The EGP1 gene may be a positive regulator of protein phosphatase type 1 in the growth control of Saccharomyces cerevisiae

Glycogen synthase activity in Candida albicans is partly controlled by the functional ortholog of Saccharomyces cerevisiae Gac1p

To adapt to various host microenvironments, the human fungal pathogen Candida albicans possesses the capacity to accumulate and store glycogen as an internal carbohydrate source. In the model yeast Saccharomyces cerevisiae, ScGlc7p and ScGac1p are the serine/threonine type 1 protein phosphatase catalytic and regulatory subunits that control glycogen synthesis by altering the phosphorylation state of the glycogen synthase Gsy2p. Despite recent delineation of the glycogen synthesis pathway in C. albicans, the molecular events driving synthase activation are currently undefined. In this study, using a combination of microbiologic and genetic techniques, we determined that the protein encoded by uncharacterized gene C1_01140C, and not the currently annotated C. albicans Gac1p, is the major regulatory subunit involved in glycogen synthesis. C1_01140Cp contains a conserved GVNK motif observed across multiple starch/glycogen-binding proteins in various species, and alanine substitution of each residue in this motif significantly impaired glycogen accumulation in C. albicans. Fluorescent protein tagging and microscopy indicated that C1_01140Cp-GFPy colocalized with CaGlc7p-tdTomato and CaGsy1p-tdTomato accordingly. Co-immunoprecipitation assays further confirmed that C1_01140Cp associates with CaGlc7p and CaGsy1p during glycogen synthesis. Lastly, c1_01140cΔ/Δ exhibited colonization defects in a murine model of vulvovaginal candidiasis. Collectively, our data indicate that uncharacterized C1_01140Cp is the functional ortholog of the PPP1R subunit ScGac1p in C. albicans.IMPORTANCEThe capacity to synthesize glycogen offers microbes metabolic flexibility, including the fungal pathogen Candida albicans. In Saccharomyces cerevisiae, dephosphorylation of glycogen synthase by the ScGlc7p-containing phosphatase is a critical rate-limiting step in glycogen synthesis. Subunits, including ScGac1p, target ScGlc7p to α-1,4-glucosyl primers for efficient ScGsy2p synthase activation. However, this process in C. albicans had not been delineated. Here, we show that the C. albicans genome encodes for two homologous phosphatase-binding subunits, annotated CaGac1p and uncharacterized C1_01140Cp, both containing a GVNK motif required for polysaccharide affinity. Surprisingly, loss of CaGac1p only moderately reduced glycogen accumulation, whereas loss of C1_01140Cp ablated it. Fluorescence microscopy and co-immunoprecipitation approaches revealed that C1_01140Cp associates with CaGlc7p and CaGsy1p during glycogen synthesis. Moreover, C1_01140Cp contributed to fungal fitness at the vaginal mucosa during murine vaginitis. Therefore, this work demonstrates that glycogen synthase regulation is conserved in C. albicans and C1_01140Cp is the functional ortholog of ScGac1p.

The phosphatase Glc7 controls the eisosomal response to starvation via post-translational modification of Pil1

The yeast (Saccharomyces cerevisiae) plasma membrane (PM) is organised into specific subdomains that regulate surface membrane proteins. Surface transporters actively uptake nutrients in particular regions of the PM where they are also susceptible to substrate-induced endocytosis. However, transporters also diffuse into distinct subdomains termed eisosomes, where they are protected from endocytosis. Although most nutrient transporter populations are downregulated in the vacuole following glucose starvation, a small pool is retained in eisosomes to provide efficient recovery from starvation. We find the core eisosome subunit Pil1, a Bin, Amphiphysin and Rvs (BAR) domain protein required for eisosome biogenesis, is phosphorylated primarily by the kinase Pkh2. In response to acute glucose starvation, Pil1 is rapidly dephosphorylated. Enzyme localisation and activity screens suggest that the phosphatase Glc7 is the primary enzyme responsible for Pil1 dephosphorylation. Defects in Pil1 phosphorylation, achieved by depletion of GLC7 or expression of phospho-ablative or phospho-mimetic mutants, correlate with reduced retention of transporters in eisosomes and inefficient starvation recovery. We propose that precise post-translational control of Pil1 modulates nutrient transporter retention within eisosomes, depending on extracellular nutrient levels, to maximise recovery following starvation.

Plasmodium berghei leucine-rich repeat protein 1 downregulates protein phosphatase 1 activity and is required for efficient oocyst development

Protein phosphatase 1 (PP1) is a key enzyme for Plasmodium development. However, the detailed mechanisms underlying its regulation remain to be deciphered. Here, we report the functional characterization of the Plasmodium berghei leucine-rich repeat protein 1 (PbLRR1), an orthologue of SDS22, one of the most ancient and conserved PP1 interactors. Our study shows that PbLRR1 is expressed during intra-erythrocytic development of the parasite, and up to the zygote stage in mosquitoes. PbLRR1 can be found in complex with PbPP1 in both asexual and sexual stages and inhibits its phosphatase activity. Genetic analysis demonstrates that PbLRR1 depletion adversely affects the development of oocysts. PbLRR1 interactome analysis associated with phospho-proteomics studies identifies several novel putative PbLRR1/PbPP1 partners. Some of these partners have previously been characterized as essential for the parasite sexual development. Interestingly, and for the first time, Inhibitor 3 (I3), a well-known and direct interactant of Plasmodium PP1, was found to be drastically hypophosphorylated in PbLRR1-depleted parasites. These data, along with the detection of I3 with PP1 in the LRR1 interactome, strongly suggest that the phosphorylation status of PbI3 is under the control of the PP1-LRR1 complex and could contribute (in)directly to oocyst development. This study provides new insights into previously unrecognized PbPP1 fine regulation of Plasmodium oocyst development through its interaction with PbLRR1.

Protein phosphatase 1: life-course regulation by SDS22 and Inhibitor-3

Protein phosphatase 1 (PP1) is expressed in all eukaryotic cells and catalyzes a sizable fraction of protein Ser/Thr dephosphorylation events. It is tightly regulated in space and time through association with a wide array of regulatory interactors of protein phosphatase one (RIPPOs). Suppressor-of-Dis2-number 2 (SDS22) and Inhibitor-3 (I3), which form a ternary complex with PP1, are the first two evolved and most widely expressed RIPPOs. Their deletion causes mitotic-arrest phenotypes and is lethal in some organisms. The role of SDS22 and I3 in PP1 regulation has been a mystery for decades as they were independently identified as both activators and inhibitors of PP1. This conundrum has largely been solved by recent reports showing that SDS22 and I3 control multiple steps of the life course of PP1. Indeed, they contribute to (a) the stabilization and activation of newly translated PP1, (b) the translocation of PP1 to the nucleus, and (c) the storage of PP1 as a reserve for holoenzyme assembly. Preliminary evidence suggests that SDS22 and I3 may also function as scavengers of released or aged PP1 for re-use in holoenzyme assembly or proteolytical degradation, respectively. Hence, SDS22 and I3 are emerging as master regulators of the life course of PP1.

