Identification of protein phosphatases-1 and 2A and inhibitor-2 in oocytes of the starfish Asterias rubens and Marthasterias glacialis

In vitro extinction learning in Hermissenda: involvement of conditioned inhibition molecules

Extinction of a conditioned association is typically viewed as the establishment of new learning rather than the erasure of the original memory. However, recent research in the nudibranch, Hermissenda crassicornis (H.c.) demonstrated that extinction training (using repeated light-alone presentations) given 15 min, but not 23 h, after memory acquisition reversed both the cellular correlates of learning (enhanced Type B cell excitability) and the behavioral changes (reduced phototaxis) produced by associative conditioning (pairings of light, CS, and rotation, US). Here, we investigated the putative molecular signaling pathways that underlie this extinction in H.c. by using a novel in vitro protocol combined with pharmacological manipulations. After intact H.c. received either light-rotation pairings (Paired), random presentations of light and rotation (Random), or no stimulation (Untrained), B cells from isolated CNSs were recorded from during exposure to extinction training consisting of two series of 15 consecutive light-steps (LSs). When in vitro extinction was administered shortly (2 h, but not 24 h) after paired training, B cells from Paired animals showed progressive and robust declines in spike frequency by the 30th LS, while control cells (Random and Untrained) did not. We found that several molecules implicated in H.c. conditioned inhibitory (CI) learning, protein phosphatase 1 (PP1) and arachidonic acid (AA)/12-lipoxygenase (12-LOX) metabolites, also contributed to the spike frequency decreases produced by in vitro extinction. Protein phosphatase 2B (PP2B) also appeared to play a role. Calyculin A (PP1 inhibitor), cyclosporin A (PP2B inhibitor), and baicalein (a 12-LOX inhibitor) all blocked the spike frequency declines in Paired B cells produced by 30 LSs. Conversely, injection of catalytically-active PP1 (caPP1) or PP2B (caPP2B) into Untrained B cells partially mimicked the spike frequency declines observed in Paired cells, as did bath-applied AA, and occluded additional LS-produced reductions in spiking in Paired cells.

High cAMP attenuation of insulin-stimulated meiotic G2-M1 transition in zebrafish oocytes: interaction between the cAMP-dependent protein kinase (PKA) and the MAPK3/1 pathways

High intra-cellular cyclic nucleotide (cAMP) ensures prophase-I arrest and prevent steroid-induced meiotic G2-M1 transition in full-grown oocytes; however, relatively less information is available for cAMP regulation of growth factor-stimulated signalling events in the oocyte model. Here using zebrafish oocytes, we show that priming with dibutyryl cAMP (dbcAMP) or cAMP modulators, e.g. adenylate cyclase activator, forskolin or phosphodiesterase inhibitors (IBMX/cilostamide) block insulin action on germinal vesicle breakdown (GVBD) and histone H1 kinase activation. Though high cAMP priming attenuates insulin-induced MAPK3/1 (ERK1/2) phosphorylation (activation), following 2h of insulin stimulation it fails to block MAPK activation and GVBD. Further, insulin stimulation promotes down regulation of phospho-PKAc (inactivation) and PKA inhibition by H89/PKI-(6-22)-amide overcomes negative regulation by cAMP and induces GVBD and MAPK activation. Moreover, MEK1/2 inhibitor U0126 has no influence on H89-induced GVBD; however, it delays GVBD response in insulin-stimulated oocytes. MAPK activation by okadaic acid (OA) promotes GVBD; however, high dbcAMP abrogates OA action suggesting cross-talk between cAMP/PKA and MAPK-mediated signalling pathways may contribute significantly in maturing zebrafish oocyte.

Regulation of protein phosphatase 1I by Cdc25C-associated kinase 1 (C-TAK1) and PFTAIRE protein kinase

Protein phosphatase 1I (PP-1I) is a major endogenous form of protein phosphatase 1 (PP-1) that consists of the core catalytic subunit PP-1c and the regulatory subunit inhibitor 2 (I-2). Phosphorylation of the Thr-72 residue of I-2 is required for activation of PP-1I. We studied the effects of two protein kinases identified previously in purified brain PP-1I by mass spectrometry, Cdc25C-associated kinase 1 (C-TAK1) and PFTAIRE (PFTK1) kinase, for their ability to regulate PP-1I. Purified C-TAK1 phosphorylated I-2 in reconstituted PP-1I (PP-1c. I-2) on Ser-71, which resulted in partial inhibition of its ATP-dependent phosphatase activity and inhibited subsequent phosphorylation of Thr-72 by the exogenous activating kinase GSK-3. In contrast, purified PFTK1 phosphorylated I-2 at Ser-86, a site known to potentiate Thr-72 phosphorylation and activation of PP-1I phosphatase activity by GSK-3. These findings indicate that brain PP-1I associates with and is regulated by the associated protein kinases C-TAK1 and PFTK1. Multisite phosphorylation of the I-2 regulatory subunit of PP-1I leads to activation or inactivation of PP-1I through bidirectional modulation of Thr-72 phosphorylation, the critical activating residue of I-2.

