Anti-immunoglobulin and Phorbol Ester Induce Phosphorylation

Protein phosphorylations are rapidly induced in intact B cells by antibodies to surface immunoglobulin (anti-IgM) and by phorbol 12-myristate 13-acetate (PMA). A comparison of the molecular weight, isoelectric points, phosphopeptides, and phosphoamino acids of the phosphoproteins induced by anti-IgM and by PMA suggests that anti-IgM acts through the activation of protein kinase C. This conclusion is strengthened by the observation that prolonged treatment with PMA ablates the ability of anti-IgM to induce phosphorylation, presumably by depleting cellular protein kinase C. Furthermore, the effects of dibutyryl cyclic AMP on protein phosphorylation are quite distinct from the effects of anti-IgM. The six most prominent phosphoproteins induced by PMA, with approximate M, values of 47, 55, 62, 68, 68, and 65-70 X lo3, are associated with the plasma membrane. Of these, four are apparently associated with the cytoskeleton, suggesting that the phosphorylation of cytoskeletal proteins may be important events early in B cell activation. Examination of protein phosphorylation in cell lines derived from different issues has identified one major B cell phosphoprotein (Mr 65-70 X los), which is absent in T cells, and two phosphoproteins (Mr 55 and 68 X lo3), which are observed in cells of hematopoietic origin but which are absent or uncommon in other cell types.