Characterization of a 60,000-dalton oncofetal protein from the plasma of tumor-bearing rats

A tumor cell-associated protein, previously shown to be present in the circulation of carcinogen-treated and tumor-bearing animals and cancer patients, has now been identified in the cytosol of embryonic tissue. This oncofetal protein, which is absent from the plasma of normal animals, has been purified from the plasma of tumor-bearing rats by a series of steps including ammonium sulfate fractionation and chromatography on Sepharose CL-6B and on CM Affi-Gel Blue. The tumor and fetal-associated 60-kd rat factors appear to be identical based on their reactivity to polyclonal antibody produced against the tumor factor. The factor, assayed by its ability to induce the transport of RNA from isolated nuclei, is a phosphoprotein with a minimum molecular weight of 60,000, as determined by polyacrylamide gel electrophoresis. In its purified form it is phosphorylated in the presence of the catalytic subunit of heart muscle protein kinase and ATP but does not exhibit auto-phosphorylating activity. 32P-orthophosphate is also incorporated into the phosphoprotein in vivo.