New ubiquitin-dependent mechanisms regulating the Aurora B-protein phosphatase 1 balance in Saccharomyces cerevisiae

Protein ubiquitylation regulates many cellular processes, including cell division. We report here a novel mutation altering the Saccharomyces cerevisiae E1 ubiquitin-activating enzyme (uba1-W928R) that suppresses the temperature sensitivity and chromosome loss phenotype of a well-characterized Aurora B mutant (ip1-2). The uba1-W928R mutation increases histone H3-S10 phosphorylation in the ipl1-2 strain, indicating that uba1-W928R acts by increasing Ipl1 activity and/or reducing the opposing protein phosphatase 1 (PP1; Glc7 in S. cerevisiae) phosphatase activity. Consistent with this hypothesis, Ipl1 protein levels and stability are elevated in the uba1-W928R mutant, likely mediated via the E2 enzymes Ubc4 and Cdc34. In contrast, the uba1-W928R mutation does not affect Glc7 stability, but exhibits synthetic lethality with several glc7 mutations. Moreover, uba1-W928R cells have an altered subcellular distribution of Glc7 and form nuclear Glc7 foci. These effects are likely mediated via the E2 enzymes Rad6 and Cdc34. Our new UBA1 allele reveals new roles for ubiquitylation in regulating the Ipl1-Glc7 balance in budding yeast. While ubiquitylation likely regulates Ipl1 protein stability via the canonical proteasomal degradation pathway, a non-canonical ubiquitin-dependent pathway maintains normal Glc7 localization and activity.This article has an associated First Person interview with the first author of the paper.

Assembly and quality control of protein phosphatase 1 holoenzyme involve Cdc48-Shp1 chaperone

Protein phosphatase 1 (PP1) controls many aspects of cell physiology, which depends on its correct targeting in the cell. Nuclear localization of Glc7, the catalytic subunit of PP1 in budding yeast, requires the AAA-ATPase Cdc48 and its adaptor Shp1 through an unknown mechanism. Herein, we show that mutations in SHP1 cause misfolding of Glc7 that co-aggregates with Hsp104 and Hsp42 chaperones and requires the proteasome for clearance. Mutation or depletion of the PP1 regulatory subunits Sds22 and Ypi1 that are involved in nuclear targeting of Glc7 also produce Glc7 aggregates, indicating that association with regulatory subunits stabilizes Glc7 conformation. Use of a substrate-trap Cdc48QQ mutant reveals that Glc7-Sds22-Ypi1 transiently associates with and is the major target of Cdc48-Shp1. Furthermore, Cdc48-Shp1 binds and prevents misfolding of PP1-like phosphatases Ppz2 and Ppq1, but not other types of phosphatases. Our data propose that Cdc48-Shp1 functions as a molecular chaperone for the structural integrity of PP1 complex in general and that it specifically promotes the assembly of Glc7-Sds22-Ypi1 for nuclear import.

Suppressors of ipl1-2 in components of a Glc7 phosphatase complex, Cdc48 AAA ATPase, TORC1, and the kinetochore

Ipl1/Aurora B is the catalytic subunit of a protein kinase complex required for chromosome segregation and nuclear division. Before anaphase, Ipl1 is required to establish proper kinetochore-microtubule associations and to regulate the spindle assembly checkpoint (SAC). The phosphatase Glc7/PP1 opposes Ipl1 for these activities. To investigate Ipl1 and Glc7 regulation in more detail, we isolated and characterized mutations in the yeast Saccharomyces cerevisiae that raise the restrictive temperature of the ipl-2 mutant. These suppressors include three intragenic, second-site revertants in IPL1; 17 mutations in Glc7 phosphatase components (GLC7, SDS22, YPI1); two mutations in SHP1, encoding a regulator of the AAA ATPase Cdc48; and a mutation in TCO89, encoding a subunit of the TOR Complex 1. Two revertants contain missense mutations in microtubule binding components of the kinetochore. rev76 contains the missense mutation duo1-S115F, which alters an essential component of the DAM1/DASH complex. The mutant is cold sensitive and arrests in G2/M due to activation of the SAC. rev8 contains the missense mutation ndc80-K204E. K204 of Ndc80 corresponds to K166 of human Ndc80 and the human Ndc80 K166E variant was previously shown to be defective for microtubule binding in vitro. In a wild-type IPL1 background, ndc80-K204E cells grow slowly and the SAC is activated. The slow growth and cell cycle delay of ndc80-K204E cells are partially alleviated by the ipl1-2 mutation. These data provide biological confirmation of a biochemically based model for the effect of phosphorylation on Ndc80 function.

The nonhistone, N-terminal tail of an essential, chimeric H2A variant regulates mitotic H3-S10 dephosphorylation

H2A.Y is an essential, divergent Tetrahymena thermophila histone variant. It has a long nonhistone N terminus that contains leucine-rich repeats (LRR) and an LRR cap domain with similarity to Sds22p, a regulator of yeast protein phosphatase 1 (PP1) activity in the nucleus. In growing cells, H2A.Y is incorporated into micronuclei only during S phase, which occurs immediately after micronuclear mitosis. Depletion of H2A.Y causes prolonged retention of mitosis-associated histone H3-S10 phosphorylation and mitotic abnormalities that mimic S10E mutation. In cells where H2A.Y is depleted, an inducible chimeric gene, in which the H2A.Y N terminus is attached to H2A.X, is shown to regulate micronuclear H3-S10 phosphorylation. H2A.Y can also be specifically coimmunoprecipitated with a Tetrahymena PP1 ortholog (Ppo1p). Taken together, these results argue that the N terminus of H2A.Y functions to regulate H3-S10 dephosphorylation. This striking in vivo case of "cross-talk" between a H2A variant and a specific post-translational modification of another histone demonstrates a novel function for a histone variant.

