Diversity in the regulatory B-subunits of protein phosphatase 2A: identification of a novel isoform highly expressed in brain

Human immunodeficiency virus type 1 Vpr-mediated G(2) cell cycle arrest: Vpr interferes with cell cycle signaling cascades by interacting with the B subunit of serine/threonine protein phosphatase 2A

The Vpr protein of primate lentiviruses arrests cell cycling at the G(2)/M phase through an inactivation of cyclin B-p34(cdc2) and its upstream regulator cdc25. We provide here biochemical and functional evidence demonstrating that human immunodeficiency virus type 1 (HIV-1) Vpr mediates G(2) arrest by forming a complex with protein phosphatase 2A (PP2A), an upstream regulator of cdc25. Vpr associates with PP2A through a specific interaction with the B55 regulatory subunit. This interaction is necessary but not sufficient for G(2) arrest. Interestingly, we found that Vpr association with B55-containing PP2A targets the enzymatic complex to the nucleus and, importantly, enhances the recruitment and dephosphorylation of the cdc25 substrate. Our data suggest that Vpr mediates G(2) arrest by enhancing the nuclear import of PP2A and by positively modulating its catalytic activity towards active phosphorylated nuclear cdc25.

Characterisation of two protein phosphatase 2A holoenzymes from maize seedlings

Two holoenzymes of protein phosphatase 2A (PP2A), designated PP2AI and PP2AII, were purified from maize seedlings. The subunit composition of maize holoenzymes generally resembled those of animal PP2A. Using SDS/PAGE and Western blots with antibodies generated against peptides derived from animal PP2A, we established the subunit composition of plant protein phosphatase 2A. In both maize holoenzymes, a 38000 catalytic (PP2Ac) and a 66000 constant regulatory subunit (A) constituting the core dimer of PP2A were present. In addition, PP2AI (180000-200000) contained a protein of 57000 which reacted with antibodies generated against the peptide (EFDYLKSLEIEE) conserved in all eukaryotic Balpha regulatory subunits. In contrast, none of the proteins visualised in PP2AII (140000-160000) by double staining reacted with these antibodies. The activity of PP2AI measured with (32)P-labelled phosphorylase a in the presence of protamine and ammonium sulfate is about two times higher than that of PP2AII. PP2AI and PP2AII displayed different patterns of activation by protamine, polylysine and histone H1 and exhibit high sensitivity toward inhibition by okadaic acid. The data obtained provide direct biochemical evidence for the existence in plants of PP2A holoenzymes composed of a catalytic subunit complexed with one or two regulatory subunits.

Molecular cloning and mapping of the brain-abundant B1gamma subunit of protein phosphatase 2A, PPP2R2C, to human chromosome 4p16

Protein phosphatase 2A (PP2A) is one kind of serine/ threonine protein phosphatase regulating mainly cell growth and division. It comprises three subunits, A, B, and C. The B-subunit is involved in enzyme activity and substrate specificity. The B-subunit family is of great diversity and is divided into three classes, the B1, B2, and B3 subfamilies. Until now, two members of the B1 subfamily, B1alpha and B1beta, have been identified in human. In this report, the third member of the sub-family, B1gamma, was identified, and its cDNA was isolated from a human brain cDNA library. This novel cDNA is 4,120 bp in length and contains an open reading frame (nt 55-1,398) encoding 447 amino acid residues. The putative protein shares 81 and 85% identity with B1alpha (PPP2R2A) and B1beta (PPP2R2B), respectively, and was named PPP2R2C for its high level of homology to the other two isoforms. One remarkable characteristic of this novel gene is that it is highly expressed in brain with a 4.7-kb transcript while it is nearly undetectable in other tissues. In addition, the PPP2R2C gene was localized to human chromosome 4p16 between markers D4S2925 and D4S3007 with 5.45 cR (LOD > 14) and 2.63 cR (LOD > 15) RH distance, respectively, by radiation hybrid panel mapping.

Isolation and characterization of par1(+) and par2(+): two Schizosaccharomyces pombe genes encoding B' subunits of protein phosphatase 2A

Protein phosphatase 2A (PP2A) is one of the major serine/threonine phosphatases found in eukaryotic cells. We cloned two genes, par1(+) and par2(+), encoding distinct B' subunits of PP2A in fission yeast. They share 52% identity at the amino acid sequence level. Neither gene is essential but together they are required for normal septum positioning and cytokinesis, for growth at both high and low temperature, and for growth under a number of stressful conditions. Immunofluorescence microscopy revealed that Par2p has a cell-cycle-related localization pattern, being localized at cell ends during interphase and forming a medial ring in cells that are undergoing septation and cytokinesis. Our analyses also indicate that Par1p is more abundant than Par2p in the cell. Cross-organism studies showed that both par1(+) and par2(+) could complement the rts1Delta allele in Saccharomyces cerevisiae, albeit to different extents, in spite of the fact that neither contains a serine/threonine-rich N-terminal domain like that found in the S. cerevisiae homolog Rts1p. Thus, while Schizosaccharomyces pombe is more similar to higher eukaryotes with respect to its complement of B'-encoding genes, the function of those proteins is conserved relative to that of Rts1p.

WD40 repeat proteins striatin and S/G(2) nuclear autoantigen are members of a novel family of calmodulin-binding proteins that associate with protein phosphatase 2A

Protein phosphatase 2A (PP2A) is a multifunctional serine/threonine phosphatase that is critical to many cellular processes including development, neuronal signaling, cell cycle regulation, and viral transformation. PP2A has been implicated in Ca(2+)-dependent signaling pathways, but how PP2A is targeted to these pathways is not understood. We have identified two calmodulin (CaM)-binding proteins that form stable complexes with the PP2A A/C heterodimer and may represent a novel family of PP2A B-type subunits. These two proteins, striatin and S/G(2) nuclear autoantigen (SG2NA), are highly related WD40 repeat proteins of previously unknown function and distinct subcellular localizations. Striatin has been reported to associate with the post-synaptic densities of neurons, whereas SG2NA has been reported to be a nuclear protein expressed primarily during the S and G(2) phases of the cell cycle. We show that SG2NA, like striatin, binds to CaM in a Ca(2+)-dependent manner. In addition to CaM and PP2A, several unidentified proteins stably associate with the striatin-PP2A and SG2NA-PP2A complexes. Thus, one mechanism of targeting and organizing PP2A with components of Ca(2+)-dependent signaling pathways may be through the molecular scaffolding proteins striatin and SG2NA.

