HUMAN TOPOISOMERASE-I IS PHOSPHORYLATED IN-VITRO ON ITS AMINO-TERMINAL DOMAIN BY PROTEIN-KINASE NII

Topoisomerase I purified from HeLa cells was phosphorylated in vitro with protein kinase NII (pkNII) purified from calf thymus: this phosphorylation was inhibited by heparin. A peptide containing a sequence corresponding to a putative pkNII phosphorylation site in topoisomerase I was synthesized and phosphorylated with pkNII. HPLC and two-dimensional analysis show identity between the synthetic phosphorylated peptide and one topoisomerase phosphopeptide indicating Ser(10) as one of the in vitro pkNII phosphorylation sites in topoisomerase I.