Identification and characterization of the nonphosphorylated precursor of pp17, a phosphoprotein associated with phorbol ester induction of growth arrest and monocytic differentiation in HL-60 promyelocytic leukemia cells.

Treatment of HL-60 promyelocytic leukemia cells with the tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), causes rapid phosphorylation and dephosphorylation of pp17, a 17-20-kDa, pI 5.5 cytosolic protein, as an early event in a response sequence leading to growth arrest and terminal differentiation into monocytes (Feuerstein, N., and Cooper, H. L., (1984) J. Biol. Chem. 259, 2782-2788). In the present study, we have identified the nonphosphorylated precursor to pp17 by tryptic peptide mapping of single proteins recovered from two-dimensional gels. The pI of the precursor, p17, was 5.9, and the apparent Mr of both p17 and pp17 was 18,400 by SDS-polyacrylamide gel electrophoresis (one dimension). p17 was shown to be a major cytosolic protein, comprising about 0.5% of steady state labeled protein in that fraction. Both p17 and pp17 were found exclusively in the cytosol (detergent-released SOL) and were not detected in membranes, cytoskeleton, or nuclei. In untreated cells, about 90% of the protein was present in the nonphosphorylated form. Upon TPA treatment, pre-existing p17 was rapidly phosphorylated to pp17. After 15 min, the two forms were nearly equal in quantity. This corresponds to phosphorylation, within that period, of about 0.2% of total cytosolic protein, represented by a single species. The maximum level of pp17 was reached within 1 h, with pp17 exceeding p17 by about 25%. Quantitatively, therefore, the phosphorylation of p17 to pp17 is one of the most prominent early biochemical responses to TPA treatment. Available data indicate that p17 predominates in rapidly proliferating cells, while phosphorylation to pp17 occurs where cell growth is modified by TPA or other agents. Thus, the p17/pp17 system is potentially a major mechanism for intracellular propagation of growth regulatory signals. We propose the name, prosolin, for this prominent cytosolic protein.