Protein phosphatase 2A1 is the major enzyme in vertebrate cell extracts that dephosphorylates several physiological substrates for cyclin-dependent protein kinases

Genetic interactions between GLC7, PPZ1 and PPZ2 in saccharomyces cerevisiae

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

The mitotic phosphorylation cycle of the cis-Golgi matrix protein GM130

The cis-Golgi matrix protein GM130 is phosphorylated in mitosis on serine 25. Phosphorylation inhibits binding to p115, a vesicle-tethering protein, and has been implicated as an important step in the mitotic Golgi fragmentation process. We have generated an antibody that specifically recognizes GM130 phosphorylated on serine 25, and used this antibody to study the temporal regulation of phosphorylation in vivo. GM130 is phosphorylated in prophase as the Golgi complex starts to break down, and remains phosphorylated during further breakdown and partitioning of the Golgi fragments in metaphase and anaphase. In telophase, GM130 is dephosphorylated as the Golgi fragments start to reassemble. The timing of phosphorylation and dephosphorylation correlates with the dissociation and reassociation of p115 with Golgi membranes. GM130 phosphorylation and p115 dissociation appear specific to mitosis, since they are not induced by several drugs that trigger nonmitotic Golgi fragmentation. The phosphatase responsible for dephosphorylation of mitotic GM130 was identified as PP2A. The active species was identified as heterotrimeric phosphatase containing the Balpha regulatory subunit, suggesting a role for this isoform in the reassembly of mitotic Golgi membranes at the end of mitosis.

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.

Induction of protein phosphatase type 2A in response to disruption of cell-matrix interactions

The proliferation of most cells is strictly dependent on cell-matrix interactions, a phenomenon called anchorage dependence. Because tumor cells are often independent of this regulation, it is important to characterize the molecular components that are involved in this control. We therefore investigated a possible role of serine/threonine protein phosphatases in the regulation of anchorage-dependent cell growth. We found that the activity of serine/threonine protein phosphatase type 2A (PP2A) and, to a lesser extent, that of type 1 (PP1), was upregulated in response to the disruption of cellular attachment. In the case of PP2A, this induction was due to the transcriptional activation of the gene and increased expression of its protein. The increase in phosphatase activity corresponded with a decrease in the phosphorylation of cellular proteins that occurred in anchorage-dependent cells, but to a much lesser degree in anchorage-independent cells. At the same time, the activity of cyclin-dependent kinases was downregulated in anchorage-dependent, but not in anchorage-independent cells. Thus, our results indicate that the balance of kinase and phosphatase activity in anchorage-dependent cells is tipped in favor of phosphatase activity, which seems to dominate the extent of reversible protein phosphorylations after cellular detachment. In contrast, anchorage-independent cells appear to neutralize elevated phosphatase activity through sustained, strong kinase activity.

The RCN1-encoded A subunit of protein phosphatase 2A increases phosphatase activity in vivo

Protein phosphatase 2A (PP2A), a heterotrimeric serine/threonine-specific protein phosphatase, comprises a catalytic C subunit and two distinct regulatory subunits, A and B. The RCN1 gene encodes one of three A regulatory subunits in Arabidopsis thaliana. A T-DNA insertion mutation at this locus impairs root curling, seedling organ elongation and apical hypocotyl hook formation. We have used in vivo and in vitro assays to gauge the impact of the rcn1 mutation on PP2A activity in seedlings. PP2A activity is decreased in extracts from rcn1 mutant seedlings, and this decrease is not due to a reduction in catalytic subunit expression. Roots of mutant seedlings exhibit increased sensitivity to the phosphatase inhibitors okadaic acid and cantharidin in organ elongation assays. Shoots of dark-grown, but not light-grown seedlings also show increased inhibitor sensitivity. Furthermore, cantharidin treatment of wild-type seedlings mimics the rcn1 defect in root curling, root waving and hypocotyl hook formation assays. In roots of wild-type seedlings, RCN1 mRNA is expressed at high levels in root tips, and accumulates to lower levels in the pericycle and lateral root primordia. In shoots, RCN1 is expressed in the apical hook and the basal, rapidly elongating cells in etiolated hypocotyls, and in the shoot meristem and leaf primordia of light-grown seedlings. Our results show that the wild-type RCN1-encoded A subunit functions as a positive regulator of the PP2A holoenzyme, increasing activity towards substrates involved in organ elongation and differential cell elongation responses such as root curling.