Impact of marine drugs on animal reproductive processes

The discovery and description of bioactive substances from natural sources has been a research topic for the last 50 years. In this respect, marine animals have been used to extract many new compounds exerting different actions. Reproduction is a complex process whose main steps are the production and maturation of gametes, their activation, the fertilisation and the beginning of development. In the literature it has been shown that many substances extracted from marine organisms may have profound influence on the reproductive behaviour, function and reproductive strategies and survival of species. However, despite the central importance of reproduction and thus the maintenance of species, there are still few studies on how reproductive mechanisms are impacted by marine bioactive drugs. At present, studies in either marine and terrestrial animals have been particularly important in identifying what specific fine reproductive mechanisms are affected by marine-derived substances. In this review we describe the main steps of the biology of reproduction and the impact of substances from marine environment and organisms on the reproductive processes.

Activation of maturation promoting factor in Bufo arenarum oocytes: injection of mature cytoplasm and germinal vesicle contents

Although progesterone is the established maturation inducer in amphibians, Bufo arenarum oocytes obtained during the reproductive period (spring-summer) resume meiosis with no need of an exogenous hormonal stimulus if deprived of their enveloping follicle cells, a phenomenon called spontaneous maturation. In this species it is possible to obtain oocytes competent and incompetent to undergo spontaneous maturation according to the seasonal period in which animals are captured. Reinitiation of meiosis is regulated by maturation promoting factor (MPF), a complex of the cyclin-dependent kinase p34cdc2 and cyclin B. Although the function and molecule of MPF are common among species, the formation and activation mechanisms of MPF differ according to species. This study was undertaken to evaluate the presence of pre-MPF in Bufo arenarum oocytes incompetent to mature spontaneously and the effect of the injection of mature cytoplasm or germinal vesicle contents on the resumption of meiosis. The results of our treatment of Bufo arenarum immature oocytes incompetent to mature spontaneously with sodium metavanadate (NaVO3) and dexamethasone (DEX) indicates that these oocytes have a pre-MPF, which activates and induces germinal vesicle breakdown (GVBD) by dephosphorylation on Thr-14/Tyr-15 by cdc25 phosphatase and without cyclin B synthesis. The injection of cytoplasm containing active MPF is sufficient to activate an amplification loop that requires the activation of cdc25 and protein kinase C, the decrease in cAMP levels, and is independent of protein synthesis. However, the injection of germinal vesicle content also induces GVBD in the immature receptor oocyte, a process dependent on protein synthesis but not on cdc25 phosphatase or PKC activity.

2-Hydroxyestradiol-17β-induced oocyte maturation in catfish (Heteropneustes fossilis) involves protein kinase C and its interaction with protein phosphatases

In vitro effects of phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, calphostin C (PKC inhibitor) and okadaic acid [OA, a protein phosphatase (PP; PP1 and PP2A) inhibitor] on 2-hydroxyestradiol-17beta (2-OHE(2))-induced oocyte maturation were investigated in the catfish Heteropneustes fossilis. Incubations of postvitellogenic follicles with PMA or OA alone did not induce oocyte maturation. However, co-incubations with 2-OHE(2) and PMA (0.05, 0.5 and 5 microM) or 2-OHE(2) and OA (0.5, 1.0 or 2.0 microM) increased germinal vesicle breakdown (GVBD) significantly over that of 2-OHE(2). Incubation of follicles with calphostin C elicited varied effects on GVBD, low (0.005 and 0.01 microM) and high (5.0 and 10.0 microM) concentrations did not affect GVBD, but medium concentrations (0.05, 0.1, 0.5, 1.0 and 2.5 microM) stimulated it. The medium concentrations elicited a biphasic stimulatory response with peak GVBD at 0.1 microM (54%). Calphostin C (>or=2.5 microM) inhibited the 2-OHE(2)-induced GVBD in a concentration-dependent manner during the 24 h incubation. Pre- or post-treatment with calphostin C inhibited the steroid-induced GVBD only at 6 h. In co-incubation studies, both PMA and OA reversed the inhibitory effect of calphostin C: the former partially and the latter fully. The results of the present study show that PKC appears to modulate the 2-OHE(2)-induced oocyte maturation. The OA-sensitive PP may be involved in the PKC modulation of steroid-induced oocyte maturation.

