Cellular mechanisms regulating protein phosphatase-1. A key functional interaction between inhibitor-2 and the type 1 protein phosphatase catalytic subunit

The PP1 regulator PPP1R2 coordinately regulates AURKA and PP1 to control centrosome phosphorylation and maintain central spindle architecture

BACKGROUND

Maintenance of centrosome number in cells is essential for accurate distribution of chromosomes at mitosis and is dependent on both proper centrosome duplication during interphase and their accurate distribution to daughter cells at cytokinesis. Two essential regulators of cell cycle progression are protein phosphatase 1 (PP1) and Aurora A kinase (AURKA), and their activities are each regulated by the PP1 regulatory subunit, protein phosphatase 1 regulatory subunit 2 (PPP1R2). We observed an increase in centrosome number after overexpression of these proteins in cells. Each of these proteins is found on the midbody in telophase and overexpression of PPP1R2 and its mutants increased cell ploidy and disrupted cytokinesis. This suggests that the increase in centrosome number we observed in PPP1R2 overexpressing cells was a consequence of errors in cell division. Furthermore, overexpression of PPP1R2 and its mutants increased midbody length and disrupted midbody architecture. Additionally, we show that overexpression of PPP1R2 alters activity of AURKA and PP1 and their phosphorylation state at the centrosome.

RESULTS

Overexpression of PPP1R2 caused an increase in the frequency of supernumerary centrosomes in cells corresponding to aberrant cytokinesis reflected by increased nuclear content and cellular ploidy. Furthermore, AURKA, PP1, phospho PPP1R2, and PPP1R2 were all localized to the midbody at telophase, and PP1 localization there was dependent on binding of PPP1R2 with PP1 and AURKA as well as its phosphorylation state. Additionally, overexpression of both PPP1R2 and its C-terminal AURKA binding site altered enzymatic activity of AURKA and PP1 at the centrosome and disrupted central spindle structure.

CONCLUSIONS

Results from our study reveal the involvement of PPP1R2 in coordinating PP1 and AURKA activity during cytokinesis. Overexpression of PPP1R2 or its mutants disrupted the midbody at cytokinesis causing accumulation of centrosomes in cells. PPP1R2 recruited PP1 to the midbody and interference with its targeting resulted in elongated and severely disrupted central spindles supporting an important role for PPP1R2 in cytokinesis.

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.

Protein phosphatases of Saccharomyces cerevisiae

The phosphorylation status of a protein is highly regulated and is determined by the opposing activities of protein kinases and protein phosphatases within the cell. While much is known about the protein kinases found in Saccharomyces cerevisiae, the protein phosphatases are much less characterized. Of the 127 protein kinases in yeast, over 90% are in the same evolutionary lineage. In contrast, protein phosphatases are fewer in number (only 43 have been identified in yeast) and comprise multiple, distinct evolutionary lineages. Here we review the protein phosphatase families of yeast with regard to structure, catalytic mechanism, regulation, and signal transduction participation.

Rif1 acts through Protein Phosphatase 1 but independent of replication timing to suppress telomere extension in budding yeast

The Rif1 protein negatively regulates telomeric TG repeat length in the budding yeast Saccharomyces cerevisiae, but how it prevents telomere over-extension is unknown. Rif1 was recently shown to control DNA replication by acting as a Protein Phosphatase 1 (PP1)-targeting subunit. Therefore, we investigated whether Rif1 controls telomere length by targeting PP1 activity. We find that a Rif1 mutant defective for PP1 interaction causes a long-telomere phenotype, similar to that of rif1Δ cells. Tethering PP1 at a specific telomere partially substitutes for Rif1 in limiting TG repeat length, confirming the importance of PP1 in telomere length control. Ablating Rif1-PP1 interaction is known to cause precocious activation of telomere-proximal replication origins and aberrantly early telomere replication. However, we find that Rif1 still limits telomere length even if late replication is forced through deletion of nearby replication origins, indicating that Rif1 can control telomere length independent of replication timing. Moreover we find that, even at a de novo telomere created after DNA synthesis during a mitotic block, Rif1-PP1 interaction is required to suppress telomere lengthening and prevent inappropriate recruitment of Tel1 kinase. Overall, our results show that Rif1 controls telomere length by recruiting PP1 to directly suppress telomerase-mediated TG repeat lengthening.

The Toxoplasma gondii inhibitor-2 regulates protein phosphatase 1 activity through multiple motifs

Toxoplasma gondii has a complex life cycle characterized by multiple differentiation steps that are essential for its survival in both human and definitive feline host. Several studies have demonstrated the importance of phosphorylations by protein kinases during the life cycle of T. gondii. However, very little is known about protein phosphatases and their regulators in the parasite. We report the molecular and functional characterization of the T. gondii ortholog of the inhibitor-2 protein, designated TgI2. We show that TgI2 encompasses conserved motifs involved in the interaction and modulation of the phosphatase activity of T. gondii protein phosphatase 1, named TgPP1. We show that a specific combination of motifs is involved in binding and/or inhibition of the TgPP1 activity. We show here that the TgI2 protein is a potent inhibitor of TgPP1 phosphatase activity. TgI2 SILK and RVxF motifs are critical for regulating the activity of TgPP1, a feature that is common with the higher eukaryotes inhibitor-2 protein.