Catalytically inactive protein phosphatase 2A can bind to polyomavirus middle tumor antigen and support complex formation with pp60(c-src)

Interaction between the heterodimeric form of protein phosphatase 2A (PP2A) and polyomavirus middle T antigen (MT) is required for the subsequent assembly of a transformation-competent MT complex. To investigate the role of PP2A catalytic activity in MT complex formation, we undertook a mutational analysis of the PP2A 36-kDa catalytic C subunit. Several residues likely to be involved in the dephosphorylation mechanism were identified and mutated. The resultant catalytically inactive C subunit mutants were then analyzed for their ability to associate with a cellular (B subunit) or a viral (MT) B-type subunit. Strikingly, while all of the inactive mutants were severely impaired in their interaction with B subunit, most of these mutants formed complexes with polyomavirus MT. These findings indicate a potential role for these catalytically important residues in complex formation with cellular B subunit, but not in complex formation with MT. Transformation-competent MT is known to associate with, and modulate the activity of, several cellular proteins, including pp60(c-src) family kinases. To determine whether association of MT with an active PP2A A-C heterodimer is necessary for subsequent association with pp60(c-src), catalytically inactive C subunits were examined for their ability to form complexes containing pp60(c-src) in MT-expressing cells. Two catalytically inactive C subunit mutants that efficiently formed complexes with MT also formed complexes that included an active pp60(c-src) kinase, demonstrating that PP2A activity is not essential in cis in MT complexes for subsequent pp60(c-src) association.

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 protein kinase cascades by protein phosphatase 2A

Many protein kinases themselves are regulated by reversible phosphorylation. Upon cell stimulation, specific kinases are transiently phosphorylated and activated. Several of these protein kinases are substrates for protein phosphatase 2A (PP2A), and PP2A appears to be the major kinase phosphatase in eukaryotic cells that downregulates activated protein kinases. This idea is substantiated by the observation that some viral proteins and naturally occurring toxins target PP2A and modulate its activity. There is increasing evidence that PP2A activity is regulated by extracellular signals and during the cell cycle. Thus, PP2A is likely to play an important role in determining the activation kinetics of protein kinase cascades.

Differential distribution of protein phosphatase 2A in human breast carcinoma cell lines and its relation to estrogen receptor status

Protein phosphatase 2A (PP2A) acts as a growth suppressor and is negatively influenced by oncogenic signals. We determined its activity in various human breast carcinoma (HBC) cell types to understand its relationship to estrogen receptor (ER) expression as well as to the distribution of protein kinase C (PKC), an opposing enzyme. PP2A activity was measured using a preferred substrate, histone H1 phosphorylated by PKC. PP2A activity was higher in both the soluble and nuclear fractions of ER-positive cell lines (MCF-7, T47D and ZR-75-1) than in the ER-negative cell lines (MDA-MB-231, Hs578T and BT-20). PP2A multiple forms (2A0, 2A1, 2A2), separated by DEAE-cellulose chromatography and immunoblot analysis of PP2A catalytic subunit, also showed similar differences in these two HBC cell types. In all cases, PP2A distribution was inversely correlated with the PKC activity profile. Moreover, PP2A activity in MCF-7 cells maintained in estrogen-depleted medium was low. Nonetheless, it was induced by a prolonged treatment with 17beta-estradiol, this induction being blocked by the antiestrogens, tamoxifen and ICI-182,780. Studies in both MCF-7 transfectants stably overexpressing ras and MDA-MB-231 transfectants stably expressing ER, suggested that a low PP2A distribution in ER-negative HBC cell types may be related to tumor progression rather than the loss of ER. Conceivably, the presence of high PP2A along with low PKC in ER-positive HBC cell types may be related to the restricted cell growth associated with the retention of a certain degree of differentiation or hormonal control. Conversely, the presence of low PP2A along with high PKC in ER-negative cell types may be related to hormone-independent enhanced cell growth.

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.

