Phosphorylation and activation of the ATP-Mg-dependent protein phosphatase by the mitogen-activated protein kinase

Role of Dopamine in the Heart in Health and Disease

Dopamine has effects on the mammalian heart. These effects can include an increase in the force of contraction, and an elevation of the beating rate and the constriction of coronary arteries. Depending on the species studied, positive inotropic effects were strong, very modest, or absent, or even negative inotropic effects occurred. We can discern five dopamine receptors. In addition, the signal transduction by dopamine receptors and the regulation of the expression of cardiac dopamine receptors will be of interest to us, because this might be a tempting area of drug development. Dopamine acts in a species-dependent fashion on these cardiac dopamine receptors, but also on cardiac adrenergic receptors. We will discuss the utility of drugs that are currently available as tools to understand cardiac dopamine receptors. The molecule dopamine itself is present in the mammalian heart. Therefore, cardiac dopamine might act as an autocrine or paracrine compound in the mammalian heart. Dopamine itself might cause cardiac diseases. Moreover, the cardiac function of dopamine and the expression of dopamine receptors in the heart can be altered in diseases such as sepsis. Various drugs for cardiac and non-cardiac diseases are currently in the clinic that are, at least in part, agonists or antagonists at dopamine receptors. We define the research needs in order to understand dopamine receptors in the heart better. All in all, an update on the role of dopamine receptors in the human heart appears to be clinically relevant, and is thus presented here.

RAF-MEK-ERK pathway in cancer evolution and treatment

The RAF-MEK-ERK signaling cascade is a well-characterized MAPK pathway involved in cell proliferation and survival. The three-layered MAPK signaling cascade is initiated upon RTK and RAS activation. Three RAF isoforms ARAF, BRAF and CRAF, and their downstream MEK1/2 and ERK1/2 kinases constitute a coherently orchestrated signaling module that directs a range of physiological functions. Genetic alterations in this pathway are among the most prevalent in human cancers, which consist of numerous hot-spot mutations such as BRAF^V600E. Oncogenic mutations in this pathway often override otherwise tightly regulated checkpoints to open the door for uncontrolled cell growth and neoplasia. The crosstalk between the RAF-MEK-ERK axis and other signaling pathways further extends the proliferative potential of this pathway in human cancers. In this review, we summarize the molecular architecture and physiological functions of the RAF-MEK-ERK pathway with emphasis on its dysregulations in human cancers, as well as the efforts made to target the RAF-MEK-ERK module using small molecule inhibitors.

Magnesium maintains the length of the circadian period in Arabidopsis

The circadian clock coordinates the physiological responses of a biological system to day and night rhythms through complex loops of transcriptional/translational regulation. It can respond to external stimuli and adjust generated circadian oscillations accordingly to maintain an endogenous period close to 24 h. However, the interaction between nutritional status and circadian rhythms in plants is poorly understood. Magnesium (Mg) is essential for numerous biological processes in plants, and its homeostasis is crucial to maintain optimal development and growth. Magnesium deficiency in young Arabidopsis thaliana seedlings increased the period of circadian oscillations of the CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1) promoter (pCCA1:LUC) activity and dampened their amplitude under constant light in a dose-dependent manner. Although the circadian period increase caused by Mg deficiency was light dependent, it did not depend on active photosynthesis. Mathematical modeling of the Mg input into the circadian clock reproduced the experimental increase of the circadian period and suggested that Mg is likely to affect global transcription/translation levels rather than a single component of the circadian oscillator. Upon addition of a low dose of cycloheximide to perturb translation, the circadian period increased further under Mg deficiency, which was rescued when sufficient Mg was supplied, supporting the model's prediction. These findings suggest that sufficient Mg supply is required to support proper timekeeping in plants.

Protein phosphatase-1: dual activity regulation by Inhibitor-2

Inhibitor-2 (I2) ranks amongst the most ancient regulators of protein phosphatase-1 (PP1). It is a small, intrinsically disordered protein that was originally discovered as a potent inhibitor of PP1. However, later investigations also characterized I2 as an activator of PP1 as well as a chaperone for PP1 folding. Numerous studies disclosed the importance of I2 for diverse cellular processes but did not describe a unifying molecular principle of PP1 regulation. We have re-analyzed the literature on I2 in the light of current insights of PP1 structure and regulation. Extensive biochemical data, largely ignored in the recent I2 literature, provide substantial indirect evidence for a role of I2 as a loader of active-site metals. In addition, I2 appears to function as a competitive inhibitor of PP1 in higher eukaryotes. The published data also demonstrate that several segments of I2 that remain unstructured in the PP1 : I2 complex are in fact essential for PP1 regulation. Together, the available data identify I2 as a dynamic activity-modulator of PP1.

Chronic β-adrenergic stimulation reverses depressed Ca handling in mice overexpressing inhibitor-2 of protein phosphatase 1

RATIONALE

A higher expression/activity of type 1 serine/threonine protein phosphatase 1 (PP1) may contribute to dephosphorylation of cardiac regulatory proteins triggering the development of heart failure.

OBJECTIVE

Here, we tested the putatively protective effects of PP1 inhibitor-2 (I₂) overexpression using a heart failure model induced by chronic β-adrenergic stimulation.

METHODS AND RESULTS

Transgenic (TG) and wild-type (WT) mice were subjected to isoprenaline (ISO) or isotonic NaCl solution supplied via osmotic minipumps for 7 days. I₂ overexpression was associated with a depressed PP1 activity. Basal contractility was unchanged in catheterized mice and isolated cardiomyocytes between TG^NaCl and WT^NaCl. TG^ISO mice exhibited more fibrosis and a higher expression of hypertrophy marker proteins as compared to WT^ISO. After acute administration of ISO, the contractile response was accompanied by a higher sensitivity in TG^ISO as compared to WT^ISO. In contrast to basal contractility, the peak amplitude of [Ca]_i and SR Ca load were reduced in TG^NaCl as compared to WT^NaCl. These effects were normalized to WT levels after chronic ISO stimulation. Cardiomyocyte relaxation and [Ca]_i decay kinetics were hastened in TG^ISO as compared to WT^ISO, which can be explained by a higher phospholamban phosphorylation at Ser¹⁶. Chronic catecholamine stimulation was followed by an enhanced expression of GSK3β, whereas the phosphorylation at Ser⁹ was lower in TG as compared to the corresponding WT group. This resulted in a higher I₂ phosphorylation that may reactivate PP1.

