Purification and characterization of maize seedling casein kinase IIB, a monomeric enzyme immunologically related to the α subunit of animal casein kinase‐2

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Abstract
Casein kinase IIB (CKIIB), a protein kinase related to animal casein kinase‐2 (CK2), has been purified to homogeneity. It appears to be a monomeric enzyme, composed by an individual 39 kDa subunit, homologous to the α/α′ subunits of animal CK2 and devoid of the autophosphorylatable 25‐kDa α subunit of animal CK2, which display an heterotetrameric α2β2/αα′β2 structure. Such a conclusion is supported by the following lines of evidence: (1) CKIIB displays an apparent 39000 Mr by gel filtration on Ultrogel AcA 34 and it gives rise to a single prominent protein band of similar Mr (38 000) upon SDS/PAGE; (2) upon incubation of the enzyme with [32P]ATP, no radiolabeled bands are detectable which might be attributable to either canonical or atypical β subunits; (3) the 39‐kDa band immunoreacts with antisera that recognize the α subunit of rat and chicken CK2; (4) conversely, no component immunologically related with the β subunit could be detected in CKIIB by Western‐blot analyses with antisera that recognize animal β subunits; (5) the recombinant β subunit of human CK2 is readily phosphorylated by CKIIB, the reaction being prevented, rather than stimulated, by polylysine, a behaviour typical of animal CK2 autophosphorylation.While the responsiveness of CKIIB to either heparin inhibition or polylysine stimulation are reminiscent of those of animal CK2, its peptide substrate specificity is significantly different and its thermolability is increased. Altogether these data would indicate that maize seedling CKIIB represents a naturally occurring monomeric form of CK2 devoid of non‐catalytic subunits. Its properties, compared to those of animal CK2, suggest that the β subunits of animal CK2 may be responsible for structural modifications conferring an altered specificity and an increased stability to the catalytic subunit.