Regulation of protein phosphatase 2A activity by caspase-3 during apoptosis

Although the available evidence suggests that whereas the caspase family plays a major role in apoptosis, they are not the sole stimulators of death. A random yeast two-hybrid screen of a lymphocyte cDNA library (using caspase-3 as the bait) found an interaction between caspase-3 and the regulatory subunit Aalpha of protein phosphatase 2A. This protein was found to be a substrate for caspase-3, but not caspase-1, and could compete effectively against either a protein or synthetic peptide substrate. In Jurkat cells induced to undergo apoptosis with anti-Fas antibody, protein phosphatase 2A (PP2A) activity increased 4.5-fold after 6 h. By 12 h, the regulatory Aalpha subunit could no longer be detected in cell lysates. There was no change in the amount of the catalytic subunit. The effects on PP2A could be prevented by the caspase family inhibitors acetyl-Asp-Glu-Val-Asp (DEVD) aldehyde or Ac-DEVD fluoromethyl ketone. The mitogen-activated protein (MAP) kinase pathway is regulated by PP2A. At 12 h after the addition of anti-Fas antibody, a decrease in the amount of the phosphorylated forms of MAP kinase was observed. Again, this loss of activated MAP kinase could be prevented by the addition of DEVD-cho or DEVD-fmk. These data are consistent with a pathway whereby induction of apoptosis activates caspase-3. This enzyme then cleaves the regulatory Aalpha subunit of PP2A, increasing its activity. These data show that the activated PP2A will then effect a change in the phosphorylation state of the cell. These data provide a link between the caspases and signal transduction pathways.

A phosphatase activity in Xenopus oocyte extracts preferentially dephosphorylates the MPM-2 epitope

MPM-2 antigens are a large family of mitotic phosphoproteins that contain similar phosphoepitopes recognized by the anti-phosphoepitope antibody MPM-2 (MPM-2 epitopes). These proteins are phosphorylated during M phase induction and dephosphorylated from the onset of anaphase through interphase. Since biochemical characterization of the MPM-2 epitope phosphatase requires a specific assay for its activity, we tested different methods for measurement of the MPM-2 epitope phosphatase activity in crude cell lysates. First, an ELISA-based assay was designed that measured the phosphatase-induced reduction of the MPM-2 reactivity in crude M phase cell lysates. Using this assay to follow the phosphatase activity during sequential chromatography of Xenopus oocyte extracts, one predominant peak of phosphatase activity was detected which was separated from the majority of PP1 and PP2A activities. This phosphatase activity dephosphorylated the MPM-2 epitope on multiple MPM-2 antigens. The second method measured dephosphorylation of cdc25, a known MPM-2 antigen. Two major peaks of cdc25 dephosphorylating activities were detected during the sequential chromatography, one that copurified with the major peak of MPM-2 epitope phosphatase activity, and the other with the major peak of PP2A activity. Finally, we examined whether GST-MPM2, a fusion protein between glutathione S-transferase and a 19-residue peptide that contained two representative MPM-2 epitope sequences, could be dephosphorylated efficiently and specifically by the major MPM-2 epitope phosphatase activity in Xenopus oocyte extracts. Neither the crude extract nor the partially purified MPM-2 epitope phosphatase activity efficiently dephosphorylated the MPM-2 epitope on GST-MPM2. These results demonstrate that the ELISA-based assay preferentially detects the MPM-2 epitope phosphatase activity in crude cell lysates which may represent a physiological MPM-2 epitope phosphatase.

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.

Distinct roles of PP1 and PP2A-like phosphatases in control of microtubule dynamics during mitosis

Assembly of a mitotic spindle requires the accurate regulation of microtubule dynamics which is accomplished, at least in part, by phosphorylation-dephosphorylation reactions. Here we have investigated the role of serine-threonine phosphatases in the control of microtubule dynamics using specific inhibitors in Xenopus egg extracts. Type 2A phosphatases are required to maintain the short steady-state length of microtubules in mitosis by regulating the level of microtubule catastrophes, in part by controlling the the microtubule-destabilizing activity and phosphorylation of Op18/stathmin. Type 1 phosphatases are only required for control of microtubule dynamics during the transitions into and out of mitosis. Thus, although both type 2A and type 1 phosphatases are involved in the regulation of microtubule dynamics, they have distinct, non-overlapping roles.

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.

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.

Protein phosphatase 2A regulates MPF activity and sister chromatid cohesion in budding yeast

BACKGROUND

Mitosis is regulated by MPF (maturation promoting factor), the active form of Cdc2/28-cyclin B complexes. Increasing levels of cyclin B abundance and the loss of inhibitory phosphates from Cdc2/28 drives cells into mitosis, whereas cyclin B destruction inactivates MPF and drives cells out of mitosis. Cells with defective spindles are arrested in mitosis by the spindle-assembly checkpoint, which prevents the destruction of mitotic cyclins and the inactivation of MPF. We have investigated the relationship between the spindle-assembly checkpoint, cyclin destruction, inhibitory phosphorylation of Cdc2/28, and exit from mitosis.

