The dual function of the regulatory β-subunit of protein kinase CK2 is highlighted by its ability to abolish calmodulin phosphorylation in contrast to its stimulatory effect on the phosphorylation of peptide substrates. Here we show that a synthetic peptide reproducing the C-terminal region of the β-subunit (β[170–215]) stimulates to a similar extent the phosphorylation of either the peptide substrate or calmodulin and also protects the catalytic α-subunit against thermal inactivation as efficiently as full-length β-subunit. These data show that the positive and negative functions of the β-subunit reside in physically separated domains and that the elements responsible for positive regulation are located in the C-terminal region.
