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

Rice protein phosphatase 1 regulatory subunits OsINH2 and OsINH3 participate actively in growth and adaptive responses under abscisic acid

Rice (Oryza sativa L.), a worldwide staple food crop, is affected by various environmental stressors that ultimately reduce yield. However, diversified physiological and molecular responses enable it to cope with adverse factors. It includes the integration of numerous signaling in which protein phosphatase 1 (PP1) plays a pivotal role. Research on PP1 has been mostly limited to the PP1 catalytic subunit in numerous cellular progressions. Therefore, we focused on the role of PP1 regulatory subunits (PP1r), OsINH2 and OsINH3, homologs of AtINH2 and AtINH3 in Arabidopsis, in rice growth and stress adaptations. Our observations revealed that these are ubiquitously expressed regulatory subunits that interacted and colocalized with their counter partners, type 1 protein phosphatase (OsTOPPs) but could not change their subcellular localization. The mutation in OsINH2 and OsINH3 reduced pollen viability, thereby affected rice fertility. They were involved in abscisic acid (ABA)-mediated inhibition of seed germination, perhaps by interacting with osmotic stress/ABA-activated protein kinases (OsSAPKs). Meanwhile, they positively participated in osmotic adjustment by proline biosynthesis, detoxifying reactive oxygen species (ROS) through peroxidases (POD), reducing malondialdehyde formation (MDA), and regulating stress-responsive genes. Moreover, their co-interaction proposed they might mediate cellular processes together or by co-regulation; however, the special behavior of two different PP1r is needed to explore. In a nutshell, this research enlightened the involvement of OsINH2 and OsINH3 in the reproductive growth of rice and adaptive strategies under stress. Hence, their genetic interaction with ABA components and deep mechanisms underlying osmotic regulation and ROS adjustment would explain their role in complex signaling. This research offers the basis for introducing stress-resistant crops.

Long-term depression-related tau phosphorylation is enhanced by methylene blue in healthy rat hippocampus

BACKGROUND

The present study examined whether inhibition of guanylate cyclase (GC) is associated with the plasticity-related microtubule-stabilizing protein tau phosphorylation in the dentate gyrus (DG) of hippocampal formation.

METHODS

To address this issue, methylene blue (MB 50 μM) or saline was infused into the DG starting from the induction of long-term potentiation (LTP) or depression (LTD) for 1 h. Then, protein phosphatase 1 alpha (PP1α), glycogen synthase kinase 3 beta (GSK3β), and tau total and phosphorylated protein levels were measured in these hippocampi using western blotting. LTP and LTD were induced by application of high- and low-frequency stimulation protocols (HFS and LFS), respectively. 5-min averages of the excitatory postsynaptic potential (EPSP) slopes and population spike amplitudes at the end of recording were averaged to measure the magnitude of LTP or LTD.

RESULTS

Low-frequency stimulation protocols was unable to phosphorylate thr¹⁸¹ and thr²³¹epitopes of tau, but possessed kinase activity similar to the HFS in phosphorylation of ser³⁹⁶ and ser⁴¹⁶ epitopes. MB infusion during LTD induction attenuated LTD, prevented EPSP/spike dissociation and increased tau phosphorylation at ser³⁹⁶ and ser⁴¹⁶ epitopes, without changing tau phosphorylation at thr¹⁸¹ and thr²³¹ epitopes. Neither LTP nor LTP-related tau phosphorylation state was changed by MB infusion.

CONCLUSION

Although MB can benefit to stabilize the balance between LTP and LTD, and to fix the increased spike wave discharges, it might trigger deregulation of tau phosphorylation, leading to the development of Alzheimer's disease by a mechanism that goes awry during induction of LTD. Thereby detailed studies to reveal more precise evidence for the use of MB in this disease are needed.

Towards Dissecting the Mechanism of Protein Phosphatase-1 Inhibition by Its C-Terminal Phosphorylation

Phosphoprotein phosphatase‐1 (PP1) is a key player in the regulation of phospho‐serine (pSer) and phospho‐threonine (pThr) dephosphorylation and is involved in a large fraction of cellular signaling pathways. Aberrant activity of PP1 has been linked to many diseases, including cancer and heart failure. Besides a well‐established activity control by regulatory proteins, an inhibitory function for phosphorylation (p) of a Thr residue in the C‐terminal intrinsically disordered tail of PP1 has been demonstrated. The associated phenotype of cell‐cycle arrest was repeatedly proposed to be due to autoinhibition of PP1 through either conformational changes or substrate competition. Here, we use PP1 variants created by mutations and protein semisynthesis to differentiate between these hypotheses. Our data support the hypothesis that pThr exerts its inhibitory function by mediating protein complex formation rather than by a direct mechanism of structural changes or substrate competition.

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.

Protein Serine/Threonine Phosphatases: Keys to Unlocking Regulators and Substrates

Protein serine/threonine phosphatases (PPPs) are ancient enzymes, with distinct types conserved across eukaryotic evolution. PPPs are segregated into types primarily on the basis of the unique interactions of PPP catalytic subunits with regulatory proteins. The resulting holoenzymes dock substrates distal to the active site to enhance specificity. This review focuses on the subunit and substrate interactions for PPP that depend on short linear motifs. Insights about these motifs from structures of holoenzymes open new opportunities for computational biology approaches to elucidate PPP networks. There is an expanding knowledge base of posttranslational modifications of PPP catalytic and regulatory subunits, as well as of their substrates, including phosphorylation, acetylation, and ubiquitination. Cross talk between these posttranslational modifications creates PPP-based signaling. Knowledge of PPP complexes, signaling clusters, as well as how PPPs communicate with each other in response to cellular signals should unlock the doors to PPP networks and signaling "clouds" that orchestrate and coordinate different aspects of cell physiology.

