Purification and characterization of a platelet-derived growth factor and heavy metal-modulated nuclear protein

Platelet-derived growth factor (PDGF), fibroblast growth factor, and heavy metal salts such as sodium arsenite stimulated BALB/c-3T3 cells to synthesize a 31-kDa protein(s) (termed p31) in a concentration-dependent manner. p31 was also synthesized in response to 12-O-tetradecanoylphorbol-13-acetate (TPA). V8 protease digestion of p31 purified from PDGF-, TPA-, and arsenite-treated cells showed identical fragmentation patterns, demonstrating that these agents modulate synthesis of the same (or a highly similar) protein. TPA-induced p31 synthesis was cell cycle-specific, occurring in density-arrested but not exponentially replicating cells. p31 was readily labeled with 35S]methionine but not with 35S]cysteine. Thus it is not a metallothionein. The protein associated with nuclei. It appears to be highly hydrophobic because solubilization required detergents or organic solvents. Reverse-phase high performance liquid chromatography (HPLC) provided further evidence of hydrophobicity. p31 has been purified to homogeneity using sodium dodecyl sulfate gels with electroelution and reverse-phase HPLC. It has an isoelectric point of 6.8. Its nuclear localization and amino acid analysis demonstrate that p31 is not heme oxygenase, a 32-kDa arsenite-induced microsomal protein. Stimulation of p31 synthesis by growth factors, PDGF and fibroblast growth factor; a tumor promoter, TPA; and heavy metal salts suggests that there is overlap in the pathways for mitogenic stimulation and heavy metal stress.