CASEIN KINASE-II STIMULATES XENOPUS-LAEVIS DNA TOPOISOMERASE-I BY PHYSICAL ASSOCIATION

A Xenopus laevis casein kinase II-like activity copurified with X. lae vis DNA topoisomerase I activity during chromatography on DEAE-cellulo se, phosphocellulose, and hydroxylapatite, but the two activities were resolved by chromatography on DNA-agarose [Kaiserman, H. B., Ingebrit sen, T. S., and Benbow, R. M. (1988) Biochemistry 27, 3216-3222]. Phos phorylation of the catalytic polypeptides of dephosphorylated X. laevi s DNA topoisomerase I by the endogenous X. laevis casein kinase II-lik e activity apparently resulted in a severalfold increase in catalytic activity. In this study, we show that incubation of purified X. laevis DNA topoisomerase I with electrophoretically homogeneous bovine brain casein kinase II and ATP strongly stimulated catalytic activity. Surp risingly, purified bovine casein kinase II stimulated X. laevis DNA to poisomerase I activity by more than an order of magnitude in the absen ce of ATP, although ATP resulted in additional stimulation. Other basi c proteins, such as histone H1 and HMG proteins, also stimulated X. la evis DNA topoisomerase I catalytic activity 2-3-fold in the absence of ATP. Modulation of catalytic activity by direct physical association (protein-protein interactions) must, therefore, be considered in addit ion to phosphorylation in assessing the physiological role of casein k inase LT and other basic proteins during regulation of X. laevis DNA t opoisomerase I activity in vivo.