Mutation in the pssZ Gene Negatively Impacts Exopolysaccharide Synthesis, Surface Properties, and Symbiosis of Rhizobium leguminosarum bv. trifolii with Clover

Rhizobium leguminosarum bv. trifolii is a soil bacterium capable of establishing a nitrogen-fixing symbiosis with clover plants (Trifolium spp.). This bacterium secretes large amounts of acidic exopolysaccharide (EPS), which plays an essential role in the symbiotic interaction with the host plant. This polymer is biosynthesized by a multi-enzymatic complex located in the bacterial inner membrane, whose components are encoded by a large chromosomal gene cluster, called Pss-I. In this study, we characterize R. leguminosarum bv. trifolii strain Rt297 that harbors a Tn5 transposon insertion located in the pssZ gene from the Pss-I region. This gene codes for a protein that shares high identity with bacterial serine/threonine protein phosphatases. We demonstrated that the pssZ mutation causes pleiotropic effects in rhizobial cells. Strain Rt297 exhibited several physiological and symbiotic defects, such as lack of EPS production, reduced growth kinetics and motility, altered cell-surface properties, and failure to infect the host plant. These data indicate that the protein encoded by the pssZ gene is indispensable for EPS synthesis, but also required for proper functioning of R. leguminosarum bv. trifolii cells.

Calcium sensitization mechanisms in detrusor smooth muscles

The contraction of detrusor smooth muscles depends on the increase in intracellular calcium. The influx of calcium from the plasma membrane calcium channels and calcium release from the sarcoplasmic reticulum give rise to intracellular calcium. Under the pathophysiological conditions, the increased sensitivity of regulatory and contractile proteins to calcium also plays an important role in maintaining the spontaneous detrusor smooth muscle activity. Many proteins have been identified to play a role in calcium sensitization. Both the protein kinase C (PKC) and Rho-kinase (ROCK) signaling pathways are responsible for the induction of calcium sensitization in the detrusor smooth muscles. The balance between the myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) regulates the intracellular calcium-contractile force relationship. The inhibition of MLCP by PKC-mediated phosphatase inhibitor (CPI-17) and myosin phosphatase target subunit (MYPT-1) phosphorylation by both the PKC and ROCK are responsible for calcium sensitization in the detrusor smooth muscles. However, the ROCK pathway predominantly participates in the calcium sensitization induction under pathophysiological situations. Many kinases are well known nowadays to play a role in calcium sensitization. This review aims to enlighten the current understanding of the regulatory mechanisms of calcium sensitization with special reference to the PKC and ROCK pathways in the detrusor smooth muscles. It will also aid in the development of new pharmacological strategies to prevent and treat bladder diseases.

Biogenesis and activity regulation of protein phosphatase 1

Protein phosphatase 1 (PP1) is expressed in all eukaryotic cells and catalyzes a substantial fraction of phosphoserine/threonine dephosphorylation reactions. It forms stable complexes with PP1-interacting proteins (PIPs) that guide the phosphatase throughout its life cycle and control its fate and function. The diversity of PIPs is huge (≈200 in vertebrates), and most of them combine short linear motifs to form large and unique interaction interfaces with PP1. Many PIPs have separate domains for PP1 anchoring, PP1 regulation, substrate recruitment and subcellular targeting, which enable them to direct associated PP1 to a specific subset of substrates and mediate acute activity control. Hence, PP1 functions as the catalytic subunit of a large number of multimeric holoenzymes, each with its own subset of substrates and mechanism(s) of regulation.

A Type-1 Phosphoprotein Phosphatase from a Dinoflagellate as a Possible Component of the Circadian Mechanism

Indicative of the importance of protein phosphorylation in the core circadian clock mechanism, chronically applied inhibitors of both protein kinases and phosphoprotein phosphatases have significant effects on the period, phase, and light-dependent regulation of circadian rhythms in the dinoflagellate Lingulodinium polyedrum. This study was aimed at identifying the presence of the affected phosphatase(s). Dephosphorylation of a PP1/PP2A-specific substrate by L. polyedrum extracts was inhibited by okadaic acid only at concentrations greater than 100 nM, as in vivo, by mammalian inhibitor-2 (I-2), and by an endogenous inhibitor with properties similar to I-2, indicating that a type-1 protein phosphatase (PP1) was predominant. A cDNA encoding a highly conserved PP1 was isolated, the 1st such signaling molecule identified in dinoflagellates. Anti-sera specific for this type of phosphatase recognized a 34 kDa protein in L. polyedrum extract, this being the same size as the PP1 encoded by the isolated cDNA. These findings are consistent with the suggestion that the L. polyedrum PP1 may be a part of the clock mechanism in this species.

Calcium Sensitization Mechanisms in Gastrointestinal Smooth Muscles

An increase in intracellular Ca²⁺ is the primary trigger of contraction of gastrointestinal (GI) smooth muscles. However, increasing the Ca²⁺ sensitivity of the myofilaments by elevating myosin light chain phosphorylation also plays an essential role. Inhibiting myosin light chain phosphatase activity with protein kinase C-potentiated phosphatase inhibitor protein-17 kDa (CPI-17) and myosin phosphatase targeting subunit 1 (MYPT1) phosphorylation is considered to be the primary mechanism underlying myofilament Ca²⁺ sensitization. The relative importance of Ca²⁺ sensitization mechanisms to the diverse patterns of GI motility is likely related to the varied functional roles of GI smooth muscles. Increases in CPI-17 and MYPT1 phosphorylation in response to agonist stimulation regulate myosin light chain phosphatase activity in phasic, tonic, and sphincteric GI smooth muscles. Recent evidence suggests that MYPT1 phosphorylation may also contribute to force generation by reorganization of the actin cytoskeleton. The mechanisms responsible for maintaining constitutive CPI-17 and MYPT1 phosphorylation in GI smooth muscles are still largely unknown. The characteristics of the cell-types comprising the neuroeffector junction lead to fundamental differences between the effects of exogenous agonists and endogenous neurotransmitters on Ca²⁺ sensitization mechanisms. The contribution of various cell-types within the tunica muscularis to the motor responses of GI organs to neurotransmission must be considered when determining the mechanisms by which Ca²⁺ sensitization pathways are activated. The signaling pathways regulating Ca²⁺ sensitization may provide novel therapeutic strategies for controlling GI motility. This article will provide an overview of the current understanding of the biochemical basis for the regulation of Ca²⁺ sensitization, while also discussing the functional importance to different smooth muscles of the GI tract.