Nuclear envelope breakdown may deliver an inhibitor of protein phosphatase 1 which triggers cyclin B translation in starfish oocytes

In vertebrates, enhanced translation of mRNAs in oocytes and early embryos entering M-phase is thought to occur through polyadenylation, involving binding, hyperphosphorylation and proteolytic degradation of Aurora-activated CPEB. In starfish, an unknown component of the oocyte nucleus is required for cyclin B synthesis following the release of G2/prophase block by hormonal stimulation. We have found that CPEB cannot be hyperphosphorylated following hormonal stimulation in starfish oocytes from which the nucleus has been removed. Activation of Aurora kinase, known to interact with protein phosphatase 1 and its specific inhibitor Inh-2, is also prevented. The microinjection of Inh-2 restores Aurora activation, CPEB hyperphosphorylation and cyclin B translation in enucleated oocytes. Nevertheless, we provide evidence that CPEB is in fact hyperphosphorylated by cdc2, without apparent involvement of Aurora or MAP kinase, and that cyclin B synthesis can be stimulated without previous degradation of phosphorylated CPEB. Thus, the regulation of cyclin B synthesis necessary for progression through meiosis can be explained by an equilibrium between CPEB phosphorylation and dephosphorylation, and both aspects of this control may rely on the sole activation of Cdc2 and subsequent nuclear breakdown.

Endogenous regulators of protein phosphatase-1 during mouse oocyte development and meiosis

Reversible phosphorylation, involving protein kinases and phosphatases (PP), is important in regulating oocyte meiosis. Okadaic acid (OA) inhibition of PP1 and/or PP2A stimulates oocyte germinal vesicle breakdown (GVB). In oocytes, PP1 is localized in the cytoplasm and nucleus, yet endogenous regulation of oocyte PP1 has not been investigated. The objectives of the study were to identify intra-oocyte mechanisms regulating PP1 during acquisition of OA-sensitive meiotic competence and meiotic resumption. Immunohistochemical studies revealed that GVB-incompetent oocytes contained equivalent cytoplasmic and nuclear PP1. Upon development of OA-sensitive meiotic competence, PP1 displayed differential intracellular localization with significantly greater nuclear staining with distinct nucleolar rimming compared with cytoplasmic staining. Germinal vesicle-intact oocytes contained neither nuclear inhibitor of PP1, nor PP1 cytoplasmic inhibitor-1 transcripts or proteins. Reverse transcription-PCR with PP1 cytoplasmic inhibitor-2 (I2) primers and oocyte RNA amplified a predicted 330-bp product with the identical sequence to mouse liver I2. Oocytes contained a heat-stable PP1 inhibitor with biochemical properties of I2. Phosphorylation of PP1 at Thr320 by cyclin dependent kinase-1 (CDK1) causes PP1 inactivation. Germinal vesicle-intact oocytes did not contain phospho-Thr320-PP1. Upon GVB, PP1 became phosphorylated at Thr320 and this phosphorylation did not occur if GVB was blocked with the CDK1 inhibitor, roscovitine (ROSC). Inhibition of oocyte GVB with ROSC was reversible and coincided with PP1 phosphorylation at Thr320. Increased oocyte staining of nuclear PP1 compared with cytoplasmic staining at a chronological stage when oocytes gain meiotic competence, and phosphorylation and inhibition of PP1 by CDK1 at or around GVB appear to be important mechanisms in regulating oocyte PP1 activity and meiosis. In addition, these studies provide further support for PP1 being the OA-sensitive PP important in the regulation of the acquisition of meiotic competence, nuclear events during meiotic arrest, and GVB.

Specific inhibition of mouse oocyte nuclear protein phosphatase-1 stimulates germinal vesicle breakdown

