Identification of two different molecular forms of Arabidopsis thaliana casein kinase II

Abstract

We have isolated and identified two different molecular forms of casein kinase II (CKIIA and CKIIB) from Arabidopsis thaliana. The use of heterologous antibodies that react with both catalytic and regulatory subunits in animal systems, revealed that CKIIB contains a single type of polypeptide with a molecular mass of 43 000 Da, whereas CKIIA is composed of two different polypeptides of 43 000 and 27 000 Da, respectively. Both forms show enzymatic characteristics typical of CKII activity, such as phosphorylation of the acidic peptide RRRDDDSDDD, use of GTP as phosphate donor, inhibition of their activity by low concentrations of heparin and stimulation by polylysine. K m values for β-casein are 0.068 mg/ml for CKIIA and 0.175 mg/ml for CKIIB. Calmodulin, a well-characterized substrate for casein kinases II, is significantly better phosphorylated by the CKIIB form than by the CKIIA form, suggesting and inhibitory effect of β-subunit on the enzymatic activity for this substrate. However, no autophosphorylation of the 27-kDa polypeptide has been detected, which can be explained by the lack of the conserved β-autophosphorylation site in Arabidopsis thaliana, as deduced from its cDNA sequence (Collinge and Walker, 1994, Plant Mol. Biol. 25, 649–658). In contrast, CKIIB form, devoid of regulatory subunit, co-purifies with a protein substrate of 15-kDa and efficiently phosphorylates it. A distinct pattern of mRNA accumulation for the catalytic and regulatory subunits suggests a different distribution of the molecular isoforms in the adult plant, that may reflect a functional specialization.