Protein phosphatase 1 is essential for Greatwall inactivation at mitotic exit

Entry into mitosis is mediated by the phosphorylation of key cell cycle regulators by cyclin-dependent kinase 1 (Cdk1). In Xenopus embryos, the M-phase-promoting activity of Cdk1 is antagonized by protein phosphatase PP2A-B55. Hence, to ensure robust cell cycle transitions, Cdk1 and PP2A-B55 must be regulated so that their activities are mutually exclusive. The mechanism underlying PP2A-B55 inactivation at mitotic entry is well understood: Cdk1-activated Greatwall (Gwl) kinase phosphorylates Ensa/Arpp19, thereby enabling them to bind to and inhibit PP2A-B55. However, the re-activation of PP2A-B55 during mitotic exit, which is essential for cell cycle progression, is less well understood. Here, we identify protein phosphatase PP1 as an essential component of the PP2A-B55 re-activation pathway in Xenopus embryo extracts. PP1 initiates the re-activation of PP2A-B55 by dephosphorylating Gwl. We provide evidence that PP1 targets the auto-phosphorylation site of Gwl, resulting in efficient Gwl inactivation. This step is necessary to facilitate subsequent complete dephosphorylation of Gwl by PP2A-B55. Thus, by identifying PP1 as the phosphatase initiating Gwl inactivation, our study provides the molecular explanation for how Cdk1 inactivation is coupled to PP2A-B55 re-activation at mitotic exit.

Phactr3/scapinin, a member of protein phosphatase 1 and actin regulator (phactr) family, interacts with the plasma membrane via basic and hydrophobic residues in the N-terminus

Proteins that belong to the protein phosphatase 1 and actin regulator (phactr) family are involved in cell motility and morphogenesis. However, the mechanisms that regulate the actin cytoskeleton are poorly understood. We have previously shown that phactr3, also known as scapinin, localizes to the plasma membrane, including lamellipodia and membrane ruffles. In the present study, experiments using deletion and point mutants showed that the basic and hydrophobic residues in the N-terminus play crucial roles in the localization to the plasma membrane. A BH analysis (http://helixweb.nih.gov/bhsearch) is a program developed to identify membrane-binding domains that comprise basic and hydrophobic residues in membrane proteins. We applied this program to phactr3. The results of the BH plot analysis agreed with the experimentally determined region that is responsible for the localization of phactr3 to the plasma membrane. In vitro experiments showed that the N-terminal itself binds to liposomes and acidic phospholipids. In addition, we showed that the interaction with the plasma membrane via the N-terminal membrane-binding sequence is required for phactr3-induced morphological changes in Cos7 cells. The membrane-binding sequence in the N-terminus is highly conserved in all members of the phactr family. Our findings may provide a molecular basis for understanding the mechanisms that allow phactr proteins to regulate cell morphogenesis.

Protein phosphatase 1 catalytic isoforms: specificity toward interacting proteins

The coordinated and reciprocal action of serine-threonine protein kinases and protein phosphatases produces transitory phosphorylation, a fundamental regulatory mechanism for many biological processes. Phosphoprotein phosphatase 1 (PPP1), a major serine-threonine phosphatase, in particular, is ubiquitously distributed and regulates a broad range of cellular functions, including glycogen metabolism, cell cycle progression, and muscle relaxation. PPP1 has evolved effective catalytic machinery but in vitro lacks substrate specificity. In vivo, its specificity is achieved not only by the existence of different PPP1 catalytic isoforms, but also by binding of the catalytic moiety to a large number of regulatory or targeting subunits. Here, we will address exhaustively the existence of diverse PPP1 catalytic isoforms and the relevance of their specific partners and consequent functions.

The molecular chaperone Hsp70 activates protein phosphatase 5 (PP5) by binding the tetratricopeptide repeat (TPR) domain

Protein phosphatase 5 (PP5) is auto-inhibited by intramolecular interactions with its tetratricopeptide repeat (TPR) domain. Hsp90 has been shown to bind PP5 to activate its phosphatase activity. However, the functional implications of binding Hsp70 to PP5 are not yet clear. In this study, we find that both Hsp90 and Hsp70 bind to PP5 using a luciferase fragment complementation assay. A fluorescence polarization assay shows that Hsp90 (MEEVD motif) binds to the TPR domain of PP5 almost 3-fold higher affinity than Hsp70 (IEEVD motif). However, Hsp70 binding to PP5 stimulates higher phosphatase activity of PP5 than the binding of Hsp90. We find that PP5 forms a stable 1:1 complex with Hsp70, but the interaction appears asymmetric with Hsp90, with one PP5 binding the dimer. Solution NMR studies reveal that Hsc70 and PP5 proteins are dynamically independent in complex, tethered by a disordered region that connects the Hsc70 core and the IEEVD-TPR contact area. This tethered binding is expected to allow PP5 to carry out multi-site dephosphorylation of Hsp70-bound clients with a range of sizes and shapes. Together, these results demonstrate that Hsp70 recruits PP5 and activates its phosphatase activity which suggests dual roles for PP5 that might link chaperone systems with signaling pathways in cancer and development.