Control of βAR- and N-methyl-D-aspartate (NMDA) Receptor-Dependent cAMP Dynamics in Hippocampal Neurons

Norepinephrine, a neuromodulator that activates β-adrenergic receptors (βARs), facilitates learning and memory as well as the induction of synaptic plasticity in the hippocampus. Several forms of long-term potentiation (LTP) at the Schaffer collateral CA1 synapse require stimulation of both βARs and N-methyl-D-aspartate receptors (NMDARs). To understand the mechanisms mediating the interactions between βAR and NMDAR signaling pathways, we combined FRET imaging of cAMP in hippocampal neuron cultures with spatial mechanistic modeling of signaling pathways in the CA1 pyramidal neuron. Previous work implied that cAMP is synergistically produced in the presence of the βAR agonist isoproterenol and intracellular calcium. In contrast, we show that when application of isoproterenol precedes application of NMDA by several minutes, as is typical of βAR-facilitated LTP experiments, the average amplitude of the cAMP response to NMDA is attenuated compared with the response to NMDA alone. Models simulations suggest that, although the negative feedback loop formed by cAMP, cAMP-dependent protein kinase (PKA), and type 4 phosphodiesterase may be involved in attenuating the cAMP response to NMDA, it is insufficient to explain the range of experimental observations. Instead, attenuation of the cAMP response requires mechanisms upstream of adenylyl cyclase. Our model demonstrates that Gs-to-Gi switching due to PKA phosphorylation of βARs as well as Gi inhibition of type 1 adenylyl cyclase may underlie the experimental observations. This suggests that signaling by β-adrenergic receptors depends on temporal pattern of stimulation, and that switching may represent a novel mechanism for recruiting kinases involved in synaptic plasticity and memory.

Noradrenaline is a stress related molecule that facilitates learning and memory when released in the hippocampus. The facilitation of memory is related to modulation of synaptic plasticity, but the mechanisms underlying this modulation are not well understood. We utilize a combination of live cell imaging and computational modeling to discover how noradrenergic receptor stimulation interacts with other molecules, such as calcium, required for synaptic plasticity and memory storage. Though prior work has shown that noradrenergic receptors and calcium interact synergistically to elevate intracellular second messengers when combined simultaneously, our results demonstrate that prior stimulation of noradrenergic receptors inhibits the elevation of intracellular second messengers. Our results further demonstrate that the inhibition may be caused by the noradrenergic receptor switching signaling pathways, thereby recruiting a different set of memory kinases. This switching represents a novel mechanism for recruiting molecules involved in synaptic plasticity and memory.

Identification of a Plasmodium falciparum inhibitor-2 motif involved in the binding and regulation activity of protein phosphatase type 1

The regulation of Plasmodium falciparum protein phosphatase type 1 (PfPP1) activity remains to be deciphered. Data from homologous eukaryotic type 1 protein phosphatases (PP1) suggest that several protein regulators should be involved in this essential process. One such regulator, named PfI2 based on its primary sequence homology with eukaryotic inhibitor 2 (I2), was recently shown to be able to interact with PfPP1 and to inhibit its phosphatase activity, mainly through the canonical 'RVxF' binding motif. The details of the structural and functional characteristics of this interaction are investigated here. Using NMR spectroscopy, a second site of interaction is suggested to reside between residues D94 and T117 and contains the 'FxxR/KxR/K' binding motif present in other I2 proteins. This site seems to play in concert/synergy with the 'RVxF' motif to bind PP1, because only mutations in both motifs were able to abolish this interaction completely. However, regarding the structure/function relationship, mutation of either the 'RVxF' or 'FxxR/KxR/K' motif is more drastic, because each mutation prevents the capacity of PfI2 to trigger germinal vesicle breakdown in microinjected Xenopus oocytes. This indicates that the tight association of the PfI2 regulator to PP1, mediated by a two-site interaction, is necessary to exert its function. Based on these results, the use of a peptide derived from the 'FxxR/KxR/K' PfI2 motif was investigated for its potential effect on Plasmodium growth. This peptide, fused at its N-terminus to a penetrating sequence, was shown to accumulate specifically in infected erythrocytes and to have an antiplasmodial effect.

PPP1R42, a PP1 binding protein, regulates centrosome dynamics in ARPE-19 cells

BACKGROUND

The centrosome is the primary site for microtubule nucleation in cells and orchestrates reorganisation of the microtubule cytoskeleton during the cell cycle. The activities of the centrosome must be closely aligned with progression of the cell cycle; misregulation of centrosome separation and duplication is a hallmark of cancer. In a subset of cells, including the developing spermatid, the centrosome becomes specialised to form the basal body thereby supporting growth of the axoneme in morphogenesis of cilia and flagella, structures critical for signalling and motility. Mammalian spermatogenesis is an excellent model system to investigate the transformations in cellular architecture that accompany these changes including formation of the flagellum. We have previously identified a leucine-rich repeat protein (PPP1R42) that contains a protein phosphatase-1 binding site and translocates from the apical nucleus to the centrosome at the base of the flagellum during spermiogenesis. In this manuscript, we examine localisation and function of PPP1R42 in a ciliated epithelial cell model as a first step in understanding the role of this protein in centrosome function and flagellar formation.

RESULTS

We demonstrate that PPP1R42 localises to the basal body in ARPE-19 retinal epithelial cells. Co-localisation and co-immunoprecipitation experiments further show that PPP1R42 interacts with γ-tubulin. Inhibition of PPP1R42 with small interfering RNAs causes accumulation of centrosomes indicating premature centrosome separation. Importantly, the activity of two signalling molecules that regulate centrosome separation, PP1 phosphatase and NEK2 kinase, changes when PPP1R42 is inhibited: PP1 activity is reduced with a corresponding increase in NEK2 activity.

CONCLUSIONS

We have identified a role for the PP1-binding protein, PPP1R42, in centrosome separation in ciliated ARPE-19 cells. Our finding that inhibition of PPP1R42 expression increases the number of centrosomes per cell is consistent with our model that PPP1R42 is a positive regulator of PP1. PPP1R42 depletion reduces the activity of PP1 leading to activation of NEK2, the kinase responsible for phosphorylation of centrosomal linker proteins promoting centrosome separation. This work identifies a new molecule localised to the centrosome and basal body with a role in the complex signalling network responsible for controlling centrosome activities.

