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 I phosphopeptide indicating Ser10 as one of the in vitro pkNII phosphorylation sites in topoisomerase I.