Functional analysis of the cyclin-dependent kinase inhibitor Pho81 identifies a novel inhibitory domain

In response to phosphate limitation, Saccharomyces cerevisiae induces trans cription of a set of genes important for survival. A phosphate-responsive s ignal transduction pathway mediates this response by controlling the activi ty of the transcription factor Pho4. Three components of this signal transd uction pathway resemble those used to regulate the eukaryotic cell cycle: a cyclin-dependent kinase (CDK), Pho85; a cyclin, Pho80; and a CDK inhibitor (CKI), Pho81. Pho81 forms a stable complex with Pho80-Pho85 under both hig h- and low-phosphate conditions, but it only inhibits the kinase when cells are starved for phosphate. Pho81 contains six tandem repeats of the ankyri n consensus domain homologous to the INK4 family of mammalian CKIs. INK4 pr oteins inhibit kinase activity through an interaction of the ankyrin repeat s and the CDK subunits. Surprisingly, we find that a region of Pho81 contai ning 80 amino acids C terminal to the ankyrin repeats is necessary and suff icient for Pho81's CKI function. The ankyrin repeats of Pho81 appear to hav e no significant role in Pho81 inhibition. Our results suggest that Pho81 i nhibits Pho80-Pho85 with a novel motif.