Membrane protein carboxyl methylation increases with human erythrocyte age. Evidence for an increase in the number of methylatable sites.

The level of carboxyl methylation of membrane proteins has been measured in intact human erythrocyte populations of different ages separated by density gradient centrifugation. Age separation was confirmed by measurement of cytosolic pyruvate kinase specific activity in each fraction. When cells of different ages were incubated with L-[methyl-3H]methionine, the steady state level of 3H radioactivity covalently bound to membrane proteins is observed to be at least 3-fold higher in older erythrocytes. Because the specific radioactivity of the methyl group donor S-adenosyl-L-[methyl-3H]methionine was identical in all age fractions, this represents an increase in the extent of modification of membrane proteins by carboxyl methylation. Of the three major methylated erythrocyte membrane proteins, this increase in carboxyl methylation with age is 4 to 7-fold for bands 2.1 and 3, while the increase in band 4.1 is 3 to 4-fold. This increase in the steady state level of methylation with age cannot be explained by changes in either the intrinsic rate of methyl transfer or by changes in the rate constant of methyl turnover. We, therefore, propose that the age-dependent change in carboxyl methylation is due to an increase in the number of available acceptor sites as the erythrocyte ages in vivo. Since methylation of acidic residues on erythrocyte membrane proteins has been detected exclusively on D-aspartic acid residues (McFadden, P. N., and Clarke, S. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 2460-2464), these results are consistent with an accumulation of D-aspartic acid in membrane protein due to spontaneous racemization a the cell ages. The relationship of these observations to possible functions of erythrocyte membrane protein carboxyl methylation is discussed.