Protein phosphatase 1γ isoforms linked interactions in the brain

Posttranslational protein modifications, in particular reversible protein phosphorylation, are important regulatory mechanisms involved in cellular signaling transduction pathways. Thousands of human proteins are phosphorylatable and the tight regulation of phosphorylation states is crucial for cell maintenance and development. Protein phosphorylation occurs primarily on serine, threonine, and tyrosine residues, through the antagonistic actions of protein kinases and phosphatases. The catalytic subunit of protein phosphatase 1 (PP1), a major Ser/Thr-phosphatase, associates with a large variety of regulatory subunits that define substrate specificity and determine specific cellular pathway responses. PP1 has been shown to bind to different proteins in the brain in order to execute key and differential functions. This work reports the identification of proteins expressed in the human brain that interact with PP1γ1 and PP1γ2 isoforms by the yeast two-hybrid method. An extensive search of PP1-binding motifs was performed for the proteins identified, revealing already known PP1 regulators but also novel interactors. Moreover, our results were integrated with the data of PP1γ interacting proteins from several public web databases, permitting the development of physical maps of the novel interactions. The PP1γ interactome thus obtained allowed for the identification of novel PP1 interacting proteins, supporting novel functions of PP1γ isoforms in the human brain.

GSK-3β activity in the hippocampus is required for memory retrieval

Several molecules were recently found to be important for the memory retrieval process in the hippocampus; however, the mechanisms underlying the memory retrieval remain poorly understood. GSK-3β has been implicated in the control of synaptic plasticity and memory formation. Here, we investigated the relationship between hippocampal GSK-3β activity and memory retrieval using behavioral and Western blotting methods. We found that GSK-3β was activated in the hippocampus after a retention session in the passive avoidance task. An intrahippocampal injection of the GSK-3β inhibitor, SB 216763, before the retention session blocked memory retrieval (but not reconsolidation) without affecting locomotor activity. These results suggest that GSK-3β activation would be essential for memory retrieval in the hippocampus.

Molecular investigations of the structure and function of the protein phosphatase 1-spinophilin-inhibitor 2 heterotrimeric complex

Regulation of the major Ser/Thr phosphatase protein phosphatase 1 (PP1) is controlled by a diverse array of targeting and inhibitor proteins. Though many PP1 regulatory proteins share at least one PP1 binding motif, usually the RVxF motif, it was recently discovered that certain pairs of targeting and inhibitor proteins bind PP1 simultaneously to form PP1 heterotrimeric complexes. To date, structural information for these heterotrimeric complexes and, in turn, how they direct PP1 activity is entirely lacking. Using a combination of NMR spectroscopy, biochemistry, and small-angle X-ray scattering (SAXS), we show that major structural rearrangements in both spinophilin (targeting) and inhibitor 2 (I-2, inhibitor) are essential for the formation of the heterotrimeric PP1-spinophilin-I-2 (PSI) complex. The RVxF motif of I-2 is released from PP1 during the formation of PSI, making the less prevalent SILK motif of I-2 essential for complex stability. The release of the I-2 RVxF motif allows for enhanced flexibility of both I-2 and spinophilin in the heterotrimeric complex. In addition, we used inductively coupled plasma atomic emission spectroscopy to show that PP1 contains two metals in both heterodimeric complexes (PP1-spinophilin and PP1-I-2) and PSI, demonstrating that PSI retains the biochemical characteristics of the PP1-I-2 holoenzyme. Finally, we combined the NMR and biochemical data with SAXS and molecular dynamics simulations to generate a structural model of the full heterotrimeric PSI complex. Collectively, these data reveal the molecular events that enable PP1 heterotrimeric complexes to exploit both the targeting and inhibitory features of the PP1-regulatory proteins to form multifunctional PP1 holoenzymes.

Temporal sensitivity of protein kinase a activation in late-phase long term potentiation

Protein kinases play critical roles in learning and memory and in long term potentiation (LTP), a form of synaptic plasticity. The induction of late-phase LTP (L-LTP) in the CA1 region of the hippocampus requires several kinases, including CaMKII and PKA, which are activated by calcium-dependent signaling processes and other intracellular signaling pathways. The requirement for PKA is limited to L-LTP induced using spaced stimuli, but not massed stimuli. To investigate this temporal sensitivity of PKA, a computational biochemical model of L-LTP induction in CA1 pyramidal neurons was developed. The model describes the interactions of calcium and cAMP signaling pathways and is based on published biochemical measurements of two key synaptic signaling molecules, PKA and CaMKII. The model is stimulated using four 100 Hz tetani separated by 3 sec (massed) or 300 sec (spaced), identical to experimental L-LTP induction protocols. Simulations show that spaced stimulation activates more PKA than massed stimulation, and makes a key experimental prediction, that L-LTP is PKA-dependent for intervals larger than 60 sec. Experimental measurements of L-LTP demonstrate that intervals of 80 sec, but not 40 sec, produce PKA-dependent L-LTP, thereby confirming the model prediction. Examination of CaMKII reveals that its temporal sensitivity is opposite that of PKA, suggesting that PKA is required after spaced stimulation to compensate for a decrease in CaMKII. In addition to explaining the temporal sensitivity of PKA, these simulations suggest that the use of several kinases for memory storage allows each to respond optimally to different temporal patterns.

The hippocampus is a part of the cerebral cortex intimately involved in learning and memory behavior. A common cellular model of learning is a long lasting form of long term potentiation (L-LTP) in the hippocampus, because it shares several characteristics with learning. For example, both learning and long term potentiation exhibit sensitivity to temporal patterns of synaptic inputs and share common intracellular events such as activation of specific intracellular signaling pathways. Therefore, understanding the pivotal molecules in the intracellular signaling pathways underlying temporal sensitivity of L-LTP in the hippocampus may illuminate mechanisms underlying learning. We developed a computational model to evaluate whether the signaling pathways leading to activation of the two critical enzymes: protein kinase A and calcium-calmodulin-dependent kinase II are sufficient to explain the experimentally observed temporal sensitivity. Indeed, the simulations demonstrate that these enzymes exhibit different temporal sensitivities, and make a key experimental prediction, that L-LTP is dependent on protein kinase A for intervals larger than 60 sec. Measurements of hippocampal L-LTP confirm this prediction, demonstrating the value of a systems biology approach to computational neuroscience.

