A specific catalytic subunit isoform of protein kinase CK2 is required for phosphorylation of the repressor Nrg1 in Saccharomyces cerevisiae

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Abstract
Protein kinase CK2 is highly conserved in eukaryotes and plays roles in many different cellular processes. CK2 is a tetramer comprising two catalytic and two regulatory subunits. Most organisms have two major isoforms of the catalytic subunit, and evidence suggests strongly overlapping function. In the yeast Saccharomyces cerevisiae, CK2 is essential for viability, and either catalytic subunit isoform, Cka1 or Cka2, suffices, but previous genetic evidence suggests that the isoforms have some distinct roles. In this work, we present evidence that the transcriptional repressor Nrg1, which regulates various stress-responsive genes, is a downstream target of CK2 containing the Cka1 isoform. We found that Nrg1 is phosphorylated in response to stress and that its phosphorylation was defective in cka1Δ, but not cka2Δ, mutants. Thus, the Cka1 catalytic subunit isoform is specifically required for phosphorylation of Nrg1 in vivo. The CK2 regulatory subunits were also required, indicating that the CK2 holoenzyme is involved. Both yeast and recombinant human CK2 phosphorylated recombinant Nrg1 in vitro. This identification of a protein whose phosphorylation requires a specific CK2 catalytic subunit isoform supports the view that the two isoforms exhibit functional specificity in vivo.