In vivo carboxyl methylation of human eruthrocyte membrane proteins

In order to study the in vivo methylation of ghost membrane proteins, human erythrocytes were incubated with L-[methyl-3H]methionine. The [3H]methyl incorporation into the membrane components was observed in freshly prepared erythrocytes. Upon treatment of the methylated membrane at pH 7.4, 100 degrees C for 5 min, a condition which is known to hydrolyze protein-carboxyl methyl ester, 80% of the incorporated [3H]methyl was recovered as methanol. Cycloleucine, an inhibitor for S-adenosylmethionine synthetase, inhibited 70% of the methylation. The analyses of the methylated proteins by sodium dodecyl sulfate/polyacrylamide gel electrophoresis revealed two major methylated protein peaks which were tentatively identified as glycophorin A and band 4.5. This methylation pattern is similar to the in vitro methylation pattern when purified human ghosts were methylated with purified protein methylase II and S-adenosylmethionine (Galletti, P., Paik, W. K., and Kim, S. (1979) Eur. J. Biochem. 97, 221-227). It is concluded that carboxyl methylation of erythrocyte membrane proteins is the major methylation reaction in vivo in these membranes.