CONCLUSION

Our findings suggest that the basal desensitization of β-adrenergic signaling and the depressed Ca handling in TG by inhibition of PP1 is restored by a GSK3β-dependent phosphorylation of I₂.

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.

Not so pseudo: the evolutionary history of protein phosphatase 1 regulatory subunit 2 and related pseudogenes

Background

Pseudogenes are traditionally considered “dead” genes, therefore lacking biological functions. This view has however been challenged during the last decade. This is the case of the Protein phosphatase 1 regulatory subunit 2 (PPP1R2) or inhibitor-2 gene family, for which several incomplete copies exist scattered throughout the genome.

Results

In this study, the pseudogenization process of PPP1R2 was analyzed. Ten PPP1R2-related pseudogenes (PPP1R2P1-P10), highly similar to PPP1R2, were retrieved from the human genome assembly present in the databases. The phylogenetic analysis of mammalian PPP1R2 and related pseudogenes suggested that PPP1R2P7 and PPP1R2P9 retroposons appeared before the great mammalian radiation, while the remaining pseudogenes are primate-specific and retroposed at different times during Primate evolution. Although considered inactive, four of these pseudogenes seem to be transcribed and possibly possess biological functions. Given the role of PPP1R2 in sperm motility, the presence of these proteins was assessed in human sperm, and two PPP1R2-related proteins were detected, PPP1R2P3 and PPP1R2P9. Signatures of negative and positive selection were also detected in PPP1R2P9, further suggesting a role as a functional protein.

Conclusions

The results show that contrary to initial observations PPP1R2-related pseudogenes are not simple bystanders of the evolutionary process but may rather be at the origin of genes with novel functions.

A versatile mass spectrometry-based method to both identify kinase client-relationships and characterize signaling network topology

While more than a thousand protein kinases (PK) have been identified in the Arabidopsis thaliana genome, relatively little progress has been made toward identifying their individual client proteins. Herein we describe the use of a mass spectrometry-based in vitro phosphorylation strategy, termed Kinase Client assay (KiC assay), to study a targeted-aspect of signaling. A synthetic peptide library comprising 377 in vivo phosphorylation sequences from developing seed was screened using 71 recombinant A. thaliana PK. Among the initial results, we identified 23 proteins as putative clients of 17 PK. In one instance protein phosphatase inhibitor-2 (AtPPI-2) was phosphorylated at multiple-sites by three distinct PK, casein kinase1-like 10, AME3, and a Ser PK-like protein. To confirm this result, full-length recombinant AtPPI-2 was reconstituted with each of these PK. The results confirmed multiple distinct phosphorylation sites within this protein. Biochemical analyses indicate that AtPPI-2 inhibits type 1 protein phosphatase (TOPP) activity, and that the phosphorylated forms of AtPPI-2 are more potent inhibitors. Structural modeling revealed that phosphorylation of AtPPI-2 induces conformational changes that modulate TOPP binding.

Identification of a regulatory subunit of protein phosphatase 1 which mediates blue light signaling for stomatal opening

Protein phosphatase 1 (PP1) is a eukaryotic serine/threonine protein phosphatase comprised of a catalytic subunit (PP1c) and a regulatory subunit that modulates catalytic activity, subcellular localization and substrate specificity. PP1c positively regulates stomatal opening through blue light signaling between phototropins and the plasma membrane H(+)-ATPase in guard cells. However, the regulatory subunit functioning in this process is unknown. We identified Arabidopsis PRSL1 (PP1 regulatory subunit2-like protein1) as a regulatory subunit of PP1c. Tautomycin, a selective inhibitor of PP1c, inhibited blue light responses of stomata in the single mutants phot1 and phot2, supporting the idea that signals from phot1 and phot2 converge on PP1c. We obtained PRSL1 based on the sequence similarity to Vicia faba PRS2, a PP1c-binding protein isolated by a yeast two-hybrid screen. PRSL1 bound to Arabidopsis PP1c through its RVxF motif, a consensus PP1c-binding sequence. Arabidopsis prsl1 mutants were impaired in blue light-dependent stomatal opening, H(+) pumping and phosphorylation of the H(+)-ATPase, but showed normal phototropin activities. PRSL1 complemented the prsl1 phenotype, but not if the protein carried a mutation in the RVxF motif, suggesting that PRSL1 functions through binding PP1c via the RVxF motif. PRSL1 did not affect the catalytic activity of Arabidopsis PP1c but it stimulated the localization of PP1c in the cytoplasm. We conclude that PRSL1 functions as a regulatory subunit of PP1 and regulates blue light signaling in stomata.

Mechanisms regulating cilia growth and cilia function in endothelial cells

The primary cilium is an important sensory organelle present in most mammalian cells. Our current studies aim at examining intracellular molecules that regulate cilia length and/or cilia function in vitro and ex vivo. For the first time, we show that intracellular cAMP and cAMP-dependent protein kinase (PKA) regulate both cilia length and function in vascular endothelial cells. Although calcium-dependent protein kinase modulates cilia length, it does not play a significant role in cilia function. Cilia length regulation also involves mitogen-activated protein kinase (MAPK), protein phosphatase-1 (PP-1), and cofilin. Furthermore, cofilin regulates cilia length through actin rearrangement. Overall, our study suggests that the molecular interactions between cilia function and length can be independent of one another. Although PKA regulates both cilia length and function, changes in cilia length by MAPK, PP-1, or cofilin do not have a direct correlation to changes in cilia function. We propose that cilia length and function are regulated by distinct, yet complex intertwined signaling pathways.

