Mutations in yeast protein phosphatase type 1 that affect targeting subunit binding

Killing two birds with one stone: how budding yeast Mps1 controls chromosome segregation and spindle assembly checkpoint through phosphorylation of a single kinetochore protein

During mitosis, the identical sister chromatids of each chromosome must attach through their kinetochores to microtubules emanating from opposite spindle poles. This process, referred to as chromosome biorientation, is essential for equal partitioning of the genetic information to the two daughter cells. Defects in chromosome biorientation can give rise to aneuploidy, a hallmark of cancer and genetic diseases. A conserved surveillance mechanism called spindle assembly checkpoint (SAC) prevents the onset of anaphase until biorientation is attained. Key to chromosome biorientation is an error correction mechanism that allows kinetochores to establish proper bipolar attachments by disengaging faulty kinetochore-microtubule connections. Error correction relies on the Aurora B and Mps1 kinases that also promote SAC signaling, raising the possibility that they are part of a single sensory device responding to improper attachments and concomitantly controlling both their disengagement and a temporary mitotic arrest. In budding yeast, Aurora B and Mps1 promote error correction independently from one another, but while the substrates of Aurora B in this process are at least partially known, the mechanism underlying the involvement of Mps1 in the error correction pathway is unknown. Through the characterization of a novel mps1 mutant and an unbiased genetic screen for extragenic suppressors, we recently gained evidence that a common mechanism based on Mps1-dependent phosphorylation of the Knl1/Spc105 kinetochore scaffold and subsequent recruitment of the Bub1 kinase is critical for the function of Mps1 in chromosome biorientation as well as for SAC activation (Benzi et al. EMBO Rep, 2020).

A common molecular mechanism underlies the role of Mps1 in chromosome biorientation and the spindle assembly checkpoint

The Mps1 kinase corrects improper kinetochore-microtubule attachments, thereby ensuring chromosome biorientation. Yet, its critical phosphorylation targets in this process remain largely elusive. Mps1 also controls the spindle assembly checkpoint (SAC), which halts chromosome segregation until biorientation is attained. Its role in SAC activation is antagonised by the PP1 phosphatase and involves phosphorylation of the kinetochore scaffold Knl1/Spc105, which in turn recruits the Bub1 kinase to promote assembly of SAC effector complexes. A crucial question is whether error correction and SAC activation are part of a single or separable pathways. Here, we isolate and characterise a new yeast mutant, mps1-3, that is severely defective in chromosome biorientation and SAC signalling. Through an unbiased screen for extragenic suppressors, we found that mutations lowering PP1 levels at Spc105 or forced association of Bub1 with Spc105 reinstate both chromosome biorientation and SAC signalling in mps1-3 cells. Our data argue that a common mechanism based on Knl1/Spc105 phosphorylation is critical for Mps1 function in error correction and SAC signalling, thus supporting the idea that a single sensory apparatus simultaneously elicits both pathways.

Essential role of GEXP15, a specific Protein Phosphatase type 1 partner, in Plasmodium berghei in asexual erythrocytic proliferation and transmission

The essential and distinct functions of Protein Phosphatase type 1 (PP1) catalytic subunit in eukaryotes are exclusively achieved through its interaction with a myriad of regulatory partners. In this work, we report the molecular and functional characterization of Gametocyte EXported Protein 15 (GEXP15), a Plasmodium specific protein, as a regulator of PP1. In vitro interaction studies demonstrated that GEXP15 physically interacts with PP1 through the RVxF binding motif in P. berghei. Functional assays showed that GEXP15 was able to increase PP1 activity and the mutation of the RVxF motif completely abolished this regulation. Immunoprecipitation assays of tagged GEXP15 or PP1 in P. berghei followed by immunoblot or mass spectrometry analyses confirmed their interaction and showed that they are present both in schizont and gametocyte stages in shared protein complexes involved in the spliceosome and proteasome pathways and known to play essential role in parasite development. Phenotypic analysis of viable GEXP15 deficient P. berghei blood parasites showed that they were unable to develop lethal infection in BALB/c mice or to establish experimental cerebral malaria in C57BL/6 mice. Further, although deficient parasites produced gametocytes they did not produce any oocysts/sporozoites indicating a high fitness cost in the mosquito. Global proteomic and phosphoproteomic analyses of GEXP15 deficient schizonts revealed a profound defect with a significant decrease in the abundance and an impact on phosphorylation status of proteins involved in regulation of gene expression or invasion. Moreover, depletion of GEXP15 seemed to impact mainly the abundance of some specific proteins of female gametocytes. Our study provides the first insight into the contribution of a PP1 regulator to Plasmodium virulence and suggests that GEXP15 affects both the asexual and sexual life cycle.

In the absence of an effective vaccine and the emerging resistance to artemisinin combination therapy, malaria is still a significant threat to human health. Increasing our understanding of the specific mechanisms of the biology of Plasmodium is essential to propose new strategies to control this infection. Here, we demonstrated that GEXP15, a specific protein in Plasmodium, was able to interact with the Protein Phosphatase 1 and regulate its activity. We showed that both proteins are implicated in common protein complexes involved in the mRNA splicing and proteasome pathways. We reported that the deletion of GEXP15 leads to a loss of parasite virulence during asexual stages and a total abolishment of the capacity of deficient parasites to develop in the mosquito. We also found that this deletion affects both protein phosphorylation status and significantly decreases the expression of essential proteins in schizont and gametocyte stages. This study characterizes for the first time a novel molecular pathway through the control of PP1 by an essential and specific Plasmodium regulator, which may contribute to the discovery of new therapeutic targets to control malaria.