Regulation of glycogen metabolism in yeast and bacteria

Microorganisms have the capacity to utilize a variety of nutrients and adapt to continuously changing environmental conditions. Many microorganisms, including yeast and bacteria, accumulate carbon and energy reserves to cope with the starvation conditions temporarily present in the environment. Glycogen biosynthesis is a main strategy for such metabolic storage, and a variety of sensing and signaling mechanisms have evolved in evolutionarily distant species to ensure the production of this homopolysaccharide. At the most fundamental level, the processes of glycogen synthesis and degradation in yeast and bacteria share certain broad similarities. However, the regulation of these processes is sometimes quite distinct, indicating that they have evolved separately to respond optimally to the habitat conditions of each species. This review aims to highlight the mechanisms, both at the transcriptional and at the post-transcriptional level, that regulate glycogen metabolism in yeast and bacteria, focusing on selected areas where the greatest increase in knowledge has occurred during the last few years. In the yeast system, we focus particularly on the various signaling pathways that control the activity of the enzymes of glycogen storage. We also discuss our recent understanding of the important role played by the vacuole in glycogen metabolism. In the case of bacterial glycogen, special emphasis is placed on aspects related to the genetic regulation of glycogen metabolism and its connection with other biological processes.

Genetic interaction network of the Saccharomyces cerevisiae type 1 phosphatase Glc7

Background

Protein kinases and phosphatases regulate protein phosphorylation, a critical means of modulating protein function, stability and localization. The identification of functional networks for protein phosphatases has been slow due to their redundant nature and the lack of large-scale analyses. We hypothesized that a genome-scale analysis of genetic interactions using the Synthetic Genetic Array could reveal protein phosphatase functional networks. We apply this approach to the conserved type 1 protein phosphatase Glc7, which regulates numerous cellular processes in budding yeast.

Results

We created a novel glc7 catalytic mutant (glc7-E101Q). Phenotypic analysis indicates that this novel allele exhibits slow growth and defects in glucose metabolism but normal cell cycle progression and chromosome segregation. This suggests that glc7-E101Q is a hypomorphic glc7 mutant. Synthetic Genetic Array analysis of glc7-E101Q revealed a broad network of 245 synthetic sick/lethal interactions reflecting that many processes are required when Glc7 function is compromised such as histone modification, chromosome segregation and cytokinesis, nutrient sensing and DNA damage. In addition, mitochondrial activity and inheritance and lipid metabolism were identified as new processes involved in buffering Glc7 function. An interaction network among 95 genes genetically interacting with GLC7 was constructed by integration of genetic and physical interaction data. The obtained network has a modular architecture, and the interconnection among the modules reflects the cooperation of the processes buffering Glc7 function.

Conclusion

We found 245 genes required for the normal growth of the glc7-E101Q mutant. Functional grouping of these genes and analysis of their physical and genetic interaction patterns bring new information on Glc7-regulated processes.

Ypi1, a positive regulator of nuclear protein phosphatase type 1 activity in Saccharomyces cerevisiae

The catalytic subunit of protein phosphatase type 1 (PP1) has an essential role in mitosis, acting in opposition to the Ipl1/Aurora B protein kinase to ensure proper kinetochore-microtubule interactions. However, the regulatory subunit(s) that completes the PP1 holoenzyme that functions in this capacity is not known. We show here that the budding yeast Ypi1 protein is a nuclear protein that functions with PP1 (Glc7) in this mitotic role. Depletion of cellular Ypi1 induces mitotic arrest due to activation of the spindle checkpoint. Ypi1 depletion is accompanied by a reduction of nuclear PP1 and by loss of nuclear Sds22, a Glc7 binding partner that is found in a ternary complex with Ypi1 and Glc7. Expression of a Ypi1 variant that binds weakly to PP1 also activates the spindle checkpoint and suppresses the temperature sensitivity of an ipl1-2 mutant. These results, together with genetic interactions among YPI1, GLC7, and SDS22 mutants, indicate that Ypi1 and Sds22 are positive regulators of the nuclear Glc7 activity that is required for mitosis.

YPI1 and SDS22 proteins regulate the nuclear localization and function of yeast type 1 phosphatase Glc7

We have recently characterized Ypi1 as an inhibitory subunit of yeast Glc7 PP1 protein phosphatase. In this work we demonstrate that Ypi1 forms a complex with Glc7 and Sds22, another Glc7 regulatory subunit that targets the phosphatase to substrates involved in cell cycle control. Interestingly, the combination of equimolar amounts of Ypi1 and Sds22 leads to an almost full inhibition of Glc7 activity. Because YPI1 is an essential gene, we have constructed conditional mutants that demonstrate that depletion of Ypi1 leads to alteration of nuclear localization of Glc7 and cell growth arrest in mid-mitosis with aberrant mitotic spindle. These phenotypes mimic those produced upon inactivation of Sds22. The fact that progressive depletion of either Ypi1 or Sds22 resulted in similar physiological phenotypes and that both proteins inhibit the phosphatase activity of Glc7 strongly suggest a common role of these two proteins in regulating Glc7 nuclear localization and function.

Characterization of Schistosoma mansoni Sds homologue, a leucine-rich repeat protein that interacts with protein phosphatase type 1 and interrupts a G2/M cell-cycle checkpoint

The suppressor of the dis2 mutant (sds22+) has been shown to be an essential regulator in cell division of fission and budding yeast where its deletion causes mitotic arrest. Its role seems to take place through the activation of PP1 (protein phosphatase type 1) in Schizosaccharomyces pombe. In the trematode Schistosoma mansoni, we have identified the Sds22 homologue (SmSds), and the PP1 (SmPP1). We showed by using a GST (glutathione S-transferase) pull-down assay that the SmSds gene product interacts with SmPP1 and that the SmSds-SmPP1 complex is present in parasite extracts. Furthermore, we observed that SmSds inhibited PP1 activity. Functional studies showed that the microinjection of SmSds into Xenopus oocytes interacted with the Xenopus PP1 and disrupted the G2/M cell-cycle checkpoint by promoting progression to GVBD (germinal vesicle breakdown). Similar results showing the appearance of GVBD were observed when oocytes were treated with anti-PP1 antibodies. Taken together, these observations suggest that SmSds can regulate the cell cycle by binding to PP1.