Decreased protein phosphatase 2A activity in hippocampal long-term potentiation

Using autophosphorylated Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) as substrate, we now find that long-term potentian (LTP) induction and maintenance are also associated with a significant decrease in calyculin A-sensitive protein phosphatase (protein phosphatase 2A) activity, without changes in Mg2+-dependent protein phosphatase (protein phosphatase 2C) activity. This decrease in protein phosphatase 2A activity was prevented when LTP induction was inhibited by treatment with calmidazolium or D-2-amino-5-phosphonopentanoic acid. In addition, the application of high-frequency stimulation to 32P-labeled hippocampal slices resulted in increases in the phosphorylation of a 55-kDa protein immunoprecipitated with anti-phosphatase 2A antibodies. Use of a specific antibody revealed that the 55-kDa protein is the B'alpha subunit of protein phosphatase 2A. Following purification of brain protein phosphatase 2A, the B'alpha subunit was phosphorylated by CaM kinase II, an event that led to the reduction of protein phosphatase 2A activity. These results suggest that the decreased activity in protein phosphatase 2A following LTP induction contributes to the maintenance of constitutively active CaM kinase II and to the long-lasting increase in phosphorylation of synaptic components implicated in LTP.

PR48, a novel regulatory subunit of protein phosphatase 2A, interacts with Cdc6 and modulates DNA replication in human cells

Initiation of DNA replication in eukaryotes is dependent on the activity of protein phosphatase 2A (PP2A), but specific phosphoprotein substrates pertinent to this requirement have not been identified. A novel regulatory subunit of PP2A, termed PR48, was identified by a yeast two-hybrid screen of a human placental cDNA library, using human Cdc6, an essential component of prereplicative complexes, as bait. PR48 binds specifically to an amino-terminal segment of Cdc6 and forms functional holoenzyme complexes with A and C subunits of PP2A. PR48 localizes to the nucleus of mammalian cells, and its forced overexpression perturbs cell cycle progression, causing a G(1) arrest. These results suggest that dephosphorylation of Cdc6 by PP2A, mediated by a specific interaction with PR48, is a regulatory event controlling initiation of DNA replication in mammalian cells.

Effects of serine/threonine protein phosphatases on ion channels in excitable membranes

This review deals with the influence of serine/threonine-specific protein phosphatases on the function of ion channels in the plasma membrane of excitable tissues. Particular focus is given to developments of the past decade. Most of the electrophysiological experiments have been performed with protein phosphatase inhibitors. Therefore, a synopsis is required incorporating issues from biochemistry, pharmacology, and electrophysiology. First, we summarize the structural and biochemical properties of protein phosphatase (types 1, 2A, 2B, 2C, and 3-7) catalytic subunits and their regulatory subunits. Then the available pharmacological tools (protein inhibitors, nonprotein inhibitors, and activators) are introduced. The use of these inhibitors is discussed based on their biochemical selectivity and a number of methodological caveats. The next section reviews the effects of these tools on various classes of ion channels (i.e., voltage-gated Ca(2+) and Na(+) channels, various K(+) channels, ligand-gated channels, and anion channels). We delineate in which cases a direct interaction between a protein phosphatase and a given channel has been proven and where a more complex regulation is likely involved. Finally, we present ideas for future research and possible pathophysiological implications.

An anchoring factor targets protein phosphatase 2A to brain microtubules

Protein phosphatase 2A (PP2A) is a ubiquitously expressed serine/threonine phosphatase composed of a heterodimeric core enzyme that associates with a variety of regulatory subunits. A fraction of brain PP2A associates with microtubules and may play a role in regulating phosphorylation of microtubule-associated proteins. We examined the isoform specificity and the mechanism involved in the association of PP2A with brain microtubules. Only the R2alpha (B/PR55alpha) and R2beta (B/PR55beta) regulatory subunits associated with endogenous neural microtubules. Neither the R2gamma (B/PR55gamma) nor members of the R5 (B'/PR56) family of regulatory subunits co-sedimented with microtubules, although abundant amounts of these proteins were detected in brain. The efficient association of PP2A with microtubules in vitro was dependent on an anchoring activity present in a brain protein fraction containing microtubule-associated and microtubule-interacting proteins. Anchoring factor-dependent association of PP2A with microtubules was specific for the heterotrimeric form of PP2A. The core dimer and the isolated subunits of PP2A had very little affinity for microtubules. Characterization of a fraction enriched in the anchoring factor showed that the activity was a heat labile protein that does not correspond to classical microtubule-associated proteins. The anchoring factor associated with microtubules independently of PP2A. These results indicate the association of PP2A with microtubules can be mediated by an anchoring factor that interacts in an isoform-specific manner with heterotrimeric forms of the phosphatase.

Cloning and characterization of B delta, a novel regulatory subunit of protein phosphatase 2A

Variable regulatory subunits of protein phosphatase 2A (PP2A) modulate activity, substrate selectivity and subcellular targeting of the enzyme. We have cloned a novel member of the B type regulatory subunit family, B delta, which is most highly related to B alpha. B delta shares with B alpha epitopes previously used to generate subunit-specific antibodies. Like B alpha, but unlike B beta and B gamma which are highly brain-enriched, B delta mRNA and protein expression in tissues is widespread. B delta is a cytosolic subunit of PP2A with a subcellular localization different from B alpha and may therefore target a pool of PP2A holoenzymes to specific substrates.

Brain protein serine/threonine phosphatases

All of the known protein serine/threonine phosphatases are expressed in the brain. These enzymes participate in a variety of signaling pathways that modulate neuronal activity. The multifunctional activity of many serine/threonine phosphatases is achieved through their association with targeting proteins. Identification and analysis of targeting molecules has led to new insights into the functions of protein phosphatases in neuronal signaling. The recent use of transgenic mice has also increased our understanding of the physiological roles of these enzymes in the brain.

