Phosphorylation of ornithine decarboxylase at both serine and threonine residues in the ODC-overproducing, Abelson virus-transformed RAW264 cell line.

Abstract

Expression of ornithine decarboxylase (ODC), the initial enzyme is polyamine biosynthesis, is essential for cell growth. The Abelson virus-transformed, 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 extracellular stimulants and followed a temporally discrete pattern similar to that in normal cells. ODC was present in RAW264 cells as two major and one minor isoelectric forms. Analysis of ODC isolated from [32P]orthophosphate metabolically radiolabeled cells demonstrated that the basic 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 RAW264 cell line, ODC is overexpressed and phosphorylated at multiple sites on the enzyme molecule.