Glucose signaling in Saccharomyces cerevisiae

Eukaryotic cells possess an exquisitely interwoven and fine-tuned series of signal transduction mechanisms with which to sense and respond to the ubiquitous fermentable carbon source glucose. The budding yeast Saccharomyces cerevisiae has proven to be a fertile model system with which to identify glucose signaling factors, determine the relevant functional and physical interrelationships, and characterize the corresponding metabolic, transcriptomic, and proteomic readouts. The early events in glucose signaling appear to require both extracellular sensing by transmembrane proteins and intracellular sensing by G proteins. Intermediate steps involve cAMP-dependent stimulation of protein kinase A (PKA) as well as one or more redundant PKA-independent pathways. The final steps are mediated by a relatively small collection of transcriptional regulators that collaborate closely to maximize the cellular rates of energy generation and growth. Understanding the nuclear events in this process may necessitate the further elaboration of a new model for eukaryotic gene regulation, called "reverse recruitment." An essential feature of this idea is that fine-structure mapping of nuclear architecture will be required to understand the reception of regulatory signals that emanate from the plasma membrane and cytoplasm. Completion of this task should result in a much improved understanding of eukaryotic growth, differentiation, and carcinogenesis.

Functional diversity of protein phosphatase-1, a cellular economizer and reset button

The protein serine/threonine phosphatase protein phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that regulates a variety of cellular processes through the dephosphorylation of dozens of substrates. This multifunctionality of PP1 relies on its association with a host of function-specific targetting and substrate-specifying proteins. In this review we discuss how PP1 affects the biochemistry and physiology of eukaryotic cells. The picture of PP1 that emerges from this analysis is that of a "green" enzyme that promotes the rational use of energy, the recycling of protein factors, and a reversal of the cell to a basal and/or energy-conserving state. Thus PP1 promotes a shift to the more energy-efficient fuels when nutrients are abundant and stimulates the storage of energy in the form of glycogen. PP1 also enables the relaxation of actomyosin fibers, the return to basal patterns of protein synthesis, and the recycling of transcription and splicing factors. In addition, PP1 plays a key role in the recovery from stress but promotes apoptosis when cells are damaged beyond repair. Furthermore, PP1 downregulates ion pumps and transporters in various tissues and ion channels that are involved in the excitation of neurons. Finally, PP1 promotes the exit from mitosis and maintains cells in the G1 or G2 phases of the cell cycle.

Protein phosphatase-1 binding to scd5p is important for regulation of actin organization and endocytosis in yeast

SCD5, an essential gene, encodes a protein important for endocytosis and actin organization in yeast. Previous two-hybrid screens showed that Scd5p interacts with Glc7p, a yeast Ser/Thr-specific protein phosphatase-1 (PP1) that participates in a variety of cellular processes. PP1 substrate specificity in vivo is regulated by association with different regulatory or targeting subunits, many of which have a consensus PP1-binding site ((V/I)XF, with a basic residue at the -1 or -2 position). Scd5p contains two of these potential PP1-binding motifs: KVDF (amino acids 240-243) and KKVRF (amino acids 272-276). Deletion analysis mapped the PP1-binding domain to a region of Scd5p containing these motifs. Therefore, the consequence of mutating these two potential PP1-binding sites was examined. Although mutation of KVDF had no effect, alteration of KKVRF dramatically reduced Scd5p interaction with Glc7p and resulted in temperature-sensitive growth. Furthermore, this mutation caused defects in fluid phase and receptor-mediated endocytosis and actin organization. Overexpression of GLC7 suppressed the temperature-sensitive growth of the KKVRF mutant and partially rescued the actin organization phenotype. These results provide evidence that Scd5p is a PP1 targeting subunit for regulation of actin organization and endocytosis or that Scd5p is a PP1 substrate, which regulates the function of Scd5p in these processes.

Binding of the concave surface of the Sds22 superhelix to the alpha 4/alpha 5/alpha 6-triangle of protein phosphatase-1

Functional studies of the protein phosphatase-1 (PP1) regulator Sds22 suggest that it is indirectly and/or directly involved in one of the most ancient functions of PP1, i.e. reversing phosphorylation by the Aurora-related protein kinases. We predict that the conserved portion of Sds22 folds into a curved superhelix and demonstrate that mutation to alanine of any of eight residues (Asp(148), Phe(170), Glu(192), Phe(214), Asp(280), Glu(300), Trp(302), or Tyr(327)) at the concave surface of this superhelix thwarts the interaction with PP1. Furthermore, we show that all mammalian isoforms of PP1 have the potential to bind Sds22. Interaction studies with truncated versions of PP1 and with chimeric proteins comprising fragments of PP1 and the yeast PP1-like protein phosphatase Ppz1 suggest that the site(s) required for the binding of Sds22 reside between residues 43 and 173 of PP1gamma(1). Within this region, a major interaction site was mapped to a triangular region delineated by the alpha4-, alpha5-, and alpha6-helices. Our data also show that well known regulatory binding sites of PP1, such as the RVXF-binding channel, the beta12/beta13-loop, and the acidic groove, are not essential for the interaction with Sds22.

Glc8 is a glucose-repressible activator of Glc7 protein phosphatase-1

Regulation of Glc7 type 1 protein phosphatase stability and activity was studied in budding yeast. We found that the Glc7 protein has a half-life of over 180min, which is sufficient for several generations. Glc7 protein stability was constant during the cell cycle and in batch culture growth. Furthermore, deletion of regulatory subunit Gac1, Reg1, Reg2, Sds22, or Glc8 had no influence on Glc7 protein half-life. The activity of Glc7 assayed as okadaic acid-resistant phosphorylase phosphatase activity was constant during the cell cycle. Deletion of the aforementioned regulatory subunits revealed that only Glc8 deletion had a significant effect in reducing Glc7 activity. Glc7 activity was induced during stationary phase in a Glc8-dependent manner. In addition, extracellular glucose repressed the induction of Glc7 activity. These results are consistent with glucose repression of Glc8 expression and favor the role of Glc8 as a major Glc7 activator.