Ser/Thr protein phosphatases in fungi: structure, regulation and function

Reversible phospho-dephosphorylation of proteins is a major mechanism for the control of cellular functions. By large, Ser and Thr are the most frequently residues phosphorylated in eukar-yotes. Removal of phosphate from these amino acids is catalyzed by a large family of well-conserved enzymes, collectively called Ser/Thr protein phosphatases. The activity of these enzymes has an enormous impact on cellular functioning. In this work we pre-sent the members of this family in S. cerevisiae and other fungal species, and review the most recent findings concerning their regu-lation and the roles they play in the most diverse aspects of cell biology.

Characterization of a Protein Phosphatase Type-1 and a Kinase Anchoring Protein in Plasmodium falciparum

With its multiple regulatory partners, the conserved Protein Phosphatase type-1 (PP1) plays a central role in many functions of the biology of eukaryotic cells, including Plasmodium falciparum. Here, we characterized a protein named PfRCC-PIP, as a major partner of PfPP1. We established its direct interaction in vitro and its presence in complex with PfPP1 in the parasite. The use of Xenopus oocyte model revealed that RCC-PIP can interact with the endogenous PP1 and act in synergy with suboptimal doses of progesterone to trigger oocyte maturation, suggesting a regulatory effect on PP1. Reverse genetic studies suggested an essential role for RCC-PIP since no viable knock-out parasites could be obtained. Further, we demonstrated the capacity of protein region containing RCC1 motifs to interact with the parasite kinase CDPK7. These data suggest that this protein is both a kinase and a phosphatase anchoring protein that could provide a platform to regulate phosphorylation/dephosphorylation processes.

Protein phosphatases of Saccharomyces cerevisiae

The phosphorylation status of a protein is highly regulated and is determined by the opposing activities of protein kinases and protein phosphatases within the cell. While much is known about the protein kinases found in Saccharomyces cerevisiae, the protein phosphatases are much less characterized. Of the 127 protein kinases in yeast, over 90% are in the same evolutionary lineage. In contrast, protein phosphatases are fewer in number (only 43 have been identified in yeast) and comprise multiple, distinct evolutionary lineages. Here we review the protein phosphatase families of yeast with regard to structure, catalytic mechanism, regulation, and signal transduction participation.

Genome wide identification of wheat and Brachypodium type one protein phosphatases and functional characterization of durum wheat TdPP1a

Reversible phosphorylation is an essential mechanism regulating signal transduction during development and environmental stress responses. An important number of dephosphorylation events in the cell are catalyzed by type one protein phosphatases (PP1), which catalytic activity is driven by the binding of regulatory proteins that control their substrate specificity or subcellular localization. Plants harbor several PP1 isoforms accounting for large functional redundancies. While animal PP1s were reported to play relevant roles in controlling multiple cellular processes, plant orthologs remain poorly studied. To decipher the role of plant PP1s, we compared PP1 genes from three monocot species, Brachypodium, common wheat and rice at the genomic and transcriptomic levels. To gain more insight into the wheat PP1 proteins, we identified and characterized TdPP1a, the first wheat type one protein phosphatase from a Tunisian durum wheat variety Oum Rabiaa3. TdPP1a is highly conserved in sequence and structure when compared to mammalian, yeast and other plant PP1s. We demonstrate that TdPP1a is an active, metallo-dependent phosphatase in vitro and is able to interact with AtI2, a typical regulator of PP1 functions. Also, TdPP1a is capable to complement the heat stress sensitivity of the yeast mutant indicating that TdPP1a is functional also in vivo. Moreover, transient expression of TdPP1a::GFP in tobacco leaves revealed that it is ubiquitously distributed within the cell, with a strong accumulation in the nucleus. Finally, transcriptional analyses showed similar expression levels in roots and leaves of durum wheat seedlings. Interestingly, the expression in leaves is significantly induced following salinity stress, suggesting a potential role of TdPP1a in wheat salt stress response.

Molecular and functional characterization of the durum wheat TdRL1, a member of the conserved Poaceae RSS1-like family that exhibits features of intrinsically disordered proteins and confers stress tolerance in yeast

Because of their fixed lifestyle, plants must acclimate to environmental changes by orchestrating several responses ranging from protective measures to growth control. Growth arrest is observed upon abiotic stress and can cause penalties to plant production. But, the molecular interface between stress perception and cell cycle control is poorly understood. The rice protein RSS1 is required at G1/S transition ensuring normal dividing activity of proliferative cells during salt stress. The role of RSS1 in meristem maintenance together with its flexible protein structure implies its key function as molecular integrator of stress signaling for cell cycle control. To study further the relevance of RSS1 and its related proteins in cereals, we isolated the durum wheat homolog, TdRL1, from Tunisian durum wheat varieties and extended our analyses to RSS1-like proteins from Poaceae. Our results show that the primary sequences of TdRL1 and the Graminae RSS1-like family members are highly conserved. In silico analyses predict that TdRL1 and other RSS1-like proteins share flexible 3-D structures and have features of intrinsically disordered/unstructured proteins (IDP). The disordered structure of TdRL1 is well illustrated by an electrophoretical mobility shift of the purified protein. Moreover, heterologous expression of TdRL1 in yeast improves its tolerance to salt and heat stresses strongly suggesting its involvement in abiotic stress tolerance mechanisms. Such finding adds new knowledge to our understanding of how IDPs may contribute as central molecular integrators of stress signaling into improving plant tolerance to abiotic stresses.