Inhibitor-2 induced M-phase arrest in Xenopus cycling egg extracts is dependent on MAPK activation

The evolutionarily-conserved protein phosphatase 1 (PP1) plays a central role in dephosphorylation of phosphoproteins during the M phase of the cell cycle. We demonstrate here that the PP1 inhibitor inhibitor-2 protein (Inh-2) induces an M-phase arrest in Xenopus cycling egg extracts. Interestingly, the characteristics of this M-phase arrest are similar to those of mitogen-activated protein kinase (p42MAPK)-induced M-phase arrest. This prompted us to investigate whether Inh-2-induced M-phase arrest was dependent on activation of the p42MAPK pathway. We demonstrate here that MAPK activity is required for Inh-2-induced M-phase arrest, as inhibition of MAPK by PD98059 allowed cycling extracts to exit M phase, despite the presence of Inh-2. We next investigated whether Inh-2 phosphorylation by the MAPK pathway was required to induce an M-phase arrest. We discovered that while p90Rsk (a MAPK protein required for M-phase arrest) is able to phosphorylate Inh-2, this phosphorylation is not required for Inh-2 function. Overall, our results suggest a novel mechanism linking p42MAPK and PP1 pathways during M phase of the cell cycle.

Molecular basis for an ancient partnership between prolyl isomerase Pin1 and phosphatase inhibitor-2

Pin1 is a prolyl isomerase that recognizes phosphorylated Ser/Thr-Pro sites, and phosphatase inhibitor-2 (I-2) is phosphorylated during mitosis at a PSpTP site that is expected to be a Pin1 substrate. However, we previously discovered I-2, but not phospho-I-2, bound to Pin1 as an allosteric modifier of Pin1 substrate specificity [Li, M., et al. (2008) Biochemistry 47, 292]. Here, we use binding assays and NMR spectroscopy to map the interactions on Pin1 and I-2 to elucidate the organization of this complex. Despite having sequences that are ∼50% identical, human, Xenopus, and Drosophila I-2 proteins all exhibited identical, saturable binding to GST-Pin1 with K(0.5) values of 0.3 μM. The (1)H-(15)N heteronuclear single-quantum coherence spectra for both the WW domain and isomerase domain of Pin1 showed distinctive shifts upon addition of I-2. Conversely, as shown by NMR spectroscopy, specific regions of I-2 were affected by addition of Pin1. A single-residue I68A substitution in I-2 weakened binding to Pin1 by half and essentially eliminated binding to the isolated WW domain. On the other hand, truncation of I-2 to residue 152 had a minimal effect on binding to the WW domain but eliminated binding to the isomerase domain. Size exclusion chromatography revealed that wild-type I-2 and Pin1 formed a large (>300 kDa) complex and I-2(I68A) formed a complex of half the size that we propose are a heterotetramer and a heterodimer, respectively. Pin1 and I-2 are conserved among eukaryotes from yeast to humans, and we propose they make up an ancient partnership that provides a means for regulating Pin1 specificity and function.

Identification and functional characterization of inhibitor-3, a regulatory subunit of protein phosphatase 1 in plants

Protein phosphatase 1 (PP1) is a eukaryotic serine/threonine protein phosphatase, and mediates diverse cellular processes in animal systems via the association of a catalytic subunit (PP1c) with multiple regulatory subunits that determine the catalytic activity, the subcellular localization, and the substrate specificity. However, no regulatory subunit of PP1 has been identified in plants so far. In this study, we identified inhibitor-3 (Inh3) as a regulatory subunit of PP1 and characterized a functional role of Inh3 in Vicia faba and Arabidopsis (Arabidopsis thaliana). We found Inh3 as one of the proteins interacting with PP1c using a yeast two-hybrid system. Biochemical analyses demonstrated that Arabidopsis Inh3 (AtInh3) bound to PP1c via the RVxF motif of AtInh3, a consensus PP1c-binding sequence both in vitro and in vivo. AtInh3 inhibited the PP1c phosphatase activity in the nanomolar range in vitro. AtInh3 was localized in both the nucleus and cytoplasm, and it colocalized with Arabidopsis PP1c in these compartments. Disruption mutants of AtINH3 delayed the progression of early embryogenesis, arrested embryo development at the globular stage, and eventually caused embryo lethality. Furthermore, reduction of AtINH3 expression by RNA interference led to a decrease in fertility. Transformation of the lethal mutant of inh3 with wild-type AtINH3 restored the phenotype, whereas that with the AtINH3 gene having a mutation in the RVxF motif did not. These results define Inh3 as a regulatory subunit of PP1 in plants and suggest that Inh3 plays a crucial role in early embryogenesis in Arabidopsis.

Activation of brain protein phosphatase-1(I) following cardiac arrest and resuscitation involving an interaction with 14-3-3 gamma

The intracellular signaling mechanisms that couple transient cerebral ischemia to cell death and neuroprotective mechanisms provide potential therapeutic targets for cardiac arrest. Protein phosphatase (PP)-1 is a major serine/threonine phosphatase that interacts with and dephosphorylates critical regulators of energy metabolism, ionic balance, and apoptosis. We report here that PP-1(I), a major regulated form of PP-1, is activated in brain by approximately twofold in vivo following cardiac arrest and resuscitation in a clinically relevant pig model of transient global cerebral ischemia and reperfusion. PP-1(I) purified to near homogeneity from either control or ischemic pig brain consisted of the PP-1 catalytic subunit, the inhibitor-2 regulatory subunit, as well as the novel constituents 14-3-3gamma, Rab GDP dissociation protein beta, PFTAIRE kinase, and C-TAK1 kinase. PP-1(I) purified from ischemic brain contained significantly less 14-3-3gamma than PP-1(I) purified from control brain, and purified 14-3-3gamma directly inhibited the catalytic subunit of PP-1 and reconstituted PP-1(I). These findings suggest that activation of brain PP-1(I) following global cerebral ischemia in vivo involves dissociation of 14-3-3gamma, a novel inhibitory modulator of PP-1(I). This identifies modulation of PP-1(I) by 14-3-3 in global cerebral ischemia as a potential signaling mechanism-based approach to neuroprotection.