RESULTS

The previously characterized budding yeast mad mutants lack the spindle-assembly checkpoint. Spindle depolymerization does not arrest them in mitosis because they cannot stabilize cyclin B. In contrast, a newly isolated mutant in the budding yeast CDC55 gene, which encodes a protein phosphatase 2A (PP2A) regulatory subunit, shows a different checkpoint defect. In the presence of a defective spindle, these cells separate their sister chromatids and leave mitosis without inducing cyclin B destruction. Despite the persistence of B-type cyclins, cdc55 mutant cells inactivate MPF. Two experiments show that this inactivation is due to inhibitory phosphorylation on Cdc28: phosphotyrosine accumulates on Cdc28 in cdc55 delta cells whose spindles have been depolymerized, and a cdc28 mutant that lacks inhibitory phosphorylation sites on Cdc28 allows spindle defects to arrest cdc55 mutants in mitosis with active MPF and unseparated sister chromatids.

CONCLUSIONS

We conclude that perturbations of protein phosphatase activity allow MPF to be inactivated by inhibitory phosphorylation instead of by cyclin destruction. Under these conditions, sister chromatid separation appears to be regulated by MPF activity rather than by protein degradation. We discuss the role of PP2A and Cdc28 phosphorylation in cell-cycle control, and the possibility that the novel mitotic exit pathway plays a role in adaptation to prolonged activation of the spindle-assembly checkpoint.

Cloning and expression of cystolic phospholipase A2 (cPLA2) and a naturally occurring variant. Phosphorylation of Ser505 of recombinant cPLA2 by p42 mitogen-activated protein kinase results in an increase in specific activity

Full-length cytosolic phospholipase A2 (cPLA2) was cloned from U937 cells and polymorphonuclear leukocytes (PMNLs) while a naturally occurring variant of cPLA2, which lacks residues Val473-Ala749 but has a C-terminal extension of ILMNLSEYMLWMSKVKRFM (DcPLA2) was cloned from PMNLs and mononuclear leukocytes. We were unable to clone DcPLA2 from U937 cells. When cPLA2 and DcPLA2 were expressed in insect cells, both proteins were detected in cell lysates by SDS/PAGE as single bands of apparent molecular masses 100 kDa and 57 kDa, respectively. Full-length cPLA2 was phosphorylated stoichiometrically by p42 mitogen-activated protein (MAP) kinase in vitro at a similar rate to other physiological substrates of this protein kinase and the major site of phosphorylation was identified by amino acid sequencing as Ser505. [32P]Ser(P)505 in cPLA2 was only dephosphorylated at a slow rate by mammalian tissue homogenates. Protein phosphatases 2A, 2B and 2C all contributed significantly to the overall dephosphorylation of cPLA2. The phosphorylation of cPLA2 by p42 MAP kinase correlated with an approximately 1.5-fold increase in specific enzyme activity which was reversed by dephosphorylation.

Evidence that the endogenous histone H1 phosphatase in HeLa mitotic chromosomes is protein phosphatase 1, not protein phosphatase 2A

Histone H1 is highly phosphorylated in mitotic HeLa cells, but is quickly dephosphorylated in vivo at the end of mitosis and in vitro following cell lysis. We show here that okadaic acid and microcystin-LR block the in vitro dephosphorylation of H1 and that they do so directly by inhibiting the histone H1 phosphatase rather than by some indirect mechanism. The concentrations of microcystin and okadaic acid required for inhibition strongly suggest that the histone H1 phosphatase is either PP1 or an unknown protein phosphatase with okadaic acid-sensitivity similar to PP1. The histone H1 phosphatase is predominantly located in chromosomes with at most one copy for every 86 nucleosomes. This tends to support its identification as PP1, since localization in mitotic chromosomes is a characteristic of PP1 but not of the other known okadaic acid-sensitive protein phosphatases. We also show that treatment of metaphase-arrested HeLa cells with staurosporine and olomoucine, inhibitors of p34cdc2 and other protein kinases, rapidly induces reassembly of interphase nuclei and dephosphorylation of histone H1 without chromosome segregation. This result indicates that protein kinase activity must remain elevated to maintain a mitotic block. Using this as a model system for the M- to G1-phase transition, we present evidence from inhibitor studies suggesting that the in vivo histone H1 phosphatase may be either PP1 or another phosphatase with similar okadaic acid-sensitivity, but not PP2A.