Vimentin dephosphorylation by protein phosphatase 2A is modulated by the targeting subunit B55

The intermediate filament protein vimentin is a major phosphoprotein in mammalian fibroblasts, and reversible phosphorylation plays a key role in its dynamic rearrangement. Selective inhibition of type 2A but not type 1 protein phosphatases led to hyperphosphorylation and concomitant disassembly of vimentin, characterized by a collapse into bundles around the nucleus. We have analyzed the potential role of one of the major protein phosphatase 2A (PP2A) regulatory subunits, B55, in vimentin dephosphorylation. In mammalian fibroblasts, B55 protein was distributed ubiquitously throughout the cytoplasm with a fraction associated to vimentin. Specific depletion of B55 in living cells by antisense B55 RNA was accompanied by disassembly and increased phosphorylation of vimentin, as when type 2A phosphatases were inhibited using okadaic acid. The presence of B55 was a prerequisite for PP2A to efficiently dephosphorylate vimentin in vitro or to induce filament reassembly in situ. Both biochemical fractionation and immunofluorescence analysis of detergent-extracted cells revealed that fractions of PP2Ac, PR65, and B55 were tightly associated with vimentin. Furthermore, vimentin-associated PP2A catalytic subunit was displaced in B55-depleted cells. Taken together these data show that, in mammalian fibroblasts, the intermediate filament protein vimentin is dephosphorylated by PP2A, an event targeted by B55.

Regulation of beta-catenin signaling by the B56 subunit of protein phosphatase 2A

Dysregulation of Wnt-beta-catenin signaling disrupts axis formation in vertebrate embryos and underlies multiple human malignancies. The adenomatous polyposis coli (APC) protein, axin, and glycogen synthase kinase 3beta form a Wnt-regulated signaling complex that mediates the phosphorylation-dependent degradation of beta-catenin. A protein phosphatase 2A (PP2A) regulatory subunit, B56, interacted with APC in the yeast two-hybrid system. Expression of B56 reduced the abundance of beta-catenin and inhibited transcription of beta-catenin target genes in mammalian cells and Xenopus embryo explants. The B56-dependent decrease in beta-catenin was blocked by oncogenic mutations in beta-catenin or APC, and by proteasome inhibitors. B56 may direct PP2A to dephosphorylate specific components of the APC-dependent signaling complex and thereby inhibit Wnt signaling.

Protein phosphatase 2A: who shall regulate the regulator?

Protein phosphatases are responsible for keeping the signaling output of stimulus-activated protein kinases in check; but protein phosphatases are also themselves targets and conveyors of biological signals. Among the major serine/threonine phosphatases, protein phosphatase 2A (PP2A) appears to play a privileged role in the regulation of cell growth and division. How PP2A is regulated is an intriguing question. This review will focus on the role of local protein-protein interactions in PP2A control. Work from a number of laboratories has shown that the catalytic activity, substrate specificity, and subcellular targeting of PP2A are regulated by a remarkably diverse range of regulatory subunits and enzyme inhibitors. On the pathological side, DNA tumor viruses subvert PP2A function by producing proteins that compete with specific regulatory subunits. By interfering with PP2A, these viral proteins can elicit changes in the activity of specific signal transduction pathways, such as the mitogen-activated protein kinase cascade. Recent data indicate that besides classical holoenzyme forms, a fraction of PP2A molecules are associated with novel partners implicated in signal transduction. PP2A biochemically and genetically interacts with the Tap42/alpha4 protein, which is part of a rapamycin-sensitive pathway that connects extracellular stimuli to the initiation of mRNA translation. PP2A also binds to CK2alpha, the catalytic subunit of CK2 (formerly casein kinase 2), and binding is sensitive to mitogenic signaling. The potent effect of quantitatively minor PP2A partners might be explained by a general requirement for docking interactions with substrates under intracellular conditions.

Molecular characterization of the B' regulatory subunit gene family of Arabidopsis protein phosphatase 2A

Type 2A serine/threonine protein phosphatases (PP2A) have been implicated as important mediators of a diverse array of reversible protein phosphorylation events in plants. We have identified a novel Arabidopsis gene (AtB' delta) which encodes a 55-kDa B' type regulatory subunit of PP2A. The protein encoded by this gene is 57-63% identical and 69-74% similar to the previously identified AtB' genes. The AtB' delta gene appears to be expressed in all Arabidopsis organs indicating its protein product has a basic housekeeping function in plant cells. Unlike certain mRNAs derived from the AtB' gamma gene, AtB' delta mRNAs do not fluctuate significantly in response to heat stress. Further analysis of cDNA sequences derived from the AtB' genes identified an alternatively spliced cDNA derived from AtB' gamma. This cDNA differs from the previously identified AtB' gamma cDNA by the absence of a 133-bp region in its 5' untranslated region. The missing 133-bp region appears to constitute an unspliced intron and its presence in the AtB' gamma gene was confirmed by PCR using Arabidopsis genomic DNA as a template. AtB' gamma mRNA containing the 133-bp intron accumulate in all Arabidopsis organs and their levels fluctuate differentially in response to heat stress. The 133-bp insert contains two short open reading frames and hence might serve as a translational control mechanism affecting AtB' gamma protein synthesis. Finally we show, using both the yeast two hybrid system and in vitro binding assays, that the B' subunit of Arabidopsis PP2A is able to associate with other PP2A subunits, supporting the notion that the B' protein serves as a regulator of PP2A activity in plants.