Docking motif-guided mapping of the interactome of protein phosphatase-1

The ubiquitous protein Ser/Thr phosphatase-1 (PP1) interacts with dozens of regulatory proteins that are structurally unrelated. However, most of them share a short, degenerate "RVxF"-type docking motif. Using a broad in silico screening based on a stringent definition of the RVxF motif, in combination with a multistep biochemical validation procedure, we have identified 78 novel mammalian PP1 interactors. A global analysis of the validated RVxF-based PP1 interactome not only provided insights into the conserved features of the RVxF motif but also led to the discovery of additional common PP1 binding elements, described as the "SILK" and "MyPhoNE" motifs. In addition to the doubling of the known mammalian PP1 interactome, our data contribute to the design of PP1 interaction networks. Notably, an interaction network linking PP1 interactors discloses a pleiotropic role of PP1 in cell polarity.

Detailed structural characterization of unbound protein phosphatase 1 inhibitors

Protein phosphatase 1 (PP1) is an essential and ubiquitous serine/threonine protein phosphatase that is regulated by more than 100 known inhibitor and targeting proteins. It is currently unclear how protein inhibitors distinctly and specifically regulate PP1 to enable rapid responses to cellular alterations. We demonstrate that two PP1 inhibitors, I-2 and DARPP-32, belong to the class of intrinsically unstructured proteins (IUPs). We show that both inhibitors have distinct preferences for transient local and long-range structure. These preferences are likely their structural signature for their interaction with PP1. Furthermore, we show that upon phosphorylation of Thr(34) in DARPP-32, which turns DARPP-32 into a potent inhibitor of PP1, neither local nor long-range structure of DARPP-32 is altered. Therefore, our data suggest a role for these transient three-dimensional topologies in binding mechanisms that enable extensive contacts with PP1's invariant surfaces. Together, these interactions enable potent and selective inhibition of PP1.

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.

Expression of Human Protein Phosphatase-1 in Saccharomyces cerevisiae Highlights the Role of Phosphatase Isoforms in Regulating Eukaryotic Functions

Human (PP1) isoforms, PP1alpha, PP1beta, PP1gamma1, and PP1gamma2, differ in primary sequences at N and C termini that potentially bind cellular regulators and define their physiological functions. The GLC7 gene encodes the PP1 catalytic subunit with >80% sequence identity to human PP1 and is essential for viability of Saccharomyces cerevisiae. In yeast, Glc7p regulates glycogen and protein synthesis, actin cytoskeleton, gene expression, and cell division. We substituted human PP1 for Glc7p in yeast to investigate the ability of individual isoforms to catalyze Glc7p functions. S. cerevisiae expressing human PP1 isoforms were viable. PP1alpha-expressing yeast grew more rapidly than strains expressing other isoforms. On the other hand, PP1alpha-expressing yeast accumulated less glycogen than PP1beta-or PP1gamma1-expressing yeast. Yeast expressing human PP1 were indistinguishable from WT yeast in glucose derepression. However, unlike WT yeast, strains expressing human PP1 failed to sporulate. Analysis of chimeric PP1alpha/beta subunits highlighted a critical role for their unique N termini in defining PP1alpha and PP1beta functions in yeast. Biochemical studies established that the differing association of PP1 isoforms with the yeast glycogen-targeting subunit, Gac1p, accounted for their differences in glycogen synthesis. In contrast to human PP1 expressed in Escherichia coli, enzymes expressed in yeast displayed in vitro biochemical properties closely resembling PP1 from mammalian tissues. Thus, PP1 expression in yeast should facilitate future structure-function studies of this protein serine/threonine phosphatase.

Structural and biochemical characterization of inhibitor-1α

Inhibitor-1alpha is one of the isoforms of human protein phosphatase inhibitor-1. It is a product of alternative splicing of inhibitor-1 gene and lacks 51 internal amino acids from residue 84 to 134 of inhibitor-1. Here we have characterized the structural and biochemical properties of inhibitor-1alpha. Structural analysis of recombinant inhibitor-1alpha by NMR spectroscopy revealed that inhibitor-1alpha adopts a predominantly random coil conformation. Excluding the region from residue 84 to 134 of inhibitor-1, the structural features of inhibitor-1 and inhibitor-1alpha are almost the same as each other. The IC(50) value of inhibitor-1alpha in inhibition of Protein phosphatase-1 (PP1) is comparable to that of inhibitor-1, indicating that inhibitor-1alpha is a potent inhibitor of PP1 when Thr-35 is phosphorylated by PKA. For phosphorylation by PKA and dephosphorylation by protein phosphatase-1, -2A, and -2B, the measured kinetic parameters of inhibitor-1alpha are very close to those of inhibitor-1. Taken together, these results suggest that inhibitor-1alpha preserves the structure of inhibitor-1, the PP1 inhibitory activity and the functional specificities toward phosphorylation by PKA and dephosphorylation by protein phosphatase-1, -2A, and -2B.