Activation of the Xenopus cyclin degradation machinery by full-length cyclin A

The entry into mitosis is dependent on the activation of mitotic forms of cdc2 kinase. In many cell types, cyclin A-associated kinase activity peaks just prior to that of cyclin B, although the precise role of cyclin A-associated kinase in the entry into mitosis is still unclear. Previous work has suggested that while cyclin B is capable of triggering cyclin destruction in Xenopus cell-free systems, cyclin A-associated kinase is not able to support this function. Here we have expressed a full-length human cyclin A in Escherichia coli and purified the protein to homogeneity by virtue of an N-terminal histidine tag. We have found that when added to Xenopus cell-free extracts free of cyclin B and incapable of protein synthesis, the temporal pattern of cyclin A-associated cdc2 kinase activity showed distinct differences that were dependent on the concentration of cyclin A added. When cyclin A was added to a concentration that generated levels of cdc2 kinase activity capable of inducing nuclear envelope breakdown, the histone H1 kinase activity profile was bi-phasic, consisting of an activation phase followed by an inactivation phase. Inactivation was found to be due to cyclin destruction, which was prevented by mos protein. Cyclin destruction was followed by nuclear reassembly and an additional round of DNA replication, indicating that there is no protein synthesis requirement for DNA replication in this embryonic system. It has been suggested that the evolutionary recruitment of cyclin A into an S phase function may have necessitated the loss of an original mitotic ability to activate the cyclin destruction pathway. The results presented here indicate that cyclin A has not lost the ability to activate its own destruction and that cyclin A-mediated activation of the cyclin destruction pathway permitted destruction of cyclin B1 as well as cyclin A, indicating that there are not distinct cyclin A and cyclin B destruction pathways. Thus the ordered progression of the cell cycle requires the careful titration of cyclin. A concentration in order to avoid activation of the cyclin destruction pathway before sufficient active cyclin B/cdc2 kinase has accumulated.

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.

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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Inhibition of interleukin-2-mediated DNA synthesis in activated human T-lymphoblasts by okadaic acid is accompanied by hyperphosphorylation of lck

We have previously shown that, during interleukin-2-driven G1/S transition in activated human T-lymphoblasts, a restricted group of cellular proteins become tyrosine phosphorylated de novo, and that p56lck is the major active tyrosine kinase at this stage of the cell cycle. We now report that okadaic acid, a potent and specific inhibitor of protein phosphatases type 1 and type 2A, inhibits S-phase entry, and that this occurs with the simultaneous disappearance of a 56 kDa tyrosine phosphoprotein. We show that this protein is the lck tyrosine kinase and that okadaic acid stimulates a mobility shift to 59 and 64 kDa forms. These two forms of lck were found to have decreased autocatalytic activity, as judged by immune-complex kinase assays. Two-dimensional phosphopeptide mapping and phosphoamino-acid analyses revealed that, in the presence of okadaic acid, lck becomes phosphorylated mainly on serine and to a lesser extent threonine, and that phosphorylation occurs at novel sites. These results show that the kinase activity of lck is at least partly regulated by protein phosphatases, and suggest a role for lck in directing growth-factor-mediated DNA synthesis during T-cell proliferation.

Protein histidine phosphatase activity in rat liver and spinach leaves

Whole cell extracts from rat liver or spinach leaves contain divalent ion-independent protein histidine phosphatase activity due to phosphatases of the PP1/PP2A family. In the rat liver extract, almost all the activity was found in the PP1, PP2A1 and PP2A2 peaks. In the spinach leaf extract, four phosphorylase phosphatase activity peaks were resolved--three containing PP1 and one containing PP2A--and all showed histidine phosphatase activity. Thus, protein histidine phosphatase activity is expressed in the cytosolic forms of protein phosphatases of the PP1/PP2A family in mammalian and plant cells.

