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 α-methyl-D-glucoside does not accept the transfer of phosphate from [γ-³²P] ATP as a substrate for endogenous protein kinase activity. The nonretarded fraction contains > 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)).