Developmental expression and co-localization of cyclin G1 and the B' subunits of protein phosphatase 2a in neurons

Cyclin G1 is a recently cloned transcriptional target of p53, it is located in neurons and ventricular ependymal cells and is elevated in neurons after axotomy and cerebral ischemia. The biological function for cyclin G1 in differentiated neurons has thus far not been elucidated. Recently, cyclin G1 has been shown to interact with the B' subunits of serine/threonine protein phosphatase 2A (PP2A) in a rat fibroblast cell line [K. Okamoto, C., Kamibayashi, M. Serrano, C. Prives, M.C. Mumby, D. Beach, p53-dependent association between cyclin G and the B' subunit of protein phosphatase 2A, Mol. Cell. Biol. 16 (1996) 6593-6602]. To further explore whether a similar interaction between cyclin G1 and PP2A B' subunits exists in the central nervous system, the present study compared the regional and developmental expression pattern, subcellular distribution and complex formation between cyclin G1 and the PP2A B' regulatory subunits in the rat brain. In situ hybridization of cyclin G1 and the B'alpha and B'beta subunits of PP2A showed an overlapping distribution in neurons of the cerebral cortex, hippocampus and thalamus at embryonic and early postnatal ages, but their developmental regulation differed. Whereas mRNA and protein levels of PP2A B' subunits were high in the cortical plate, subiculum, hippocampal areas and thalamus at E20 and decreased with age, those of cyclin G1 increased with age and were maximal in the adult cortex and hippocampus. In rat 14-day-old embryonic cortical cultures, cyclin G1 and PP2A B'alpha protein co-localized in nuclear and perinuclear areas of neurons, and both proteins were highly expressed in nuclei of cortical and hippocampal pyramidal cells and the mitral cell layer of the neonatal olfactory bulb. Both cyclin G1 and the PP2A regulatory B'alpha subunits were specifically expressed in neurons and not in glial cells. Antibodies raised against the B'alpha subunits of PP2A immunoprecipitated cyclin G1 in adult cortical lysates, indicating the presence of a complex involving cyclin G1 and the B'alpha subunits of PP2A. This study shows that the regional and subcellular localization of PP2A B' regulatory subunits and cyclin G1 are very similar at early postnatal stages. We discuss the possible functions of a cyclin G1-PP2A B'alpha complex in neurons.

Binding specificity of protein phosphatase 2A core enzyme for regulatory B subunits and T antigens

The core enzyme of protein phosphatase 2A is composed of a regulatory subunit A and a catalytic subunit C. It is controlled by three types of regulatory B subunits (B, B', and B") and by tumor (T) antigens, which are unrelated by sequence but bind to overlapping regions on the A subunit. To find out whether the different B subunits and T antigens bind to identical or distinct amino acids of the A subunit, mutants were generated and their abilities to bind B subunits and T antigens were tested. We found that some amino acids are involved in the binding of all types of B subunits, whereas others are specifically involved in the binding of one or two types of B subunits. T-antigen-binding specificity does not correlate with that of a particular type of B subunit.

Protein phosphatase 2A is required for the initiation of chromosomal DNA replication

Protein phosphatase 2A (PP2A) is an abundant, multifunctional serine/threonine-specific phosphatase that stimulates simian virus 40 DNA replication. The question as to whether chromosomal DNA replication also depends on PP2A was addressed by using a cell-free replication system derived from Xenopus laevis eggs. Immunodepletion of PP2A from Xenopus egg extract resulted in strong inhibition of DNA replication. PP2A was required for the initiation of replication but not for the elongation of previously engaged replication forks. Therefore, the initiation of chromosomal DNA replication depends not only on phosphorylation by protein kinases but also on dephosphorylation by PP2A.

Linkage mapping of the Bra, Brb and Brg genes for rat protein phosphatase 2A 55 kDa B-regulatory subunit isotypes

We previously identified the rat Bra, Brb and Brg genes, which encode alpha, beta and gamma isotypes of the 55 kDa B-regulatory subunit of protein phosphatase 2A. Polymerase chain reaction-single strand conformation polymorphism analysis in the present study identified polymorphisms in Bra, Brb and Brg between the ACI and BUF, ZI and TM, and BN and WTC strains, respectively. Linkage analysis using mapping panels composed of F2 or back-crosses of these strains allowed Bra, Brb and Brg to be assigned to chromosomes 15, 18 and 14, respectively. Furthermore, it was revealed that Bra is located close to the Rb1 locus. Using polymorphism in Bra, loss of heterozygosity (LOH) was analyzed for rat mammary tumors induced in (SD x F344) F1 female rats by a food-borne carcinogen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, and a typical mammary carcinogen, 7,12-dimethylbenz[a]anthracene. No LOH was detected at the Bra locus.

RNA and protein interactions modulated by protein arginine methylation

This review summarizes the current status of protein arginine N-methylation reactions. These covalent modifications of proteins are now recognized in a number of eukaryotic proteins and their functional significance is beginning to be understood. Genes that encode those methyltransferases specific for catalyzing the formation of asymmetric dimethylarginine have been identified. The enzyme modifies a number of generally nuclear or nucleolar proteins that interact with nucleic acids, particularly RNA. Postulated roles for these reactions include signal transduction, nuclear transport, or a direct modulation of nucleic acid interactions. A second methyltransferase activity that symmetrically dimethylates an arginine residue in myelin basic protein, a major component of the axon sheath, has also been characterized. However, a gene encoding this activity has not been identified to date and the cellular function for this methylation reaction has not been clearly established. From the analysis of the sequences surrounding known arginine methylation sites, we have determined consensus methyl-accepting sequences that may be useful in identifying novel substrates for these enzymes and may shed further light on their physiological role.

Increasing the ratio of PP2A core enzyme to holoenzyme inhibits Tat-stimulated HIV-1 transcription and virus production

We demonstrated previously that PP2A exists in many cell types as two abundant forms: (1) holoenzyme composed of two regulatory subunits, A and B, and a catalytic subunit C; and (2) core enzyme consisting of the A and C subunits. These two forms have different substrate specificities. Since published data suggested that HIV-1 transcription may be regulated by a cellular protein phosphatase, it was of interest to determine whether changing the ratio between PP2A core and holoenzyme affects HIV-1 gene expression. This question was addressed by expression in COS cells of an N-terminal mutant of the A subunit, A delta 5, which binds the C but not the B subunit. This resulted in an increase in the amount of core enzyme and a decrease in the amount of holoenzyme concomitant with the expected change in phosphatase activity. Tat-stimulated transcription from the HIV-1 LTR was inhibited 5-fold by mutant A delta 5, whereas mRNA synthesis directed by the actin promoter was not affected. Furthermore, virus production in COS, HeLa, and Jurkat T cells was inhibited 45-, 5-, and 3-fold, respectively, by mutant A delta 5. These results demonstrate that the balance between PP2A holoenzyme and core enzyme is important for HIV-1 gene expression and virus production.

