The SIT4 Protein Phosphatase Functions in Late G₃ for Progression into S Phase

Abstract

Saccharomyces cerevisiae strains containing temperature-sensitive mutations in the SIT4 protein phosphatase arrest in late G₁ at the nonpermissive temperature. Order-of-function analysis shows that SIT4 is required in late G₁ for progression into S phase. While the levels of SIT4 do not change in the cell cycle, SIT4 associates with two high-molecular-weight phosphoproteins in a cell-cycle-dependent fashion. In addition, we have identified a polymorphic gene, SSD1, that in some versions can suppress the lethality due to a deletion of SIT4 and can also partially suppress the phenotypic defects due to a null mutation in BCY1. The SSD1 protein is implicated in G₁ control and has a region of similarity to the dis3 protein of Schizosaccharomyces pombe. We have also identified a gene, PPH2α, that in high copy number can partially suppress the growth defect of sit4 strains. The PPH2α gene encodes a predicted protein that is 80% identical to the catalytic domain of mammalian type 2A protein phosphatases but also has an acidic amino-terminal extension not present in other phosphatases.