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.

Protein phosphatase PP1/GLC7 interaction domain in yeast eIF2γ bypasses targeting subunit requirement for eIF2α dephosphorylation

Whereas the protein kinases GCN2, HRI, PKR, and PERK specifically phosphorylate eukaryotic translation initiation factor 2 (eIF2α) on Ser51 to regulate global and gene-specific mRNA translation, eIF2α is dephosphorylated by the broadly acting serine/threonine protein phosphatase 1 (PP1). In mammalian cells, the regulatory subunits GADD34 and CReP target PP1 to dephosphorylate eIF2α; however, as there are no homologs of these targeting subunits in yeast, it is unclear how GLC7, the functional homolog of PP1 in yeast, is recruited to dephosphorylate eIF2α. Here, we show that a novel N-terminal extension on yeast eIF2γ contains a PP1-binding motif (KKVAF) that enables eIF2γ to pull down GLC7 and target it to dephosphorylate eIF2α. Truncation or point mutations designed to eliminate the KKVAF motif in eIF2γ impair eIF2α dephosphorylation in vivo and in vitro and enhance expression of GCN4. Replacement of the N terminus of eIF2γ with the GLC7-binding domain from GAC1 or fusion of heterologous dimerization domains to eIF2γ and GLC7, respectively, maintained eIF2α phosphorylation at basal levels. Taken together, these results indicate that, in contrast to the paradigm of distinct PP1-targeting or regulatory subunits, the unique N terminus of yeast eIF2γ functions in cis to target GLC7 to dephosphorylate eIF2α.

The budding yeast Cdc48(Shp1) complex promotes cell cycle progression by positive regulation of protein phosphatase 1 (Glc7)

The conserved, ubiquitin-selective AAA ATPase Cdc48 regulates numerous cellular processes including protein quality control, DNA repair and the cell cycle. Cdc48 function is tightly controlled by a multitude of cofactors mediating substrate specificity and processing. The UBX domain protein Shp1 is a bona fide substrate-recruiting cofactor of Cdc48 in the budding yeast S. cerevisiae. Even though Shp1 has been proposed to be a positive regulator of Glc7, the catalytic subunit of protein phosphatase 1 in S. cerevisiae, its cellular functions in complex with Cdc48 remain largely unknown. Here we show that deletion of the SHP1 gene results in severe growth defects and a cell cycle delay at the metaphase to anaphase transition caused by reduced Glc7 activity. Using an engineered Cdc48 binding-deficient variant of Shp1, we establish the Cdc48^Shp1 complex as a critical regulator of mitotic Glc7 activity. We demonstrate that shp1 mutants possess a perturbed balance of Glc7 phosphatase and Ipl1 (Aurora B) kinase activities and show that hyper-phosphorylation of the kinetochore protein Dam1, a key mitotic substrate of Glc7 and Ipl1, is a critical defect in shp1. We also show for the first time a physical interaction between Glc7 and Shp1 in vivo. Whereas loss of Shp1 does not significantly affect Glc7 protein levels or localization, it causes reduced binding of the activator protein Glc8 to Glc7. Our data suggest that the Cdc48^Shp1 complex controls Glc7 activity by regulating its interaction with Glc8 and possibly further regulatory subunits.

Modulation of yeast alkaline cation tolerance by Ypi1 requires calcineurin

Ypi1 was discovered as an essential protein able to act as a regulatory subunit of the Saccharomyces cerevisiae type 1 protein phosphatase Glc7 and play a key role in mitosis. We show here that partial depletion of Ypi1 causes lithium sensitivity and that high levels of this protein confer a lithium-tolerant phenotype to yeast cells. Remarkably, this phenotype was independent of the role of Ypi1 as a Glc7 regulatory subunit. Lithium tolerance in cells overexpressing Ypi1 was caused by a combination of increased efflux of lithium, mediated by augmented expression of the alkaline cation ATPase ENA1, and decreased lithium influx through the Trk1,2 high-affinity potassium transporters. Deletion of CNB1, encoding the regulatory subunit of the calcineurin phosphatase, blocked Ypi1-induced expression of ENA1, normalized Li(+) fluxes, and abolished the Li(+) hypertolerant phenotype of Ypi1-overexpressing cells. These results point to a complex role of Ypi1 on the regulation of cation homeostasis, largely mediated by the calcineurin phosphatase.

Function of protein phosphatase-1, Glc7, in Saccharomyces cerevisiae

Budding yeast, Saccharomyces cerevisiae, and its close relatives are unique among eukaryotes in having a single gene, GLC7, encoding protein phosphatase-1 (PP1). This enzyme with a highly conserved amino acid sequence controls many processes in all eukaryotic cells. Therefore, the study of Glc7 function offers a unique opportunity to gain a comprehensive understanding of this critical regulatory enzyme. This review summarizes our current knowledge of how Glc7 function modulates processes in the cytoplasm and nucleus. Additionally, global Glc7 regulation is described.

Cell signalling and radiation survival: the impact of protein phosphatases

PURPOSE

This review will examine the role of phosphatases in cancer cell signalling and also outline emerging findings regarding the influence of phosphatases on tumor cell survival after ionising radiation.

