Recombinant human casein kinase II: A study with the complete set of subunits (α, α' and β), site-directed autophosphorylation mutants and a bicistronically expressed holoenzyme

Human casein kinase II (CKII) is a ubiquitous and multipotential Ser/Thr kinase involved in the regulation of cell growth and differentiation. Biochemically, two characteristics are particularly notable; first, the tetrameric composition of two catalytic subunits (alpha and/or alpha') and two regulatory subunits (beta); second, the autophosphorylation of the holoenzyme at the N-terminus of CKII beta, suspected to be involved in tuning of the kinase activity. Whether CKII alpha and CKII alpha' reconstitute comparably with CKII beta to form holoenzyme is unclear. For a systematic investigation, the complete set of recombinant CKII subunits and of autophosphorylation mutants of CKII beta were expressed in Escherichia coli and comparative reconstitutions carried out. At 1:1 molar ratio, CKII beta stimulated both catalytic subunits roughly fivefold with phosvitin as a substrate. The level of activity reached with both of the reconstituted CKII isoforms was of the same order of magnitude as that of holoenzyme isolated from human placenta. It was also similar to a recombinant alpha 2 beta 2 holoenzyme whose expression had been attained in E. coli with a bicistronic construct containing the coding regions of CKII beta and CKII alpha in a tandem arrangement. Both Ser2 and Ser3 were identified as the autophosphorylation sites; replacement of one of these with Ala by oligonucleotide-mediated site-directed mutagenesis influenced only the extent of CKII beta autophosphorylation, replacement of both resulted in a loss of autophosphorylation. Despite these differences, the stimulatory effect of all the CKII beta mutants was comparable both to each other and to that of wild-type CKII beta. This was also obtained when substrates other than phosvitin were employed such as tubulin, or upstream-binding factor (UBF). However, the degree of stimulation was substrate specific and ranged from 2-5-fold with no major differences between CKII alpha and CKII alpha' stimulation. Calmodulin phosphorylation by both CKII alpha and CKII alpha' was decreased similarly by CKII beta and the CKII beta mutants. Proteins such as cAMP-responsive-element-binding protein (CREB), HPV16 E7 or Jun were not phosphorylated by either catalytic subunit but became substrates of both in the presence of CKII beta or CKII beta mutants. The data suggest that CKII alpha and CKII alpha' form similar CKII holoenzymes and that the tuning of holoenzyme activity is independent of the autophosphorylation status of C