The possible role of protein phosphatase 2A in the sodium sensitivity of the receptor binding of opiate antagonists naloxone and naltrindole

In striatal membrane preparation used for receptor binding experiments high levels of protein phosphatase 1 and 2A activities were detected using [32P]phosphorylase a as substrate. Sodium chloride decreased the activity of protein phosphatase 2A and increased the activity of protein phosphatase 1 in a concentration-dependent manner. Sodium chloride facilitated the saturation binding of naloxone and naltrindole in rat striatal membrane preparation preincubated with ATP (50 microM) and MgCl2 (5 mM). Preincubation with calyculin A (1 nM) further increased the binding of naloxone. Addition of okadaic acid in a concentration of 2 nM, which is specific for the inhibition of protein phosphatase 2A, augmented the number of binding sites of naloxone or naltrindole. The results suggest a protein phosphatase-dependent regulation of the binding of opiate ligands in the striatum.

Molecular genetic analysis of Rts1p, a B' regulatory subunit of Saccharomyces cerevisiae protein phosphatase 2A

The Saccharomyces cerevisiae gene RTS1 encodes a protein homologous to a variable B-type regulatory subunit of the mammalian heterotrimeric serine/threonine protein phosphatase 2A (PP2A). We present evidence showing that Rts1p assembles into similar heterotrimeric complexes in yeast. Strains in which RTS1 has been disrupted are temperature sensitive (ts) for growth, are hypersensitive to ethanol, are unable to grow with glycerol as their only carbon source, and accumulate at nonpermissive temperatures predominantly as large-budded cells with a 2N DNA content and a nondivided nucleus. This cell cycle arrest can be overcome and partial suppression of the ts phenotype of rts1-null cells occurs if the gene CLB2, encoding a Cdc28 kinase-associated B-type cyclin, is expressed on a high-copy-number plasmid. However, CLB2 overexpression has no suppressive effects on other aspects of the rts1-null phenotype. Expression of truncated forms of Rts1p can also partially suppress the ts phenotype and can fully suppress the inability of cells to grow on glycerol and the hypersensitivity of cells to ethanol. By contrast, the truncated forms do not suppress the accumulation of large-budded cells at high temperatures. Coexpression of truncated Rts1p and high levels of Clb2p fully suppresses the ts phenotype, indicating that the inhibition of growth of rts1-null cells at high temperatures is due to both stress-related and cell cycle-related defects. Genetic analyses show that the role played by Rts1p in PP2A regulation is distinctly different from that played by the other known variable B regulatory subunit, Cdc55p, a protein recently implicated in checkpoint control regulation.

Molecular heterogeneity of the cDNA encoding a 74-kDa regulatory subunit (B" or delta) of human protein phosphatase 2A

Two cDNAs for possible splicing variants of a 74-kDa regulatory subunit (B" or delta) of human protein phosphatase 2A, were isolated. These variants were identified from human cerebral cortex by library screening and PCR, and designated delta1 and delta3 isoforms, while the previously reported isoform [Tanabe et al. (1996) FEBS Lett. 379, 107-1111 was designated delta2. Compared with the delta2 isoform, the delta1 isoform contained a 32-residue insertion beginning at residue 84, and consisted of 602 amino acids in all. The delta3 isoform lacked a 74-residue sequence corresponding to residues 1083 of the delta2 isoform, and consisted of 496 amino acids. Using isoform-specific antipeptide antisera, the 74-kDa subunit (B" or delta) originally purified from human erythrocytes was identified as the delta1 isoform.

Saccharomyces cerevisiae homologs of mammalian B and B' subunits of protein phosphatase 2A direct the enzyme to distinct cellular functions

Protein phosphatase 2A (PP2A) is a major cellular serine/threonine protein phosphatase, present in the cell in a variety of heterotrimeric forms that differ in their associated regulatory B-subunit. Cloning of the mammalian B' subunit has allowed the identification of a highly homologous Saccharomyces cerevisiae gene, RTS1. Disruption of the gene results in a temperature-sensitive growth defect that can be suppressed by expression of rabbit B'alpha or B'gamma isoforms. The B'alpha subunit is much more effective in restoring normal growth at 37 degrees C than B'gamma. Immunoprecipitated Rts1p was found associated with type 2A-specific protein phosphatase activity that is sensitive to 2 nM okadaic acid, but not to 100 nM phosphatase inhibitor-2, and to be phosphorylated in vivo. However, overexpression of RTS1 was unable to suppress the cold sensitivity, defective cytokinesis, and abnormal cell morphology resulting from defects in the CDC55 gene, which encodes the yeast homolog of a different B subunit of another form of 2A phosphatase, PP2A1. These results indicate that Rts1p is a yeast homolog of the mammalian B' subunit and that the various regulatory B-subunits of PP2A are not functionally redundant but direct the enzyme to distinct cellular functions.

cDNA cloning of a novel B subunit of Xenopus protein phosphatase 2A and its biological activity in oocytes

We have cloned a cDNA encoding a novel B regulatory subunit of protein phosphatase 2A (PP2A) from a Xenopus oocyte cDNA library. The novel B subunit, termed B beta', shows the strongest overall sequence similarity to, but a distinct N-terminal sequence from, the beta isoform of the human/rat B subunit. When expressed ectopically in Xenopus oocytes, the B beta' isoform can augment the endogenous PP2A activity and inhibit oocyte maturation induced by progesterone. These results suggest that the B beta' isoform can form a complex with other PP2A subunits to make a trimeric PP2A holoenzyme in Xenopus oocytes and may negatively control the initiation of oocyte maturation.