A series of double disruptants for protein phosphatase genes in Saccharomyces cerevisiae and their phenotypic analysis

Thirty-two protein phosphatase (PPase) genes were identified in Saccharomyces cerevisiae based on the nucleotide sequences of the entire genome. In an effort to understand the role of PPases and their functional redundancy in the cellular physiology of one of the reference eukaryotic organisms, a series of single and double PPase gene disruptants were constructed in the W303 strain background. Two single disruptants for the CDC14 and GLC7 genes were lethal. Double disruptants for 30 non-essential PPase genes were constructed in all possible 435 combinations. No double disruptant showed synthetic lethality. Several phenotypes of the viable 30 single and 435 double disruptants were examined; temperature-sensitive growth, utilization of carbon sources and sensitivity to cations and drugs. Four double disruptants exhibited synthetic phenotypes in addition to eight single ones: the pph21 pph22 double disruptant showed slow growth on complete medium, as did the sit4 and yvh1 single ones. In addition to the ptc1, ynr022c and ycr079w single disruptants, the ppz1 ppz2 double disruptant showed temperature-sensitive slow growth. The msg5 ptp2 double disruptant, like the ynr022c single one, did not grow on complete medium containing 0.3 M CaCl(2). The double msg5 ptc2 disruptant failed to grow on medium containing 1.0 M NaCl and, like the ynr022c single deletion, also could not grow on medium containing 0.3 M CaCl(2). The synthetic phenotypes in the two latter cases where each of the PPases is categorized in a different phosphatase family led us to discuss the novel mechanism involved in the functional redundancy of the PPases.

Essential functions of Sds22p in chromosome stability and nuclear localization of PP1

Sds22p is a conserved, leucine-rich repeat protein that interacts with the catalytic subunit of protein phosphatase 1 (PP1(C)) and which has been proposed to regulate one or more functions of PP1(C) during mitosis. Here we show that Saccharomyces cerevisiae Sds22p is a largely nuclear protein, most of which is present as a sTable 1:1 complex with yeast PP1(C) (Glc7p). Temperature-sensitive (Ts(-)) S. cerevisiae sds22 mutants show profound chromosome instability at elevated growth temperatures but do not confer a cell cycle stage-specific arrest. In the sds22-6 Ts(-) mutant, nuclear Glc7p is both reduced in level and aberrantly localized at 37 degrees C and the interaction between Glc7p and Sds22p in vitro is reduced at higher temperatures, consistent with the in vivo Ts(-) growth defect. Like some glc7 mutations, sds22-6 can suppress the Ts(-) growth defect associated with ipl1-2, a loss of function mutation in a protein kinase that is known to work in opposition to PP1 on at least two nuclear substrates. This, together with reciprocal genetic interactions between GLC7 and SDS22, suggests that Sds22p functions positively with Glc7p to promote dephosphorylation of nuclear substrates required for faithful transmission of chromosomes during mitosis, and this role is at least partly mediated by effects of Sds22p on the nuclear distribution of Glc7p

A sds22 homolog that is associated with the testis-specific serine/threonine protein phosphatase 1gamma2 in rat testis

Two cDNAs sequences (1320 bp and 1180 bp) of the 55-kDa subunit associated with a testis-specific serine/threonine protein phosphatase 1gamma2 (PP1gamma2) were cloned. They were the same up to 1180 bp, suggesting that they may be generated by alternative splicing. Sequence studies showed that the 1320 bp-cDNA is a homolog of the human sds22alpha(1) (thus, named rat sds22alpha(1)). The 1180 bp-cDNA is a new splice-variant since its sequence at the 3' end has not been identified in human sds22 genes (named rat sds22alpha(3)). The 1320 bp-cDNA is ubiquitously expressed in various tissues including the immature testis. However, the expression of 1180 bp-cDNA was only observed in the testis after puberty. This expression pattern matches very well with that of PP1gamma2, suggesting that 1180 bp-cDNA may encode the 55-kDa subunit to associate with PP1gamma2 in rat testis and is involved in spermatogenesis by controlling PP1gamma2 activity.

Glc7p protein phosphatase inhibits expression of glutamine-fructose-6-phosphate transaminase from GFA1

Inhibitor-1 (I-1) is a specific inhibitor of protein phosphatase-1 (PP1). We assayed the ability of I-1 to inhibit Saccharomyces cerevisiae PP1, Glc7p, in vivo. Glc7p like other PP1 catalytic subunits associates with a variety of noncatalytic subunits, and Glc7p holoenzymes perform distinct physiological roles. Our results show that I-1 inhibits Glc7p holoenzymes that regulate transcription and mitosis, but holoenzymes responsible for meiosis and glycogen metabolism were unaffected. Additionally, we exploited a genetic screen for mutants that were dependent on I-1 to grow. This scheme can identify processes that are negatively regulated by Glc7p-catalyzed dephosphorylation. In this paper I-1-dependent gfa1 mutations were analyzed in detail. GFA1 encodes glutamine-fructose-6-phosphate transaminase. One or more phosphorylated proteins activate GFA1 transcription because the pheromone response and Pkc1p/mitogen-activated protein kinase pathways positively regulate GFA1 transcription. Our findings show that an I-1-sensitive Glc7p holoenzyme reduces GFA1 transcription. Therefore, GFA1 is a member of a growing list of genes that are negatively regulated by Glc7p dephosphorylation.

Genetic interactions between GLC7, PPZ1 and PPZ2 in saccharomyces cerevisiae

GLC7 encodes an essential serine/threonine protein type I phosphatase in Saccharomyces cerevisiae. Three other phosphatases (Ppz1p, Ppz2p, and Sal6p) share >59% identity in their catalytic region with Glc7p. ppz1 ppz2 null mutants have no apparent growth defect on rich media. However, null alleles of PPZ1 and PPZ2, in combination with mutant alleles of GLC7, confer a range of growth defects varying from slow growth to lethality. These results indicate that Glc7p, Ppz1p, and Ppz2p may have overlapping functions. To determine if this overlap extends to interaction with targeting subunits, Glc7p-binding proteins were tested for interaction in the two-hybrid system with the functional catalytic domain of Ppz1p. Ppz1p interacts strongly with a number of Glc7p regulatory subunits, including Glc8p, a protein that shares homology with mammalian PP1 inhibitor I2. Genetic data suggest that Glc8p positively affects both Glc7p and Ppz1p functions. Together our data suggest that Ppz1p and Ppz2p may have overlapping functions with Glc7p and that all three phosphatases may act through common regulatory proteins.

