Spermine induced phosphorylation of a 42 kD/pI 5.9 nuclear protein in different human tumor cell lines

In vitro phosphorylation of 5 M urea extracts from nuclei obtained from different human tumor cell lines leads to incorporation of phosphate from 32P-gamma-ATP in more than 20 polypeptides with an acidic pI. Whereas heparin at a concentration of 1 microgram had no effect on the phosphorylation pattern, spermine stimulated the total phosphorylation up to twofold. Furthermore, in the presence of this polyamine, the two-dimensional polyacrylamide gel revealed an additional phosphoprotein with an apparent pI of 5.9 and a relative molecular mass of 42 000. Phosphoamino acid analysis of the most prominent phosphoproteins showed serine and threonine as phosphoacceptors.