MOLECULAR-GENETIC ANALYSIS OF RTS1P, A B' REGULATORY SUBUNIT OF SACCHAROMYCES-CEREVISIAE PROTEIN PHOSPHATASE 2A

The Saccharomyces cerevisiae gene RTS1 encodes a protein homologous to a variable B-type regulatory subunit of the mammalian heterotrimeric serine/threonine protein phosphatase 2A (PP2A). We present evidence sh elving that Rts1p assembles into similar heterotrimeric complexes in y east. Strains in which RTS1 has been disrupted are temperature sensiti ve (ts) for growth, are hypersensitive to ethanol, are unable to grow with glycerol as their only carbon source, and accumulate at nonpermis sive temperatures predominantly as large-budded cells with a 2N DNA co ntent and a nondivided nucleus. This cell cycle arrest can be overcome and partial suppression of the ts phenotype of rts1-null cells occurs if the gene CLB2, encoding a Cdc28 kinase-associated B-type cyclin, i s expressed on a high-copy-number plasmid. However, CLB2 overexpressio n has no suppressive effects on other aspects of the rts1-null phenoty pe. Expression of truncated forms of Rts1p can also partially suppress the ts phenotype and can fully suppress the inability of cells to gro w on glycerol and the hypersensitivity of cells to ethanol. By contras t, the truncated forms do not suppress the accumulation of large-budde d cells at high temperatures. Coexpression of truncated Rts1p and high levels of Clb2p fully suppresses the ts phenotype, indicating that th e inhibition of growth of rts1-null cells at high temperatures is due to both stress-related and cell cycle-related defects. Genetic analyse s show that the role played by Rts1p in PP2A regulation is distinctly different from that played by the other known variable B regulatory su bunit, Cdc55p, a protein recently implicated in checkpoint control reg ulation.