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Microscopic screening of a collection of cold-sensitive mutants of Saccharomyces cerevisiae led to the identification of a new gene, CDC55, which appears to be involved in the morphogenetic events of the cell cycle. CDC55 maps between CDC43 and CHC1 on the left arm of chromosome VII. At restrictive temperature, the original cdc55 mutant produces abnormally elongated buds and displays a delay or partial block of septation and/or cell separation. A cdc55 deletion mutant displays a cold-sensitive phenotype like that of the original isolate. Sequencing of CDC55 revealed that it encodes a protein of about 60 kDa, as confirmed by Western immunoblots using Cdc55p-specific antibodies. This protein has greater than 50% sequence identity to the B subunits of rabbit skeletal muscle type 2A protein phosphatase; the latter sequences were obtained by analysis of peptides derived from the purified protein, a polymerase chain reaction product, and cDNA clones. An extragenic suppressor of the cdc55 mutation lies in BEM2, a gene previously identified on the basis of an apparent role in bud emergence.

Association of the catalytic subunit (C2) with a variety of regulatory subunits is believed to modulate the activity and specificity of protein phosphatase 2A (PP2A). In this study we report the cloning and expression of a new family of B-subunit, the B, associated with the PP2A 0 form. Polymerase chain reactions and cDNA library screening have identified at least seven cDNA isotypes, designated ␣, ␤1, ␤2, ␤3, ␤4, ␥, and ␦. The different ␤ subtypes appear to be generated by alternative splicing. The deduced amino acid sequences of the ␣, ␤2, ␤3, ␤4 and ␥ isoforms predict molecular weights of 57,600, 56,500, 60,900, 52,500, and 68,000, respectively. The proteins are 60 -80% identical and differ mostly at their termini. Two of the isoforms, B␤3 and B␥, contain a bipartite nuclear localization signal in their COOH terminus. No homology was found with other B-or Brelated subunits. Northern analyses indicate a tissuespecific expression of the isoforms. Expression of B␣ protein in Escherichia coli generated a polypeptide of ϳ53 kDa, similar to the size of the B subunit present in the purified PP2A 0 . The recombinant protein was recognized by antibody raised against native B and interacted with the dimeric PP2A (A•C2) to generate a trimeric phosphatase. The deduced amino acid sequences of the B isoforms show significant homology to mammalian, fungal, and plant nucleotide sequences of unknown function present in the data bases. Notably, a high degree of homology (55-66%) was found with a yeast gene, RTS1, encoding a multicopy suppressor of a rox3 mutant. Our data indicate that at least seven B subunit isoforms may participate in the generation of a large number of PP2A 0 holoenzymes that may be spatially and/or functionally targeted to different cellular processes.Protein phosphatase 2A (PP2A) 1 is one of the major serine/ threonine protein phosphatases present in the cell and is involved in the control of many cellular functions and metabolic pathways (reviewed by Cohen (1989), Mumby and Walter (1993), DePaoli-Roach et al. (1994), and Mayer-Jaekel and Hemmings (1994)). The Ser/Thr protein phosphatases, with the exception of PP2C, consist of multimeric structures. Their catalytic subunit associates with specific proteins, which serve as targeting/regulatory subunits and play substantial roles in the control of phosphatase activity.PP2A is a family of holoenzymes containing a common core of a 36-kDa catalytic (C2) subunit and a 63-kDa A subunit associated with a variety of regulatory B-subunits (B, BЈ, and BЉ) to form the trimeric PP2A 1 , PP2A 0 , and polycation-stimulated protein phosphatase M, respectively (Tung et al., 1985;Waelkens et al., 1987;Mumby et al., 1987;Zolnierowicz et al., 1994). Takeda and co-workers (Usui et al., 1988) also isolated from human erythrocytes a PP2A form that contained a polypeptide of 74 kDa associated with the A⅐C2 core. Molecular cloning has identified in mammals two isoforms each of the C2 (da Cruz e Silva and Cohen, 1987;Green et al., 1987;Stone et al., 1987) and the A (Walter et al....

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