Expression of a novel T-complex testis expressed 5 (Tctex5) in mouse testis, epididymis, and spermatozoa

Expression of T-complex testis expressed 5 (Tctex5), an orthologue of protein phosphatase-1 inhibitor-3 (PPP1R11), was enhanced in mouse testis and was also expressed in epididymis and spermatozoa. There were three transcripts of Tctex5 including one brain specific and two common transcripts dominant in mouse testis. Tctex5 protein isoforms (75, 52, 32, 25, and 14.3 kDa) were identified. Isoforms of 75 and 52 kDa were spermatogenic-specific and were found in protein fraction containing nuclei, mitochondria, and flagellum accessory, and also in protein fraction containing mainly membranes. Tctex5 was localized in nuclei of pachytene spermatocytes, round spermatocytes, cytoplasm of Sertoli cells in testis; cilia, secretion bodies and nuclei of epithelial cells and interstitium smooth muscle cells in epididymis; and head and principal piece of tail in epididymal spermatozoa. The results suggested that Tctex5 might be a specific protein phosphatase-1 inhibitor in sperm; various Tctex5 transcripts and isoforms and cellular locations imply its different roles in spermatogenesis. Nuclei-type isoforms (75 and 52 kDa) might take part in nucleus remodeling during spermatogenesis whilst membrane-type isoform (52 kDa) might be responsible for dephosphorylation of proteins during capacitation. The other isoforms might play general roles for all kinds of cell types.

N-acetyl-cysteine abolishes hydrogen peroxide-induced modification of eukaryotic initiation factor 4F activity via distinct signalling pathways

During the oxidative stress generated by hydrogen peroxide (H2O2) in nerve growth factor (NGF)-differentiated PC12 cells, eIF4E binding protein (4E-BP1) and initiation factor 4E (eIF4E) phosphorylated levels decrease significantly, and an enhancement of the association of 4E-BP1 to eIF4E, which in turn decreases eIF4F formation is observed. The treatment with N-acetyl-cysteine (NAC) completely abolishes the H2O2-induced decrease in eIF4E phosphorylated levels, whereas the decrease in 4E-BP1 phosphorylated levels and eIF4F activity inhibition are significantly but not fully reversed. Rapamycin, the mammalian target of rapamycin (FRAP/mTOR) inhibitor, prevents the effect of NAC on H2O2-induced eIF4F complex formation inhibition. Besides the inhibitor induces a similar decrease in 4E-BP1 phosphorylated levels to that promote by H2O2. However, rapamycin has no effect on the NAC-induced recovery in phosphorylated eIF4E levels. Neither the MAP kinase inhibitors, PD98056 and SB203580, or the protein phosphatase 2A inhibitor, okadaic acid, mimic NAC effect on the H2O2-induced eIF4E dephosphorylation. Altogether our findings suggest that the effects caused by oxidative stress on eIF4s factors depends on two MAP kinase-independent signal transduction pathways, being at least one of them rapamycin-dependent.

Importance of a Surface Hydrophobic Pocket on Protein Phosphatase-1 Catalytic Subunit in Recognizing Cellular Regulators

Cellular functions of protein phosphatase-1 (PP1), a major eukaryotic serine/threonine phosphatase, are defined by the association of PP1 catalytic subunits with endogenous protein inhibitors and regulatory subunits. Many PP1 regulators share a consensus RVXF motif, which docks within a hydrophobic pocket on the surface of the PP1 catalytic subunit. Although these regulatory proteins also possess additional PP1-binding sites, mutations of the RVXF sequence established a key role of this PP1-binding sequence in the function of PP1 regulators. WT PP1alpha, the C-terminal truncated PP1alpha-(1-306), a chimeric PP1alpha containing C-terminal sequences from PP2A, another phosphatase, PP1alpha-(1-306) with the RVXF-binding pocket substitutions L289R, M290K, and C291R, and PP2A were analyzed for their regulation by several mammalian proteins. These studies established that modifications of the RVXF-binding pocket had modest effects on the catalytic activity of PP1, as judged by recognition of substrates and sensitivity to toxins. However, the selected modifications impaired the sensitivity of PP1 to the inhibitor proteins, inhibitor-1 and inhibitor-2. In addition, they impaired the ability of PP1 to bind neurabin-I, the neuronal regulatory subunit, and G(M), the skeletal muscle glycogen-targeting subunit. These data suggested that differences in RVXF interactions with the hydrophobic pocket dictate the affinity of PP1 for cellular regulators. Substitution of a distinct RVXF sequence in inhibitor-1 that enhanced its binding and potency as a PP1 inhibitor emphasized the importance of the RVXF sequence in defining the function of this and other PP1 regulators. Our studies suggest that the diversity of RVXF sequences provides for dynamic physiological regulation of PP1 functions in eukaryotic cells.

Functional diversity of protein phosphatase-1, a cellular economizer and reset button

The protein serine/threonine phosphatase protein phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that regulates a variety of cellular processes through the dephosphorylation of dozens of substrates. This multifunctionality of PP1 relies on its association with a host of function-specific targetting and substrate-specifying proteins. In this review we discuss how PP1 affects the biochemistry and physiology of eukaryotic cells. The picture of PP1 that emerges from this analysis is that of a "green" enzyme that promotes the rational use of energy, the recycling of protein factors, and a reversal of the cell to a basal and/or energy-conserving state. Thus PP1 promotes a shift to the more energy-efficient fuels when nutrients are abundant and stimulates the storage of energy in the form of glycogen. PP1 also enables the relaxation of actomyosin fibers, the return to basal patterns of protein synthesis, and the recycling of transcription and splicing factors. In addition, PP1 plays a key role in the recovery from stress but promotes apoptosis when cells are damaged beyond repair. Furthermore, PP1 downregulates ion pumps and transporters in various tissues and ion channels that are involved in the excitation of neurons. Finally, PP1 promotes the exit from mitosis and maintains cells in the G1 or G2 phases of the cell cycle.