Recurrent reciprocal RNA chimera involving YPEL5 and PPP1CB in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is the most common form of leukemia in adults in the Western hemisphere. Tumor-specific chromosomal translocations, characteristic findings in several human malignancies that directly lead to malignant transformation, have not been identified in CLL. Using paired-end transcriptome sequencing, we identified recurrent and reciprocal RNA chimeras involving yippee like 5 (YPEL5) and serine/threonine-protein phosphatase PP1-beta-catalytic subunit (PPP1CB) in CLL. Two of seven index cases (28%) harbored the reciprocal RNA chimeras in our initial screening. Using quantitative real-time PCR (q real-time PCR), YPEL5/PPP1CB and PPP1CB/YPEL5 fusion transcripts were detected in 97 of 103 CLL samples (95%) but not in paired normal samples, benign lymphocytes, or various unrelated cancers. Whole-genome sequencing and Southern blotting demonstrated no evidence for a genomic fusion between YPEL5 and PPP1CB. YPEL5/PPP1CB chimera, when introduced into mammalian cells, expressed a truncated PPP1CB protein that demonstrated diminished phosphatase activity. PPP1CB silencing resulted in enhanced proliferation and colony formation of MEC1 and JVM3 cells, implying a role in the pathogenesis of mature B-cell leukemia. These studies uncover a potential role for recurrent RNA chimeras involving phosphatases in the pathogenesis of a common form of leukemia.

The PP1 binding code: a molecular-lego strategy that governs specificity

Ser/Thr protein phosphatase 1 (PP1) is a single-domain hub protein with nearly 200 validated interactors in vertebrates. PP1-interacting proteins (PIPs) are ubiquitously expressed but show an exceptional diversity in brain, testis and white blood cells. The binding of PIPs is mainly mediated by short motifs that dock to surface grooves of PP1. Although PIPs often contain variants of the same PP1 binding motifs, they differ in the number and combination of docking sites. This molecular-lego strategy for binding to PP1 creates holoenzymes with unique properties. The PP1 binding code can be described as specific, universal, degenerate, nonexclusive and dynamic. PIPs control associated PP1 by interference with substrate recruitment or access to the active site. In addition, some PIPs have a subcellular targeting domain that promotes dephosphorylation by increasing the local concentration of PP1. The diversity of the PP1 interactome and the properties of the PP1 binding code account for the exquisite specificity of PP1 in vivo.

Identification of NO induced and capacitation associated tyrosine phosphoproteins in buffalo (Bubalus bubalis) spermatozoa

To acquire the fertilizing competence, spermatozoa must undergo a cascade of physiological and biochemical changes collectively defined as capacitation. Compelling evidence signifies that the global increase in protein tyrosine phosphorylation is the driving factor for capacitation. In our laboratory, we previously demonstrated that nitric oxide (NO) induces capacitation in buffalo sperm and is associated with an increase in protein tyrosine phosphorylation. The aim of the present study is to identify the proteins undergo tyrosine phosphorylation during NO induced buffalo sperm capacitation using 2-D immunoblotting and mass spectrometry. The percentage of progressively motile and capacitated sperm was more in presence of l-arginine. Along with known tyrosine phosphoproteins like ATP synthase subunit beta, pyruvate dehydrogenase E1 component subunit beta, GST mu 3, F-actin capping protein subunit beta 2, GPD2 and VDAC2, interestingly novel tyrosine phosphoprotein substrates such as actin, serine/threonine-protein phosphatase PP1-gamma catalytic subunit, and glutamine synthetase were also identified which might be specific to the NO induced signaling and also emphasizes the species specificity with respect to tyrosine phosphorylation of proteins during capacitation. In conclusion, this study forms an essential step in delineating the proteins undergo tyrosine phosphorylation in response to NO induced signaling pathways during capacitation of buffalo sperm.

Protein phosphatase 1 (PP-1)-dependent inhibition of insulin secretion by leptin in INS-1 pancreatic β-cells and human pancreatic islets

Leptin inhibits insulin secretion from pancreatic β-cells, and in turn, insulin stimulates leptin biosynthesis and secretion from adipose tissue. Dysfunction of this adipoinsular feedback loop has been proposed to be involved in the development of hyperinsulinemia and type 2 diabetes mellitus. At the molecular level, leptin acts through various pathways, which in combination confer inhibitory effects on insulin biosynthesis and secretion. The aim of this study was to identify molecular mechanisms of leptin action on insulin secretion in pancreatic β-cells. To identify novel leptin-regulated genes, we performed subtraction PCR in INS-1 β-cells. Regulated expression of identified genes was confirmed by RT-PCR and Northern and Western blotting. Furthermore, functional impact on β-cell function was characterized by insulin-secretion assays, intracellular Ca²(+) concentration measurements, and enzyme activity assays. PP-1α, the catalytic subunit of protein phosphatase 1 (PP-1), was identified as a novel gene down-regulated by leptin in INS-1 pancreatic β-cells. Expression of PP-1α was verified in human pancreatic sections. PP-1α mRNA and protein expression is down-regulated by leptin, which culminates in reduction of PP-1 enzyme activity in β-cells. In addition, glucose-induced insulin secretion was inhibited by nuclear inhibitor of PP-1 and calyculin A, which was in part mediated by a reduction of PP-1-dependent calcium influx into INS-1 β-cells. These results identify a novel molecular pathway by which leptin confers inhibitory action on insulin secretion, and impaired PP-1 inhibition by leptin may be involved in dysfunction of the adipoinsular axis during the development of hyperinsulinemia and type 2 diabetes mellitus.

Function of protein phosphatase-1, Glc7, in Saccharomyces cerevisiae

Budding yeast, Saccharomyces cerevisiae, and its close relatives are unique among eukaryotes in having a single gene, GLC7, encoding protein phosphatase-1 (PP1). This enzyme with a highly conserved amino acid sequence controls many processes in all eukaryotic cells. Therefore, the study of Glc7 function offers a unique opportunity to gain a comprehensive understanding of this critical regulatory enzyme. This review summarizes our current knowledge of how Glc7 function modulates processes in the cytoplasm and nucleus. Additionally, global Glc7 regulation is described.