CONCLUSION

The exposure of tumour cells to clinically relevant doses of ionising radiation causes DNA damage and rapidly activates a series of signaling cascades involved in cell survival (reviewed in (Valerie et al. 2007 )). The role of kinases in this signalling has been extensively studied, but the role of phosphatases is less well defined. There is an abundance of literature implicating phosphatases in cell cycle control, cell growth and survival but there has been much less reported on the involvement of these enzymes as determinants of radiosensitivity. Recent studies, however, suggest that phosphatases may modulate tumor cell radiosensitivity and may be targets for the enhancement of radiotherapy.

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.

Saccharomyces cerevisiae Afr1 protein is a protein phosphatase 1/Glc7-targeting subunit that regulates the septin cytoskeleton during mating

Glc7, the type1 serine/threonine phosphatase in the yeast Saccharomyces cerevisiae, is targeted by auxiliary subunits to numerous locations in the cell, where it regulates a range of physiological pathways. We show here that the accumulation of Glc7 at mating projections requires Afr1, a protein required for the formation of normal projections. AFR1-null mutants fail to target Glc7 to projections, and an Afr1 variant specifically defective in binding to Glc7 [Afr1(V546A F548A)] forms aberrant projections. The septin filaments in mating projections of AFR1 mutants initiate normally but then rearrange asymmetrically as the projection develops, suggesting that the Afr1-Glc7 holoenzyme may regulate the maintenance of septin complexes during mating. These results demonstrate a previously unknown role for Afr1 in targeting Glc7 to mating projections and in regulating the septin architecture during mating.

The Glc7 phosphatase subunit of the cleavage and polyadenylation factor is essential for transcription termination on snoRNA genes

Glc7, the yeast protein phosphatase 1, is a component of the cleavage and polyadenylation factor (CPF). Here we show that downregulation of Glc7, or its dissociation from CPF in the absence of CPF subunits Ref2 or Swd2, results in similar snoRNA termination defects. Overexpressing a C-terminal fragment of Sen1, a superfamily I helicase required for snoRNA termination, suppresses the growth and termination defects associated with loss of Swd2 or Ref2, but not Glc7. Suppression by Sen1 requires nuclear localization and direct interaction with Glc7, which can dephosphorylate Sen1 in vitro. The suppressing fragment, and in a similar manner full-length Sen1, copurifies with the snoRNA termination factors Nrd1 and Nab3, suggesting loss of Glc7 from CPF can be compensated by recruiting Glc7 to Nrd1-Nab3 through Sen1. Swd2 is also a subunit of the Set1c histone H3K4 methyltransferase complex and is required for its stability and optimal methyltransferase activity.

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.

Access denied: Snf1 activation loop phosphorylation is controlled by availability of the phosphorylated threonine 210 to the PP1 phosphatase

Phosphorylation of the Saccharomyces cerevisiae Snf1 kinase activation loop is determined by the integration of two reaction rates: the rate of phosphorylation by upstream kinases and the rate of dephosphorylation by Glc7. The activities of the Snf1-activating kinases do not appear to be glucose-regulated, since immune complex kinase assays with each of the three Snf1-activating kinases show similar levels of activity when prepared from cells grown in either high or low glucose. In contrast, the dephosphorylation of the Snf1 activation loop was strongly regulated by glucose. When de novo phosphorylation of Snf1 was inhibited, phosphorylation of the Snf1 activation loop was found to be stable in low glucose but rapidly lost upon the addition of glucose. A greater than 10-fold difference in the rates of Snf1 activation loop dephosphorylation was detected. However, the activity of the Glc7-Reg1 phosphatase may not itself be directly regulated by glucose, since the Glc7-Reg1 enzyme was active in low glucose toward another substrate, the transcription factor Mig1. Glucose-mediated regulation of Snf1 activation loop dephosphorylation is controlled by changes in the ability of the Snf1 activation loop to act as a substrate for Glc7.

NOM1 targets protein phosphatase I to the nucleolus

Protein phosphatase I (PP1) is an essential eukaryotic serine/threonine phosphatase required for many cellular processes, including cell division, signaling, and metabolism. In mammalian cells there are three major isoforms of the PP1 catalytic subunit (PP1alpha, PP1beta, and PP1gamma) that are over 90% identical. Despite this high degree of identity, the PP1 catalytic subunits show distinct localization patterns in interphase cells; PP1alpha is primarily nuclear and largely excluded from nucleoli, whereas PP1gamma and to a lesser extent PP1beta concentrate in the nucleoli. The subcellular localization and the substrate specificity of PP1 catalytic subunits are determined by their interaction with targeting subunits, most of which bind PP1 through a so-called "RVXF" sequence. Although PP1 targeting subunits have been identified that direct PP1 to a number of subcellular locations and/or substrates, no targeting subunit has been identified that localizes PP1 to the nucleolus. Identification of nucleolar PP1 targeting subunit(s) is important because all three PP1 isoforms are included in the nucleolar proteome, enzymatically active PP1 is present in nucleoli, and PP1gamma is highly concentrated in nucleoli of interphase cells. In this study, we identify NOM1 (nucleolar protein with MIF4G domain 1) as a PP1-interacting protein and further identify the NOM1 RVXF motif required for its binding to PP1. We also define the NOM1 nucleolar localization sequence. Finally, we demonstrate that NOM1 can target PP1 to the nucleolus and show that a specific NOM1 RVXF motif and the NOM1 nucleolar localization sequence are required for this targeting activity. We therefore conclude that NOM1 is a PP1 nucleolar targeting subunit, the first identified in eukaryotic cells.