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

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

Reversible inhibition of the protein phosphatase 1 by hydrogen peroxide. Potential regulation of eIF2 alpha phosphorylation in differentiated PC12 cells

Oxidative inactivation of protein tyrosine phosphatases and calcineurin is a well established mechanism; however, little information with regard to the effect of oxidants on PP1 and PP2A activity is available. Herein, we show that PP1 activity is inhibited by H(2)O(2) treatment in differentiated PC12 cells both in vitro and in vivo experiments. Thiol-antioxidant N-acetyl-cysteine (NAC) and reduced glutathione (GSH), when added in vitro to lysates from H(2)O(2)-treated cells, reversed PP1 inhibition. H(2)O(2) treatment increased eIF2 alpha phosphorylated levels (eIF2 alpha P) in a time- and dose-dependent fashion and promoted protein synthesis inhibition. Interestingly, NAC pretreatment protected cells from H(2)O(2)-induced PP1 inactivation and, consequently, it abolished increased H(2)O(2)-induced eIF2 alpha phosphorylation and protein synthesis inhibition. In addition, PP1 inhibitor tautomycin prevented both NAC-induced PP1 reactivation and eIF2 alpha P dephosphorylation in H(2)O(2)-treated cells. Taken together, our findings support a role for PP1 in eIF2 alpha phosphorylation and oxidative stress-triggered translation down regulation.

Degeneracy and function of the ubiquitous RVXF motif that mediates binding to protein phosphatase-1

Most interactors of protein phosphatase-1 (PP1) contain a variant of a so-called "RVXF" sequence that binds to a hydrophobic groove of the catalytic subunit. A combination of sequence alignments and site-directed mutagenesis has enabled us to further define the consensus sequence for this degenerate motif as [RK]-X(0-1)-[VI]-[P]-[FW], where X denotes any residue and [P] any residue except Pro. Naturally occurring RVXF sequences differ in their affinity for PP1, and we show by swapping experiments that this binding affinity is an important determinant of the inhibitory potency of the regulators NIPP1 and inhibitor-1. Also, inhibition by NIPP1-(143-224) was retained when the RVXF motif (plus the preceding Ser) was swapped for either of two unrelated PP1-binding sequences from human inhibitor-2, i.e. KGILK or RKLHY. Conversely, the KGILK motif of inhibitor-2 could be functionally replaced by the RVXF motif of NIPP1. Our data provide additional evidence for the view that the RVXF and KGILK motifs function as anchors for PP1 and thereby promote the interaction of secondary binding sites that determine the activity and substrate specificity of the enzyme.

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.

Expression of alpha-ENaC2 is dependent on an upstream Sp1 binding motif and is modulated by protein phosphatase 1 in lung epithelial cells

The amiloride-sensitive Na(+) channel ENaC is expressed in lung epithelium and plays a pivotal role in lung fluid clearance in the newborn. Multiple splice variants of the ENaC alpha-subunit have been reported. Among them, alpha-ENaC2 accounts for a considerable portion of alpha-ENaC transcripts in human lung and kidney, possesses channel functions similar to alpha-ENaC1, and is driven by a downstream promoter. In the current study, we examine the regulation of alpha-ENaC2 transcription in lung epithelial cells. We found that transcription factors Sp1 and Sp3 activate alpha-ENaC2 transcription through a GC-rich element (Sp1-binding site) in the promoter. Because alpha-ENaC expression and Sp1 phosphorylation are both significantly up-regulated in the perinatal lung, we then examined the possible connection between Sp1/Sp3 phosphorylation and alpha-ENaC2 expression. We found that protein phosphatase 1 (PP1) dephosphorylates Sp1 and Sp3 in lung epithelial cells, reduces their binding to the alpha-ENaC2 promoter, and decreases Sp1/Sp3-mediated promoter activity. Our results suggest that Sp1 and Sp3 are essential for alpha-ENaC2 transcription in lung epithelial cells and that dephosphorylation of the Sp transcription factors by PP1 suppresses alpha-ENaC2 expression. The significance of these findings in the regulation of gene expression in perinatal lung is discussed.

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.

Characterisation and expression of a PP1 serine/threonine protein phosphatase (PfPP1) from the malaria parasite, Plasmodium falciparum: demonstration of its essential role using RNA interference

BACKGROUND

Reversible protein phosphorylation is relatively unexplored in the intracellular protozoa of the Apicomplexa family that includes the genus Plasmodium, to which belong the causative agents of malaria. Members of the PP1 family represent the most highly conserved protein phosphatase sequences in phylogeny and play essential regulatory roles in various cellular pathways. Previous evidence suggested a PP1-like activity in Plasmodium falciparum, not yet identified at the molecular level.

RESULTS

We have identified a PP1 catalytic subunit from P. falciparum and named it PfPP1. The predicted primary structure of the 304-amino acid long protein was highly similar to PP1 sequences of other species, and showed conservation of all the signature motifs. The purified recombinant protein exhibited potent phosphatase activity in vitro. Its sensitivity to specific phosphatase inhibitors was characteristic of the PP1 class. The authenticity of the PfPP1 cDNA was further confirmed by mutational analysis of strategic amino acid residues important in catalysis. The protein was expressed in all erythrocytic stages of the parasite. Abrogation of PP1 expression by synthetic short interfering RNA (siRNA) led to inhibition of parasite DNA synthesis.

CONCLUSIONS

The high sequence similarity of PfPP1 with other PP1 members suggests conservation of function. Phenotypic gene knockdown studies using siRNA confirmed its essential role in the parasite. Detailed studies of PfPP1 and its regulation may unravel the role of reversible protein phosphorylation in the signalling pathways of the parasite, including glucose metabolism and parasitic cell division. The use of siRNA could be an important tool in the functional analysis of Apicomplexan genes.

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.