Intra-amygdalar okadaic acid enhances conditioned taste aversion learning and CREB phosphorylation in rats

Protein phosphatases (PPs) regulate many substrates implicated in learning and memory. Conditioned taste aversion (CTA) learning, in which animals associate a novel taste paired with a toxin and subsequently avoid the taste, is dependent on several serine/threonine phosphatase substrates and the PP1-binding protein spinophilin. In order to examine the effects of PP1/2A blockade on CTA acquisition and extinction, rats received bilateral infusions of okadaic acid (OA) (100nM, 1microl/hemisphere) or vehicle (0.15M NaCl) into the amygdala either 5min prior to, or 5min after, a single pairing of sodium saccharin (0.125%, 10-min access) and LiCl or NaCl (0.15M, 3ml/kg i.p.). Two-bottle, 24-h preference tests were conducted for 13days to measure CTA expression and extinction. Rats conditioned with saccharin and LiCl showed a decreased preference for saccharin, and OA administered before (but not after) the pairing of saccharin and LiCl resulted in a significantly stronger CTA that did not extinguish over 13days. The enhancement of the CTA was not due to aversive effects of OA, because rats given OA and a pairing of saccharin and NaCl did not acquire a CTA. Finally, OA administration increased levels of phosphorylated CREB immunoreactivity following a CTA trial. Together, these results suggest a critical role for PP1/2A during normal CTA learning. Because CTA learning was enhanced only when OA was given prior to conditioning, phosphatase activity may be a constraint on learning during the taste-toxin interval but not during acquisition and consolidation processes that occur after toxin administration.

The extended PP1 toolkit: designed to create specificity

Protein Ser/Thr phosphatase-1 (PP1) catalyzes the majority of eukaryotic protein dephosphorylation reactions in a highly regulated and selective manner. Recent studies have identified an unusually diversified PP1 interactome with the properties of a regulatory toolkit. PP1-interacting proteins (PIPs) function as targeting subunits, substrates and/or inhibitors. As targeting subunits, PIPs contribute to substrate selection by bringing PP1 into the vicinity of specific substrates and by modulating substrate specificity via additional substrate docking sites or blocking substrate-binding channels. Many of the nearly 200 established mammalian PIPs are predicted to be intrinsically disordered, a property that facilitates their binding to a large surface area of PP1 via multiple docking motifs. These novel insights offer perspectives for the therapeutic targeting of PP1 by interfering with the binding of PIPs or substrates.

Phosphatase inhibitor 2 promotes acetylation of tubulin in the primary cilium of human retinal epithelial cells

BACKGROUND

Primary cilia are flagella-like projections from the centriole of mammalian cells that have a key role in cell signaling. Human diseases are linked to defects in primary cilia. Microtubules make up the axoneme of cilia and are selectively acetylated and this is thought to contribute to the stability of the structure. However, mechanisms to regulate tubulin acetylation in cilia are poorly understood.

RESULTS

Endogenous phosphatase inhibitor-2 (I-2) was found concentrated in cilia of human epithelial cells, and was localized to cilia early in the process of formation, prior to the full acetylation of microtubules. Knockdown of I-2 by siRNA significantly reduced the acetylation of microtubules in cilia, without a net decrease in whole cell tubulin acetylation. There was a reduction in the percentage of I-2 knockdown cells with a primary cilium, but no apparent alteration in the cilium length, suggesting no change in microtubule-based transport processes. Inhibition of either histone deacetylases with trichostatin A, or protein phosphatase-1 with calyculin A in I-2 knockdown cells partially rescued the acetylation of microtubules in cilia and the percentage of cells with a primary cilium.

CONCLUSION

The regulatory protein I-2 localizes to the primary cilium where it affects both Ser/Thr phosphorylation and is required for full tubulin acetylation. Rescue of tubulin acetylation in I-2 knockdown cells by different chemical inhibitors shows that deacetylases and phosphatases are functionally interconnected to regulate microtubules. As a multifunctional protein, I-2 may link cell cycle progression to structure and stability of the primary cilium.

Identification of the interaction sites of Inhibitor-3 for protein phosphatase-1

Inhibitor-3 is a potent inhibitor of protein phosphatase-1, with an IC(50) in the nanomolar range for the inhibition of the dephosphorylation of phosphorylase a. Human Inhibitor-3 possesses a putative protein phosphatase-1 binding motif, (39)KKVEW(43). We provide direct evidence that this sequence is involved in PP1 interaction by examining the effects of site-directed mutations of Inhibitor-3 on its ability to inhibit protein phosphatase-1. A second interaction site whose deletion led to loss of inhibitory potency was identified between residues 65 and 77. The existence of two interaction sites is consistent with the high inhibitory potency of Inhibitor-3, and with current models for other inhibitor and targeting proteins that interact with protein phosphatase-1 with high affinity.

Phosphatase inhibitor-2 balances protein phosphatase 1 and aurora B kinase for chromosome segregation and cytokinesis in human retinal epithelial cells

Mitosis in Saccharomyces cerevisiae depends on IPL1 kinase, which genetically interacts with GLC8. The metazoan homologue of GLC8 is inhibitor-2 (I-2), but its function is not understood. We found endogenous and ectopic I-2 localized to the spindle, midzone, and midbody of mitotic human epithelial ARPE-19 cells. Knockdown of I-2 by RNA interference produced multinucleated cells, with supernumerary centrosomes, multipolar spindles and lagging chromosomes during anaphase. These defects did not involve changes in levels of protein phosphatase-1 (PP1), and the multinuclear phenotype was rescued by overexpression of I-2. Appearance of multiple nuclei and supernumerary centrosomes required progression through the cell cycle and I-2 knockdown cells failed cytokinesis, as observed by time-lapse microscopy. Inhibition of Aurora B by hesperadin produced multinucleated cells and reduced H3S10 phosphorylation. I-2 knockdown enhanced this latter effect. Partial knockdown of PP1Calpha prevented multiple nuclei caused by either knockdown of I-2 or treatment with hesperadin. Expression of enhanced green fluorescent protein-I-2 or hemagglutinin-I-2 made cells resistant to hesperadin. We propose that I-2 acts to enhance Aurora B by inhibiting specific PP1 holoenzymes that dephosphorylate Aurora B substrates necessary for chromosome segregation and cytokinesis. Conserved together throughout eukaryotic evolution, I-2, PP1 and Aurora B function interdependently during mitosis.