Okadaic acid (OA)-induced germinal vesicle breakdown (GVBD) and localization of protein phosphatase-1 (PP1) in oocyte nuclei are suggestive of PP1's role in regulating oocyte GVBD. To explore this possibility, we microinjected protein phosphatase (PP) inhibitors OA, anti-PP1 antibody (anti-PP1), PP1 inhibitor I2, and anti-PP2A antibody (anti-PP2A) into nuclei of roscovitine (ROSC)-arrested mouse oocytes. Oocytes were also injected with recombinant PP1 in the absence of ROSC. Oocytes were assessed for GVBD and metaphase II (MII) development at 2 and 18 hr post-injection. Data were analyzed using Cochran-Mantel-Haenszel Statistics adjusted for time. Microinjection of OA significantly enhanced GVBD in comparison to controls at 2 and 18 hr (P < 0.01), yet had no effect on MII development. Similarly, microinjection of anti-PP1 resulted in significantly higher levels of GVBD compared to controls at 2 and 18 hr (P < 0.01). Interestingly, anti-PP1 microinjection also tended to enhance MII development at 18 hr in comparison to controls (P < 0.09). Microinjection of I2, anti-PP2A, and PP1 had no effect on GVBD or MII development. If reduction of PP1 activity was important for GVBD, one would anticipate an endogenous means of regulating PP1 activity at this developmental stage. In somatic cells, phosphorylation of PP1 at Thr320 causes PP1 inactivation. Germinal vesicle-intact oocytes did not contain phosphorylated PP1, as determined using a specific Thr320-Phospho-PP1 antibody, Western blot analysis, and confocal immunocytochemistry. At or around the time of GVBD, oocyte PP1 became phosphorylated at Thr320, which remained phosphorylated through MII development. These data indicate that inhibition of intra-nuclear PP1, through specific antibody neutralization, mimics OA-stimulated GVBD, providing the first direct evidence that nuclear PP1 is involved in regulation of oocyte nuclear membrane integrity. In addition, phosphorylation of PP1 occurs at/or around GVBD indicating that inactivation of PP1 is an important intracellular event in regulation of nuclear envelope dissolution at GVBD.

Characterization of protein phosphatases in mouse oocytes

Okadaic acid (OA) enhances the resumption of meiosis in mouse oocytes, indicating that serine/threonine protein phosphatase-1 (PP1) and/or PP2A is involved. However, specific identification of PP1 and/or PP2A in mouse oocytes has not been reported. Here we demonstrate that fully grown germinal vesicle-intact (GVI) mouse oocytes contain mRNA corresponding to two isotypes of PP1, PP1alpha and PP1gamma. In addition, the transcript for PP2A was also present. At the protein level only PP1alpha and PP2A were recognized in fully grown GVI oocytes by Western blot analysis. Neither of the PP1gamma spliced variant proteins, PP1gamma1 and PP1gamma2, was detectable. Immunohistochemical analysis of ovarian tissue from gonadotropin-stimulated adult mice resulted in subcellular localization of both PP1alpha and PP2A, but not PP1gamma, in oocytes from all stages of folliculogenesis. In primordial oocytes, PP1alpha and PP2A were present in the cytoplasm. In more advanced stages of oogenesis, PP1alpha, although still present in the cytoplasm, was highly concentrated in the nucleus, whereas PP2A was predominantly cytoplasmic with a distinct reduction in the nuclear area. Both PP1alpha and PP2A were immunodetectable in oocytes during the prepubertal period. Eleven-day-old mouse oocytes, considered OA-insensitive and germinal vesicle breakdown (GVB)-incompetent, displayed both PP1alpha and PP2A predominantly in the cytoplasm. By 15 days of age mouse oocytes, which are beginning to acquire OA sensitivity and GVB competence, showed a relocation of PP1alpha into the nucleoplasm while PP2A remained predominantly cytoplasmic. This is the first specific identification of PP1alpha and PP2A in mouse oocytes. The differential localization of PP1alpha and PP2A, in addition to the relocation of PP1alpha during the acquisition of meiotic competence, suggests that these PPs have distinct regulatory roles during the resumption of meiosis.

Regulation of chromosome segregation by Glc8p, a structural homolog of mammalian inhibitor 2 that functions as both an activator and an inhibitor of yeast protein phosphatase 1

The Ipl1 protein kinase is essential for proper chromosome segregation and cell viability in the budding yeast Saccharomyces cerevisiae. We have previously shown that the temperature-sensitive growth phenotype of conditional ipl1-1ts mutants can be suppressed by a partial loss-of-function mutation in the GLC7 gene, which encodes the catalytic subunit (PP1C) of protein phosphatase 1, thus suggesting that this enzyme acts in opposition to the Ipl1 protein kinase in regulating yeast chromosome segregation. We report here that the Glc8 protein, which is related in primary sequence to mammalian inhibitor 2, also participates in this regulation. Like inhibitor 2, the Glc8 protein is heat stable, exhibits anomalous electrophoretic mobility, and functions in vitro as an inhibitor of yeast as well as rabbit skeletal muscle PP1C. Interestingly, overexpression as well as deletion of the GLC8 gene results in a partial suppression of the temperature-sensitive growth phenotype of ipl1ts mutants and also moderately reduces the amount of protein phosphatase 1 activity which is assayable in crude yeast lysates. In addition, the chromosome missegregation phenotype caused by an increase in the dosage of GLC7 is totally suppressed by the glc8-delta 101::LEU2 deletion mutation. These findings together suggest that the Glc8 protein is involved in vivo in the activation of PP1C and that when the Glc8 protein is overproduced, it may also inhibit PP1C function. Furthermore, site-directed mutagenesis studies of GLC8 suggest that Thr-118 of the Glc8 protein, which is equivalent to Thr-72 of inhibitor 2, may play a central role in the ability of this protein to activate and/or inhibit PP1C in vivo.

