12 Reversible methylation of glutamate residues in the receptor proteins of bacterial sensory systems

Abstract

The methyltransferase CheR catalyzes methyl group transfer from S-adenosyl-l-methionine to specific glutamic acid side chains of bacterial chemoreceptors, referred to as the methyl-accepting chemotaxis proteins (MCPs). A second enzyme, the methylesterase CheB, catalyzes ester hydrolysis. Together, CheR and CheB facilitate a reversible receptor methylation process that is essential for sensory adaptation. This property of adaptation has been most extensively studied in free-swimming Escherichia coli and Salmonella, where it serves as a rudimentary short-term memory during chemotaxis in gradients of attractants and repellents. The methylation-demethylation process allows the bacterium to compare and respond to changes in the current concentration relative to thoseof the past three to four seconds. The feedback loop in which CheR participates facilitates perfect or near-perfect adaptation over a large range of chemoeffector concentrations, generating the means by which the cell remains responsive to small changes in chemoeffector concentration. The structures of Salmonella CheR and the methyl-accepting domain of the serine receptor (Tsr) from E. coli, with biochemical data, paint a relatively detailed picture of receptor methylation, which proceeds by transmethylation in E. coli and Salmonella. Beyond E. coli and Salmonella, the diversity of receptor organization and the roles of methylation are still emerging. This diversity is likely to be large, given the wide range of ecological niches that prokaryotes occupy.