Maternal phosphatase inhibitor-2 is required for proper chromosome segregation and mitotic synchrony during Drosophila embryogenesis

Protein phosphatase-1 (PP1) is a major Ser/Thr phosphatase conserved among all eukaryotes, present as the essential GLC7 gene in yeast. Inhibitor-2 (I-2) is an ancient PP1 regulator, named GLC8 in yeast, but its in vivo function is unknown. Unlike mammals with multiple I-2 genes, in Drosophila there is a single I-2 gene, and here we describe its maternally derived expression and required function during embryogenesis. During oogenesis, germline expression of I-2 results in the accumulation of RNA and abundant protein in unfertilized eggs; in embryos, the endogenous I-2 protein concentrates around condensed chromosomes during mitosis and also surrounds interphase nuclei. An I-2 loss-of-function genotype is associated with a maternal-effect phenotype that results in drastically reduced progeny viability, as measured by reduced embryonic hatch rates and larval lethality. Embryos derived from I-2 mutant mothers show faulty chromosome segregation and loss of mitotic synchrony in cleavage-stage embryos, patchy loss of nuclei in syncytial blastoderms, and cuticular pattern defects in late-stage embryos. Transgenic expression of wild-type I-2 in mutant mothers gives dose-dependent rescue of the maternal effect on embryo hatch rate. We propose that I-2 is required for proper chromosome segregation during Drosophila embryogenesis through the coordinated regulation of PP1 and Aurora B.

Paliperidone as a mood stabilizer: a pre-frontal cortex synaptoneurosomal proteomics comparison with lithium and valproic acid after chronic treatment reveals similarities in protein expression

A series of recent studies has demonstrated that the molecules involved in regulation of neuronal plasticity are also involved in the mode of action of antidepressants and mood stabilizer drugs. Intracellular calcium signaling, energy metabolism, and neuronal plasticity can be influenced by inducing axonal remodeling and increasing levels of certain synaptic proteins. Because antipsychotic drugs are used as mood stabilizers our studies focused on a newly-marketed antipsychotic drug, paliperidone. We determined changes in rat synaptoneurosomal proteins after chronic treatment with paliperidone, lithium salt, or valproic acid in order to find similarities or differences between the mode of action of paliperidone and these two classical mood stabilizers. We determined differential protein expression profiles in prefrontal cortex (PFC) of male Sprague-Dawley rats (n = 4/group). Synaptoneurosomal-enriched preparations were obtained from PFC after chronic treatment with these three drugs. Proteins were separated by 2D-DIGE and identified by nano-LC-MS/MS. We observed similar protein expression profiles at the synaptoneurosomal level, suggesting that the mode of action for paliperidone is similar to that of lithium and valproic acid. However, the expression profile for paliperidone was more similar to that of lithium. Pathways affected in common by these two drugs included oxidative phosphorylation, electron transport, carbohydrate metabolism, and post-synaptic cytokinesis implicating the effects of these drugs in signaling pathways, energy metabolism, and synaptic plasticity.

Role(s) of the serine/threonine protein phosphatase 1 on mammalian sperm motility

Mammalian spermatozoa acquire the capacity for motility and fertilization during the transit through the epididymis under the control of different factors, such as cAMP, intracellular pH, intracellular calcium and phosphorylation of sperm proteins. As the acquisition of functional competence including gaining motility during epididymal transit occurs in the complete absence of contemporaneous gene transcription and translation on the part of the spermatozoa, it is widely accepted that post-translational modifications are the only means by which spermatozoa can acquire functionality. Serine-threonine protein phosphatase 1 (PP1) together with their testis/sperm-specific interacting proteins might be involved in this regulatory mechanism. PP1alpha, PP1beta/delta, PP1gamma1 and PP1gamma2 are all expressed in the testis whereas PP1gamma2 is the only isoform expressed on spermatozoa. I2, I3, sds22, 14-3-3 and hsp90 are associated with PP1gamma2 in spermatozoa located on the sperm head and tail. Activity of PP1gamma2 and the binding pattern to these regulatory proteins changes in spermatozoa recruited from the caput and those from the cauda part of the epididymis. In this review, we summarize the possible roles of PP1 on spermatozoa during spermatogenesis and flagellar motility control. We suggest that PP1 might take part in the inhibition of the sperm motility activation by interacting with AKAPs and CAMKII. A hypothesized signaling pathway of mammalian sperm motility activation and PP1's function has been proposed.

A Toxoplasma gondii leucine-rich repeat protein binds phosphatase type 1 protein and negatively regulates its activity

We have characterized the Toxoplasma gondii protein phosphatase type 1 (TgPP1) and a potential regulatory binding protein belonging to the leucine-rich repeat protein family, designated TgLRR1. TgLRR1 is capable of binding to TgPP1 to inhibit its activity and to override a G(2)/M cell cycle checkpoint in Xenopus oocytes. In the parasite, TgLRR1 mRNA and protein are both highly expressed in the rapidly replicating and virulent tachyzoites, while only low levels are detected in the slowly dividing and quiescent bradyzoites. TgPP1 mRNA and protein levels are equally abundant in tachyzoites and bradyzoites. Affinity pull down and immunoprecipitation experiments reveal that the TgLRR1-TgPP1 interaction takes place in the nuclear subcompartment of tachyzoites. These results are consistent with those of localization studies using both indirect immunofluorescence with specific polyclonal antibody and transient transfection of T. gondii vector expressing TgLRR1 and TgPP1. The inability to obtain stable transgenic tachyzoites suggested that overexpression of TgLRR1 and TgPP1 may impair the parasite's growth. Together with the activation of Xenopus oocyte meiosis reinitiation, these data indicate that TgLRR1 protein could play a role in the regulation of the T. gondii cell cycle through the modulation of phosphatase activity.

