Dosage suppressors of pds1 implicate ubiquitin-associated domains in checkpoint control

In budding yeast, anaphase initiation is controlled by ubiquitin-dependent degradation of Pds1p, Analysis of pds1 mutants implicated Pds1p in the DNA damage, spindle assembly, and S-phase checkpoints. Though some components o f these pathways are known, others remain to be identified. Moreover, the e ssential function of Pds1p, independent of its role in checkpoint control, has not been elucidated. To identify loci that genetically interact with PD S1, we screened for dosage suppressors of a temperature-sensitive pds1 alle le, pds1-128, defective for checkpoint control at the permissive temperatur e and essential for viability at 37 degreesC. Genetic and functional intera ctions of two suppressors are described, RAD23 and DDI1 suppress the temper ature and hydroxyurea, but not radiation or nocodazole, sensitivity of pds1 -128, rad23 and ddi1 mutants are partially defective in S-phase checkpoint control but are proficient in DNA damage and spindle assembly checkpoints. Therefore, Rad23p and Ddi1p participate in a subset of Pds1p-dependent cell cycle controls. Both Rad23p and Ddi1p contain ubiquitin-associated (UBA) d omains which are required for dosage suppression of pds1-128, UBA domains a re found in several proteins involved in ubiquitin-dependent proteolysis, t hough no function has been assigned to them. Deletion of the UBA domains of Rad23p and Ddi1p renders cells defective in S-phase checkpoint control, im plicating UBA domains in checkpoint signaling. Since Pds1p destruction, and thus checkpoint regulation of mitosis, depends on ubiquitin-dependent prot eolysis, we propose that the UBA domains functionally interact with the ubi quitin system to control Pds1p degradation in response to checkpoint activa tion.