Recombinant interferon-gamma (IFN-γ) in contact with human embryonic fibroblasts or with a great variety of cells from different animal species was phosphorylated in the presence of [γ-P]ATP and magnesium ions by a protein kinase released in the culture medium. Using SDS-polyacrylamide gel electrophoresis, we found that both the monomeric (17000 to 18000) and dimeric (34000 to 35000) molecular weight forms of IFN-γ became intensely radioactive. Serine, but not threonine or tyrosine, was phosphorylated. It is of interest that the kinase released from reputedly insensitive cells also phosphorylated IFN-γ. The process did not noticeably degrade the antiviral functions of the molecule nor did it affect, at least in a detectable manner, its anti-proliferative effect on WISH or Daudi cells. Furthermore, the antigenic structure and its capacity to react with monoclonal antibodies were also unaltered. It is presently not known which biological function is regulated by the phosphorylating process.


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