Expression and glycogenic effect of glycogen-targeting protein phosphatase 1 regulatory subunit GL in cultured human muscle

Glycogen-targeting PP1 (protein phosphatase 1) subunit G(L) (coded for by the PPP1R3B gene) is expressed in human, but not rodent, skeletal muscle. Its effects on muscle glycogen metabolism are unknown. We show that G(L) mRNA levels in primary cultured human myotubes are similar to those in freshly excised muscle, unlike subunits G(M) (gene PPP1R3A) or PTG (protein targeting to glycogen; gene PPP1R3C), which decrease strikingly. In cultured myotubes, expression of the genes coding for G(L), G(M) and PTG is not regulated by glucose or insulin. Overexpression of G(L) activates myotube GS (glycogen synthase), glycogenesis in glucose-replete and -depleted cells and glycogen accumulation. Compared with overexpressed G(M), G(L) has a more potent activating effect on glycogenesis, while marked enhancement of their combined action is only observed in glucose-replete cells. G(L) does not affect GP (glycogen phosphorylase) activity, while co-overexpression with muscle GP impairs G(L) activation of GS in glucose-replete cells. G(L) enhances long-term glycogenesis additively to glucose depletion and insulin, although G(L) does not change the phosphorylation of GSK3 (GS kinase 3) on Ser9 or its upstream regulator kinase Akt/protein kinase B on Ser473, nor its response to insulin. In conclusion, in cultured human myotubes, the G(L) gene is expressed as in muscle tissue and is unresponsive to glucose or insulin, as are G(M) and PTG genes. G(L) activates GS regardless of glucose, does not regulate GP and stimulates glycogenesis in combination with insulin and glucose depletion.

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.

A complex of catalytically inactive protein phosphatase-1 sandwiched between Sds22 and inhibitor-3

Protein Ser/Thr phosphatase-1 (PP1) associates with a host of proteins to form substrate-specific holoenzymes. Sds22 and Inhibitor-3 (I3) are two independently described ancient interactors of PP1. We show here by various approaches that Sds22 and I3 form a heterotrimeric complex with PP1, both in cell lysates and after purification. The stability of the complex depended on functional PP1 interaction sites in Sds22 and I3, indicating that PP1 is sandwiched between Sds22 and I3. Intriguingly, I3 could not be replaced in this complex by another PP1 interactor with the same PP1 binding motif. In vitro, Sds22 and I3 were potent inhibitors of PP1, but with only some substrates. The inhibition by Sds22 could be reproduced with synthetic Sds22 fragments comprising leucine-rich repeats (LRR) 2 and 5. Sds22 and LRR5 also slowly converted PP1 into a conformation that was inactive with all tested substrates. Cell lysates that were prepared under conditions that prevented the Sds22-induced inactivation of PP1 contained a catalytically inactive complex of Sds22, PP1, and I3, indicating that this complex exists in vivo. Therefore, our studies show that a pool of PP1 is complexly controlled by both Sds22 and I3.

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.

The translation initiation factor eIF2beta is an interactor of protein phosphatase-1

It is reasonably well understood how the initiation of translation is controlled by reversible phosphorylation of the eukaryotic translation initiation factors eIF2alpha, eIF2Bepsilon and eIF4E. Other initiation factors, including eIF2beta, are also established phosphoproteins but the physiological impact of their phosphorylation is not known. Using a sequence homology search we found that the central region of eIF2beta contains a putative PP1-(protein phosphatase-1) binding RVxF-motif. The predicted eIF2beta-PP1 interaction was confirmed by PP1 binding and co-immunoprecipitation assays on cell lysates as well as with the purified components. Site-directed mutagenesis showed that eIF2beta contains, in addition to an RVxF-motif, at least one other PP1-binding site in its C-terminal half. eIF2beta functioned as an inhibitor for the dephosphorylation of glycogen phosphorylase and Ser51 of eIF2alpha by PP1, but did not affect the dephosphorylation of Ser464 of eIF2Bepsilon by this phosphatase. Strikingly, eIF2beta emerged as an activator of its own dephosphorylation (Ser2, Ser67, Ser218) by associated PP1, since the substrate quality of eIF2beta was decreased by the mere mutation of its RVxF-motif. These results make eIF2beta an attractive candidate substrate for associated PP1 in vivo. The overexpression of wild-type eIF2beta or eIF2beta with a mutated RVxF-motif did not differentially affect the rate of translation, indicating that the binding of PP1 is not rate-limiting for translation under basal conditions.

Regulation of protein phosphatase 1gamma activity in hypoxia through increased interaction with NIPP1: implications for cellular metabolism

Eukaryotic cells sense decreased oxygen levels and respond by altering their metabolic strategy to sustain non-respiratory ATP production through glycolysis, and thus promote cell survival in a hypoxic environment. Protein phosphatase 1 (PP1) has been recently implicated in the governance of the rational use of energy when metabolic substrates are abundant and contributes to cellular recovery following metabolic stress. Under conditions of hypoxia, the expression of the gamma isoform of PP1 (PP1gamma), is diminished, an event we have hypothesized to be involved in the adaptive cellular response to hypoxia. Decreased PP1gamma activity in hypoxia has a profound impact on the activity of the cAMP response element binding protein (CREB), a major transcriptional regulator of metabolic genes and processes. Here, we demonstrate a further mechanism leading to inhibition of PP1 activity in hypoxia which occurs at least in part through increased association with the nuclear inhibitor of PP1 (NIPP1), an event dependent upon decreased basal cAMP/PKA-dependent signaling. Using a dominant negative NIPP1 construct, we provide evidence that NIPP1 plays a major role in the regulation of both CREB protein expression and CREB-dependent transcription in hypoxia. Furthermore, we demonstrate functional sequellae of such events including altered gene expression and recovery of cellular ATP levels. In summary, we demonstrate that interaction with NIPP1 mediates decreased PP1gamma activity in hypoxia, an event which may constitute an inherent part of the cellular oxygen-sensing machinery and may play a role in physiologic adaptation to hypoxia.

