Bacterial chemoreceptors as membrane-spanning allosteric enzymes
Michael D. Manson
from: Sensory Mechanisms in Bacteria: Molecular Aspects of Signal Recognition (Edited by: Stephen Spiro and Ray Dixon). Caister Academic Press, U.K. (2010)
This review approaches bacterial chemoreceptors from the standpoint that they are allosteric enzymes. What is conventionally thought of as the receptor itself, the transmembrane homodimer, is the regulatory subunit. The allosteric ligand binds to an extracellular (or periplasmic, in the case of Gram-negative bacteria) domain of a homodimeric protein, usually at the interface of the two subunits. This binding triggers a transmembrane signal that is propagated to the distal cytoplasmic tip of the dimer to modulate the activity of the catalytic subunit, a histidine protein kinase (HPK) known as CheA. The ligand can be either a negative or positive regulator of kinase activity. A schematic view of the overall structure is shown in Figure 1. It is also becoming increasingly clear that higher-order allosteric interactions among larger groups of receptors are essential for achieving the high sensitivity and sophisticated signal integration characteristic of bacterial chemotaxis (Hazelbauer et al., 2008). The best-studied receptors are those of the enteric bacteria Escherichia coli and Salmonella enterica. For that reason, and because the author works with E. coli, this review will focus on those two species. However, important differences in chemoreceptor structure and function in other bacteria will be noted from time to time. A nice summary of the diversity - and the underlying unity - that exists across the bacterial domain can be found in Alexander and Zhulin (2007). Macnab (1996) and Stock and Surette (1996) provide comprehensive reviews of the older work on motility and chemotaxis in E. coli and S. enterica, and the roles of chemoreceptors in transmembrane signaling have also been reviewed in detail (Falke and Hazelbauer, 2001; Miller and Falke, 2004) read more ...