Protein kinase CK2 is a protein serine/threonine kinase that exhibits elevated expression in a number of cancers and displays oncogenic activity in mice. The regulatory CK2b subunit has a central role in assembly of functional tetrameric CK2 complexes where it participates in modulation of catalytic activity and substrate speci®city. Since overexpression of CK2b results in elevated levels of CK2 activity, we investigated the molecular mechanisms that control its degradation since perturbations in these pathways could contribute to elevated CK2 in cancer. In this study, we demonstrate that CK2b is degraded by a proteasome-dependent pathway and that it is ubiquitinated. We have also investigated the role of phosphorylation and a putative destruction box in regulating its stability in cells. Importantly, replacement of three serine residues within the autophosphorylation site of CK2b with glutamic acid residues resulted in a signi®cant decrease in its degradation indicating that autophosphorylation is involved in regulating its stability. Notably, although the autophosphorylation site of CK2b is remarkably conserved between species, this is the ®rst functional role ascribed to this site. Furthermore, based on these results, we speculate that alterations in the phosphorylation or dephosphorylation of the regulatory CK2b subunit could underlie the elevated expression of CK2 that is observed in cancer cells.

The convergence of caspase and protein kinase signaling pathways has become increasingly evident, as illustrated by the protection of caspase substrates from cleavage upon undergoing phosphorylation at or near to their caspase recognition motifs. To investigate the global role of phosphorylation in the regulation of caspase signaling, we designed a peptide match program to identify sequences from the human proteome that contained overlapping recognition motifs for caspases and kinases. We identified the protein kinase CK2 as the most prominent kinase with a consensus site for phosphorylation that overlapped with caspase recognition motifs. We then evaluated potential targets of CK2 and caspases by combining peptide array target screens with identification of caspase substrates. We identified numerous shared candidate targets of CK2 and caspases, including procaspase-3, which functions at a level at which both intrinsic and extrinsic apoptotic signals converge. Together, these data support a role for CK2-dependent phosphorylation as a global mechanism for inhibiting caspase signaling pathways.

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Protein kinase CK2 (formerly casein kinase II) exhibits elevated expression in a variety of cancers, induces lymphocyte transformation in transgenic mice, and collaborates with Ha-Ras in fibroblast transformation. To systematically examine the cellular functions of CK2, human osteosarcoma U2-OS cells constitutively expressing a tetracycline-regulated transactivator were stably transfected with a bidirectional plasmid encoding either catalytic isoform of CK2 (i.e. CK2␣ or CK2␣) together with the regulatory CK2␤ subunit in order to increase the cellular levels of either CK2 isoform. To interfere with either CK2 isoform, cells were also transfected with kinase-inactive CK2␣ or CK2␣ (i.e. GK2␣ (K68M) or CK2␣(K69M)) together with CK2␤. In these cells, removal of tetracycline from the growth medium stimulated coordinate expression of catalytic and regulatory CK2 subunits. Increased expression of active forms of CK2␣ or CK2␣ resulted in modest decreases in cell proliferation, suggesting that optimal levels of CK2 are required for optimal proliferation. By comparison, the effects of induced expression of kinase-inactive CK2␣ differed significantly from the effects of induced expression of kinase-inactive CK2␣. Of particular interest is the dramatic attenuation of proliferation that is observed following induction of CK2␣(K69M), but not following induction of CK2␣(K68M). These results provide evidence for functional specialization of CK2 isoforms in mammalian cells. Moreover, cell lines exhibiting regulatable expression of CK2 will facilitate efforts to systematically elucidate its cellular functions. Protein kinase CK21 (formerly casein kinase II) is a ubiquitously distributed and highly conserved protein serine/threonine kinase that is essential for viability in eukaryotes (1, 2). Although its precise functions remain poorly understood, there is mounting evidence to suggest that CK2 plays an important role in the control of cell proliferation and transformation (3-6). Alterations in the expression of CK2 have been observed in a variety of tumor or leukemic cells (7-13), and in the lymphocytes of cattle that develop T cell lymphomas following infection with the parasite Theileria parva (14). Furthermore, the targeted overexpression of CK2␣ in the T cells of transgenic mice results in the development of lymphomas (15). By crossing different lines of transgenic mice, there is evidence for collaboration between the dysregulated expression of CK2␣ and the c-Myc and Tal-1 oncogenes in lymphoma development (16). Accelerated lymphomagenesis is also observed when the mice that overexpress CK2 in T cells are crossed with mice deficient in the functional expression of p53 (17). There is also evidence that CK2 contributes to the transformation of primary and established fibroblasts. Overexpression of either catalytic isoform of CK2 (i.e. CK2␣ or CK2␣Ј) exhibited cooperativity with Ha-Ras in the transformation of rat embryo fibroblasts and Balb/c 3T3 cells (18). All of these observations suggest that the increased expression of CK2...