Identification of Aspartic Acid As a Site of Methylation in Human Erythrocyte Membrane Proteins

Aspartic acid beta-[3H]methyl ester has been isolated from proteolytic digests of [3H]methylated human red blood cell membranes. The digestion product was identified by its co-elution with an ion exchange chromatography, gel filtration, and thin layer chromatography. The rate of hydrolysis of the methyl group of the isolated compound was determined at serveral pH values and was found to be identical with that of aspartic acid beta-methyl ester. This radioactive compound could be isolated from membranes prepared from broken cells incubated with S-adenosyl-L-[methyl-3H]methionine or from intact cells incubated with L-[methyl-3H]methionine. NO evidence was obtained for the presence of glutamic acid gamma-methyl ester in these digests. We suggest on the basis of these results that a major site of protein methylation in human red blood cell membrane proteins is at aspartyl residues.