Cloning and characterization of human phosphatase inhibitor-2 (IPP-2) sequences

cDNA clones similar to rabbit muscle phosphatase inhibitor-2 (IPP-2) were isolated from human libraries. On Northern blots two transcripts of approximately 2kbp and approximately 4kbp were detected in all tissues tested. Analysis of cDNA sequences showed that the longer transcripts were similar to the shorter clones but contained extended 3' ends. The human nucleotide sequence was highly homologous (94% identity) to the rabbit IPP-2 sequence and encoded a peptide of 205 amino acids. IPP-2 sequences were highly conserved throughout vertebrates. Southern hybridization results were consistent with the existence of a family of related IPP-2 sequences in the human genome. Most of these are likely to be pseudogenes, since all of the cDNA clones examined could have originated from a single gene. By in situ hybridization IPP-2 sequences were mapped to several different human chromosomes. We sequenced one gene located in the major histocompatibility complex (MHC) on Chromosome (Chr) 6 that contained the entire coding region of IPP-2.

Inhibitor-2 functions like a chaperone to fold three expressed isoforms of mammalian protein phosphatase-1 into a conformation with the specificity and regulatory properties of the native enzyme

Three isoforms of mammalian protein phosphatase-1 (PP1 alpha, PP1 beta and PP1 gamma) were expressed in Escherichia coli and purified to near homogeneity. The activities of all isoforms towards phosphorylase, phosphorylase kinase and myosin and their sensitivities to inhibitor-2 were similar to the native PP1 catalytic subunit (PP1C) isolated from vertebrate tissues. Like PP1C, they each formed a complex with the glycogen-targetting(G) subunit which directs PP1C to glycogen particles in skeletal muscle. However, other properties differed strikingly from native PP1C. The expressed isoforms were 100-600-fold less sensitive to inhibitor-1, 3-5-fold less sensitive to okadaic acid, 5-100-fold less sensitive to microcystin-LR and approximately 20-fold more active in dephosphorylating histone H1 than native PP1C. Although PP1 gamma (like PP1C) was active in the absence of Mn2+, expressed PP1 alpha and PP1 beta were completely dependent on Mn2+ for activity. PP1 beta, like PP1C, interacted with the myofibrillar-targetting(M) complexes from skeletal-muscle and smooth-muscle producing species with enhanced myosin-phosphatase activity, whereas expressed PP1 alpha and PP1 gamma did not. The expressed isoforms of PP1 combined with inhibitor-2 to form an inactive complex (PP1I) that could be reactivated by the glycogen-synthase-kinase-3(GSK3)-catalysed phosphorylation of inhibitor-2. This procedure transformed the properties of all three expressed isoforms to those of native PP1C. Their sensitivities to inhibitor-1, okadaic acid and microcystin-LR were increased greatly, their histone-phosphatase activities decreased and the activities of PP1 alpha and PP1 beta became independent of Mn2+. Furthermore PP1 alpha and PP1 gamma now interacted with the M complexes in a similar manner to PP1 beta and PP1C. Conversely, incubation of native PP1C with 50 mM NaF caused conversion to a Mn(2+)-dependent form with properties similar to those of the expressed isozymes. The G subunit from skeletal muscle or the M complex from smooth muscle could displace PP1C from activated PP1I, but not inactive PP1I, to form G-subunit/PP1C and M-complex/PP1C heterodimeric complexes. Inhibitor-2 was also found to be essential for the reactivation of PP1C from 6 M guanidinium chloride in the absence of Mn2+. Taken together, the results suggest that inhibitor-2 is critical for the correct folding of nascent PP1C polypeptides, that its function is similar to that of a molecular chaperone and that it acts as a cytosolic reservoir of PP1C molecules which can be directed to the required subcellular locations following the synthesis of specific targetting subunits.

Mitotic regulation of protein phosphatases by the fission yeast sds22 protein

BACKGROUND

Cell cycle progression requires the activity of protein kinases and phosphatases at critical points in the cell cycle in all eukaryotes. We have previously reported that the dis2(+) and sds2(+) genes of fission yeast encode redundant catalytic subunits of a type 1-like protein phosphatase. The sds22(+) gene was shown to be essential for cell viability and to interact genetically with dis2(+) and sds21(+).