Structural basis for regulation of protein phosphatase 1 by inhibitor-2

The functional specificity of type 1 protein phosphatases (PP1) depends on the associated regulatory/targeting and inhibitory subunits. To gain insights into the mechanism of PP1 regulation by inhibitor-2, an ancient and intrinsically disordered regulator, we solved the crystal structure of the complex to 2.5A resolution. Our studies show that, when complexed with PP1c, I-2 acquires three regions of order: site 1, residues 12-17, binds adjacent to a region recognized by many PP1 regulators; site 2, amino acids 44-56, interacts along the RVXF binding groove through an unsuspected sequence, KSQKW; and site 3, residues 130-169, forms alpha-helical regions that lie across the substrate-binding cleft. Specifically, residues 148-151 interact at the catalytic center, displacing essential metal ions, accounting for both rapid inhibition and slower inactivation of PP1c. Thus, our structure provides novel insights into the mechanism of PP1 inhibition and subsequent reactivation, has broad implications for the physiological regulation of PP1, and highlights common inhibitory interactions among phosphoprotein phosphatase family members.

The extracellular signal-regulated kinase: multiple substrates regulate diverse cellular functions

The extracellular signal-regulated kinase (ERK) cascade is a central pathway that transmits signals from many extracellular agents to regulate cellular processes such as proliferation, differentiation and cell cycle progression. The signaling via the ERK cascade is mediated by sequential phosphorylation and activation of protein kinases in the different tiers of the cascade. Although the main core phosphorylation chain of the cascade includes Raf kinases, MEK1/2, ERK1/2 (ERKs) and RSKs, other alternatively spliced forms and distinct components exist in the different tiers, and participate in ERK signaling under specific conditions. These components enhance the complexity of the ERK cascade and thereby, enable the wide variety of functions that are regulated by it. Another factor that is important for the dissemination of ERKs' signals is the multiplicity of the cascade's substrates, which include transcription factors, protein kinases and phosphatases, cytoskeletal elements, regulators of apoptosis, and a variety of other signaling-related molecules. About 160 substrates have already been discovered for ERKs, and the list of these substrates, as well as the function and mechanism of activation of representative substrates, are described in the current review. Many of these substrates are localized in the nucleus, and seem to participate in the regulation of transcription upon stimulation. However, other substrates are found in the cytosol as well as in other cellular organelles, and those are responsible for processes such as translation, mitosis and apoptosis. Understanding of these processes may provide a full picture of the distinct, and even opposing cellular processes that are regulated by the ERK cascade.

Phosphorylation of the Pro-X-Thr-Pro site in phosphatase inhibitor-2 by cyclin-dependent protein kinase during M-phase of the cell cycle

Protein phosphorylation serves as a primary mechanism for triggering events during mitosis and depends on coordinated regulation of kinases and phosphatases. Protein Ser-Thr phosphatase-1 (PP1) activity is essential for the metaphase to anaphase transition and the most ancient regulator of PP1 conserved from yeast to human is inhibitor-2 (I-2), an unstructured heat-stable protein. A unique sequence motif in I-2 from various species surrounds a phosphorylation site PXTP that can be phosphorylated in biochemical assays by GSK3, MAPK and CDK kinases. Here we used a phosphosite specific antibody to investigate the phosphorylation of I-2. We fractioned extracts from HeLa cells arrested with nocodazole and assayed for PXTP kinases using recombinant I-2. One major and two minor peaks of kinase activity were identified and the major peak contained both active MAPK and cdk1::cyclinB1, confirmed by immunoblotting. Cells released from a double thymidine block synchronously progressed through mitosis and immunoblotting revealed transient phosphorylation of endogenous I-2 in cells only during mitosis, and corresponding phosphorylation of histone H3 (Ser10) and PP1 (Thr320). Activation of cdk1::cyclinB1 was coincident with I-2 phosphorylation, but neither MAPK nor GSK3 were phosphorylated at this time, so we concluded that in living cells only cdk1::cyclinB1 phosphorylated the PXTP site in I-2. Immunofluorescent staining of cells with the PXTP phosphosite antibody revealed highly specific staining of mitotic cells prior to anaphase, at which point the staining disappeared. Thus, phosphorylation of I-2 is catalyzed by cdk1::cyclinB1 and staining with a specific antibody should prove useful as a selective marker of cells in the early stages of mitosis.

A positive feedback loop between glycogen synthase kinase 3beta and protein phosphatase 1 after stimulation of NR2B NMDA receptors in forebrain neurons

N-methyl-D-aspartate receptors (NMDARs) are critical for neuronal plasticity and survival, whereas their excessive activation produces excitotoxicity and may accelerate neurodegeneration. Here, we report that stimulation of NMDARs in cultured rat hippocampal or cortical neurons and in the adult mouse brain in vivo disinhibited glycogen synthase kinase 3beta (GSK3beta) by protein phosphatase 1(PP1)-mediated dephosphorylation of GSK3beta at the serine 9 residue. NMDA-triggered GSK3beta activation was mediated by NMDAR that contained the NR2B subunit. Interestingly, GSK3beta inhibition reduced inhibitory phosphorylation of the PP1 inhibitor 2 (I2) and attenuated serine 9 dephosphorylation by PP1. These data suggest existence of a feedback loop between GSK3beta and PP1 that results in amplification of PP1 activation by GSK3beta. In addition, GSK3beta inhibition decreased PP1-mediated dephosphorylation of the cAMP-response element-binding protein (CREB) at the serine 133 residue in NMDA-stimulated neurons. Conversely, overexpression of GSK3beta abolished non-NR2B-mediated activation of CRE-driven transcription. These data suggest that cross-talk between GSK3beta and PP1 contributes to NR2B NMDAR-induced inhibition of CREB signaling by non-NR2B NMDAR. The excessive activation of NR2B-PP1-GSK3beta-PP1 circuitry may contribute to the deficits of CREB-dependent neuronal plasticity in neurodegenerative diseases.