The interconversion of protein phosphatase 2A between PP2A1 and PP2A0 during retinoic acid-induced granulocytic differentiation and a modification on the catalytic subunit in S phase of HL-60 cells

Alterations in protein phosphatase 2A (PP2A) during retinoic acid-induced differentiation of HL-60 cells have been investigated. PP2A activity of HL-60 cells for phosphorylated myelin basic protein showed a sharp and transient increase after 18-h treatment with 1 microM retinoic acid, which corresponded to G1/S boundary of the cell cycle. This PP2A of the 18-h treated cells was eluted from a DEAE-Sepharose column with 0.13 M NaCl, while PP2A from control cells was eluted with 0.23 M NaCl. The phosphorylase phosphatase activity of PP2A in the 0.13 M eluate was greatly enhanced in the presence of protamine compared with that of the later eluting PP2A. Immunoblot analyses with antisera against B' and B alpha subunits showed that the PP2A in the 0.13 M NaCl eluate from 18-h retinoic acid-treated cells was PP2A0 (AC-B'), whereas the PP2A eluted with 0.23 M NaCl from 24-h retinoic acid-treated cells and 0-, 18-, and 24-h control cells was PP2A1 (AC-B alpha). These results strongly suggest that PP2A undergoes a transient and reversible interconversion of holoenzyme forms during the initial stage of retinoic acid-induced granulocytic differentiation. PP2A activity assayed after dissociation of the catalytic subunit, for phosphorylase as substrate, showed a sharp and transient decrease in S phase of HL-60 cells irrespective of the presence or absence of retinoic acid. Immunoblot analyses with antisera against C-terminus and N-terminus of the catalytic subunit of PP2A suggested that a modification at the C-terminus is responsible for the decrease in PP2A activity. Immunoreactivity to the C-terminal antibody was restored after treatments of the S-phase extract with alkali or ethanol, the conditions which remove the methyl group from the C-terminus. These results suggest that the C-terminus of PP2A catalytic subunit is transiently methylated in S phase of HL-60 cells.

Separation of PP2A core enzyme and holoenzyme with monoclonal antibodies against the regulatory A subunit: abundant expression of both forms in cells

Protein phosphatase 2A (PP2A) holoenzyme is composed of a catalytic subunit, C, and two regulatory subunits, A and B. The A subunit is rod shaped and consists of 15 nonidentical repeats. According to our previous model, the B subunit binds to repeats 1 through 10 and the C subunit binds to repeats 11 through 15 of the A subunit. Another form of PP2A, core enzyme, is composed only of subunits A and C. It is generally believed that core enzyme does not exist in cells but is an artifact of enzyme purification. To study the structure and relative abundance of different forms of PP2A, we generated monoclonal antibodies against the native A subunit. Two antibodies, 5H4 and 1A12, recognized epitopes in repeat 1 near the N terminus and immunoprecipitated free A subunit and core enzyme but not holoenzyme. Another antibody, 6G3, recognized an epitope in repeat 15 at the C terminus and precipitated only the free A subunit. Monoclonal antibodies against a peptide corresponding to the N-terminal 11 amino acids of the A alpha subunit (designated 6F9) precipitated free A subunit, core enzyme, and holoenzyme. 6F9, but not 5H4, recognized holoenzymes containing either B, B', or B" subunits. These results demonstrate that B subunits from three unrelated gene families all bind to repeat 1 of the A subunit, and the results confirm and extend our model of the holoenzyme. By sequential immunoprecipitations with 5H4 or 1A12 followed by 6F9, core enzyme and holoenzyme in cytoplasmic extracts from 10T1/2 cells were completely separated and they exhibited the expected specificities towards phosphorylase a and retinoblastoma peptide as substrates. Quantitative analysis showed that under conditions which minimized proteolysis and dissociation of holoenzyme, core enzyme represented at least one-third of the total PP2A. We conclude that core enzyme is an abundant form in cells rather than an artifact of isolation. The biological implications of this finding are discussed.

The variable subunit associated with protein phosphatase 2A0 defines a novel multimember family of regulatory subunits

Two protein phosphatase 2A (PP2A) holoenzymes were isolated from rabbit skeletal muscle containing, in addition to the catalytic and PR65 regulatory subunits, proteins of apparent molecular masses of 61 and 56 kDa respectively. Both holoenzymes displayed low basal phosphorylase phosphatase activity, which could be stimulated by protamine to an extent similar to that of previously characterized PP2A holoenzymes. Protein micro-sequencing of tryptic peptides derived from the 61 kDa protein, termed PR61, yielded 117 residues of amino acid sequence. Molecular cloning by enrichment of specific mRNAs, followed by reverse transcription-PCR and cDNA library screening, revealed that this protein exists in multiple isoforms encoded by at least three genes, one of which gives rise to several splicing variants. Comparisons of these sequences with the available databases identified one more human gene and predicted another based on a rabbit cDNA-derived sequence, thus bringing the number of genes encoding PR61 family members to five. Peptide sequences derived from PR61 corresponded to the deduced amino acid sequences of either alpha or beta isoforms, indicating that the purified PP2A preparation was a mixture of at least two trimers. In contrast, the 56 kDa subunit (termed PR56) seems to correspond to the epsilon isoform of PR61. Several regulatory subunits of PP2A belonging to the PR61 family contain consensus sequences for nuclear localization and might therefore target PP2A to nuclear substrates.

A specific protein carboxyl methylesterase that demethylates phosphoprotein phosphatase 2A in bovine brain

Phosphoprotein phosphatase 2A (PP2A) is one of the four major protein serine/threonine phosphatases found in all eukaryotic cells. We have shown that the 36-kDa catalytic subunit of PP2A is carboxyl methylated in eukaryotic cells, and we have previously identified and purified a novel methyltransferase (MTase) that is responsible for this modification. Here, we describe a novel protein carboxyl methyl-esterase (MEase) from bovine brain that demethylates PP2A. The enzyme has been purified to homogeneity as a monomeric 46-kDa soluble protein. The MEase is highly specific for PP2A. It does not catalyze the demethylation of other protein or peptide methylesters. Moreover, MEase activity is dramatically inhibited by nanomolar concentrations of okadaic acid, a specific inhibitor of PP2A. From these results, we conclude that PP2A methylation is controlled by two specific enzymes, a MTase and a MEase. Since PP2A methylation is highly conserved in eukaryotes ranging from human to yeast, it is likely that this system plays an important role in phosphatase regulation.

The myeloid leukemia-associated protein SET is a potent inhibitor of protein phosphatase 2A

Two potent heat-stable protein phosphatase 2A (PP2A) inhibitor proteins designated I1PP2A and I2PP2A have been purified to apparent homogeneity from extracts of bovine kidney (Li, M., Guo, H., and Damuni, Z. (1995) Biochemistry 34, 1988-1996). N-terminal and internal amino acid sequencing indicated that I2PP2A was a truncated form of SET, a largely nuclear protein that is fused to nucleoporin Nup214 in acute non-lymphocytic myeloid leukemia. Experiments using purified preparations of recombinant human SET confirmed that this protein inhibited PP2A. Half-maximal inhibition of the phosphatase occurred at about 2 nM SET. By contrast, SET (up to 20 nM) did not affect the activities of purified preparations of protein phosphatases 1, 2B, and 2C. The results indicate that SET is a potent and specific inhibitor of PP2A and suggest that impaired regulation of PP2A may contribute to acute myeloid leukemogenesis.

