Phosphorylation of the major leukocyte surface sialoglycoprotein, leukosialin, is increased by phorbol 12-myristate 13-acetate.

Leukosialin (CD43) is a heavily O-glycosylated membrane glycoprotein present on all leukocytes and on platelets. We found that leukosialin is phosphorylated in erythroid, myeloid, and T-lymphoid cell lines, as well as in platelets and peripheral blood lymphocytes. Leukosialin phosphorylation was increased 2.5-15-fold following phorbol ester treatment. The phosphorylation could be inhibited with the protein kinase C inhibitor staurosporine but not with HA 1004 that inhibits cAMP- or cGMP-dependent protein kinases. The phosphoamino acid analysis showed that serine residues were exclusively phosphorylated, either with or without phorbol ester treatment. Two-dimensional peptide maps of phosphorylated leukosialin from K562 and Jurkat cells gave almost identical patterns. The number of labeled peptides increased after treatment with phorbol ester, indicating that new sites were phosphorylated. The major phosphorylation site on leukosialin was identified as Ser-332 in a region of the cytoplasmic domain located 73 amino acids from the transmembrane portion.