Surface structure recognized for covalent modification of the aspartate receptor in chemotaxis

The aspartate receptor involved in chemotaxis is modified by methyl esterification at four distinct glutamate residues during the adaptive response of this receptor. To explain the high degree of specificity of this modification, it has been proposed that the methyltransferase recognizes the sequence Glu-Glu-Xaa-Xaa-Ala-Ser/Thr in an alpha-helical conformation and methylates the second glutamate in this sequence. This hypothesis is strengthened here by localized mutagenesis studies. By reversing the alanine-threonine sequence to threonine-alanine at the principal site of methylation, Glu-309, a factor of 4 decrease in reactivity was achieved. Thus, the rate of methylation of this site is sensitive to the reversal of two residues of similar structure. These residues are somewhat distant in sequence from the glutamate that is modified but are adjacent in space if an alpha-helical structure is present. The other sites of modification, Glu-295, Glu-302, and Glu-491, are slightly increased in reactivity in the mutant. The 4-fold change in reactivity of the major site of methylation obtained with a relatively subtle change supports the recognition sequence hypothesis, including its structural implications. It is noted, in addition, that chemotaxis of bacteria expressing the mutant receptor does not seem to be greatly altered. This might be explained by the observation that the overall methylation levels of the mutant and wild-type receptors are similar.