13 DNA Topoisomerase Modification

Abstract

I. INTRODUCTION
The first reports of topoisomerase modification were published in 1982 and 1983 (Mills et al. 1982; Durban et al. 1983; Ferro et al. 1983; Jongstra-Bilen et al. 1983). Although a wide variety of posttranslational modifications of DNA topoisomerases may occur, this chapter focuses only on phosphorylation and poly(ADP-ribosylation), which have been observed both in vitro and in vivo. The experimental data suggest a regulatory role, but the precise cellular functions of these DNA topoisomerase modifications remain undefined at the present time.

A. Phosphorylation of DNA Topoisomerase at Serine Residues
The initial reports of phosphorylation in topoisomerases involved topoisomerase I purified from Novikoff hepatoma cells. A phosphoprotein of approximately 110 kD with a pI of 8.4 purified from cells labeled with ³²P (Durban et al. 1981) was subsequently identified as DNA topoisomerase I (Durban et al. 1983). A form of topoisomerase I could also be labeled in vitro by incubation with serine-specific protein kinases and [³²P]ATP. A protein kinase from an African Burkitt’s lymphoma cell line that appeared to exhibit high affinity for topoisomerase I was purified (Mills et al. 1982). This serine kinase had an apparent K_m of approximately 0.3 μM for topoisomerase I; phosphorylation stimulated the DNA-relaxing activity of the enzyme three- to fivefold. The discovery of phosphorylation of topoisomerase I was followed by reports of phosphorylation of topoisomerase II. Sander et al. (1984) found a protein kinase activity that remained associated with Drosophila topoisomerase II through four purification steps. Phosphoserine was the predominant species modified by...