Nucleocytoplasmic trafficking is required for functioning of the adaptor protein Sla1p in endocytosis

Dual localization of proteins at the plasma membrane and within the nucleus has been reported in mammalian cells. Among these proteins are those involved in cell adhesion structures and in clathrin-mediated endocytosis. In the case of endocytic proteins, trafficking to the nucleus is not known to play a role in their endocytic function. Here, we show localization of the yeast endocytic adaptor protein Sla1p to the nucleus as well as to the cell cortex and we demonstrate the importance of specific regions of Sla1p for this nuclear localization. A role for specific karyopherins (importins and exportins) in Sla1p nuclear localization is revealed. Furthermore, endocytosis of Sla1p-dependent cargo is defective in three strains with karyopherin mutations. Finally, we investigate possible functions for nuclear trafficking of endocytic proteins. Our data reveal for the first time that nuclear transport of endocytic proteins is important for functional endocytosis in Saccharomyces cerevisiae. We determine the mechanism, involving an alpha/beta importin pair, that facilitates uptake of Sla1p and demonstrate that nuclear transport is required for the functioning of Sla1p during endocytosis.

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.

Glc7/protein phosphatase 1 regulatory subunits can oppose the Ipl1/aurora protein kinase by redistributing Glc7

Faithful chromosome segregation depends on the opposing activities of the budding yeast Glc7/PP1 protein phosphatase and Ipl1/Aurora protein kinase. We explored the relationship between Glc7 and Ipl1 and found that the phosphorylation of the Ipl1 substrate, Dam1, was altered by decreased Glc7 activity, whereas Ipl1 levels, localization, and kinase activity were not. These data strongly suggest that Glc7 ensures accurate chromosome segregation by dephosphorylating Ipl1 targets rather than regulating the Ipl1 kinase. To identify potential Glc7 and Ipl1 substrates, we isolated ipl1-321 dosage suppressors. Seven genes (SDS22, BUD14, GIP3, GIP4, SOL1, SOL2, and PEX31) encode newly identified ipl1 dosage suppressors, and all 10 suppressors encode proteins that physically interact with Glc7. The overexpression of the Gip3 and Gip4 suppressors altered Glc7 localization, indicating they are previously unidentified Glc7 regulatory subunits. In addition, the overexpression of Gip3 and Gip4 from the galactose promoter restored Dam1 phosphorylation in ipl1-321 mutant cells and caused wild-type cells to arrest in metaphase with unsegregated chromosomes, suggesting that Gip3 and Gip4 overexpression impairs Glc7's mitotic functions. We therefore propose that the overexpression of Glc7 regulatory subunits can titrate Glc7 away from relevant Ipl1 targets and thereby suppress ipl1-321 cells by restoring the balance of phosphatase/kinase activity.

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.

Glc7-Reg1 phosphatase signals to Yck1,2 casein kinase 1 to regulate transport activity and glucose-induced inactivation of Saccharomyces maltose permease

The Saccharomyces casein kinase 1 isoforms encoded by the essential gene pair YCK1 and YCK2 control cell growth and morphogenesis and are linked to the endocytosis of several membrane proteins. Here we define roles for the Yck1,2 kinases in Mal61p maltose permease activation and trafficking, using a yck1delta yck2-2(ts) (yck(ts)) strain with conditional Yck activity. Moreover, we provide evidence that Glc7-Reg1 phosphatase acts as an upstream activator of Yck1,2 kinases in a novel signaling pathway that modulates kinase activity in response to carbon source availability. The yck(ts) strain exhibits significantly reduced maltose transport activity despite apparently normal levels and cell surface localization of maltose permease protein. Glucose-induced internalization and rapid loss of maltose transport activity of Mal61/HAp-GFP are not observed in the yck(ts) strain and maltose permease proteolysis is blocked. We show that a reg1delta mutant exhibits a phenotype remarkably similar to that conferred by yck(ts). The reg1delta phenotype is not enhanced in the yck(ts) reg1delta double mutant and is suppressed by increased Yck1,2p dosage. Further, although Yck2p localization and abundance do not change in the reg1delta mutant, Yck1,2 kinase activity, as assayed by glucose-induced HXT1 expression and Mth1 repressor stability, is substantially reduced in the reg1delta strain.

The Set1 methyltransferase opposes Ipl1 aurora kinase functions in chromosome segregation

A balance in the activities of the Ipl Aurora kinase and the Glc7 phosphatase is essential for normal chromosome segregation in yeast. We report here that this balance is modulated by the Set1 methyltransferase. Deletion of SET1 suppresses chromosome loss in ipl1-2 cells. Conversely, combination of SET1 and GLC7 mutations is lethal. Strikingly, these effects are independent of previously defined functions for Set1 in transcription initiation and histone H3 methylation. We find that Set1 is required for methylation of conserved lysines in a kinetochore protein, Dam1. Biochemical and genetic experiments indicate that Dam1 methylation inhibits Ipl1-mediated phosphorylation of flanking serines. Our studies demonstrate that Set1 has important, unexpected functions in mitosis. Moreover, our findings suggest that antagonism between lysine methylation and serine phosphorylation is a fundamental mechanism for controlling protein function.