Differential methylation and altered conformation of cytoplasmic and nuclear forms of protein phosphatase 2A during cell cycle progression

Protein phosphatase 2A (PP2A) appears to be involved in the regulation of many cellular processes. Control mechanisms that lead to the activation (and deactivation) of the various holoenzymes to initiate appropriate dephosphorylation events remain obscure. The core components of all PP2A holoenzymes are the catalytic (PP2Ac) and 63-65-kD regulatory (PR65) subunits. Monospecific and affinity-purified antibodies against both PP2Ac and PR65 show that these proteins are ubiquitously localized in the cytoplasm and the nucleus in nontransformed fibroblasts. As determined by quantitative immunofluorescence the core subunits of PP2A are twofold more concentrated in the nucleus than in the cytoplasm. Detailed analysis of synchronized cells reveals striking changes in the nuclear to cytoplasmic ratio of PP2Ac-specific immunoreactivity albeit the total amounts of neither PP2Ac nor PR65 in each compartment alters significantly during the cell cycle. Our results imply that differential methylation of PP2Ac occurs at the G0/G1 and G1/S boundaries. Specifically a demethylated form of PP2Ac is found in the cytoplasm of G1 cells, and in the nucleus of S and G2 cells. In addition nuclear PP2A holoenzymes appear to undergo conformational changes at the G0/G1 and G1/S boundaries. During mitosis PP2A is lost from the nuclear compartment, and unlike protein phosphatase 1 shows no specific association with the condensed chromatin.

A novel pool of protein phosphatase 2A is associated with microtubules and is regulated during the cell cycle

Immunofluorescence microscopy revealed the presence of protein phosphatase 2A (PP2A) on microtubules in neuronal and nonneuronal cells. Interphase and mitotic spindle microtubules, as well as centrosomes, were all labeled with antibodies against individual PP2A subunits, showing that the AB alpha C holoenzyme is associated with microtubules. Biochemical analysis showed that PP2A could be reversibly bound to microtubules in vitro and that approximately 75% of the PP2A in cytosolic extracts could interact with microtubules. The activity of microtubule-associated PP2A was differentially regulated during the cell cycle. Enzymatic activity was high during S phase and intermediate during G1, while the activity in G2 and M was 20-fold lower than during S phase. The amount of microtubule-bound PP2A remained constant throughout the cell cycle, implying that cell cycle regulation of its enzymatic activity involves factors other than microtubules. These results raise the possibility that PP2A regulates cell cycle-dependent microtubule functions, such as karyokinesis and membrane transport.

Inactivation of p42 MAP kinase by protein phosphatase 2A and a protein tyrosine phosphatase, but not CL100, in various cell lines

BACKGROUND

Mitogen-activated protein (MAP) kinase is central to a signal transduction pathway that triggers cell proliferation or differentiation. Activation of the p42mapk isoform requires its phosphorylation at two residues, Thr 183 and Tyr 185, and this phosphorylation is catalysed by MAP kinase kinase (MAPKK). Relatively little is known, however, about the enzymes that dephosphorylate these residues, thereby inactivating the pathway. Recently, the CL100 phosphatase has been shown to inactivate p42mapk in vitro by dephosphorylating Thr 183 and Tyr 185 at similar rates. CL100, the product of an immediate early gene, is synthesized within one hour of stimulating cells with growth factors or exposure to oxidative stress or heat shock. Incubation of NIH 3T3 fibroblasts with cycloheximide prevents both synthesis of CL100 and inactivation of p42mapk after stimulation with serum.

RESULTS

Depleting cells of CL100 and preventing its induction using cycloheximide stopped the inactivation of p42mapk in Swiss 3T3 fibroblasts following stimulation with epidermal growth factor (EGF), but had no effect on the rapid inactivation of p42mapk in response to EGF in adipose (3T3-L1) or chromaffin (PC12) cells or in response to platelet-derived growth factor (PDGF) in endothelial (PAE) cells. Moreover, maximal induction of CL100 mRNA and a CL100-like activity did not trigger inactivation of p42mapk, which was sustained at a high level after stimulation of PC12 cells with nerve growth factor, PAE cells with serum, or Swiss 3T3 cells with PDGF. Dephosphorylation of Tyr 185 but not Thr 183 of p42mapk was suppressed by vanadate in EGF-stimulated PC12 cells; dephosphorylation of Thr 183, by contrast, was elicited by a vanadate-insensitive activity. Protein phosphatase-2A was the only vanadate-insensitive phosphatase acting on Thr 183 of p42mapk or on MAPKK to be detected in PC12 cell extracts. Phosphorylation of Thr 183 also inhibited the dephosphorylation of Tyr 185 in vitro by the major vanadate-sensitive Tyr 185-specific phosphatase, explaining why dephosphorylation of Thr 183 is rate-limiting for p42mapk inactivation in PC12 cells after stimulation with EGF.