Reduced inhibitor 1 and 2 activity is associated with increased protein phosphatase type 1 activity in left ventricular myocardium of one-kidney, one-clip hypertensive rats

In failing hearts, although protein phosphatase type 1 (PP1) activity has increased, information about the regulation and status of PP1 inhibitor-1 (INH-1) and inhibitor-2 (INH-2) is limited. In this study, we examined activity and protein expression of PP1, INH-1 and INH-2 and phosphorylation of sarcoplasmic reticulum (SR) phospholamban (PLB), a substrate of PP1 and modulator of SR Ca2+-ATPase activity, in failing and non-failing hearts. These studies were performed in LV myocardium of seven rats with chronic renal hypertension produced by Goldblatt's one-kidney, one-clip procedure and seven age-matched sham-operated normal controls (CTR). Eight weeks after surgery, LV ejection fraction, LV hypertrophy, and pulmonary congestion were determined in all rats. PP1 activity (nmol 32P/min/mg non-collagen protein) was assessed in LV homogenates using 32P-labeled phosphorylase a as substrate. INH-1 and INH-2 activity was determined in the immunoprecipitate of LV homogenates and expressed as percentage inhibitory activity. Using a specific antibody, LV tissue levels of PP1C and calsequestrin (CSQ), a SR calcium binding protein, which is not altered in failing hearts, were also determined. Further, total and phosphorylated PLB, INH-1 and INH-2 protein levels were determined in the LV homogenate and phosphoprotein-enriched fraction, respectively. The band density of each protein was quantified in densitometric units and normalized to CSQ.

RESULTS

rats with chronic renal hypertension exhibited significantly reduced LV ejection fraction and increased LV hypertrophy and pulmonary congestion, characteristics of chronic heart failure (CHF). We found that compared to CTR, (1) both INH-1 (10.2+/-2 versus 57.5+/-1; p < 0.05) and INH-2 activity (3.8+/-0.4 versus 36.2+/-4; p < 0.05) were reduced, (2) total and phosphorylated PLB amount reduced, (3) protein level of phosphorylated INH-1 was reduced (2.32+/-0.1 versus 0.73+/-0.04; p < 0.05) whereas that of phosphorylated INH-2 increased (3.05+/-0.3 versus 1.42+/-0.1; p < 0.05), and (4) PP1 activity was increased approximately 2.6-fold in rats with CHF (1.59+/-0.05 versus 0.61+/-0.01; p < 0.05) while protein level of the catalytic subunit of PP1 (PP1C) increased 3.85-fold (0.77+/-0.05 versus 0.20+/-0.02; p < 0.05). These results suggest that reduced inhibitory INH-1 and INH-2 activity, increased PP1C protein level, and reduced PLB phosphorylation are associated with increased PP1 activity in failing hearts.

Dephosphorylation of cofilin is regulated through Ras and requires the combined activities of the Ras-effectors MEK and PI3K

Remodeling of the actin cytoskeleton is crucial for a multitude of cellular functions including cell movement, intracellular transport as well as signal transduction and gene expression processes. Cofilin has been identified as a key mediator of actin reorganization. Its activity is regulated via reversible phosphorylation of ser-3. In a variety of cell types stimulation through particular surface receptors fastly induces the dephosphorylation/activation of cofilin. Yet, the signal transduction cascades linking receptor stimulation with cofilin activation have not been identified so far. Here we show that the GTPase Ras acts as a central regulator of the cofilin dephosphorylation pathway. Thus, stimulation of Ras through platelet-derived growth factor (PDGF) or transient expression of activated Ras-proteins induces the dephosphorylation of cofilin. Importantly, the cooperation of two Ras-initiated signaling pathways is required to induce cofilin dephosphorylation: a Ras-Raf-MAPkinase/Erk-kinase (MEK)- and a Ras-phosphatidylinositol-3-kinase (PI3K)-effector cascade.

The synergistic effects of NGF and IGF-1 on neurite growth in adult sensory neurons: convergence on the PI 3-kinase signaling pathway

Nerve growth factor (NGF) and insulin-like growth factor-1 (IGF-1) play an important role in promoting axonal growth from dorsal root ganglion (DRG) neurons. Adult DRG neurons exhibit neurotrophin-independent survival, providing an excellent system with which to study trophic factor effects on neurite growth in the absence of significant survival effects. Using young adult rat DRG neurons we have demonstrated a synergistic effect of NGF plus IGF (N + I), compared with either factor alone, in promoting neurite growth. Not only does the presence of NGF and IGF-1 enhance neurite initiation, it also significantly augments the extent of neurite branching and elongation. We have also examined potential mechanism(s) underlying this synergistic effect. Immunoblotting experiments of classical growth factor intermediary signalling pathways (PI 3-K-Akt-GSK-3 and Ras-Raf-MAPK) were performed using phospho-specific antibodies to assess activation state. We found that activation of Akt and MAPK correlated with neurite elongation and branching. However, using pharmacological inhibitors, we observed that a PI 3-K pathway involving both Akt and GSK-3 appeared to be more important for neurite extension and branching than MAPK-dependent signalling. In fact, inhibition of activation of MAPK with U0126 resulted in increased neuritic branching, possibly as a result of the concomitant increase observed in phospho-Akt. Furthermore, inhibition of GSK3 (which is negatively regulated by phosphorylation on S9/S21) also resulted in increased growth. Our data point to signalling convergence upon the PI 3-K-Akt-GSK-3 pathway that underlies the NGF plus IGF synergism. In addition, to our knowledge, this is the first report in primary neurons that inhibition of GSK3 results in an enhanced neurite growth.

