Multiple methylation of methyl-accepting chemotaxis proteins during adaptation of E. coli to chemical stimuli

In bacterial chemotaxis, adaptation is correlated with methylation or demethylation of methyl-accepting chemotaxis proteins (MCPs). Each protein migrates as a characteristic set of multiple bands in sodium dodecylsulfate polyacrylamide gel electrophoresis. The changes in MCP methylation that accompany adaptation are not the same for all bands of a set. Adaptation to a type II repellent stimulus results in an overall decrease in MCP II methylation, but also in an increase in the amount of radioactive methyl groups in the upper band of the set. We demonstrate that this increase is not due to new methylation, but rather to reduced electrophoretic mobility of previously methylated molecules that have lost some but not all of their methyl groups. We suggest that the pattern of multiple bands is a direct reflection of multiple sites for methylation on MCP molecules, and that the distribution of radiolabel among the bands is determined by the total extent of methylation. The patterns of methylated peptides produced by limited proteolysis of different MCP bands imply that methylation of the multiple sites on a molecule may occur in a specific order.