Characterization of DNA sequences encoding a novel isoform of the 55 kDa B regulatory subunit of the type 2A protein serine/threonine phosphatase of Arabidopsis thaliana

We have identified a novel gene (AtB beta) encoding a previously uncharacterized isoform of the B regulatory subunit of the type 2A serine/threonine protein phosphatase (PP2A) of Arabidopsis, and show that mRNA derived from the AtB beta gene accumulates in all Arabidopsis organs. In addition, we examined the expression of the three genes encoding the A regulatory subunit of Arabidopsis PP2A and show these genes are expressed in all organs as well. Taken together, our results suggest a myriad of PP2A subunit combinations, possibly with distinct substrate specificities, may occur within each Arabidopsis cell.

Identification of a novel protein phosphatase 2A regulatory subunit highly expressed in muscle

Differential association of regulatory B subunits with a core heterodimer, composed of a catalytic (C) and a structural (A) subunit, is an important mechanism that regulates protein phosphatase 2A (PP2A). We have isolated and characterized three novel cDNAs related to the B' subunit of bovine cardiac PP2A. Two human (B'alpha1 and B'alpha2) and a mouse (B'alpha3) cDNA encode for alternatively spliced variants of the B subunit. The deduced primary sequences of these clones contain 12 of 15 peptides derived from the purified bovine B' subunit. Differences between the deduced sequences of the B alpha splice variants and the cardiac peptide sequences suggest the existence of multiple isoforms of the B' subunit. Comparison of the protein and nucleotide sequences of the cloned cDNAs show that all three forms of B'alpha diverge at a common splice site near the 3'-end of the coding regions. Northern blot and reverse transcription-polymerase chain reaction analyses revealed that the B'alpha transcripts (4.3-4.4 kb) are widely expressed and very abundant in heart and skeletal muscle. The expressed human and mouse B'alpha proteins readily associated with the PP2A core enzyme in both in vitro and in vivo complex formation assays. Immunofluorescence microscopy revealed that epitope-tagged B'alpha was localized in both the cytosol and nuclei of transiently transfected cells. The efficiency of binding of all three expressed proteins to a glutathione S-transferase-A subunit fusion protein was greatly enhanced by the addition of the C subunit. Expression of the B'alpha subunits in insect Sf9 cells resulted in formation of AC.B'alpha heterotrimers with the endogenous insect A and C subunits. These results show that the B' subunit, which is the predominant regulatory subunit in cardiac PP2A, is a novel protein whose sequence is unrelated to other PP2A regulatory subunits. The nuclear localization of expressed B'alpha suggests that some variants of the B' subunit are involved in the nuclear functions of PP2A.

High complexity in the expression of the B' subunit of protein phosphatase 2A0. Evidence for the existence of at least seven novel isoforms

Association of the catalytic subunit (C2) with a variety of regulatory subunits is believed to modulate the activity and specificity of protein phosphatase 2A (PP2A). In this study we report the cloning and expression of a new family of B-subunit, the B', associated with the PP2A0 form. Polymerase chain reactions and cDNA library screening have identified at least seven cDNA isotypes, designated alpha, beta 1, beta 2, beta 3, beta 4, gamma, and delta. The different beta subtypes appear to be generated by alternative splicing. The deduced amino acid sequences of the alpha, beta 2, beta 3, beta 4 and gamma isoforms predict molecular weights of 57,600, 56,500, 60,900, 52,500, and 68,000, respectively. The proteins are 60-80% identical and differ mostly at their termini. Two of the isoforms, B' beta 3 and B' gamma, contain a bipartite nuclear localization signal in their COOH terminus. No homology was found with other B- or B- related subunits. Northern analyses indicate a tissue-specific expression of the isoforms. Expression of B' alpha protein in Escherichia coli generated a polypeptide of approximately 53 kDa, similar to the size of the B' subunit present in the purified PP2A0. The recombinant protein was recognized by antibody raised against native B' and interacted with the dimeric PP2A (A.C2) to generate a trimeric phosphatase. The deduced amino acid sequences of the B' isoforms show significant homology to mammalian, fungal, and plant nucleotide sequences of unknown function present in the data bases. Notably, a high degree of homology (55-66%) was found with a yeast gene, RTS1, encoding a multicopy suppressor of a rox3 mutant. Our data indicate that at least seven B' subunit isoforms may participate in the generation of a large number of PP2A0 holoenzymes that may be spatially and/or functionally targeted to different cellular processes.

Molecular cloning of a 74-kDa regulatory subunit (B" or delta) of human protein phosphatase 2A

Based on amino acid sequence data of a 74-kDa regulatory subunit (B" or delta) of a human heterotrimeric protein phosphatase 2A, a cDNA encoding the subunit was isolated from a human cerebral cortex library. The cDNA had an open reading frame encoding an M(r) 66,138 protein of 570 amino acids. Bacterial expression of the cDNA yielded a protein immunoreactive with antisera specific to the 74-kDa subunit. The predicted primary structure of the subunit had no similarity to already reported sequences of PP2A regulatory subunits including A, B, and PR72. Potential phosphorylation sites for protein kinases A and C, a bipartite motif of putative nuclear localization signal, and SH3 accessible proline-rich domain, and a unique PQ repeat were found in the sequence. The subunit mRNA of about 2.9 kb was ubiquitously expressed in rat tissues.