Phosphorylation of phosphatase inhibitor-2 at centrosomes during mitosis

Inhibitor-2 (I-2) is a regulator of protein phosphatase type-1 (PP1), known to be phosphorylated in vitro by multiple kinases. In particular Thr72 is a Thr-Pro phosphorylation site conserved from yeast to human, but there is no evidence that this phosphorylation responds to any physiological signals. Here, we used electrophoretic mobility shift and immunoblotting with a site-specific phospho-Thr72 antibody to establish Thr72 phosphorylation in HeLa cells and show a 25-fold increase in phosphorylation during mitosis. Mass spectrometry demonstrated I-2 in actively growing HeLa cells was also phosphorylated at three other sites, Ser120, Ser121, and an additional Ser located between residues 70 and 90. In vitro kinase assays using recombinant I-2 as a substrate showed that the Thr72 kinase(s) was activated during mitosis, and sensitivity to kinase inhibitors indicated that the principal I-2 Thr72 kinase was not GSK3 but instead a member of the cyclin-dependent protein kinase family. Immunocytochemistry confirmed Thr72 phosphorylation of I-2 during mitosis, with peak intensity at prophase, and revealed subcellular concentration of the phospho-Thr72 I-2 at centrosomes. Together, the data show dynamic changes in I-2 phosphorylation during mitosis and localization of phosphorylated I-2 at centrosomes, suggesting involvement in mammalian cell division.

Pho85 phosphorylates the Glc7 protein phosphatase regulator Glc8 in vivo

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

Inhibitor-2 regulates protein phosphatase-1 complexed with NimA-related kinase to induce centrosome separation

Centrosome separation is regulated by balance of in situ protein kinase/phosphatase activities during the cell cycle. The mammalian NimA-related kinase Nek2 forms a complex with the catalytic subunit of protein phosphatase-1 (PP1C). This complex is located at centrosomes and has been implicated in regulation of the cycle of duplication and separation. Inhibitor-2 (Inh2) is an inhibitor protein specific for PP1C, and its expression level fluctuates during the cell cycle. Here we report cellular regulation of the Nek2.PP1C complex by Inh2. PP1C-binding segments of Nek2 were isolated by yeast two-hybrid screening using Inh2 bait. Inh2 indirectly associates with Nek2 via PP1C, which binds to both proteins, forming a bridged heterotrimeric complex. Double Ala mutation of the PP1C-binding site (KVHF) in Nek2 eliminated both PP1C and Inh2 interactions in both a yeast conjugation assay and an in vitro binding assay. The kinase activity of Nek2.PP1C was enhanced 2-fold by addition of recombinant Inh2, with EC(50) = 10 nm. Immunofluorescence showed concentration of endogenous Inh2 at centrosomes and in a region surrounding the centrosomes. Transient expression of wild-type Inh2 increased by 5-fold dispersed/split centrosomes in fibroblasts, mimicking the phenotype produced by overexpression of Nek2. Deletion of the Inh2 C-terminal domain yielded Inh2-(1-118), which failed to interact with or activate the Nek2.PP1C complex, suggesting that the C-terminal region of Inh2 is required for regulation of the Nek2.PP1C complex. Thus, Inh2 can enhance the kinase activity of the Nek2.PP1C complex via inhibition of phosphatase activity to initiate centrosome separation.

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

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

Protein phosphatase 1 – targeted in many directions

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

Neuronal Cdc2-like protein kinase (Cdk5/p25) is associated with protein phosphatase 1 and phosphorylates inhibitor-2

Protein phosphatase 1 (PP1) is complexed with inhibitor 2 (I-2) in the cytosol. In rabbit muscle extract PP1.I-2 is activated upon preincubation with ATP/Mg. This activation is caused by phosphorylation of I-2 on Thr(72) by glycogen synthase kinase 3 (GSK3). We have found that PP1.I-2 in bovine brain extract is also activated upon preincubation with ATP/Mg. However, blocking GSK3 action by LiCl inhibited only approximately 29% of PP1 activity and indicated that GSK3 is not the sole PP1.I-2 activator in the brain. When bovine brain extract was analyzed by gel filtration PP1.I-2 and neuronal Cdc2-like protein kinase (NCLK), a heterodimer of Cdk5 and the regulatory p25 subunit, co-eluted as a approximately 450-kDa size species. The NCLK from the eluted column fractions bound to PP1-specific microcystin-Sepharose and glutathione S-transferase (GST)-I-2-coated glutathione-agarose beads. Similarly, PP1 from the eluted column fractions was pulled down with GST-Cdk5-coated glutathione-agarose beads. In vitro, NCLK phosphorylated I-2 on Thr(72) and activated PP1.I-2 in an ATP/Mg-dependent manner. NCLK bound to PP1 through its Cdk5 subunit and the PP1 binding region was localized to Cdk5 residues 28-41. Our data demonstrate that in brain extract PP1.I-2 and NCLK are associated within a complex of approximately 450 kDa and suggest that NCLK is one of the PP1.I-2-activating kinases in the mammalian brain.

