Purification and Characterization of Protein Kinase CK2 fromCandida albicans:Evidence for the Presence of Two Distinct Regulatory Subunits β and β′

Protein kinase CK2 ofCandida albicanshas been purified to near homogeneity by a procedure which involves chromatography on DEAE-cellulose, phosphocellulose, Q-Sepharose, and heparin–agarose. The purified enzyme has the characteristic properties of animal and yeast CK2, i.e., it utilizes ATP as well as GTP as phosphate donor, phosphorylates serine and threonine residues on casein, is inhibited by low concentrations of heparin, and is stimulated by NaCl and polycationic compounds such as polylysine, spermine, and spermidine. The native form of the enzyme exhibits a molecular mass of 159 kDa, and SDS–PAGE analysis indicates that it is composed of four polypeptides with relative molecular masses of 44, 39, 37, and 36 kDa. The 39- and 37-kDa polypeptides were identified as distinct catalytic subunits α and α′ on the basis ofin situphosphorylation assays and immunological recognition with heterologous antibodies. The purified kinase undergoes autophosphorylation on the 44- and 36-kDa polypeptides, a characteristic of the β subunits from other species. Antibodies raised against the β subunit ofDrosophila melanogasterand human CK2 crossreact only with the 36-kDa polypeptide. The 44-kDa polypeptide was identified as an unusually large β′ subunit by Western blotting with an antibody raised against the β′ subunit ofSaccharomyces cerevisiae.All these data suggest thatC. albicansCK2 has an αα′ββ′ heterotetrameric composition similar to that found inS. cerevisiae.