Phosphorylation of calmodulin by the catalytic subunit of casein kinase II is inhibited by the regulatory subunit.

Casein kinase II (CKII) is composed of a catalytic subunit (alpha) and a regulatory subunit (beta) that combine to form an alpha 2 beta 2 holoenzyme. The alpha-subunit monomer is enzymatically active, albeit kinetically attenuated relative to the holoenzyme, and the addition of purified beta subunit stimulates its activity against casein (C. Cochet and E. M. Chambaz, 1983, J. Biol. Chem. 258, 1403-1406). Here we report a kinetic analysis of the phosphorylation of various protein and peptide substrates by the alpha subunit and the holoenzyme of Drosophila melanogaster CKII. We demonstrate that the alpha subunit, like the holoenzyme, is competent to phosphorylate typical physiological substrates such as the regulatory (RII) subunit of cAMP-dependent protein kinase (cAMPdPK), as well as artificial substrates such as alpha-casein and the synthetic peptide RRREEETEEE. The Km of the alpha subunit in each case is similar to that of the holoenzyme, whereas the Vmax is 5- to 60-fold lower. In contrast, calmodulin, a protein that is significantly phosphorylated by the holoenzyme only in the presence of polybasic compounds, is readily phosphorylated by the alpha subunit alone. While the Km values of the alpha subunit and the holoenzyme for calmodulin are similar, the Vmax of the alpha subunit is at least 10-fold higher than that of the holoenzyme. These results suggest that while the alpha subunit contains the necessary determinants for CKII substrate specificity, the beta subunit can either inhibit or activate it, in a substrate-dependent manner. Finally, we also demonstrate that polybasic compounds stimulate not only the holoenzyme but, to a lesser extent, the alpha subunit as well.