Sds22p is a subunit of a stable isolatable form of protein phosphatase 1 (Glc7p) from Saccharomyces cerevisiae

Protein phosphatase 1 (PP1) is one of the major protein phosphatases in eukaryotic cells. PP1 activity is believed to be controlled by the interaction of PP1 catalytic subunit with various regulatory subunits. The essential gene GLC7 encodes the PP1 catalytic subunit in Saccharomyces cerevisiae. In this study, full-length GLC7(1-312), C-terminal deletion mutants, and C-terminally poly-his tagged mutants were constructed and expressed in a GLC7 knockout strain of S. cerevisiae. Viability studies of the GLC7 knockout strains carrying the plasmids expressing GLC7 C-terminal deletion mutants and their tagged forms showed that the mutants 1-295 and 1-304 were functional, whereas the mutant 1-245 was not. The C-terminally poly-his tagged Glc7p with and without an N-terminal hemagglutinin (HA) tag was partially purified by immobilized Ni(2+) affinity chromatography and further analyzed by gel filtration and ion exchange chromatography. Phosphatase activity assays, SDS-PAGE, and Western blot analyses of the chromatographic fractions suggested that the Glc7p associated with regulatory subunits in vivo. A 40-kDa protein was copurified with tagged Glc7p through several chromatographic procedures. Monoclonal antibody against the HA tag coimmunoprecipitated the tagged Glc7p and the 40-kDa protein. This protein was further purified by a reverse phase HPLC column. Analysis by CNBr digestion, peptide sequencing, and electrospray mass spectrometry showed that this 40-kDa protein is Sds22p, one of the proteins proposed to be a regulatory subunit of Glc7. These results demonstrate that Sds22p forms a complex with Glc7p and that Sds22p:Glc7p is a stable isolatable form of yeast PP1.

Dynamic localization of protein phosphatase type 1 in the mitotic cell cycle of Saccharomyces cerevisiae

Protein phosphatase type I (PP1), encoded by the single essential gene GLC7 in Saccharomyces cerevisiae, functions in diverse cellular processes. To identify in vivo subcellular location(s) where these processes take place, we used a functional green fluorescent protein (GFP)-Glc7p fusion protein. Time-lapse fluorescence microscopy revealed GFP-Glc7p localizes predominantly in the nucleus throughout the mitotic cell cycle, with the highest concentrations in the nucleolus. GFP-Glc7p was also observed in a ring at the bud neck, which was dependent upon functional septins. Supporting a role for Glc7p in bud site selection, a glc7-129 mutant displayed a random budding pattern. In alpha-factor treated cells, GFP-Glc7p was located at the base of mating projections, again in a septin-dependent manner. At the start of anaphase, GFP-Glc7p accumulated at the spindle pole bodies and remained there until cytokinesis. After anaphase, GFP-Glc7p became concentrated in a ring that colocalized with the actomyosin ring. A GFP-Glc7-129 fusion was defective in localizing to the bud neck and SPBs. Together, these results identify sites of Glc7p function and suggest Glc7p activity is regulated through dynamic changes in its location.

Regulatory interactions between the Reg1-Glc7 protein phosphatase and the Snf1 protein kinase

Protein phosphatase 1, comprising the regulatory subunit Reg1 and the catalytic subunit Glc7, has a role in glucose repression in Saccharomyces cerevisiae. Previous studies showed that Reg1 regulates the Snf1 protein kinase in response to glucose. Here, we explore the functional relationships between Reg1, Glc7, and Snf1. We show that different sequences of Reg1 interact with Glc7 and Snf1. We use a mutant Reg1 altered in the Glc7-binding motif to demonstrate that Reg1 facilitates the return of the activated Snf1 kinase complex to the autoinhibited state by targeting Glc7 to the complex. Genetic evidence indicated that the catalytic activity of Snf1 negatively regulates its interaction with Reg1. We show that Reg1 is phosphorylated in response to glucose limitation and that this phosphorylation requires Snf1; moreover, Reg1 is dephosphorylated by Glc7 when glucose is added. Finally, we show that hexokinase PII (Hxk2) has a role in regulating the phosphorylation state of Reg1, which may account for the effect of Hxk2 on Snf1 function. These findings suggest that the phosphorylation of Reg1 by Snf1 is required for the release of Reg1-Glc7 from the kinase complex and also stimulates the activity of Glc7 in promoting closure of the complex.

A series of protein phosphatase gene disruptants in Saccharomyces cerevisiae

Thirty-two protein phosphatase (PPase) genes were identified in the genome nucleotide sequence of Saccharomyces cerevisiae. We constructed S. cerevisiae disruptants for each of the PPase genes and examined their growth under various conditions. The disruptants of six putative PPase genes, i.e. of YBR125c, YCR079w, YIL113w, YJR110w, YNR022c and YOR090c, were created for the first time in this study. The glc7, sit4 and cdc14 disruptants were lethal in our strain background. The remaining 29 PPase gene disruptants were viable at 30 degrees C and 37 degrees C, but only one disruptant, yvh1, showed intrinsic cold-sensitive growth at 13 degrees C. Transcription of the YVH1 gene was induced at 13 degrees C, consistent with an idea that Yvh1p has a specific role for growth at a low temperature. The viable disruptants grew normally on nutrient medium containing sucrose, galactose, maltose or glycerol as carbon sources. The ppz1 disruptant was tolerant to NaCl and LiCl, while the cmp2 disruptant was sensitive to these salts, as reported previously, and none of the other viable PPase disruptants exhibited the salt sensitivity. When the viable disruptants were tested for sensitivity to drugs, i.e. benomyl, caffeine and hydroxyurea, ppz1 and ycr079w disruptants exhibited sensitivity to caffeine.

Functional analysis of the yeast Glc7-binding protein Reg1 identifies a protein phosphatase type 1-binding motif as essential for repression of ADH2 expression

In Saccharomyces cerevisiae, the protein phosphatase type 1 (PP1)-binding protein Reg1 is required to maintain complete repression of ADH2 expression during growth on glucose. Surprisingly, however, mutant forms of the yeast PP1 homologue Glc7, which are unable to repress expression of another glucose-regulated gene, SUC2, fully repressed ADH2. Constitutive ADH2 expression in reg1 mutant cells did require Snf1 protein kinase activity like constitutive SUC2 expression and was inhibited by unregulated cyclic AMP-dependent protein kinase activity like ADH2 expression in derepressed cells. To further elucidate the functional role of Reg1 in repressing ADH2 expression, deletions scanning the entire length of the protein were analyzed. Only the central region of the protein containing the putative PP1-binding sequence RHIHF was found to be indispensable for repression. Introduction of the I466M F468A substitutions into this sequence rendered Reg1 almost nonfunctional. Deletion of the central region or the double substitution prevented Reg1 from significantly interacting with Glc7 in two-hybrid analyses. Previous experimental evidence had indicated that Reg1 might target Glc7 to nuclear substrates such as the Snf1 kinase complex. Subcellular localization of a fully functional Reg1-green fluorescent protein fusion, however, indicated that Reg1 is cytoplasmic and excluded from the nucleus independently of the carbon source. When the level of Adr1 was modestly elevated, ADH2 expression was no longer fully repressed in glc7 mutant cells, providing the first direct evidence that Glc7 can repress ADH2 expression. These results suggest that the Reg1-Glc7 phosphatase is a cytoplasmic component of the machinery responsible for returning Snf1 kinase activity to its basal level and reestablishing glucose repression. This implies that the activated form of the Snf1 kinase complex must cycle between the nucleus and the cytoplasm.

