A proteomics analysis of CK2ß(-/-) C2C12 cells provides novel insights into the biological functions of the non-catalytic ß subunit.

The acronym CK2 (derived from the misnomer 'casein kinase-2') denotes a pleiotropic acidophilic protein kinase implicated in a plethora of cellular functions, whose abnormally high expression correlates with malignancy. CK2 holoenzyme is composed of two catalytic (α and/or α') and two noncatalytic ß-subunits. The ß-subunits are not responsible for either activation or inactivation of the catalytic ones. Hence, to gain additional information about the roles of the individual CK2 subunits, we have generated C2C12 myoblasts entirely devoid either of both catalytic subunits, or of the ß-subunit. Here, we show that while CK2α/α'(-/-) cells grow similarly to wild-type cells, the growth of CK2ß(-/-) cells is severely impaired, consistent with the hypothesis that not all cellular functions of the ß-subunit are mediated by CK2 holoenzyme. To get a deeper insight into the functional implications of the ß-subunit, a quantitative proteomics study of CK2ß(-/-) cells was performed, leading to the identification and quantification of more than 1200 proteins. Of these, 187 showed a significantly altered expression (fold change ≥ 1.5 or ≤ -1.5) as compared to wild-type cells. A functional analysis of these proteins discloses the implication of CK2ß in many processes, for example, cell cycle, proliferation, transport, metabolic processes, etc., and in some of which the catalytic subunits of CK2 do not seem to play a relevant role. On the other hand, the pool of ecto-CK2 is not apparently affected by the lack of the ß-subunit. Collectively, our data corroborate the concept that the cellular functions of the ß-subunit of CK2 are partially independent of CK2 holoenzyme.

Re-evaluation of protein kinase CK2 pleiotropy: new insights provided by a phosphoproteomics analysis of CK2 knockout cells.

CK2 denotes a ubiquitous and pleiotropic protein kinase whose holoenzyme is composed of two catalytic (α and/or α') and two regulatory ß subunits. The CK2 consensus sequence, S/T-x-x-D/E/pS/pT is present in numerous phosphosites, but it is not clear how many of these are really generated by CK2. To gain information about this issue, advantage has been taken of C2C12 cells entirely deprived of both CK2 catalytic subunits by the CRISPR/Cas9 methodology. A comparative SILAC phosphoproteomics analysis reveals that, although about 30% of the quantified phosphosites do conform to the CK2 consensus, only one-third of these are substantially reduced in the CK2α/α'(-/-) cells, consistent with their generation by CK2. A parallel study with C2C12 cells deprived of the regulatory ß subunit discloses a role of this subunit in determining CK2 targeting. We also find that phosphosites notoriously generated by CK2 are not fully abrogated in CK2α/α'(-/-) cells, while some phosphosites unrelated to CK2 are significantly altered. Collectively taken our data allow to conclude that the phosphoproteome generated by CK2 is not as ample and rigidly pre-determined as it was believed before. They also show that the lack of CK2 promotes phosphoproteomics perturbations attributable to kinases other than CK2.

Exploring the CK2 Paradox: Restless, Dangerous, Dispensable.

The history of protein kinase CK2 is crowded with paradoxes and unanticipated findings. Named after a protein (casein) that is not among its physiological substrates, CK2 remained in search of its targets for more than two decades after its discovery in 1954, but it later came to be one of the most pleiotropic protein kinases. Being active in the absence of phosphorylation and/or specific stimuli, it looks unsuitable to participate in signaling cascades, but its "lateral" implication in a variety of signaling pathways is now soundly documented. At variance with many "onco-kinases", CK2 is constitutively active, and no oncogenic CK2 mutant is known; still high CK2 activity correlates to neoplasia. Its pleiotropy and essential role may cast doubts on the actual "druggability" of CK2; however, a CK2 inhibitor is now in Phase II clinical trials for the treatment of cancer, and cell clones viable in the absence of CK2 are providing information about the mechanism by which cancer becomes addicted to high CK2 levels. A phosphoproteomics analysis of these CK2 null cells suggests that CK2 pleiotropy may be less pronounced than expected and supports the idea that the phosphoproteome generated by this kinase is flexible and not rigidly pre-determined.