Catalyzes the methylation of several Htr transducer proteins (methyl-accepting chemotaxis proteins) to form gamma-glutamyl methyl ester residues. L-glutamyl-[protein] + S-adenosyl-L-methionine = [protein]-L-glutamate 5-O-methyl ester + S-adenosyl-L-homocysteine Mutants show lack of Htr methylation.