No abstract

A number of cancers are characterized by elevated expression of CK2 (formerly casein kinase II), which has been implicated as a key component in cell proliferation and transformation. Two lines of evidence, (a) deregulated expression of CK2 and (b) CK2␤ ubiquitination and degradation of these in a proteasomedependent manner prompted further investigation of the regulation of CK2␤ protein stability. We demonstrate that mutating six surface-exposed lysine residues to arginine (6KR) to interfere with ubiquitin attachment can stabilize CK2␤. Examination of 6KR expression in cells revealed increased stability over time and increased its steady-state expression level compared with CK2␤. In cells, 6KR was no longer sensitive to proteasome inhibition but maintained an elevated expression level. In our studies, 6KR functioned as a normal CK2 regulatory subunit, because it participated in CK2␤ dimerization, associated with catalytic subunits, was autophosphorylated, and formed active, stable CK2 tetramers. The physiological role of CK2␤ stabilization was investigated in cell proliferation assays, which showed a significant decrease in proliferation in cells expressing 6KR compared with CK2␤. Overall, our results indicate that a stabilized form of CK2␤ can be used to inhibit cell proliferation.Fundamental cellular processes such as proliferation and survival involve regulation by CK2 (formerly casein kinase II), 3 a serine/threonine protein kinase that is ubiquitously expressed in eukaryotic cells (1). Further evidence for its critical role is revealed in the absolute requirement for CK2 for viability in yeast and slime mold and in the requirements for CK2 in the G 1 /S and G 2 /M cell cycle transitions in yeast and mammalian cells (2-7).Given the diverse, yet essential, roles of CK2 within the cell, it is important to understand the mechanisms regulating CK2, which are equally as diverse and critical. Furthermore, perturbations in expression or activity of CK2 are associated with human disease. Abnormally high levels of CK2 have been observed in cancers of the breast, prostate, lung, head and neck, and kidney (8 -12). Overexpression of catalytic CK2 subunits led to increased proliferation and transformation. By comparison, overexpression of the regulatory CK2 subunit has been associated with decreased proliferation in yeast and mammalian cells, although this inhibitory role has not been universally observed (13-15). Collectively, these results indicate that CK2 has a profound effect on cell proliferation and suggests that individual CK2 subunits may exert competing effects.CK2 has typically been viewed as a tetrameric complex consisting of two catalytic subunits, CK2␣ and CK2␣Ј, and one regulatory subunit, CK2␤ (16 -20). Studies investigating CK2 tetramer assembly determined that a dimer of CK2␤ subunits forms the core of the enzyme, and the catalytic subunits subsequently bind to the CK2␤ core (21-26).CK2␤ is phosphorylated at serine 209 in a cell-cycle-dependent manner by p34 cdc2 in vitro and in mammalian cells (27)(28...