Protein kinase CK2 (also known as casein kinase II) is a ubiquitous eukaryotic ser/thr protein kinase present in the nucleus and cytoplasm. CK2 is known to phosphorylate more than 100 substrates, many of which are involved in the control of cell division and in signal transduction. The review centers on the structure and function of CK2 alpha and beta subunits and on the regulation of its activity, a topic that remains to be elucidated. An analogy is drawn between CK2 and the cyclin-dependent kinases (cdks); both types of protein kinases share many substrates and are activated by regulatory subunits.

Protein kinase casein kinase 1 (CK1) phosphorylates Ser-45 of ␤-catenin, ''priming'' the subsequent phosphorylation by glycogen synthase-3 of residues 41, 37, and 33. This concerted phosphorylation of ␤-catenin signals its degradation and prevents its function in triggering cell division. The sequence around Ser-45 does not conform to the canonical consensus for CK1 substrates, which prescribes either phosphoamino acids or acidic residues in position n؊3 from the target serine. However, the ␤-catenin sequence downstream from Ser-45 is very similar to a sequence recognized by CK1 in nuclear factor for activated T cells 4. The common features include an SLS motif followed two to five residues downstream by a cluster of acidic residues. Synthetic peptides reproducing residues 38 -65 of ␤-catenin were assayed with purified rat liver CK1 or recombinant CK1␣ and CK1␣L from zebrafish. The results demonstrate that SLS and acidic cluster motifs are crucial for CK1 recognition. Pro-44 and Pro-52 are also important for efficient phosphorylation. Similar results were obtained with the different isoforms of CK1. Phosphorylation of mutants of full-length recombinant ␤-catenin from zebrafish confirmed the importance of the SLS and acidic cluster motifs. A search for proteins with similar motifs yielded, among other proteins, adenomatous polyposis coli, previously found to be phosphorylated by CK1. There is a strong correlation of ␤-catenin mutations found in thyroid tumors with the motifs recognized by CK1 in this protein.consensus sequence ͉ nuclear factor for activated T cells 4 ͉ adenomatous polyposis coli ͉ Wnt signaling ͉ thyroid tumor mutations

A variety of synthetic peptides derived from either the inhibitor-2 (I-2) phosphoacceptor sites or the optimal sequences selected in an oriented peptide library have been compared for their susceptibility to phosphorylation by protein kinase CK1 (also termed casein kinase-1). The I-2-derived peptides are by far preferred over the library peptides by both rat liver CK1 (and by the alpha/beta, gamma and delta/epsilon isoforms immunoprecipitated from it) and recombinant Xenopus laevis CK1 alpha. The superiority of the I-2-derived peptides over the library ones is reflected by Vmax values one to two orders of magnitude higher while the Km values are comparable. Individual substitutions of any of the aspartic acids with alanine in the I-2-derived peptide RRKHAAIGDDDDAYSITA is detrimental, producing both a fall in Vmax and an increase in Km which are more pronounced at position n -3, but also quite significant at positions n -4, n -5 and, to a lesser extent, n -6. The unfavourable effect of these substitutions is more evident with rat liver CK1 than with recombinant Xenopus laevis CK1 alpha. The chimeric peptide IGDDDDAY-S-IIIFFA, resulting from the combination of the N-terminal acidic sequence of the I-2 (Ser86) site and the C-terminal hydrophobic cluster selected in the library peptides (MAEFDTG-S-IIIFFAKKK and MAYYDAA-S-IIIFFAKKK) is phosphorylated as efficiently as the I-2-derived peptide in terms of both Km and Vmax. These combined data strongly support the conclusion that, at variance with the optimal sequences selected in the library, optimal non-phosphate-directed phosphorylation of peptide substrates by CK1 critically relies on the presence of a cluster of acidic residues (preferably aspartic acid) upstream from position n -2, while the highly hydrophobic region downstream from serine selected in the library appears to be dispensable. The reason for these discrepancies remains unclear. The possibility that the library data are biased by the invariant elements forming its scaffold (MA-x-x-x-x-x-SI-x-x-x-x-AKKK) would be consistent with the observation that the library-selected peptides, despite their low Km values, fail to compete against the phosphorylation of protein and peptide substrates by CK1, suggesting that they bind to elements partially distinct from those responsible for substrate recognition.

Protein kinase CK1 (previously known as casein kinase I) conforms to a subgroup of the great protein kinase family found in eukaryotic organisms. The CK1 subgroup of vertebrates contains seven members known as alpha, beta, gamma1, gamma2, gamma3, delta, and epsilon. The CK1alpha gene can generate four variants (CK1alpha, CK1alphaS, CK1alphaL, and CK1alphaLS) through alternate splicing, characterized by the presence or absence of two additional coding sequences. Exon "L" encodes a 28-amino acid stretch that is inserted after lysine 152, in the center of the catalytic domain. The "S" insert encodes 12 amino acid residues and is located close to the carboxyl terminus of the protein. This work reports some biochemical and cellular properties of the four CK1alpha variants found to be expressed in zebrafish (Danio rerio). The results obtained indicate that the presence of the "L" insert affects several biochemical properties of CK1alpha: (a) it increases the apparent Km for ATP twofold, from approximately 30 to approximately 60 microM; (b) it decreases the sensitivity to the CKI-7 inhibitor, raising the I50 values from 113 to approximately 230 microM; (c) it greatly decreases the heat stability of the enzyme at 40 degrees C. In addition, the insertion of the "L" fragment exerts very important effects on some cellular properties of the enzyme. CK1alphaL concentrates in the cell nucleus, excluding nucleoli, while the CK1alpha variant is predominantly cytoplasmic, although some presence is observed in the nucleus. This finding supports the thesis that the basic-rich region found in the "L" insert acts as a nuclear localization signal. The "L" insert-containing variant was also found to be more rapidly degraded (half-life of 100 min) than the CK1alpha variant (half-life of 400 min) in transfected Cos-7 cells.