Identification of a new family of protein phosphatase 2A regulatory subunits

Protein phosphatase 2A (PP2A) is a major intracellular protein phosphatase that regulates multiple aspects of cell growth and metabolism. The ability of this widely distributed heterotrimeric enzyme to act on a diverse array of substrates is largely controlled by the nature of its regulatory B subunit. Only two gene families encoding endogenous B subunits have been cloned to date, although the existence of several additional regulatory subunits is likely. We have identified by two-hybrid interaction a new human gene family encoding PP2A B subunits. This family, denoted B56, contains three distinct genes, one of which is differentially spliced. B56 polypeptides co-immunoprecipitate with PP2A A and C subunits and with an okadaic acid-inhibitable, heparin-stimulated phosphatase activity. The three B56 family members are 70% identical to each other but share no obvious homology with previously identified B subunits. These phosphatase regulators are differentially expressed, with B56 alpha and B56 gamma highly expressed in heart and skeletal muscle and B56 beta highly expressed in brain. The identification of this novel phosphatase regulator gene family will facilitate future studies on the control of protein dephosphorylation and the role of PP2A in cellular function.

Serine/threonine protein phosphatases

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Deregulation of translational control of the 65-kDa regulatory subunit (PR65 alpha) of protein phosphatase 2A leads to multinucleated cells

Efficient translation of the mRNA encoding the 65-kDa regulatory subunit (PR65 alpha) of protein phosphatase 2A (PP2A) is prevented by an out of frame upstream AUG and a stable stem-loop structure (delta G = -55.9 kcal/mol) in the 5'-untranslated region (5'-UTR). Deletion of the 5'-UTR allows efficient translation of the PR65 alpha message in vitro and overexpression in COS-1 cells. Insertion of the 5'-UTR into the beta-galactosidase leader sequence dramatically inhibits translation of the beta-galactosidase message in vitro and in vivo, confirming that this sequence functions as a potent translation regulatory sequence. Cells transfected or microinjected with a PR65 alpha expression vector lacking the 5'-UTR, express high levels of PR65 alpha, accumulating in both nucleus and cytoplasm. PR65 alpha overexpressing rat embryo fibroblasts (REF-52 cells) become multinucleated. These data and previous results (Mayer-Jaekel, R. E., Ohkura, H., Gomes, R., Sunkel, C. E., Baumgartner, S., Hemmings, B. A., and Glover, D. M. (1993) Cell 72, 621-633) suggest that PP2A participates in the regulation of both mitosis and cytokinesis.

The G-protein-coupled receptor phosphatase: a protein phosphatase type 2A with a distinct subcellular distribution and substrate specificity

Phosphorylation of G-protein-coupled receptors plays an important role in regulating their function. In this study the G-protein-coupled receptor phosphatase (GRP) capable of dephosphorylating G-protein-coupled receptor kinase-phosphorylated receptors is described. The GRP activity of bovine brain is a latent oligomeric form of protein phosphatase type 2A (PP-2A) exclusively associated with the particulate fraction. GRP activity is observed only when assayed in the presence of protamine or when phosphatase-containing fractions are subjected to freeze/thaw treatment under reducing conditions. Consistent with its identification as a member of the PP-2A family, the GRP is potently inhibited by okadaic acid but not by I-2, the specific inhibitor of protein phosphatase type 1. Solubilization of the membrane-associated GRP followed by gel filtration in the absence of detergent yields a 150-kDa peak of latent receptor phosphatase activity. Western blot analysis of this phosphatase reveals a likely subunit composition of AB alpha C. PP-2A of this subunit composition has previously been characterized as a soluble enzyme, yet negligible soluble GRP activity was observed. The subcellular distribution and substrate specificity of the GRP suggests significant differences between it and previously characterized forms of PP-2A.

Characterization of a cDNA encoding the 55 kDa B regulatory subunit of Arabidopsis protein phosphatase 2A

Type 2A serine/threonine protein phosphatases (PP2A) are key components in the regulation of signal transduction and control of cell metabolism. The activity of these protein phosphatases is modulated by regulatory subunits. While PP2A activity has been characterized in plants, little is known about its regulation. We used the polymerase chain reaction to amplify a segment of a cDNA encoding the B regulatory subunit of PP2A from Arabidopsis. The amplified DNA fragment of 372 nucleotides was used as a probe to screen an Arabidopsis cDNA library and a full-length clone (AtB alpha) of 2.1 kbp was isolated. The predicted protein encoded by AtB alpha is 43 to 46% identical and 53 to 56% similar to its yeast and mammalian counterparts, and contains three unique regions of amino acid insertions not present in the animal B regulatory subunit. Genomic Southern blots indicate the Arabidopsis genome contains at least two genes encoding the B regulatory subunit. In addition, other plant species also contain DNA sequences homologous to the B regulatory subunit, indicating that regulation of PP2A activity by the 55 kDa B regulatory subunit is probably ubiquitous in plants. Northern blots indicate the AtB alpha mRNA accumulates in all Arabidopsis tissues examined, suggesting the protein product of the AtB alpha gene performs a basic housekeeping function in plant cells.

Identification of soluble protein phosphatases that dephosphorylate voltage-sensitive sodium channels in rat brain

Rat brain sodium channels are phosphorylated at multiple serine residues by cAMP-dependent protein kinase. We have identified soluble rat brain phosphatases that dephosphorylate purified sodium channels. Five separable forms of sodium channel phosphatase activity were observed. Three forms (two, approximately 234 kDa and one, 192 kDa) are identical or related to phosphatase 2A, since they were 85-100% inhibited by 10 nM okadaic acid and contained a 36-kDa polypeptide recognized by a monoclonal antibody directed against the catalytic subunit of phosphatase 2A. Immunoblots performed using antibodies specific for isoforms of the B subunit of phosphatase 2A indicate that the two major peaks of phosphatase 2A-like activity, A1 and B1, are enriched in either B' or B alpha. The remaining two activities (approximately 100 kDa each) probably represent calcineurin. Each was relatively insensitive to okadaic acid, was active only in the presence of CaCl2 and calmodulin, and contained a 19-kDa polypeptide recognized by a monoclonal antibody raised against the B subunit of calcinerurin. Treatment of synaptosomes with okadaic acid to inhibit phosphatase 2A or cyclosporin A to inhibit calcineurin increased apparent phosphorylation of sodium channels at cAMP-dependent phosphorylation sites, as assayed by back phosphorylation. These results indicate that phosphatase 2A and calcineurin dephosphorylate sodium channels in brain, and thus may counteract the effect of cAMP-dependent phosphorylation on sodium channel activity.