Interaction of inhibitor-2 with the catalytic subunit of type 1 protein phosphatase. Identification of a sequence analogous to the consensus type 1 protein phosphatase-binding motif

Inhibitor-2 (I-2) is the regulatory subunit of a cytosolic type 1 Ser/Thr protein phosphatase (PP1) and potently inhibits the activity of the free catalytic subunit (CS1). Previous work from the laboratory had proposed that the interaction of I-2 with CS1 involved multiple sites (Park, I. K., and DePaoli-Roach, A. A. (1994) J. Biol. Chem. 269, 28919-28928). The present study refines the earlier analysis and arrives at a more detailed model for the interaction between I-2 and CS1. Although the NH(2)-terminal I-2 regions containing residues 1-35 and 1-64 have no inhibitory activity on their own, they increase the IC(50) for I-2 by approximately 30-fold, indicating the presence of a CS1-interacting site. Based on several experimental approaches, we have also identified the sequence Lys(144)-Leu-His-Tyr(147) as a second site of interaction that corresponds to the RVXF motif present in many CS1-binding proteins. The peptide I-2(135-151) significantly increases the IC(50) for I-2 and attenuates CS1 inhibition. Replacement of Leu and Tyr with Ala abolishes the ability to counteract inhibition by I-2. The I-2(135-151) peptide, but not I-2(1-35), also antagonizes inhibition of CS1 by DARPP-32 in a pattern similar to that of I-2. Furthermore, a peptide derived from the glycogen-binding subunit, R(GL)/G(M)(61-80), which contains a consensus CS1-binding motif, completely counteracts CS1 inhibition by I-2 and DARPP-32. The NH(2)-terminal 35 residues of I-2 bind to CS1 at a site that is specific for I-2, whereas the KLHY sequence interacts with CS1 at a site shared with other interacting proteins. Other results suggest the presence of yet more sites of interaction. A model is presented in which multiple "anchoring interactions" serve to position a segment of I-2 such that it sterically occludes the catalytic pocket but need not make high affinity contacts itself.

Actin depolymerizing factor and cofilin phosphorylation dynamics: response to signals that regulate neurite extension

The actin assembly-regulating activity of actin depolymerizing factor (ADF)/ cofilin is inhibited by phosphorylation. Studies were undertaken to characterize the signaling pathways and phosphatases involved in activating phosphorylated ADF (pADF), emphasizing signals related to neuronal process extension. Western blots using antibodies to ADF and cofilin, as well as an ADF/cofilin phosphoepitope-specific antibody characterized in this paper, were used to measure changes in the phosphorylation state and phosphate turnover of ADF/cofilin in response to inhibitors and agents known to influence growth cone motility. Increases in both [Ca2+]i and cAMP levels induced rapid pADF dephosphorylation in HT4 and cortical neurons. Calcium-dependent dephosphorylation depended on the activation of protein phosphatase 2B (PP2B), while cAMP-dependent dephosphorylation was likely through activation of PP1. Growth factors such as NGF and insulin also induced rapid pADF/pcofilin dephosphorylation, with NGF-stimulated dephosphorylation in PC12 cells correlated with the translocation of ADF/cofilin to ruffling membranes. Of special interest was the finding that the rate of phosphate turnover on both pADF and pcofilin could be enhanced by growth factors without changing net pADF levels, demonstrating that growth factors can activate bifurcating pathways that promote both phosphorylation and dephosphorylation of ADF/cofilin. All experimental results indicated that dynamics of phosphorylation on ADF and cofilin are coordinately regulated. Signals that decreased pADF levels are associated with increased process extension, while agents that increased pADF levels, such as lysophosphatidic acid, inhibit process extension. These data indicate that dephosphorylation/activation of pADF is a significant response to the activation of signal pathways that regulate actin dynamics and alter cell morphology and neuronal outgrowth.

Mpl Ligand Enhances the Transcription of the Cyclin D3 Gene: A Potential Role for Sp1 Transcription Factor

Cyclin D3 plays a major role in the development of polyploidy in megakaryocytes. The expression of cyclin D3 gene and the level of cyclin D3 protein are increased by the Mpl ligand in the Y10/L8057 megakaryocytic cell line, as indicated by Northern and Western blot analyses, and by nuclear run-on assays and transfection experiments with cyclin D3 promoter constructs. DNase I footprinting of the promoter region showed protected segments, at −75 to −60 bp and at −134 to −92 bp, which display binding sites for the Sp family of transcription factors. Gel mobility shift assay and supershifts with specific antibodies indicate that Sp1 binds to these regions in the cyclin D3 promoter and that Sp1 binding activity is significantly increased by Mpl ligand. Mutation of either Sp1 site both decreases the basal promoter activity and eliminates the induction by Mpl ligand. We find that the nonphosphorylated form of SP1 has greater affinity for the cyclin D3 promoter and that the majority of Sp1 in the cells is nonphosphorylated. Mpl ligand treatment results in increased levels of Sp1 protein, which also appears as nonphosphorylated. Okadaic acid, which inhibits protein phosphatase 1 (PP1) and shifts Sp1 to a phosphorylated form, decreases cyclin D3 gene expression and suppresses Mpl ligand induction. Our data point to the potential of Mpl ligand to activate at once several Sp1-dependent genes during megakaryopoiesis.

Mpl Ligand Enhances the Transcription of the Cyclin D3 Gene: A Potential Role for Sp1 Transcription Factor

Cyclin D3 plays a major role in the development of polyploidy in megakaryocytes. The expression of cyclin D3 gene and the level of cyclin D3 protein are increased by the Mpl ligand in the Y10/L8057 megakaryocytic cell line, as indicated by Northern and Western blot analyses, and by nuclear run-on assays and transfection experiments with cyclin D3 promoter constructs. DNase I footprinting of the promoter region showed protected segments, at −75 to −60 bp and at −134 to −92 bp, which display binding sites for the Sp family of transcription factors. Gel mobility shift assay and supershifts with specific antibodies indicate that Sp1 binds to these regions in the cyclin D3 promoter and that Sp1 binding activity is significantly increased by Mpl ligand. Mutation of either Sp1 site both decreases the basal promoter activity and eliminates the induction by Mpl ligand. We find that the nonphosphorylated form of SP1 has greater affinity for the cyclin D3 promoter and that the majority of Sp1 in the cells is nonphosphorylated. Mpl ligand treatment results in increased levels of Sp1 protein, which also appears as nonphosphorylated. Okadaic acid, which inhibits protein phosphatase 1 (PP1) and shifts Sp1 to a phosphorylated form, decreases cyclin D3 gene expression and suppresses Mpl ligand induction. Our data point to the potential of Mpl ligand to activate at once several Sp1-dependent genes during megakaryopoiesis.

