PHOSPHORYLATION OF ORNITHINE DECARBOXYLASE AT BOTH SERINE AND THREONINE RESIDUES IN THE ODC-OVERPRODUCING, ABELSON VIRUS-TRANSFORMED RAW264CELL-LINE

Expression of ornithine decarboxylase (ODC), the initial enzyme in pol yamine biosynthesis, is essential for cell growth. The Abelson virus-t ransformed, murine macrophage derived RAW264 cell line, overexpresses ODC activity and enzyme protein at a level 100-1000-fold greater than in normal cells. Expression of ODC was completely dependent on extrace llular stimulants and followed a temporally discrete pattern similar t o that in normal cells. ODC was present in RAW264 cells as two major a nd one minor isoelectric forms. Analysis of ODC isolated from [P-32]or thophosphate metabolically radiolabeled cells demonstrated that the ba sic isoelectric enzyme form was unphosphorylated, the two more acidic forms were phosphorylated, and both phosphoserine and phosphothreonine residues were present in the phosphorylated ODC. Therefore, in the RA W264 cell line, ODC is overexpressed and phosphorylated at multiple si tes on the enzyme molecule.