Regulation of yeast mRNA 3' end processing by phosphorylation

Recent studies have found that the phosphatase Glc7 associates with the yeast cleavage/polyadenylation factor (CPF), but the role of Glc7 in 3' end processing has not been investigated. Here, we report that depletion of Glc7 causes shortened poly(A) tails in vivo and accumulation of phosphorylated Pta1, a CPF subunit. Removal of Glc7 also gives extract defective for poly(A) addition but normal for cleavage at the poly(A) site. Polyadenylation is rescued by addition of Glc7 or Pta1, but not by phosphorylated Pta1. Moreover, Ypi1, a Glc7-specific inhibitor, or the Cka1 kinase blocks poly(A) addition in wild-type (wt) extract. Pta1 interacts physically and genetically with Glc7, suggesting that Pta1 may also regulate Glc7 or recruit it to CPF. A weakened association of Fip1 with phosphorylated CPF may explain the specific effect on polyadenylation. These results support a model in which poly(A) synthesis is controlled by cycles of phosphorylation and dephosphorylation that require the action of Glc7.

Incorporation of flexibility into rigid-body docking: Applications in rounds 3-5 of CAPRI

We have submitted models for all 9 targets in Rounds 3-5 of CAPRI and have predicted at least 30% of the correct contacts for 4 of the targets and at least 10% of the correct contacts for another 4 targets. We have employed a variety of techniques but have had the greatest success by combining established rigid-body docking with a variety of initial conformations generated by molecular dynamics.

Importance of a Surface Hydrophobic Pocket on Protein Phosphatase-1 Catalytic Subunit in Recognizing Cellular Regulators

Cellular functions of protein phosphatase-1 (PP1), a major eukaryotic serine/threonine phosphatase, are defined by the association of PP1 catalytic subunits with endogenous protein inhibitors and regulatory subunits. Many PP1 regulators share a consensus RVXF motif, which docks within a hydrophobic pocket on the surface of the PP1 catalytic subunit. Although these regulatory proteins also possess additional PP1-binding sites, mutations of the RVXF sequence established a key role of this PP1-binding sequence in the function of PP1 regulators. WT PP1alpha, the C-terminal truncated PP1alpha-(1-306), a chimeric PP1alpha containing C-terminal sequences from PP2A, another phosphatase, PP1alpha-(1-306) with the RVXF-binding pocket substitutions L289R, M290K, and C291R, and PP2A were analyzed for their regulation by several mammalian proteins. These studies established that modifications of the RVXF-binding pocket had modest effects on the catalytic activity of PP1, as judged by recognition of substrates and sensitivity to toxins. However, the selected modifications impaired the sensitivity of PP1 to the inhibitor proteins, inhibitor-1 and inhibitor-2. In addition, they impaired the ability of PP1 to bind neurabin-I, the neuronal regulatory subunit, and G(M), the skeletal muscle glycogen-targeting subunit. These data suggested that differences in RVXF interactions with the hydrophobic pocket dictate the affinity of PP1 for cellular regulators. Substitution of a distinct RVXF sequence in inhibitor-1 that enhanced its binding and potency as a PP1 inhibitor emphasized the importance of the RVXF sequence in defining the function of this and other PP1 regulators. Our studies suggest that the diversity of RVXF sequences provides for dynamic physiological regulation of PP1 functions in eukaryotic cells.

Interactor-mediated nuclear translocation and retention of protein phosphatase-1

Protein Ser/Thr phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that controls numerous cellular processes by the dephosphorylation of key regulatory proteins. PP1 is expressed in various cellular compartments but is most abundant in the nucleus. We have examined the determinants for the nuclear localization of enhanced green fluorescent protein-tagged PP1 in COS1 cells. Our studies show that PP1gamma(1) does not contain a functional nuclear localization signal and that its nuclear accumulation does not require Sds22, which has previously been implicated in the nuclear accumulation of PP1 in yeast (Peggie, M. W., MacKelvie, S. H., Bloecher, A., Knatko, E. V., Tatchell, K., and Stark, M. J. R. (2002) J. Cell Sci. 115, 195-206). However, the nuclear targeting of PP1 isoforms was alleviated by the mutation of their binding sites for proteins that interact via an RVXF motif. Moreover, one of the mutants with a cytoplasmic accumulation and decreased affinity for RVXF motifs (PP1gamma(1)-F257A) could be re-targeted to the nucleus by the overexpression of nuclear interactors (NIPP1 (nuclear inhibitor of PP1) and PNUTS (PP1 nuclear targeting subunit)) with a functional RVXF motif. Also, the addition of a synthetic RVXF-containing peptide to permeabilized cells resulted in the loss of nuclear enhanced green fluorescent protein-PP1gamma(1). Finally, NIPP1(-/-) mouse embryos showed a nuclear hyperphosphorylation on threonine, consistent with a role for NIPP1 in the nuclear targeting and/or retention of PP1. Our data suggest that both the nuclear translocation and the nuclear retention of PP1 depend on its binding to interactors with an RVXF motif.