CONCLUSIONS

The rapid inactivation of p42mapk initiated five minutes after stimulation of endothelial, adipose and chromaffin cells with growth factor is not catalysed by CL100, but rather by protein phosphatase 2A and by a protein tyrosine phosphatase distinct from CL100. Induction of CL100 is not accompanied by the inactivation of p42mapk in a number of situations.

Protein phosphatase 1 regulates the cytoplasmic dynein-driven formation of endoplasmic reticulum networks in vitro

Interphase Xenopus egg extracts form extensive tubular membrane networks in vitro. These networks are identified here as endoplasmic reticulum by the presence of ER resident proteins, as shown by immunofluorescence, and by the presence of single ribosomes and polysomes, as shown by electron microscopy. The effect of phosphorylation on ER movement in interphase was tested using the phosphatase inhibitor, okadaic acid. Okadaic acid treatment resulted in an increase of up to 27-fold in the number of ER tubules moving and in the extent of ER networks formed compared to control extracts. This activation was blocked by the broad-specificity kinase inhibitor 6-dimethylaminopurine. Okadaic acid had no effect, however, on the direction of ER tubule movement, which occurred towards the minus end of microtubules, and was sensitive to low concentrations of vanadate. Inhibition of phosphatases also had no effect on the speed or duration of ER tubule extensions, and did not stimulate the activity of soluble cytoplasmic dynein. The sensitivity of ER movement to okadaic acid closely matched that of protein phosphatase 1. Although the amount of ER motility was greatly increased by inhibiting protein phosphatase 1 (PP1), the amount of cytoplasmic dynein associated with the membrane was not altered. The data support a model in which phosphorylation regulates ER movement by controlling the activity of cytoplasmic dynein bound to the ER membrane.

Chromosome condensation induced by fostriecin does not require p34cdc2 kinase activity and histone H1 hyperphosphorylation, but is associated with enhanced histone H2A and H3 phosphorylation

Chromosome condensation at mitosis correlates with the activation of p34cdc2 kinase, the hyperphosphorylation of histone H1 and the phosphorylation of histone H3. Chromosome condensation can also be induced by treating interphase cells with the protein phosphatase 1 and 2A inhibitors okadaic acid and fostriecin. Mouse mammary tumour FT210 cells grow normally at 32 degrees C, but at 39 degrees C they lose p34cdc2 kinase activity and arrest in G2 because of a temperature-sensitive lesion in the cdc2 gene. The treatment of these G2-arrested FT210 cells with fostriecin or okadaic acid resulted in full chromosome condensation in the absence of p34cdc2 kinase activity or histone H1 hyperphosphorylation. However, phosphorylation of histones H2A and H3 was strongly stimulated, partly through inhibition of histone H2A and H3 phosphatases, and cyclins A and B were degraded. The cells were unable to complete mitosis and divide. In the presence of the protein kinase inhibitor starosporine, the addition of fostriecin did not induce histone phosphorylation and chromosome condensation. The results show that chromosome condensation can take place without either the histone H1 hyperphosphorylation or the p34cdc2 kinase activity normally associated with mitosis, although it requires a staurosporine-sensitive protein kinase activity. The results further suggest that protein phosphatases 1 and 2A may be important in regulating chromosome condensation by restricting the level of histone phosphorylation during interphase, thereby preventing premature chromosome condensation.

Purification of type 1 protein (serine/threonine) phosphatases by microcystin-Sepharose affinity chromatography

A microcystin (MC)-Sepharose column was prepared by addition of 2-aminoethanethiol to the alpha, beta-unsaturated carbonyl of the N-methyldehydroalanine residue of MC-LR, followed by reaction of the introduced amino group with N-hydroxysuccinimide-activated CH-Sepharose. The MC-Sepharose bound protein phosphatase-1 (PP1) with high capacity and purified human PP1 gamma in one step from E. coli extracts. It was also used to purify forms of PP1 bound to myofibrils from skeletal muscle. Two of these comprised PP1 complexed to N-terminal fragments of the M-subunit which enhance its myosin phosphatase activity, while the third comprised PP1 and an N-terminal fragment of the glycogen-binding (G)-subunit.

