The phosphorylation of nuclear proteins in normal and transformed cells.

Normal (Rat 1) and transformed cells (middle T antigen-transformed derivative 3C3) were grown in the presence of 32P-orthophosphate. 32P-labelled nuclear proteins were fractionated by means of two-dimensional gel electrophoresis and detected by autoradiography. The comparative analysis of the autoradiographs of the normal and transformed cells revealed differences in the phosphorylation patterns of histone and low-molecular-mass high mobility group proteins (HMG). Three of the HMG proteins were highly phosphorylated in the transformed cells, and the analysis of their phosphorylation sites showed that these HMG proteins were phosphorylated on serine and threonine but not on tyrosine residues.