RESULTS

Here we show by immunoprecipitation that the sds22 protein physically interacts with the dis2 and sds21 proteins, and that sds22-associated phosphatase activity has altered substrate specificity, The loss of sds22 function by a temperature sensitive mutation leads to cell cycle arrest at mid-mitosis, at which point cdc2-dependent histone Hl kinase activity is high while sds22-dependent H1 phosphatase activity is low. To examine the unusual properties of sds22 protein structure, we analyzed a collection of sds22 deletion and point mutants by a variety of functional criteria.

CONCLUSION

We propose that sds22 is a regulatory subunit of the dis2/sds21 phosphatase catalytic subunits and that sds22-bound phosphatase carries a key phosphatase activity essential for the progression from metaphase to anaphase. Mutational analysis indicates that dis2/sds21 interacts with the central repetitive domain of sds22, while the C-terminal and central regions of sds22 may be involved in subcellular targeting and the N-terminus is important for stability.

Purification and characterization of three distinct types of protein phosphatase catalytic subunits in bovine platelets

The catalytic subunits of bovine platelet protein phosphatases were separated into three distinct forms by chromatography on heparin-Sepharose. Each phosphatase was further purified to apparent homogeneity as judged in sodium dodecyl sulfate-polyacrylamide gel yielding single protein bands of 37, 41, and 36 kDa. The 37-kDa phosphatase was excluded from heparin-Sepharose and preferentially dephosphorylated the alpha-subunit of phosphorylase kinase. It was stimulated by polycations (polybrene or histone H1) and was inhibited by okadaic acid (IC50 = 0.3 nM), but its activity was not influenced by inhibitor-2 or heparin. The 41-kDa phosphatase was eluted from heparin-Sepharose by 0.20-0.25 M NaCl and preferentially dephosphorylated the beta-subunit of phosphorylase kinase. It was stimulated by polycations and inhibited by okadaic acid (IC50 = 2 nM), but its activity was not affected by inhibitor-2 or heparin. The 36-kDa phosphatase was eluted from heparin-Sepharose by 0.45-0.50 M NaCl and preferentially dephosphorylated the beta-subunit of phosphorylase kinase. It was inhibited by inhibitor-2, heparin, histone H1, and okadaic acid (IC50 = 70 nM). The 37- and 36-kDa phosphatases can be classified as type-2A and type-1 enzymes, respectively. The 41-kDa phosphatase does not precisely fit the criteria of either type, showing only partial similarities to both type-1 and type-2A enzymes and it may represent a novel type of protein phosphatase in bovine platelets.

The structure, role, and regulation of type 1 protein phosphatases

Type 1 protein phosphatases (PP-1) comprise a group of widely distributed enzymes that specifically dephosphorylate serine and threonine residues of certain phosphoproteins. They all contain an isoform of the same catalytic subunit, which has an extremely conserved primary structure. One of the properties of PP-1 that allows one to distinguish them from other serine/threonine protein phosphatases is their sensitivity to inhibition by two proteins, termed inhibitor 1 and inhibitor 2, or modulator. The latter protein can also form a 1:1 complex with the catalytic subunit that slowly inactivates upon incubation. This complex is reactivated in vitro by incubation with MgATP and protein kinase FA/GSK-3. In the cell the type 1 catalytic subunit is associated with noncatalytic subunits that determine the activity, the substrate specificity, and the subcellular location of the phosphatase. PP-1 plays an essential role in glycogen metabolism, calcium transport, muscle contraction, intracellular transport, protein synthesis, and cell division. The activity of PP-1 is regulated by hormones like insulin, glucagon, alpha- and beta-adrenergic agonists, glucocorticoids, and thyroid hormones.

Evidence for the regulation of exocytic transport by protein phosphorylation

We investigated the effects of the protein phosphatase inhibitors okadaic acid and microcystin-LR upon transport of newly synthesized proteins through the exocytic pathway. Treatment of CHO cells with 1 microM okadaic acid rapidly inhibited movement of a marker protein (vesicular stomatitis virus G protein) from the endoplasmic reticulum to the Golgi compartment. Both okadaic acid and microcystin-LR also inhibited transport in an in vitro assay reconstituting movement to the Golgi compartment, at concentrations equivalent to those required to inhibit phosphorylase phosphatase activity. Inhibition both in vivo and in vitro could be antagonized by protein kinase inhibitors, suggesting that protein phosphorylation was directly responsible for this effect. An early stage in the transport reaction associated with vesicle formation or targeting was inhibited by protein phosphorylation, which could be reversed by fractions enriched in protein phosphatase 2A. Protein kinase antagonists did not inhibit transport between sequential compartments of the exocytic pathway in vitro, suggesting that protein phosphorylation is not itself required for vesicular transport. During mitosis, vesicular transport is inhibited simultaneous to the activation of maturation-promoting factor. It is proposed that the inhibition caused by okadaic acid and microcystin-LR involves a similar mechanism to that responsible for the mitotic arrest of vesicular transport.