Structure and splice products of the human gene encoding sds22, a putative mitotic regulator of protein phosphatase-1

sds22 is a regulatory subunit of protein phosphatase-1 that is required for the completion of mitosis in yeast. It consists largely of 11 tandem leucine-rich repeats of 22 residues that are expected to mediate interactions with other polypeptides, including protein phosphatase-1. In this paper, we report on the structure of the human gene encoding sds22, designated PPP1R7. This gene (33 kb) comprises 11 exons, but these do not coincide with the sequences encoding the leucine-rich repeats. Up to six splice variants can be generated by exon skipping and alternative polyadenylation, as revealed by expressed sequence tag database analysis, RT-PCR and Northern blot analysis. The sds22 transcripts are expected to encode four different polypeptides. sds22alpha1 corresponds to the variant cloned previously from human brain [Renouf et al. (1995) FEBS Lett. 375, 75-78]. Sds22beta1 is truncated within the ninth repeat and has a short and different C-terminus. Both variants also exist without the sequence corresponding to exon 2, and these are termed sds22alpha2 and sds22beta2. The 5'-flanking region of PPP1R7 contains two NF-Y-binding CCAAT boxes near the transcription start site and potential binding sites for the transcription factors c-Myb, Ik-2 and NF-1, which are conserved in the mouse gene.

Regulation of yeast glycogen metabolism and sporulation by Glc7p protein phosphatase

Glc7p is an essential serine/threonine type 1 protein phosphatase (PP1) from the yeast Saccharomyces cerevisiae, which has a role in many processes including cell cycle progression, sporulation, glycogen accumulation, translation initiation, and glucose repression. Two hallmarks of PP1 enzymes are very high amino acid sequence conservation and association of the catalytic subunit with a variety of noncatalytic, regulatory subunits. We tested the hypothesis that PP1 sequence conservation was the result of each PP1 residue playing a role in multiple intermolecular interactions. Analysis of 24 glc7 mutants, isolated primarily by their glycogen accumulation traits, revealed that every mutated Glc7p residue altered many noncatalytic subunit affinities and conferred unselected sporulation traits to various degrees. Furthermore, quantitative analysis showed that Glc7p affinity for the glycogen-binding noncatalytic subunit Gac1p was not the only parameter that determines the glycogen accumulation by a glc7 mutant. Sds22p is one Glc7p noncatalytic subunit that is essential for mitotic growth. Surprisingly, several mutant Glc7p proteins had undetectable affinity for Sds22p, yet grew apparently normally. The characterization of glc7 diploid sporulation revealed that Glc7p has at least two meiotic roles. Premeiotic DNA synthesis was undetectable in glc7 mutants with the poorest sporulation. In the glc7 diploids examined, expression of the meiotic inducer IME1 was proportional to the glc7 diploid sporulation frequency. Moreover, IME1 hyperexpression could not suppress glc7 sporulation traits. The Glc7p/Gip1p holoenzyme may participate in completion of meiotic divisions or spore packaging because meiotic dyads predominate when some glc7 diploids sporulate.

Yeast protein serine/threonine phosphatases: multiple roles and diverse regulation

Since the isolation of the first yeast protein phosphatase genes in 1989, much progress has been made in understanding this important group of proteins. Yeast contain genes encoding all the major types of protein phosphatase found in higher eukaryotes and the ability to use genetic approaches will complement the wealth of biochemical information available from other systems. This review will summarize recent progress in understanding the structure, function and regulation of the PPP family of protein serine-threonine phosphatases, concentrating on the budding yeast Saccharomyces cerevisiae.

Alanine-scanning mutagenesis of protein phosphatase type 1 in the yeast Saccharomyces cerevisiae

Protein phosphatase type 1, encoded by GLC7 in Saccharomyces cerevisiae, is an essential serine/threonine phosphatase implicated in the regulation of a diverse array of physiological functions. We constructed and examined 20 mutant alleles of GLC7 in which codons encoding clusters of charged residues were changed to alanine codons. Three of 20 mutant alleles alter residues in the active site of the phosphatase and are unable to rescue the lethality of a glc7::LEU2 disruption. The 17 alleles that support growth confer a range of mutant traits including cell cycle arrest, 2-deoxyglucose resistance, altered levels of glycogen, sensitivity to high salt, and sporulation defects. For some traits, such as 2-deoxyglucose resistance and cell cycle arrest, the mutated residues map to specific regions of the protein whereas the mutated residues in glycogen-deficient mutants and sporulation-defective mutants are more widely distributed over the protein surface. Many mutants have complex phenotypes, each displaying a diverse range of defects. The wide range of phenotypes identified from the collection of mutant alleles is consistent with the hypothesis that Glc7p-binding proteins, which are thought to regulate the specificity of Glc7p, have overlapping binding sites on the surface of Glc7p. This could account for the high level of sequence conservation found among type 1 protein phosphatases from different species.

Requirement for PP1 phosphatase and 20S cyclosome/APC for the onset of anaphase is lessened by the dosage increase of a novel gene sds23+

Ubiquitin-dependent proteolysis is required for the onset of anaphase. We show that protein dephosphorylation by protein phosphatase 1 (PP1) is also essential for initiating anaphase in fission yeast. PP1 may directly or indirectly regulate the 20S cyclosome/APC (anaphase-promoting complex) required for anaphase-promoting proteolysis. Using anti-phosphopeptide antibodies, PP1 is shown to be dephosphorylated at the C-terminus, upon the onset of anaphase, for reactivation. sds23+, a novel gene, is a multicopy suppressor for mutations in PP1 and the 20S cyclosome/APC, implying that the gene dosage increase can relieve the requirement for PP1 and the cyclosome/APC for the onset of anaphase. The sds23+ gene is not essential for cell viability, but a mutant with the gene deleted cannot form colonies at 22 and 36 degrees C. In the sds23 deletion mutant, the progression of anaphase and cytokinesis is retarded and cell shape is aberrant. These defects are overcome by plasmids carrying the genes encoding subunits of the 20S cyclosome/APC or PP1. These results demonstrate functions other than promoting anaphase for the components of the 20S cyclosome/APC and also a close functional relationship of Sds23 with PP1 and 20S cyclosome/APC.

