Abstract

The IPLJ gene is required for high-fidelity chromosome segregation in the budding yeast Saccharomyces cerevisiae. Conditional ipll ' mutants missegregate chromosomes severely at 37TC. Here, we report that IPLJ encodes an essential putative protein kinase whose function is required during the later part of each cell cycle. At 260C, the permissive growth temperature, ipil mutant cells are defective in the recovery from a transient G2/M-phase arrest caused by the antimicrotubule drug nocodazole. In an effort to identify additional gene products that participate with the Ipll protein kinase in regulating chromosome segregation in yeast, a truncated version of the previously identified DIS2S1/GLC7 gene was isolated as a dosage-dependent suppressor of ipllts mutations. DIS2S1/GLC7 is predicted to encode a catalytic subunit (PP1c) of type 1 protein phosphatase. Overexpression of the full-length DIS2S1/GLC7 gene results in chromosome missegregation in wild-type cells and exacerbates the mutant phenotype in ipl] cells. In addition, the gkc7-1 mutation can partially suppress the ipll-1 mutation. These results suggest that type 1 protein phosphatase acts in opposition to the Ipll protein kinase in vivo to ensure the high fidelity of chromosome segregation. As a eukaryotic cell progresses through the cell cycle and divides, a complete set of chromosomes is segregated faithfully to each of the two progeny cells. The fidelity of this chromo-