Regulation of synaptic strength by protein phosphatase 1

We investigated the role of postsynaptic protein phosphatase 1 (PP1) in regulating synaptic strength by loading CA1 pyramidal cells either with peptides that disrupt PP1 binding to synaptic targeting proteins or with active PP1. The peptides blocked synaptically evoked LTD but had no effect on basal synaptic currents mediated by either AMPA or NMDA receptors. They did, however, cause an increase in synaptic strength following the induction of LTD. Similarly, PP1 had no effect on basal synaptic strength but enhanced LTD. In cultured neurons, synaptic activation of NMDA receptors increased the proportion of PP1 localized to synapses. These results suggest that PP1 does not significantly regulate basal synaptic strength. Appropriate NMDA receptor activation, however, allows PP1 to gain access to synaptic substrates and be recruited to synapses where its activity is necessary for sustaining LTD.

Defining the structural determinants and a potential mechanism for inhibition of myosin phosphatase by the protein kinase C-potentiated inhibitor protein of 17 kDa

Contractility of smooth muscle and non-muscle microfilaments involves phosphorylation of myosin II light chain. Myosin light chain phosphatase (MLCP) is specifically inhibited by the protein kinase C-potentiated inhibitor protein of 17 kDa, called CPI-17, as part of Ca(2+) sensitization of vascular smooth muscle contraction. Phosphorylation of Thr(38) in CPI-17 enhances inhibitory potency toward MLCP over 1000-fold. In this study we mapped regions of CPI-17 required for inhibition and investigated the mechanism using deletion and point mutants. Deletion of either the N-terminal 34 residues or C-terminal 27 residues gave no change in the IC(50) of either phospho- or unphospho-CPI-17. However, further deletion to give CPI-17 proteins of 1-102, 1-89, 1-76, and 1-67, resulted in much higher IC(50) values. The results indicate there is a minimal inhibitory domain between residues 35 and 120. A single Ala substitution at Tyr(41) eliminated phosphorylation-dependent inhibition, and phospho-Thr(38) in the Y41A protein was efficiently dephosphorylated by MLCP itself. The wild type CPI-17 expressed in fibroblast-induced bundling and contraction of actomyosin filaments, whereas expression of the Y41A protein had no obvious effects. Thus, a central domain of CPI-17(35-120) including phospho-Thr(38) is necessary for recognition by myosin phosphatase and Tyr(41) arrests dephosphorylation, thereby producing inhibition.

Growth arrest and DNA damage-inducible protein GADD34 assembles a novel signaling complex containing protein phosphatase 1 and inhibitor 1

The growth arrest and DNA damage-inducible protein, GADD34, was identified by its interaction with human inhibitor 1 (I-1), a protein kinase A (PKA)-activated inhibitor of type 1 protein serine/threonine phosphatase (PP1), in a yeast two-hybrid screen of a human brain cDNA library. Recombinant GADD34 (amino acids 233 to 674) bound both PKA-phosphorylated and unphosphorylated I-1(1-171). Serial truncations mapped the C terminus of I-1 (amino acids 142 to 171) as essential for GADD34 binding. In contrast, PKA phosphorylation was required for PP1 binding and inhibition by the N-terminal I-1(1-80) fragment. Pulldowns of GADD34 proteins expressed in HEK293T cells showed that I-1 bound the central domain of GADD34 (amino acids 180 to 483). By comparison, affinity isolation of cellular GADD34/PP1 complexes showed that PP1 bound near the C terminus of GADD34 (amino acids 483 to 619), a region that shows sequence homology with the virulence factors ICP34.5 of herpes simplex virus and NL-S of avian sarcoma virus. While GADD34 inhibited PP1-catalyzed dephosphorylation of phosphorylase a, the GADD34-bound PP1 was an active eIF-2alpha phosphatase. In brain extracts from active ground squirrels, GADD34 bound both I-1 and PP1 and eIF-2alpha was largely dephosphorylated. In contrast, the I-1/GADD34 and PP1/GADD34 interactions were disrupted in brain from hibernating animals, in which eIF-2alpha was highly phosphorylated at serine-51 and protein synthesis was inhibited. These studies suggested that modification of the I-1/GADD34/PP1 signaling complex regulates the initiation of protein translation in mammalian tissues.

Combinatorial control of protein phosphatase-1

The catalytic subunit of the type 1 Ser/Thr protein phosphatases (PP1) can interact with many different regulatory (R) subunits. These R subunits function as activity-modulators, targeting subunits and/or substrates. The specificity of the R subunits can be accounted for by their interaction with specific subsets of binding pockets on the catalytic subunit and by the presence of subcellular targeting sequences. Hormones, growth factors and metabolites control the function of PP1 holoenzymes mainly by modulating the interaction of the subunits.

Mutations in yeast protein phosphatase type 1 that affect targeting subunit binding

Protein phosphatase type 1 (PP1) is a major Ser/Thr protein phosphatase that is involved in many cellular processes. The activity of PP1 is controlled by regulatory subunits, many of which are thought to bind to a hydrophobic groove in PP1 via a short consensus sequence termed the V/IXF motif. To test this hypothesis, 11 variants of yeast PP1 (Glc7) were constructed in which one or more of the residues comprising the groove were changed to alanine. These variants were tested for their biological activity in vivo, for their biochemical activity in vitro, and for their ability to associate with three PP1 binding proteins. Five variants are unable to complement the essential function of PP1 in vivo although they are catalytically active in vitro. Many of the mutants are deficient in binding two V/IXF-containing subunits, Gac1 and Reg1, which regulate glycogen accumulation and glucose repression, respectively, but all retain the ability to associate with Sds22, a regulatory subunit that lacks this motif. The subcellular locations at which PP1 normally accumulates (bud neck, nucleolus, spindle pole body) were not occupied by one PP1 variant. Additionally, we provide evidence that mutations in the hydrophobic groove of PP1 affect substrate specificity. Together, these results demonstrate the importance of the hydrophobic groove for the interaction with regulatory subunits, for the proper subcellular localization of PP1 and for the substrate specificity of PP1.

