Expression of wild-type and mutated forms of the catalytic (alpha) subunit of Caenorhabditis elegans casein kinase II in Escherichia coli

A full-length Caenorhabditis elegans cDNA that encodes the alpha subunit of casein kinase II was inserted into the inducible bacterial expression vector pET3a to generate the plasmid pCK alpha. Escherichia coli DE21 lysozyme S that was transformed with pCK alpha expressed soluble, catalytically active casein kinase II alpha upon induction with isopropyl beta-D-thiogalactopyranoside. The expressed alpha subunit was purified to homogeneity with a 60% yield by chromatography on CM-Sephadex, P-11 phosphocellulose, and heparin-agarose. The Mr values estimated from sodium dodecyl sulfate-polyacrylamide gel electrophoresis (Mr = 42,000) or calculated from hydrodynamic measurements (s20,w = 3.3 S, Stokes radius = 2.8 nm, Mr = 37,000) were similar, thereby indicating that the expressed enzyme is monomeric. The native holoenzyme and the expressed alpha subunit exhibited several similar properties including the utilization of both ATP and GTP as substrates and the susceptibility to inhibition of phosphotransferase activity by low concentrations of heparin. However, the kcat for E. coli-derived alpha was only 9% of the kcat for the native holoenzyme, and catalytic activity was not stimulated by polyamines. Recombinant casein kinase II alpha aggregates at low ionic strength, and the aggregation is partially reversible. A mutant alpha subunit in which Lys74 and Lys75 were substituted by glutamic acid residues was constructed by site-directed mutagenesis. The mutant enzyme was not inhibited by typically effective concentrations of heparin (e.g. IC50 = 0.3 micrograms/ml) because the affinity of modified recombinant casein kinase II Glu-74Glu-75 for heparin decreased approximately 70-fold. Thus, Lys74 and Lys75 are implicated in the heparin binding, inhibitory domain. The successful expression of casein kinase II alpha in E. coli will facilitate the analysis of the structural basis for functional domains in this enzyme.