Dual modulation of phosphorylation of a 60 kDa nuclear protein in interferon-alpha treated Daudi cells

To explore the modulation of phosphorylation and/or kinase activity of cellular proteins in interferon-alpha (IFN-alpha) treated Daudi cells, experiments were performed to determine the effect of IFN-alpha on the ability of cellular proteins to undergo autophosphorylation reaction in-vitro. Treatment of cells with IFN-alpha significantly inhibited the amount of phosphorylation of a nuclear protein of about 60 kDa [+/- 3000 KDa, (P60)]. Results of phosphoamino acids analysis indicated that the nuclear P60 was a phosphotyrosine containing protein and was predominantly phosphorylated on threonine and serine in the absence of IFN-alpha. Treatment with IFN-alpha had a dual effect on relative phosphoamino acids content of P60: inhibited the phosphorylation on threonine residue, and enhanced the phosphorylation on serine and tyrosine residues. The nuclear P60 is not one of earlier described IFN-responsive signaling protein as it was not translocated from the cytosol.