Ischemia-induced inhibition of the initiation factor 2alpha phosphatase activity in the rat brain

Rats were subjected to the standard four-vessel occlusion model of brain transient ischemia for 30 min. Following different recirculation periods, the level of phosphorylation of the initiation factor 2 subunit alpha (eIF2alpha) and the eIF2alpha kinase/s and phosphatase/s activity were determined. eIF2alpha phosphorylation significantly increased very early during reperfusion (10-30 min), recovering at 4 h of reperfusion. Activation of any eIF2alpha kinases studied during ischemia or reperfusion was not noted. Conversely, eIF2alpha phosphatase activity significantly decreased at 10-15 min of reperfusion, reaching values even higher than in controls at 2-4 h of reperfusion. Our results support the hypothesis that the reperfusion-induced phosphorylated eIF2alpha changes are at least a result of the transiently eIF2alpha phosphatase inhibition.

Multiple interactions within the AKAP220 signaling complex contribute to protein phosphatase 1 regulation

The phosphorylation status of cellular proteins is controlled by the opposing actions of protein kinases and phosphatases. Compartmentalization of these enzymes is critical for spatial and temporal control of these phosphorylation/dephosphorylation events. We previously reported that a 220-kDa A-kinase anchoring protein (AKAP220) coordinates the location of the cAMP-dependent protein kinase (PKA) and the type 1 protein phosphatase catalytic subunit (PP1c) (Schillace, R. V., and Scott, J. D. (1999) Curr. Biol. 9, 321-324). We now demonstrate that an AKAP220 fragment is a competitive inhibitor of PP1c activity (K(i) = 2.9 +/- 0.7 micrometer). Mapping studies and activity measurements indicate that several protein-protein interactions act synergistically to inhibit PP1. A consensus targeting motif, between residues 1195 and 1198 (Lys-Val-Gln-Phe), binds but does not affect enzyme activity, whereas determinants between residues 1711 and 1901 inhibit the phosphatase. Analysis of truncated PP1c and chimeric PP1/2A catalytic subunits suggests that AKAP220 inhibits the phosphatase in a manner distinct from all known PP1 inhibitors and toxins. Intermolecular interactions within the AKAP220 signaling complex further contribute to PP1 inhibition as addition of the PKA regulatory subunit (RII) enhances phosphatase inhibition. These experiments indicate that regulation of PP1 activity by AKAP220 involves a complex network of intra- and intermolecular interactions.

Protein phosphatase 1 regulation by inhibitors and targeting subunits

Regulation of protein phosphatase 1 (PP1) by protein inhibitors and targeting subunits has been previously studied through the use of recombinant protein expressed in Escherichia coli. This preparation is limited by several key differences in its properties compared with native PP1. In the present study, we have analyzed recombinant PP1 expressed in Sf9 insect cells using baculovirus. Sf9 PP1 exhibited properties identical to those of native PP1, with respect to regulation by metals, inhibitor proteins, and targeting subunits, and failure to dephosphorylate a phosphotyrosine-containing substrate or phospho-DARPP-32 (Dopamine and cAMP-regulated phosphoprotein, M(r) 32,000). Mutations at Y272 in the beta12/beta13 loop resulted in a loss of activity and reduced the sensitivity to thiophospho-DARPP-32 and inhibitor-2. Mutations of Y272 also increased the relative activity toward a phosphotyrosine-containing substrate or phospho-DARPP-32. Mutation of acidic groove residues caused no change in sensitivity to thiophospho-DARPP-32 or inhibitor-2, but one mutant (E252A:D253A:E256R) exhibited an increased K(m) for phosphorylase a. Several PP1/PP2A chimeras were prepared in which C-terminal sequences of PP2A were substituted into PP1. Replacement of residues 274-330 of PP1 with the corresponding region of PP2A resulted in a large loss of sensitivity to thiophospho-DARPP-32 and inhibitor-2, and also resulted in a loss of interaction with the targeting subunits, spinophilin and PP1 nuclear targeting subunit (PNUTS). More limited alterations in residues in beta12, beta13, and beta14 strands highlighted a key role for M290 and C291 in the interaction of PP1 with thiophospho-DARPP-32, but not inhibitor-2.

Identification and characterization of a novel protein inhibitor of type 1 protein phosphatase

We have isolated human cDNA for a novel type 1 protein phosphatase (PP1) inhibitory protein, named inhibitor-4 (I-4), from a cDNA library of germ cell tumors. I-4, composed of 202 amino acids, is 44% identical to a PP1 inhibitor, inhibitor-2 (I-2). I-4 conserves functionally important structure of I-2 and exhibited similar biochemical properties. I-4 inhibited activity of the catalytic subunit of PP1 (PP1C), specifically with an IC(50) of 0.2 nM, more potently than I-2 with an IC(50) of 2 nM. I-4 weakly inhibited the activity of myosin-associated phosphates (PP1M). However, the level of inhibition of PP1M was increased during preincubation of PP1M with I-4, suggesting that the inhibition is caused by interaction of I-4 with PP1C in such a manner that it competes with the M subunit of PP1M. Gel overlay experiments showed that I-4 binds PP1C directly. Three I-4 peptides containing the N-terminal residues 1-123, 1-131, and 1-142 all showed strong binding ability to PP1C but did not show PP1 inhibitory activity, whereas an I-2 peptide (residues 1-134), lacking the corresponding C-terminal residues, potently inhibited PP1C activity as previously reported. Removal of the 18 N-terminal amino acid residues from I-4 dramatically reduced the PP1 binding activity with a correlated loss of inhibitory activity, whereas removal of the 10 N-terminal residues had only a little effect. The two peptides GST-I-4(19-131) and GST-I-4(132-202) showed ability to bind to PP1C, albeit very weakly. These results strongly suggest a multiple-point interaction between I-4 and PP1C, which is thought to cause the inhibition of I-4 which is stronger than the inhibition of I-2.

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.