Substrate heterogeneity of component a of the human erythrocyte membrane.

Component a of the erythrocyte membrane is a specific substrate for endogenous protein kinase activity and its phosphorylation is significantly decreased under assay conditions in myotonic muscular dystrophy (Roses, A.D., and Appel, S.H.J. Membr. Biol 20:51-58 (1975)). We have demonstrated substrate heterogeneity of two fractions of component a separated by concanavalin A (Con-A) sepharose chromatography. The fraction of component a that is retarded by Con A and eluted with alpha-methyl-D-glucoside does not accept the transfer of phosphate from [gamma-32 P] ATP as a substrate for endogenous protein kinase activity. The nonretarded fraction contains greater than 90% of the radioactive label. These experiments also confirm the carbohydrate heterogeneity of component a (Findley, J.B.C., J. Biol. Chem. 249:4398 (1974).