The Reg1-interacting proteins, Bmh1, Bmh2, Ssb1, and Ssb2, have roles in maintaining glucose repression in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, a type 1 protein phosphatase complex composed of the Glc7 catalytic subunit and the Reg1 regulatory subunit represses expression of many glucose-regulated genes. Here we show that the Reg1-interacting proteins Bmh1, Bmh2, Ssb1, and Ssb2 have roles in glucose repression. Deleting both BMH genes causes partially constitutive ADH2 expression without significantly increasing the level of Adr1 protein, the major activator of ADH2 expression. Adr1 and Bcy1, the regulatory subunit of cAMP-dependent protein kinase, are both required for this effect indicating that constitutive expression in Deltabmh1Deltabmh2 cells uses the same activation pathway that operates in Deltareg1 cells. Deletion of both BMH genes and REG1 causes a synergistic relief from repression, suggesting that Bmh proteins also act independently of Reg1 during glucose repression. A two-hybrid interaction with the Bmh proteins was mapped to amino acids 187-232, a region of Reg1 that is conserved in different classes of fungi. Deleting this region partially releases SUC2 from glucose repression. This indicates a role for the Reg1-Bmh interaction in glucose repression and also suggests a broad role for Bmh proteins in this process. An in vivo Reg1-Bmh interaction was confirmed by copurification of Bmh proteins with HA(3)-TAP-tagged Reg1. The nonconventional heat shock proteins Ssb1 and Ssb2 are also copurified with HA(3)-TAP-tagged Reg1. Deletion of both SSB genes modestly decreases repression of ADH2 expression in the presence of glucose, suggesting that Ssb proteins, perhaps through their interaction with Reg1, play a minor role in glucose repression.

Nuclear protein phosphatases with Kelch-repeat domains modulate the response to brassinosteroids in Arabidopsis

Perception of the plant steroid hormone brassinolide (BL) by the membrane-associated receptor kinase BRI1 triggers the dephosphorylation and accumulation in the nucleus of the transcriptional modulators BES1 and BZR1. We identified bsu1-1D as a dominant suppressor of bri1 in A abidopsis. BSU1 encodes a nuclear-localized serine-threonine protein phosphatase with an N-terminal Kelch-repeat domain, and is preferentially expressed in elongating cells. BSU1 is able to modulate the phosphorylation state of BES1, counter acting the action of the glycogen synthase kinase-3 BIN2, and leading to inc eased steady-state levels of dephosphorylated BES1. BSU1 belongs to a small gene family; loss-of-function analyses unravel the extent of functional overlap among members of the family and confirm the role of these phosphatases in the control of cell elongation by BL. Our data indicate that BES1 is subject to antagonistic phosphorylation and dephosphorylation reactions in the nucleus, which fine-tune the amplitude of the response to BL.

The type 1 phosphatase Reg1p-Glc7p is required for the glucose-induced degradation of fructose-1,6-bisphosphatase in the vacuole

Protein phosphatases play an important role in vesicular trafficking and membrane fusion processes. The type 1 phosphatase Glc7p and its regulatory subunit Reg1p were identified as required components in the glucose-induced targeting of the key gluconeogenic enzyme fructose-1,6-bisphosphatase (FBPase) to the vacuole for degradation. The interaction of Reg1p with Glc7p was important for the transport of FBPase from intermediate vacuole import and degradation (Vid) vesicles to vacuoles. The glc7-T152K mutant strain exhibited a reduced Reg1p binding along with defects in FBPase degradation and Vid vesicle trafficking to the vacuole. In this mutant, Vid vesicles were the most defective components, whereas the vacuole was also defective. Shp1p and Glc8p regulate Glc7p phosphatase activity and are required for FBPase degradation. In the Deltashp1 and Deltaglc8 strains, Reg1p-Glc7p interaction was not affected, suggesting that phosphatase activity is also necessary for FBPase degradation. Similar to those seen in the glc7-T152K mutant, the Deltashp1 and Deltaglc8 mutants exhibited severely defective Vid vesicles, but partially defective vacuoles. Taken together, our results suggest that Reg1p-Glc7p interaction and Glc7p phosphatase activity play a required role in the Vid vesicle to vacuole-trafficking step along the FBPase degradation pathway.

Detection of multiple splice variants of the nuclear protein phosphatase 1 regulator sds22 in rat liver nuclei

Antipeptide antibodies generated against the N terminus of the protein phosphatase 1 (PP1) binding protein sds22 detected at least four forms of the protein in a rat liver nuclear extract. Four of these immunoreactive bands likely correspond to four predicted forms of sds22 that are generated by alternative splicing. These four proteins are expressed at different levels and appear to be localized exclusively in the nucleus, and two of these proteins copurify with PPI on the protein phosphatase affinity matrix microcystin-Sepharose. Two higher molecular mass nuclear proteins that are immunoreactive with the sds22 antibodies also copurify on microcystin-Sepharose and may be novel forms of sds22 expressed in mammalian cells.

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.

Neurabins recruit protein phosphatase-1 and inhibitor-2 to the actin cytoskeleton

Inhibitor-2 (I-2) bound protein phosphatase-1 (PP1) and several PP1-binding proteins from rat brain extracts, including the actin-binding proteins, neurabin I and neurabin II. Neurabins from rat brain lysates were sedimented by I-2 and its structural homologue, I-4. The central domain of both neurabins bound PP1 and I-2, and mutation of a conserved PP1-binding motif abolished neurabin binding to both proteins. Microcystin-LR, a PP1 inhibitor, also attenuated I-2 binding to neurabins. Immunoprecipitation of neurabin I established its association with PP1 and I-2 in HEK293T cells and suggested that PP1 mediated I-2 binding to neurabins. The C terminus of I-2, although not required for PP1 binding, facilitated PP1 recruitment by neurabins, which also targeted I-2 to polymerized F-actin. Mutations that attenuated PP1 binding to I-2 and neurabin I suggested distinct and overlapping sites for these two proteins on the PP1 catalytic subunit. Immunocytochemistry in epithelial cells and cultured hippocampal neurons showed that endogenous neurabin II and I-2 colocalized at actin-rich structures, consistent with the ability of neurabins to target the PP1.I-2 complex to actin cytoskeleton and regulate cell morphology.

