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.     

Pho85 phosphorylates the Glc7 protein phosphatase regulator Glc8 in vivo

The budding yeast Glc7 serine/threonine protein phosphatase-1 is regulated by Glc8, the yeast ortholog of mammalian phosphatase inhibitor-2. In this work, we demonstrated that similarly to inhibitor-2, Glc8 function is regulated by phosphorylation. The cyclin-dependent protein kinase, Pho85, in conjunction with the related cyclins Pcl6 and Pcl7 comprise the major Glc8 kinase in vivo and in vitro. Several glc7 mutations are dependent on the presence of Glc8 for viability. For example, glc7 alleles R121K, R142H, and R198D are lethal in combination with a glc8 deletion. We found that glc7-R121K is lethal in combination with a pho85 deletion. This finding indicates that Pho85 is the sole Glc8 kinase in vivo. Furthermore, glc7-R121K is also lethal when combined with deletions of pcl6, plc7, pcl8, and pcl10, indicating that these related cyclins redundantly activate Pho85 for Glc8 phosphorylation in vivo. In vitro kinase assays and genetic results indicate that Pho85 cyclins Pcl6 and Pcl7 comprise the predominant Glc8 kinase.     

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.     

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.     

Lack of tension at kinetochores activates the spindle checkpoint in budding yeast

The spindle checkpoint delays the onset of anaphase until all pairs of sister chromatids are attached to the mitotic spindle. The checkpoint could monitor the attachment of microtubules to kinetochores, the tension that results from the two sister chromatids attaching to opposite spindle poles, or both. We tested the role of tension by allowing cells to enter mitosis without a prior round of DNA replication. The unreplicated chromatids are attached to spindle microtubules but are not under tension since they lack a sister chromatid that could attach to the opposite pole. Because the spindle checkpoint is activated in these cells, we conclude that the absence of tension at the yeast kinetochore is sufficient to activate the spindle checkpoint in mitosis.     

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.     

Tamoxifen activates smooth muscle BK channels through the regulatory beta 1 subunit.

Estrogen (17beta-estradiol; 17betaE) and xenoestrogens, estrogenic compounds that are not steroid hormones, have non-genomic actions at plasma membrane receptors unrelated to the nuclear estrogen receptor. The open probability (P(o)) of large conductance Ca(2+)/voltage-sensitive k(+)(BK) channels is increased by 17betaE through the regulatory beta1 subunit. The pharmacological nature of the putative membrane binding site is unclear. We probed the site by determining whether tamoxifen ((Z)-1-(p-dimethylaminoethoxy-phenyl)-1,2-diphenyl-1-butene; Tx), a chemotherapeutic xenoestrogen, increased P(o) in clinically relevant concentrations (0.1-10 microm). In whole cell patch clamp recordings on canine colonic myocytes, which express the beta1 subunit, Tx activated charybdotoxin-sensitive K(+) current. In single channel experiments, Tx increased the NP(o) (P(o) x number channels; N) and decreased the unitary conductance (gamma) of BK channels. Tx increased NP(o) (EC(50) = 0.65 microm) in excised membrane patches independent of Ca(2+) changes. The Tx mechanism of action requires the beta1 subunit, as Tx increased the NP(o) of Slo alpha expressed in human embryonic kidney cells only in the presence of the beta1 subunit. Tx decreased gamma of the alpha subunit expressed alone, without effect on NP(o). Our data indicate that Tx increases BK channel activity in therapeutic concentrations and reveal novel pharmacological properties attributable to the alpha and beta1 subunits. These data shed light on BK channel structure and function, non-genomic mechanisms of regulation, and physiologically and therapeutically relevant effects of xenoestrogens.     

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.     

Molecular characterization of Ypi1, a novel Saccharomyces cerevisiae type 1 protein phosphatase inhibitor

The Saccharomyces cerevisiae open reading frame YFR003c encodes a small (155-amino acid) hydrophilic protein that we identified as a novel, heat-stable inhibitor of type 1 protein phosphatase (Ypi1). Ypi1 interacts physically in vitro with both Glc7 and Ppz1 phosphatase catalytic subunits, as shown by pull-down assays. Ypi1 inhibits Glc7 but appears to be less effective toward Ppz1 phosphatase activity under the conditions tested. Ypi1 contains a 48RHNVRW53 sequence, which resembles the characteristic consensus PP1 phosphatase binding motif. A W53A mutation within this motif abolishes both binding to and inhibition of Glc7 and Ppz1 phosphatases. Deletion of YPI1 is lethal, suggesting a relevant role of the inhibitor in yeast physiology. Cells overexpressing Ypi1 display a number of phenotypes consistent with an inhibitory role of this protein on Glc7, such as decreased glycogen content and an increased growth defect in a slt2/mpk1 mitogen-activated protein kinase-deficient background. Taking together, these results define Ypi1 as the first inhibitory subunit of Glc7 identified in budding yeast.     

