Mutations of the CK2 phosphorylation site of Sic1 affect cell size and S-Cdk kinase activity in Saccharomyces cerevisiae.

By sequence analysis we found an amino acid stretch centred on Serine 201 matching a stringent CK2 consensus site within the C-terminal, inhibitory domain of Sic1. Here we show by direct mass spectrometry analysis that Sic1, but not a mutant protein whose CK2 phospho-acceptor site has been mutated to alanine, Sic1 S201A , is actually phosphorylated in vitro by CK2 on Serine 201. Mutation of Serine 201 alters the coordination between growth and cell cycle progression. A significant increase of average protein content and of the average protein content at the onset of DNA synthesis is observed for exponentially growing cells harbouring the Sic1 S201A protein. A strong reduction of the same parameters is observed in cells harbouring Sic1 S201E . The deregulated coordination between cell size and cell cycle is also apparent at the level of SCdk activity.