Yeast protein serine/threonine phosphatases: multiple roles and diverse regulation

Since the isolation of the first yeast protein phosphatase genes in 1989, much progress has been made in understanding this important group of proteins. Yeast contain genes encoding all the major types of protein phosphatase found in higher eukaryotes and the ability to use genetic approaches will complement the wealth of biochemical information available from other systems. This review will summarize recent progress in understanding the structure, function and regulation of the PPP family of protein serine-threonine phosphatases, concentrating on the budding yeast Saccharomyces cerevisiae.

Protein phosphatase type 1 interacts with proteins required for meiosis and other cellular processes in Saccharomyces cerevisiae

Protein phosphatase type I (PP1) is involved in diverse cellular processes, and its activity toward specific substrates is thought to be controlled by different regulatory or targeting subunits. To identify regulatory subunits and substrates of the Saccharomyces cerevisiae PP1, encoded by GLC7, we used the two-hybrid system to detect interacting proteins. Among the many proteins identified were Gac1, a known glycogen regulatory subunit, and a protein with homology to Gac1. We also characterized a new gene designated GIP1, for Glc7-interacting protein. We show that a Gip1 fusion protein coimmunoprecipitates with PP1 from cell extracts. Molecular and genetic analyses indicate that GIP1 is expressed specifically during meiosis, affects transcription of late meiotic genes, and is essential for sporulation. Thus, the Gip1 protein is a candidate for a meiosis-specific substrate or regulator of PP1. Finally, we recovered two genes, RED1 and SCD5, with roles in meiosis and the vesicular secretory pathway, respectively. These results provide strong evidence implicating PP1 function in meiosis. In addition, this study indicates that the two-hybrid system offers a promising approach to understanding the multiple roles and interactions of PP1 in cellular regulation.

The REG2 gene of Saccharomyces cerevisiae encodes a type 1 protein phosphatase-binding protein that functions with Reg1p and the Snf1 protein kinase to regulate growth

The GLC7 gene of Saccharomyces cerevisiae encodes the catalytic subunit of type 1 protein phosphatase (PP1) and is essential for cell growth. We have isolated a previously uncharacterized gene, REG2, on the basis of its ability to interact with Glc7p in the two-hybrid system. Reg2p interacts with Glc7p in vivo, and epitope-tagged derivatives of Reg2p and Glc7p coimmunoprecipitate from cell extracts. The predicted protein product of the REG2 gene is similar to Reg1p, a protein believed to direct PP1 activity in the glucose repression pathway. Mutants with a deletion of reg1 display a mild slow-growth defect, while reg2 mutants exhibit a wild-type phenotype. However, mutants with deletions of both reg1 and reg2 exhibit a severe growth defect. Overexpression of REG2 complements the slow-growth defect of a reg1 mutant but does not complement defects in glycogen accumulation or glucose repression, two traits also associated with a reg1 deletion. These results indicate that REG1 has a unique role in the glucose repression pathway but acts together with REG2 to regulate some as yet uncharacterized function important for growth. The growth defect of a reg1 reg2 double mutant is alleviated by a loss-of-function mutation in the SNF1-encoded protein kinase. The snf1 mutation also suppresses the glucose repression defects of reg1. Together, our data are consistent with a model in which Reg1p and Reg2p control the activity of PP1 toward substrates that are phosphorylated by the Snf1p kinase.

Genetic interactions between REG1/HEX2 and GLC7, the gene encoding the protein phosphatase type 1 catalytic subunit in Saccharomyces cerevisiae

Mutations in GLC7, the gene encoding the type 1 protein phosphatase catalytic subunit, cause a variety of abberrant phenotypes in yeast, such as impaired glycogen synthesis and relief of glucose repression of the expression of some genes. Loss of function of the REG1/HEX2 gene, necessary for glucose repression of several genes, was found to suppress the glycogen-deficient phenotype of the glc7-1 allele. Deletion of REG1 in a wild-type background led to overaccumulation of glycogen as well as slow growth and an enlarged cell size. However, loss of REG1 did not suppress other phenotypes associated with GLC7 mutations, such as inability to sporulate or, in cells bearing the glc7Y-170 allele, lack of growth at 14 degrees. The effect of REG1 deletion on glycogen accumulation is not simply due to derepression of glucose-repressed genes, although it does require the presence of SNF1, which encodes a protein kinase essential for expression of glucose-repressed genes and for glycogen accumulation. We propose that REG1 has a role in controlling glycogen accumulation.

Molecular cloning of a human polypeptide related to yeast sds22, a regulator of protein phosphatase-1

sds22 is a regulatory polypeptide of protein phosphatase-1 that is required for the completion of mitosis in both fission and budding yeast. We report here the cDNA cloning of a human polypeptide that is 46% identical to yeast sds22. The human homolog of sds22 consists of 360 residues, has a calculated molecular mass of 41.6 kDa and shows a tandem array of 11 leucine-rich repeat structures of 22 residues. Northern analysis revealed a major transcript of 1.39 kb in all 8 investigated human tissues. sds22 was detected by western analysis in both the cytoplasm and the nucleus of rat liver cells as a polypeptide of 44 kDa.

The Saccharomyces cerevisiae gene SDS22 encodes a potential regulator of the mitotic function of yeast type 1 protein phosphatase

In higher eukaryotes, the activity and specificity of the type 1 protein serine-threonine phosphatase (PP1) catalytic subunit is thought to be controlled by its association with a number of regulatory or targeting subunits. Here we describe the characterization of a gene encoding one such potential polypeptide in the yeast Saccharomyces cerevisiae. The gene which we have isolated (termed SDS22) encodes a product with a high degree of sequence identity to the fission yeast sds22 protein, a known regulator of the mitotic function of PP1 in Schizosaccharomyces pombe. Using two different criteria, we have demonstrated that Sds22p and the catalytic subunit of PP1 (Glc7p) interact in yeast cells. We have also generated a temperature-sensitive allele of GLC7 (glc7-12) which causes a block to the completion of mitosis at the restrictive temperature. Additional copies of SDS22 lead to allele-specific suppression of the glc7-12 mutant, strongly suggesting that the interaction between the two proteins is of functional significance. Sds22p is therefore likely to be the second example of a PP1 regulatory subunit identified in S. cerevisiae.