Casein kinase II-like ectokinase activity on RBL-2H3 cells

We studied the properties of the ectokinase activity on the outer cell surfaces of RBL-2H3 cells and examined the phosphorylation of exogenous substrates to clarify the substrate specificity of the ectokinases on RBL-2H3 cells. Among the several protein substrates tested, casein was the most strongly phosphorylated with [gamma-P-32]ATP, and the net incorporation of P-32 into casein was 0.65 pmol P/50 mu g/10(6) cells. Casein kinase II peptide was also phosphorylated with [y-P-32]ATP. The phosphorylation of casein and casein kinase II peptide was almost completely inhibited by the addition of 3 mu g/ml of cell-impermeable K252b. Phosphorylation of casein and casein kinase II peptide was also observed by [gamma-P-32]GTP. Western blot analysis using anti-casein kinase II antibody revealed a 44-kDa casein kinase band in the membrane fraction and Fc epsilon RI complexes. The immunofluorescence microscopic analysis using anti-casein kinase II antibody showed the existence of casein kinase II on the surface of the cells. This is the first report about the existence of ectokinase on mast cells. (C) 1999 Elsevier Science B.V. All rights reserved.