Characterization of neuronal protein phosphatases in Aplysia californica

Biochemical properties of neuronal protein phosphatases from Aplysia californica were characterized. Dephosphorylation of phosphorylase alpha by extracts of abdominal ganglia and clusters of sensory neurons from pleural ganglia was demonstrated. Type-1 protein phosphatase (PrP-1) was identified in these extracts by the dephosphorylation of the beta-subunit of phosphorylase kinase and its inhibition by the protein, inhibitor-2. Type-2A protein phosphatase (PrP-2A) was demonstrated by the dephosphorylation of the alpha-subunit of phosphorylase kinase, which was insensitive to inhibitor-2. As in vertebrate tissues, only four enzymes, PrP-1 (47%), PrP-2A (42%), PrP-2B (11%), and PrP-2C (less than 1%), accounted for all the cellular protein phosphatase activity dephosphorylating phosphorylase kinase. Aplysia PrP-1 and PrP-2A were potently inhibited by okadaic acid, with PrP-1 being approximately 20-fold more sensitive than PrP-2A. By comparison, purified PrP-2A from rabbit skeletal muscle was 15- to 20-fold more sensitive to okadaic acid than PrP-1 from the same source. Only PrP-1 was associated with the particulate fractions from Aplysia neurons, whereas PrP-1 and PrP-2A, -2B, and -2C were all present in the cytosol. Extraction of the particulate PrP-1 decreased its sensitivity to okadaic acid by sixfold, suggesting that cellular factor(s) affect its sensitivity to this inhibitor. In most respects, protein phosphatases from Aplysia neurons resemble their mammalian counterparts, and their biochemical characterization sets the stage for examining the role of these enzymes in neuronal plasticity, and in learning and memory.

Chromatographic separation of four Ser/Thr-protein phosphatases from solubilized ciliary membranes of Paramecium tetraurelia by heparin-sepharose

Chromatography of deoxycholate-solubilized proteins from Paramecium ciliary membranes on heparin-Sepharose resolved three peaks of protein phosphatase activities: one type 2A-like and a type 2C phosphatase in the flow-through fractions, another type 2A-like enzyme in the 0.1 M NaCl eluate and type 1 protein phosphatase in the 0.5 M NaCl eluate. The differential sensitivity of the two type 2A-like phosphatases to heparin and protamine further substantiated the existence of distinct isozymes. Once solubilized, none of these ciliary phosphatases required detergent to remain soluble. The molecular mass as determined by chromatography on Superose 6 was in the range 30,000-45,000 dalton for all four protein phosphatases.

A review of echinoderm oogenesis

This review of the anatomical, histological, biochemical, and molecular biological literature on echinoderm oogenesis includes the entire developmental history of oocytes; from their inception to the time they become ova. This is done from a comparative perspective, with reference to members of the five extant echinoderm classes; crinoids, holothurians, asteroids, ophiuroids, and echinoids. I describe the anatomy and fine structure of the echinoderm ovary, with emphasis on both the cellular relationships of the germ line cells to the somatic cells of the inner epithelium, and on the neuromuscular systems. I review the literature on the growth of oogonia into fully formed oocytes, including the process of vitellogenesis, presenting an ultrastructural analysis of the organelles and extracellular structures found in fully formed echinoderm oocytes. Echinoderm oocyte maturation is reviewed and a description of the ultrastructural, biochemical and molecular biological changes thought to occur during this process is presented. Finally, I discuss oocyte ovulation, the severing of cellular connections between the oocyte and its surrounding somatic epithelial cells.

In vivo progesterone regulation of protein phosphatase activity in Xenopus oocytes

Exogenous beta casein, previously phosphorylated in vitro by protein kinase A and casein kinase II, was microinjected into Xenopus oocytes to monitor in vivo protein phosphatase activities. Phosphatase activities were 1.6 and 3.4 fmol/min/oocyte, respectively, for beta casein phosphorylated by casein kinase II and beta casein phosphorylated by protein kinase A. Progesterone induced an early decrease (35% after 10 min) in phosphatase activity restricted to the protein kinase A sites of beta casein.