Phosphorylation-dependent inhibition of protein phosphatase-1 by G-substrate. A Purkinje cell substrate of the cyclic GMP-dependent protein kinase

G-substrate, a specific substrate of the cGMP-dependent protein kinase, has previously been localized to the Purkinje cells of the cerebellum. We report here the isolation from mouse brain of a cDNA encoding G-substrate. This cDNA was used to localize G-substrate mRNA expression, as well as to produce recombinant protein for the characterization of G-substrate phosphatase inhibitory activity. Brain and eye were the only tissues in which a G-substrate transcript was detected. Within the brain, G-substrate transcripts were restricted almost entirely to the Purkinje cells of the cerebellum, although transcripts were also detected at low levels in the paraventricular region of the hypothalamus and the pons/medulla. Like the native protein, the recombinant protein was preferentially phosphorylated by cGMP-dependent protein kinase (Km = 0.2 microM) over cAMP-dependent protein kinase (Km = 2.0 microM). Phospho-G-substrate inhibited the catalytic subunit of native protein phosphatase-1 with an IC50 of 131 +/- 27 nM. Dephospho-G-substrate was not found to be inhibitory. Both dephospho- and phospho-G-substrate were weak inhibitors of native protein phosphatase-2A1, which dephosphorylated G-substrate 20 times faster than the catalytic subunit of protein phosphatase-1. G-substrate potentiated the action of cAMP-dependent protein kinase on a cAMP-regulated luciferase reporter construct, consistent with an inhibition of cellular phosphatases in vivo. These results provide the first demonstration that G-substrate inhibits protein phosphatase-1 and suggest a novel mechanism by which cGMP-dependent protein kinase I can regulate the activity of the type 1 protein phosphatases.

Characterisation of a novel Drosophila melanogaster testis specific PP1 inhibitor related to mammalian inhibitor-2: identification of the site of interaction with PP1

A novel Drosophila melanogaster protein, termed inhibitor-t, that bears 41% sequence similarity to human protein phosphatase inhibitor-2 has been identified using human protein phosphatase 1 (PP1) in the yeast two hybrid system. Inhibitor-t mRNA is detected in adult males, larvae and pupae and the 184 amino acid thermostable protein located only in testis. The gene for inhibitor-t maps to cytological location 86F1 on the third chromosome. Bacterially expressed inhibitor-t specifically inhibits both mammalian and D. melanogaster PP1 catalytic subunits with an IC50 of approximately 200 nM. A motif -FEX1X2RK-, conserved between inhibitor-t, inhibitor-2 and its Saccharomyces cerevisiae homologue Glc8, is demonstrated to be required for binding to PP1.

Multisite phosphorylation and the nuclear localization of phosphatase inhibitor 2-green fluorescent protein fusion protein during S phase of the cell growth cycle

Human phosphatase inhibitor 2 (Inh2) is a phosphoprotein that complexes with type 1 protein phosphatase, and its expression peaks during S phase and mitosis during the cell cycle. Localization of Inh2 was visualized in HS68 human fibroblasts by fusing Inh2 to green fluorescent protein (GFP). During G1 phase, Inh2-GFP was localized in the cytoplasm, and as cells progressed into S phase Inh2-GFP accumulated in the nucleus. Known phosphorylation sites of Inh2 at Thr-72, Ser-86, and Ser-120/121 were each replaced with alanine. None of the mutated Inh2-GFP proteins accumulated in the nucleus during S phase, indicating that all of these phosphorylation sites were required. Mutation of two lysine residues in a putative nuclear localization sequence in Inh2 also prevented the Inh2-GFP fusion protein from accumulating in the nucleus during S phase. Recombinant Inh2 was phosphorylated by kinases in cytosols prepared from G1 and S phase cells. The amount of Inh2 kinase attributed to casein kinase 2, based on inhibition by heparin, increased 2.6-fold from G1 to S phase. In addition, kinases in G1 versus S phase cytosols produced distinct Inh2 phosphopeptides. The results indicate that changes in phosphorylation of Inh2 are involved in intracellular redistribution of the protein during the cell cycle.

Activation of the ATP.Mg-dependent type 1 protein phosphatase by the Fe2+/ascorbate system

The ATP.Mg-dependent type 1 protein phosphatase is inactive as isolated but can be activated in several different ways. In this report, we show that the phosphatase can also be activated by the Fe2+/ascorbate system. Activation of the phosphatase requires both Fe2+ ion and ascorbate and the level of activation is dependent on the concentrations of Fe2+ ion and ascorbate. In the presence of 20 mM ascorbate, the Fe2+ ion concentrations required for half-maximal and maximal activation are about 0.3 and 3 mM, respectively. Several common divalent metal ions, including CO2+, Ni2+, Cu2+, Mg2+, and Ca2+ ions, cannot cooperate with ascorbate to activate the phosphatase, and SH-containing reducing agents such as 2-mercaptoethanol and dithiothreitol cannot cooperate with Fe2+ ion to activate the phosphatase, indicating that activation of the phosphatase by the Fe2+/ascorbate system is a specific process. Moreover, H2O2, a strong oxidizer, could significantly diminish the phosphatase activation by the Fe2+/ascorbate system, suggesting that reduction mechanism other than SH-SS interchange is a prerequisite for the Fe2+/ascorbate-mediated phosphatase activation. Taken together, the present study provides initial evidence for a new mode of type 1 protein phosphatase activation mechanism.