Selective methyl esterification of erythrocyte membrane proteins by protein methylase II

Methyl esterification of erythrocyte membrane proteins have been demonstrated by incubating the isolated membrane with purified protein methylase II (S-adenosyl-methionine:protein-carboxyl O-methyltransferase, EC 2.1.1.24) and S-adenosyl-L-[methyl-14C]methionine. Methyl esterification of membrane-bound proteins occurred selectively to proteins corresponding to bands 3 (mol wt 97 000), 4 (mol wt 75 000), and 4.5 (mol wt 48 000) [designated according to Steck, T. L. (1974), J. Cell Biol. 62, 1] as identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Mild alkali treated depleted vesicles which lacked bands 1, 2, 5, and 6 had a higher methyl accepting capacity; 500 pmol of methyl groups/mg of depleted vesicle proteins vs. 200 pmol of methyl groups/mg of intact membrane proteins. Alkali-extractable membrane components were not methylated.