Mutation at the CK2 phosphorylation site on Cdc28 affects kinase activity and cell size in Saccharomyces cerevisiae

We have recently reported that protein kinase CK2 phosphorylates both in vivo and in vitro residue serine-46 of the cell cycle regulating protein Cdc28 of budding yeast Saccharomyces cerevisiae, confirming a previous observation that the same site is phosphorylated in Cdc2/Cdk1, the human homolog of Cdc28. In addition, S. cerevisiae in which serine-46 of Cdc28 has been mutated to alanine show a decrease of 33% in both cell volume and protein content, providing the genetic evidence that CK2 is involved in the regulation of budding yeast cell division cycle, and suggesting that this regulation may be brought about in G1 phase of the mammalian cell cycle. Here, we extended this observation reporting that the mutation of serine-46 of Cdc28 to glutamic acid doubles, at least in vitro, the H1-kinase activity of the Cdc28/cyclin A complex. Since this mutation has only little effects on the cell size of the cells, we hypothesize multiple roles of yeast CK2 in regulating the G1 transition in budding yeast.