Mutations in the C-terminus of the conserved NDR kinase, Cbk1p of Saccharomyces cerevisiae, make the protein independent of upstream activators

In Saccharomyces cerevisiae, the RAM network is involved in cell separation after cytokinesis, cell integrity and cell polarity. The key function of this network is the regulation of the activity of the protein kinase Cbk1p, which is a member of the conserved NDR kinase family. Cbk1p function is controlled by its sub-cellular localization and at least two phosphorylation events: an auto phosphorylation in the kinase domain (S570) and the phosphorylation of a C-terminal hydrophobic motif by an upstream kinase (T743). After a UV mutagenesis, we have isolated 115 independent extragenic suppressors of four ∆ram mutations: ∆tao3, ∆hym1, ∆kic1 and ∆sog2. Over 50% of the suppressors affect a single residue in Cbk1p (S745F), which is close to the phosphorylation site in the hydrophobic motif. Our results show that the CBK1-S745F allele leads to a constitutively active form of Cbk1p that is independent of the upstream RAM network. We hypothesize that the mutant Cbk1-S745Fp mimics the effect of the phosphorylation of T743.