Characterisation and expression of a PP1 serine/threonine protein phosphatase (PfPP1) from the malaria parasite, Plasmodium falciparum: demonstration of its essential role using RNA interference

BACKGROUND

Reversible protein phosphorylation is relatively unexplored in the intracellular protozoa of the Apicomplexa family that includes the genus Plasmodium, to which belong the causative agents of malaria. Members of the PP1 family represent the most highly conserved protein phosphatase sequences in phylogeny and play essential regulatory roles in various cellular pathways. Previous evidence suggested a PP1-like activity in Plasmodium falciparum, not yet identified at the molecular level.

RESULTS

We have identified a PP1 catalytic subunit from P. falciparum and named it PfPP1. The predicted primary structure of the 304-amino acid long protein was highly similar to PP1 sequences of other species, and showed conservation of all the signature motifs. The purified recombinant protein exhibited potent phosphatase activity in vitro. Its sensitivity to specific phosphatase inhibitors was characteristic of the PP1 class. The authenticity of the PfPP1 cDNA was further confirmed by mutational analysis of strategic amino acid residues important in catalysis. The protein was expressed in all erythrocytic stages of the parasite. Abrogation of PP1 expression by synthetic short interfering RNA (siRNA) led to inhibition of parasite DNA synthesis.

CONCLUSIONS

The high sequence similarity of PfPP1 with other PP1 members suggests conservation of function. Phenotypic gene knockdown studies using siRNA confirmed its essential role in the parasite. Detailed studies of PfPP1 and its regulation may unravel the role of reversible protein phosphorylation in the signalling pathways of the parasite, including glucose metabolism and parasitic cell division. The use of siRNA could be an important tool in the functional analysis of Apicomplexan genes.

Protein phosphatase type 1 regulates ion homeostasis in Saccharomyces cerevisiae

Protein phosphatase type 1 (PP1) is encoded by the essential gene GLC7 in Saccharomyces cerevisiae. glc7-109 (K259A, R260A) has a dominant, hyperglycogen defect and a recessive, ion and drug sensitivity. Surprisingly, the hyperglycogen phenotype is partially retained in null mutants of GAC1, GIP2, and PIG1, which encode potential glycogen-targeting subunits of Glc7. The R260A substitution in GLC7 is responsible for the dominant and recessive traits of glc7-109. Another mutation at this residue, glc7-R260P, confers only salt sensitivity, indicating that the glycogen and salt traits of glc7-109 are due to defects in distinct physiological pathways. The glc7-109 mutant is sensitive to cations, aminoglycosides, and alkaline pH and exhibits increased rates of l-leucine and 3,3'-dihexyloxacarbocyanine iodide uptake, but it is resistant to molar concentrations of sorbitol or KCl, indicating that it has normal osmoregulation. KCl suppresses the ion and drug sensitivities of the glc7-109 mutant. The CsCl sensitivity of this mutant is suppressed by recessive mutations in PMA1, which encodes the essential plasma membrane H(+)ATPase. Together, these results indicate that Glc7 regulates ion homeostasis by controlling ion transport and/or plasma membrane potential, a new role for Glc7 in budding yeast.

Regulator-driven functional diversification of protein phosphatase-1 in eukaryotic evolution

We have used the (nearly) completed eukaryotic genome sequences to trace the evolution of thirteen families of established vertebrate regulators of type-1 protein phosphatases (PP1). Two of these families are present in all lineages of the eukaryotic crown and therefore qualify as candidate primordial regulators that determined the surface of PP1. The set of regulators of PP1 has continued to expand ever since, often in response to functional innovations in different eukaryotic lineages. In particular, the development of metazoan multicellularity was accompanied by an explosive increase in the number of regulators of PP1. The further increase in the functional diversity of PP1 in the vertebrate lineage was mainly achieved by the duplication of genes for regulatory subunits and by the conversion of already existing proteins into regulators of PP1. Unexpectedly, our analysis has also enabled us to classify nine poorly characterized proteins as likely regulators of PP1.

Protein phosphatase 1 – targeted in many directions

Protein phosphatase 1 (PP1) is a major eukaryotic protein serine/threonine phosphatase that regulates an enormous variety of cellular functions through the interaction of its catalytic subunit (PP1c) with over fifty different established or putative regulatory subunits. Most of these target PP1c to specific subcellular locations and interact with a small hydrophobic groove on the surface of PP1c through a short conserved binding motif – the RVxF motif – which is often preceded by further basic residues. Weaker interactions may subsequently enhance binding and modulate PP1 activity/specificity in a variety of ways. Several putative targeting subunits do not possess an RVxF motif but nevertheless interact with the same region of PP1c. In addition, several ‘modulator’ proteins bind to PP1c but do not possess a domain targeting them to a specific location. Most are potent inhibitors of PP1c and possess at least two sites for interaction with PP1c, one of which is identical or similar to the RVxF motif.Regulation of PP1c in response to extracellular and intracellular signals occurs mostly through changes in the levels, conformation or phosphorylation status of targeting subunits. Understanding of the mode of action of PP1c complexes may facilitate development of drugs that target particular PP1c complexes and thereby modulate the phosphorylation state of a very limited subset of proteins.

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 (PP1C) and which has been proposed to regulate one or more functions of PP1C 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 PP1C (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°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