Involvement of protein phosphatases 1 and 2A in the control of M phase-promoting factor activity in starfish

Specific inhibition of types 1 and 2A protein phosphatases by microinjection of okadaic acid (OA) into starfish oocytes induced germinal vesicle breakdown and activation of M phase-promoting factor (MPF) and histone H1 kinase. The effects were evident in immature oocytes arrested at first meiotic prophase as well as in fully mature oocytes arrested at the pronucleus stage. In addition, MPF and histone H1 kinase were stabilized for several hours and protected from inactivation by inhibition of type 1 protein phosphatases with either OA or specific anti-phosphatase antibodies. Microinjection of okadaic acid was associated with unusual changes of the microtubule network, including the disappearance of spindles and extension of the cytoplasmic array of microtubules. MPF activation after OA injection was associated with dephosphorylation of phosphothreonine and phosphoserine residues in cdc2, showing that neither type 1 nor 2A protein phosphatases catalyzes these dephosphorylations. The effects of OA on MPF activation and inactivation appeared to involve the cyclin subunit. OA did not induce MPF activation in the absence of protein synthesis and it prevented degradation of cyclin. Therefore protein phosphatases types 1 and 2A appear to be involved in activation and inactivation of MPF involving mechanisms that operate after cyclin synthesis and before its degradation.

Characterization of polyclonal antibodies to brain protein phosphatase 2A and immunohistochemical localization of the enzyme in rat brain

Polyclonal antibodies to the catalytic subunit of protein phosphatase 2A (PrP-2A) from bovine brain were prepared by immunizing rabbits and then purified by antigen-affinity column chromatography. The purified antibodies recognized only PrP-2A among proteins examined, including calcineurin and PrP-1. The antibodies cross-reacted only with a protein in the crude homogenate from rat brain, which comigrated with the catalytic subunit of PrP-2A on SDS-PAGE. The antibodies completely inhibited the activity of PrP-2A, and immunoprecipitated the purified enzyme. Immunoblot analysis demonstrates that, among the subcellular fractions from rat brain, the cytosol fraction and the synaptosomal cytosol fraction show high immunoreactivities, and that any of examined regions of rat brain shows immunoreactivity more or less, in which the caudatoputamen was highest. Immunohistochemical studies demonstrate that the enzyme is distributed widely in various regions of rat brain and that the immunoreactivity is localized mainly in neurons. In general, immunostaining of neurons was strong in neurites as well as somata. It was noted that intracerebellar nuclei were strongly stained in both neuronal somata and dendrites. Amygdaloid complex, thalamus, neocortex, hippocampal formation, and caudatoputamen were moderately stained.

Purification of MPF from starfish: identification as the H1 histone kinase p34cdc2 and a possible mechanism for its periodic activation

MPF extracted from starfish oocytes copurifies with an M phase-specific H1 histone kinase encoded by a homolog of the fission yeast cell cycle control gene cdc2+. The most purified preparations contain p34cdc2 as the only major protein. Activation of the p34cdc2 kinase is correlated with appearance of the MPF activity both in vivo and in vitro. The increase in protein kinase activity is associated with p34cdc2 dephosphorylation and the decrease in protein kinase activity on leaving M phase with rephosphorylation. Microinjection of a peptide perfectly conserved in p34cdc2 from yeast to humans induces meiotic maturation, suggesting that an inhibitory component in G2 arrested oocytes interacts with this region of the p34cdc2 kinase. We propose that initiation of M phase is brought about by the dephosphorylation of p34cdc2, leading to increase in its protein kinase activity.

The myosin-bound form of protein phosphatase 1 (PP-1M) is the enzyme that dephosphorylates native myosin in skeletal and cardiac muscles

The myosin-bound form of protein phosphatase 1 (PP-1M) and the glycogen-bound form (PP-1G) together account for virtually all the phosphatase activity in rabbit skeletal muscle extracts towards native myosin. PP-1M has a 3-fold higher activity towards native myosin than does PP-1G and accounts for at least 60% of the myosin phosphatase activity in rabbit skeletal muscle. PP-1M accounts for 90% of the myosin phosphatase activity in bovine cardiac muscle, where PP-1G is essentially absent. The high activity of PP-1M towards native myosin appears to arise from interaction of the catalytic subunit with the putative myosin-binding subunit, since chymotryptic digestion liberates a catalytic subunit having the same characteristics as that released by limited proteolysis of PP-1G. Protein phosphatase 2A in skeletal and cardiac muscles is very active towards the isolated myosin P-light chain, but ineffective in dephosphorylating native myosin. The results suggest that PP-1M is the enzyme that dephosphorylates myosin in skeletal and cardiac muscle.