S-Adenosyl-L-Methionine-Dependent Methylation of Chloroplast Proteins

Abstract

It is well-known that the functional activity of proteins can be altered by post-translational modification, perhaps most notably by reversible phosphorylation (1,2). One such modification is the incorporation of methyl groups into proteins from S-adenosyl-L-methionine (SAM) resulting in carboxyl methylester formation (for reviews, see 3–6). Methyltransferase activity appears to be an almost ubiquitous phenomenon in both procaryotes and eucaryotes. At least two classes of methyltransferase exist which catalyse the methylation of distinct amino acids and which have different physiological roles. Class I enzymes catalyse the methylation of carboxylic acid residues of normal proteins and have thus far only been detected in bacteria where they catalyse the formation of L-glutamyl 7-methylesters on specific membrane chemoreceptors involved in Chemotaxis. In these cases the differential action of methylated and demethylated chemoreceptors controls the processing of sensory input. Demethylation is catalysed by L-glutamyl γ-methylesterases; such a protein has been isolated in pure form from Salmonella typhimurium (7) and from Bacillus subtilis (8).