S. cerevisiae cells are capable of responding to mating pheromone only prior to their exit from the G1 phase of the cell cycle. Ste5 scaffold protein is essential for pheromone response because it couples pheromone receptor stimulation to activation of the appropriate MAPK cascade. In naive cells, Ste5 resides primarily in the nucleus. Upon pheromone treatment, Ste5 is rapidly exported from the nucleus and accumulates at the tip of the mating projection via its association with multiple plasma membrane-localized molecules. We found that concomitant with its nuclear export, the rate of Ste5 turnover is markedly reduced. Preventing nuclear export destabilized Ste5, whereas preventing nuclear entry stabilized Ste5, indicating that Ste5 degradation occurs mainly in the nucleus. This degradation is ubiquitin- and proteasome-dependent. We show that Ste5 ubiquitinylation is mediated by the SCF(Cdc4) ubiquitin ligase and requires phosphorylation by the G1 CDK. Inability to efficiently degrade Ste5 resulted in pathway activation and cell-cycle arrest in the absence of pheromone. These findings reveal that maintenance of this MAPK scaffold at an appropriately low level depends on its compartment-specific and cell cycle-dependent degradation. Overall, this mechanism provides a novel means for helping to prevent inadvertent stimulus-independent activation of a response and for restricting and maximizing the signaling competence of the cell to a specific cell cycle stage, which likely works hand-in-hand with the demonstrated role that G1 Cdk1-dependent phosphorylation of Ste5 has in preventing its association with the plasma membrane.
|Evidence ID||Analyze ID||Interactor||Interactor Systematic Name||Interactor||Interactor Systematic Name||Type||Assay||Annotation||Action||Modification||Phenotype||Source||Reference||Note|
|Evidence ID||Analyze ID||Gene||Gene Systematic Name||Gene Ontology Term||Gene Ontology Term ID||Qualifier||Aspect||Method||Evidence||Source||Assigned On||Annotation Extension||Reference|
|Evidence ID||Analyze ID||Gene||Gene Systematic Name||Phenotype||Experiment Type||Experiment Type Category||Mutant Information||Strain Background||Chemical||Details||Reference|
|Evidence ID||Analyze ID||Regulator||Regulator Systematic Name||Target||Target Systematic Name||Experiment||Assay||Construct||Conditions||Strain Background||Reference|