Different oligomeric forms of protein phosphatase 2A activate and inhibit simian virus 40 DNA replication

The ability of simian virus 40 (SV40) large T antigen to catalyze the initiation of viral DNA replication is regulated by its phosphorylation state. Previous studies have identified the free catalytic subunit of protein phosphatase 2A (PP2Ac) as the cellular phosphatase which can remove inhibitory phosphoryl groups from serines 120 and 123. The catalytic C subunit exists in the cell complexed with a 65-kDa A subunit and one of several B subunits. To determine if any of the holoenzymes could activate T antigen, we tested the ability of the heterodimeric AC and two heterotrimeric ABC forms to stimulate T-antigen function in unwinding the origin of SV40 DNA replication. Only free catalytic subunit C and the heterotrimeric form with a 72-kDa B subunit (PP2A-T72) could stimulate T-antigen-dependent origin unwinding. Both the dimeric form (PP2A-D) and the heterotrimer with a 55-kDa B subunit (PP2A-T55) actively inhibited T-antigen function. We found that PP2A-T72 activated T antigen by dephosphorylating serines 120 and 123, while PP2A-D and PP2A-T55 inactivated T antigen by dephosphorylating the p34cdc2 target site, threonine 124. Thus, alterations in the subunit composition of PP2A holoenzymes have significant functional consequences for the initiation of in vitro SV40 DNA replication. The regulatory B subunits of PP2A may play a role in regulating SV40 DNA replication in infected cells as well.

Different oligomeric forms of protein phosphatase 2A activate and inhibit simian virus 40 DNA replication

The ability of simian virus 40 (SV40) large T antigen to catalyze the initiation of viral DNA replication is regulated by its phosphorylation state. Previous studies have identified the free catalytic subunit of protein phosphatase 2A (PP2Ac) as the cellular phosphatase which can remove inhibitory phosphoryl groups from serines 120 and 123. The catalytic C subunit exists in the cell complexed with a 65-kDa A subunit and one of several B subunits. To determine if any of the holoenzymes could activate T antigen, we tested the ability of the heterodimeric AC and two heterotrimeric ABC forms to stimulate T-antigen function in unwinding the origin of SV40 DNA replication. Only free catalytic subunit C and the heterotrimeric form with a 72-kDa B subunit (PP2A-T72) could stimulate T-antigen-dependent origin unwinding. Both the dimeric form (PP2A-D) and the heterotrimer with a 55-kDa B subunit (PP2A-T55) actively inhibited T-antigen function. We found that PP2A-T72 activated T antigen by dephosphorylating serines 120 and 123, while PP2A-D and PP2A-T55 inactivated T antigen by dephosphorylating the p34cdc2 target site, threonine 124. Thus, alterations in the subunit composition of PP2A holoenzymes have significant functional consequences for the initiation of in vitro SV40 DNA replication. The regulatory B subunits of PP2A may play a role in regulating SV40 DNA replication in infected cells as well.

Alternative cell fate choice induced by low-level expression of a regulator of protein phosphatase 2A in the Drosophila peripheral nervous system

The Drosophila gene twins encodes the regulatory B subunit of type 2A protein phosphatase. Here we report that its partial loss-of-function mutations caused abnormal morphogenesis in the adult peripheral nervous system. In wild-type flies, the mechanoreceptor, one major class of sensory organs, is composed of four specialized cells (one neuron and three accessory cells) that are derived from a single precursor cell. The hypomorphic twins mutations did not block division of this precursor, but most likely altered cell fate in this lineage to produce only accessory cells that form sensory structures. Stepwise reductions of twins protein enhanced this transformation. In these mutants, another regulatory subunit, A, and the catalytic subunit, C, of the phosphatase were expressed at normal levels. Therefore, the modulation of the phosphatase activity by the B subunit appears to be crucial for specification of neural cell identity.

PPQ, a novel protein phosphatase containing a Ser + Asn-rich amino-terminal domain, is involved in the regulation of protein synthesis

The sequence of a Saccharomyces cerevisiae cDNA encoding a novel 61-kDa protein serine/threonine phosphatase, termed PPQ1, is presented. The protein consists of two distinct domains: the carboxy-terminal phosphatase domain is approximately 60% identical to either PP1 or the carboxy-terminal domains of PPZ1 and PPZ2, while the amino-terminal region is rich in serine and asparagine. Deletion of the gene encoding PPQ1 reduces cell growth on several carbon sources, and lowers cell density in the stationary phase. Cells in which PPQ1 gene has been deleted show altered morphology from wild-type cells in the stationary phase in the absence of an essential amino acid and a reduced rate of protein synthesis in the exponential phase. They are hypersensitive to the protein synthesis inhibitors, cycloheximide and G418, implicating PPQ1 in the regulation of translation.