Overexpression of yeast PAM1 gene permits survival without protein phosphatase 2A and induces a filamentous phenotype.

Protein phosphatase 2A (PP2A) is an essential enzyme which is present in all eukaryotic cells. PP2A has been implicated in regulating various metabolic processes and also in the control of cell cycle progression. In the yeast Saccharomyces cerevisiae, the catalytic subunit of PP2A is encoded by two duplicated genes, PPH21 and PPH22. A third related gene, PPH3, also contributes some PP2A activity. We have used a yeast strain in which a single functional PP2A gene is expressed from a regulated promoter to screen for high copy number suppressors of PP2A depletion. A new gene was cloned, PAM1 (PP2A multicopy suppressor), which in high copy number can bypass the need for a PP2A catalytic subunit. The PAM1 gene encodes a hydrophilic 93-kDa protein that contains two coiled coil motifs and has a highly basic C-terminal tail. High level overexpression of PAM1 inhibits growth and induces a filamentous phenotype.