Structural features underlying the unusual mode of calmodulin phosphorylation by protein kinase CK2: A study with synthetic calmodulin fragments

To shed light on the paradoxical behaviour of calmodulin, whose phosphoryla tion is inhibited by the regulatory beta-subunit of protein kinase CK2, a s eries of peptides encompassing the phosphoacceptor sites of calmodulin have been synthesized and assayed as substrates of CK2 alpha-subunit either alo ne or combined with the beta-subunit. The shortest peptide whose phosphoryl ation is reduced instead of being enhanced by the beta-subunit encompasses the sequence 68-106, including the central helix and the Ca2+-binding loop- III. In contrast, the phosphorylation of a peptide encompassing loop II and the central helix (54-92) is stimulated, like that of several shorter pept ides, by the beta-subunit. Our data localize to the C-terminal domain of ca lmodulin the structural elements that are responsible for inverted suscepti bility to beta-subunit regulation. (C) 1999 Academic Press.