allow chemoreceptors to drive large changes in CheA kinase activity in response to small changes in stimuli concentration. As a result, E. coli can rapidly adapt to changing environments. Since then, researchers have determined the functional relevance of chemoreceptor clustering, linked cell cycle control to their dynamic localization, and demonstrated conservation of these mechanisms across many bacterial species.
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