Scd5p mediates phosphoregulation of actin and endocytosis by the type 1 phosphatase Glc7p in yeast

Pan1p plays essential roles in both actin and endocytosis in yeast. It interacts with, and regulates the function of, multiple endocytic proteins and actin assembly machinery. Phosphorylation of Pan1p by the kinase Prk1p down-regulates its activity, resulting in disassembly of the endocytic vesicle coat complex and termination of vesicle-associated actin polymerization. In this study, we focus on the mechanism that acts to release Pan1p from phosphorylation inhibition. We show that Pan1p is dephosphorylated by the phosphatase Glc7p, and the dephosphorylation is dependent on the Glc7p-targeting protein Scd5p, which itself is a phosphorylation target of Prk1p. Scd5p links Glc7p to Pan1p in two ways: directly by interacting with Pan1p and indirectly by interacting with the Pan1p-binding protein End3p. Depletion of Glc7p from the cells causes defects in cell growth, actin organization, and endocytosis, all of which can be partially suppressed by deletion of the PRK1 gene. These results suggest that Glc7p antagonizes the activity of the Prk1p kinase in regulating the functions of Pan1p and possibly other actin- and endocytosis-related proteins.     

Sip5 interacts with both the Reg1/Glc7 protein phosphatase and the Snf1 protein kinase of Saccharomyces cerevisiae

The Snf1 protein kinase is an essential component of the glucose starvation signalling pathway in Saccharomyces cerevisiae. We have used the two-hybrid system to identify a new protein, Sip5, that interacts with the Snf1 kinase complex in response to glucose limitation. Coimmunoprecipitation studies confirmed the association of Sip5 and Snf1 in cell extracts. We found that Sip5 also interacts strongly with Reg1, the regulatory subunit of the Reg1/Glc7 protein phosphatase 1 complex, in both two-hybrid and coimmunoprecipitation assays. Previous work showed that Reg1/Glc7 interacts with the Snf1 kinase under glucose-limiting conditions and negatively regulates its activity. Sip5 is the first protein that has been shown to interact with both Snf1 and Reg1/Glc7. Genetic analysis showed that the two-hybrid interaction between Reg1 and Snf1 is reduced threefold in a sip5Delta mutant. These findings suggest that Sip5 facilitates the interaction between the Reg1/Glc7 phosphatase and the Snf1 kinase.     

The checkpoint clamp activates Mec1 kinase during initiation of the DNA damage checkpoint

Yeast Mec1/Ddc2 protein kinase, the ortholog of human ATR/ATRIP, plays a central role in the DNA damage checkpoint. The PCNA-like clamp Rad17/Mec3/Ddc1 (the 9-1-1 complex in human) and its loader Rad24-RFC are also essential components of this signal transduction pathway. Here we have studied the role of the clamp in regulating Mec1, and we delineate how the signal generated by DNA lesions is transduced to the Rad53 effector kinase. The checkpoint clamp greatly activates the kinase activity of Mec1, but only if the clamp is appropriately loaded upon partial duplex DNA. Activated Mec1 phosphorylates the Ddc1 and Mec3 subunits of the clamp, the Rad24 subunit of the loader, and the Rpa1 and Rpa2 subunits of RPA. Phosphorylation of Rad53, and of human PHAS-1, a nonspecific target, also requires a properly loaded clamp. Phosphorylation and binding studies with individual clamp subunits indicate that the Ddc1 subunit mediates the functional interactions with Mec1.     

Genetic interaction between the Ras-CAMP pathway and the Dis2s1/Glc7 protein phosphatase in Saccharomyces cerevisiae

The Saccharomyces cerevisiae DIS2S1/GLC7 gene encodes a type 1 protein phosphatase indispensable for cell proliferation. We found that introduction of a multicopy DIS2S1 plasmid impaired growth of cells with reduced activity of the cAMP-dependent protein kinase. In order to understand further the interaction between the two enzymes, a temperature-sensitive mutation in the DIS2S1 gene was isolated. The mutant accumulated less glycogen than wild type at the permissive temperature, indicating that activity of the Dis2s1 protein phosphatase is attenuated by the mutation. Furthermore, the dis2s1 ^ts mutation was shown to be suppressed by a multicopy plasmid harboring PDE2, a gene for cAMP phosphodiesterase. These results indicate that the Ras-cAMP pathway interacts genetically with the DIS2S1/GLC7 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.     

