Multiple forms of protein kinase CK2 present in leukemic cells: In vitro study of its origin by proteolysis

Human recombinant CK2 subunits were incubated for different times with the two main cytosolic proteases m-calpain and 20 S proteasome. Both, m-calpain in a calcium dependent manner and the 20 S proteasome, were able to degrade CK subunits in vitro. In both cases, CK2α' was more resistant to these proteases than CK2α. When these proteases were assayed on the reconstituted (α2β2 holoenzyme, a 37 kDa α-band, analogues to that observed in AML extracts, was generated which was resistant to further degradation. No degradation was observed when the 26 S proteasome was assayed on free subunits. Studies with CK2α deletion mutants showed that m-calpain and the 20 S proteasome acted on the C-terminus end of CK2α. These results pointed to cytosolic proteases as agents involved in the control of the amount of free CK2 subunits within the cell, which becomes evident when CK2 is overexpressed as in AML cells.