Activated protein kinase C directly phosphorylates the CD34 antigen on hematopoietic cells

The CD34 antigen is a human leukocyte membrane protein expressed specifically by lymphohematopoietic progenitor cells. We found that CD34 is a phosphoprotein and therefore examined the regulation of its phosphorylation. Activation of protein kinase C (PKC) enhanced CD34 phosphorylation. The PKC activators, 12-O-tetradecanoylphorbol-13-acetate and bryostatin-1, induced rapid, stoichiometric hyperphosphorylation of CD34 protein in cells, resulting in a 5-fold increase in CD34 phosphorylation. In vitro kinase studies revealed that purified PKC could directly phosphorylate purified CD34. Only serine phosphorylation was detected in the CD34 molecule. Two-dimensional phosphopeptide mapping experiments indicated that PKC induces the phosphorylation of identical serine residue(s) in vitro and in vivo (in KG1 cells). These are newly phosphorylated serine residue(s), which are not detectably phosphorylated in CD34 from exponentially growing KG1 cells. These data indicate that the developmental stage-specific molecule, CD34, is a phosphorylation target for activated PKC. Furthermore, these findings raise the possibility that PKC activation and phosphorylation of the CD34 molecule may play a role in signal transduction during early lymphohematopoiesis.