The MyD116 African swine fever virus homologue interacts with the catalytic subunit of protein phosphatase 1 and activates its phosphatase activity

The DP71L protein of African swine fever virus (ASFV) shares sequence similarity with the herpes simplex virus ICP34.5 protein over a C-terminal domain. We showed that the catalytic subunit of protein phosphatase 1 (PP1) interacts specifically with the ASFV DP71L protein in a yeast two-hybrid screen. The chimeric full-length DP71L protein, from ASFV strain Badajoz 71 (BA71V), fused to glutathione S-transferase (DP71L-GST) was expressed in Escherichia coli and shown to bind specifically to the PP1-alpha catalytic subunit expressed as a histidine fusion protein (6xHis-PP1alpha) in E. coli. The functional effects of this interaction were investigated by measuring the levels of PP1 and PP2A in ASFV-infected Vero cells. This showed that infection with wild-type ASFV strain BA71V activated PP1 between two- and threefold over that of mock-infected cells. This activation did not occur in cells infected with the BA71V isolate in which the DP71L gene had been deleted, suggesting that expression of DP71L leads to PP1 activation. In contrast, no effect was observed on the activity of PP2A following ASFV infection. We showed that infection of cells with wild-type BA71V virus resulted in decreased phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2alpha). ICP34.5 recruits PP1 to dephosphorylate the alpha subunit of eukaryotic translational initiation factor 2 (also known as eIF-2alpha); possibly the ASFV DP71L protein has a similar function.     

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.     

Structural basis for the recognition of regulatory subunits by the catalytic subunit of protein phosphatase 1.

The diverse forms of protein phosphatase 1 in vivo result from the association of its catalytic subunit (PP1c) with different regulatory subunits, one of which is the G-subunit (G(M)) that targets PP1c to glycogen particles in muscle. Here we report the structure, at 3.0 A resolution, of PP1c in complex with a 13 residue peptide (G(M[63-75])) of G(M). The residues in G(M[63-75]) that interact with PP1c are those in the Arg/Lys-Val/Ile-Xaa-Phe motif that is present in almost every other identified mammalian PP1-binding subunit. Disrupting this motif in the G(M[63-75]) peptide and the M(110[1-38]) peptide (which mimics the myofibrillar targeting M110 subunit in stimulating the dephosphorylation of myosin) prevents these peptides from interacting with PP1. A short peptide from the PP1-binding protein p53BP2 that contains the RVXF motif also interacts with PP1c. These findings identify a recognition site on PP1c, invariant from yeast to humans, for a critical structural motif on regulatory subunits. This explains why the binding of PP1 to its regulatory subunits is mutually exclusive, and suggests a novel approach for identifying the functions of PP1-binding proteins whose roles are unknown.     

GAC1 may encode a regulatory subunit for protein phosphatase type 1 in Saccharomyces cerevisiae.

Elevated dosage of the GAC1 gene from the yeast Saccharomyces cerevisiae causes hyperaccumulation of glycogen whereas a gene disruption of GAC1 results in reduced glycogen levels. Glycogen synthase is almost entirely in the active, glucose 6-phosphate-independent, form in cells with increased gene dosage of GAC1 whereas the enzyme is mostly in the inactive form in strains lacking GAC1. GAC1 encodes an 88 kDa protein that is similar to the regulatory subunit (RG1) of phosphoprotein phosphatase type 1 (PP-1) from skeletal muscle that targets PP-1 to glycogen particles. Taken together, these results suggest that GAC1 encodes a regulatory subunit of PP-1. As previously shown for glycogen phosphorylase (GPH1), GAC1 RNA accumulates concomitantly with the appearance of glycogen. A strain with a mutation in the regulatory subunit of the cAMP-dependent protein kinase (bcy1) fails to accumulate GPH1 and GAC1 RNA. These results point to coordinate regulation of enzymes involved in glycogen metabolism at the level of RNA accumulation and indicate that at least part of this control is exerted by the RAS-cAMP pathway.