Iminosugars as Potential Inhibitors of Glycogenolysis: Structural Insights into the Molecular Basis of Glycogen Phosphorylase Inhibition

Iminosugars DAB (5), isofagomine (9), and several N-substituted derivatives have been identified as potent inhibitors of liver glycogen phosphorylase a (IC(50) = 0.4-1.2 microM) and of basal and glucagon-stimulated glycogenolysis (IC(50) = 1-3 microM). The X-ray structures of 5, 9, and its N-3-phenylpropyl analogue 8 in complex with rabbit muscle glycogen phosphorylase (GPb) shows that iminosugars bind tightly at the catalytic site in the presence of the substrate phosphate and induce conformational changes that characterize the R-state conformation of the enzyme. Charged nitrogen N1 is within hydrogen-bonding distance with the carbonyl oxygen of His377 (5) and in ionic contact with the substrate phosphate oxygen (8 and 9). Our findings suggest that the inhibitors function as oxocarbenium ion transition-state analogues. The conformational change to the R state provides an explanation for previous findings that 5, unlike inhibitors that favor the T state, promotes phosphorylation of GPb in hepatocytes with sequential inactivation of glycogen synthase.

Regulation of protein phosphatase type 1 and cell cycle progression by PfLRR1, a novel leucine-rich repeat protein of the human malaria parasite Plasmodium falciparum

The protein called 'suppressor of the dis2 mutant (sds22+)' is an essential regulator of cell division in fission and budding yeasts, where its deletion causes mitotic arrest. Its role in cell cycle control appears to be mediated through the activation of protein phosphatase type 1 (PP1) in Schizosaccharomyces pombe. We have identified the Plasmodium falciparum Sds22 orthologue, which we designated PfLRR1 as it belongs to the leucine-rich repeat protein family. We showed by glutathione-S-transferase pull-down assay that the PfLRR1 gene product interacts with PfPP1, that the PfLRR1-PfPP1 complex is present in parasite extracts and that PfLRR1 inhibits PfPP1 activity. Functional studies in Xenopus oocytes revealed that PfLRR1 interacted with endogenous PP1 and overcame the G2/M cell cycle checkpoint by promoting progression to germinal vesicle breakdown (GVBD). Confirmatory results showing the appearance of GVBD were observed when oocytes were treated with anti-PP1 antibodies or okadaic acid. Taken together, these observations suggest that PfLRR1 can regulate the cell cycle by binding to PP1 and regulating its activity.

Identification of a differentially expressed gene PPP1CB between porcine Longissimus dorsi of Meishan and Large WhitexMeishan hybrids

To study the molecular basis of heterosis, suppression subtractive hybridization was used to investigate the differences in gene expression between porcine Longissimus dorsi of F1 hybrids Large WhitexMeishan and their female parents Meishan. From two specific subtractive cDNA libraries, the clones selected by reverse Northern high-density blot screening were chosen to clone full-length cDNA by rapid amplification of cDNA ends. An expression-upregulated gene for Meishan skeletal muscle, designated protein phosphatase 1, catalytic subunit, beta isoform (PPP1CB), was identified. Porcine PPP1CB contains an open reading frame encoding 327 amino acid residues with 13 and 1763 nucleotides in the 5' and 3' untranslated regions, respectively. A DNA fragment of 721 nucleotides was amplified and a mutation that creates/disrupts a restriction site for endonuclease RsaI was found. The derived amino acid sequence of PPP1CB has high homology with the PPP1CB of three species, Mus musculus (99%), human (99%) and mouse (100%). The tissue expression analysis indicated that the swine PPP1CB gene is generally expressed in most tissues. The possible role of PPP1CB and its relation to porcine heterosis are discussed.

Identification of interactive gene networks: a novel approach in gene array profiling of myometrial events during guinea pig pregnancy

OBJECTIVE

The transition from myometrial quiescence to activation is poorly understood, and the analysis of array data is limited by the available data mining tools. We applied functional analysis and logical operations along regulatory gene networks to identify molecular processes and pathways underlying quiescence and activation.

STUDY DESIGN

We analyzed some 18,400 transcripts and variants in guinea pig myometrium at stages corresponding to quiescence and activation, and compared them to the nonpregnant (control) counterpart using a functional mapping tool, MetaCore (GeneGo, St Joseph, MI) to identify novel gene networks composed of biological pathways during mid (MP) and late (LP) pregnancy.

RESULTS

Genes altered during quiescence and or activation were identified following gene specific comparisons with myometrium from nonpregnant animals, and then linked to curated pathways and formulated networks. The MP and LP networks were subtracted from each other to identify unique genomic events during those periods. For example, changes 2-fold or greater in genes mediating protein biosynthesis, programmed cell death, microtubule polymerization, and microtubule based movement were noted during the transition to LP.

CONCLUSION

We describe a novel approach combining microarrays and genetic data to identify networks associated with normal myometrial events. The resulting insights help identify potential biomarkers and permit future targeted investigations of these pathways or networks to confirm or refute their importance.

Protein phosphatase 1, a Plasmodium falciparum essential enzyme, is exported to the host cell and implicated in the release of infectious merozoites

The malarial parasite Plasmodium falciparum transposes a Golgi-like compartment, referred to as Maurer's clefts, into the cytoplasm of its host cell, the erythrocyte, and delivering parasite molecules to the host cell surface. We report here a novel role of the Maurer's clefts implicating a parasite protein phosphatase 1 (PP1) and related to the phosphorylation status of P. falciparum skeleton-binding protein 1 (PfSBP1), a trans-membrane protein of the clefts interacting with the host cell membrane via its carboxy-terminal domain. Based on co-immunoprecipitation and inhibition studies, we show that the parasite PP1 type phosphatase modulates the phosphorylation status of the amino-terminal domain of PfSBP1 in the lumen of Maurer's clefts. Importantly, the addition of a PP1 inhibitor, calyculin A, to late schizonts results in the hyperphosphorylation of PfSBP1 and prevents parasite release from the host cell. We propose that the hyperphosphorylation of PfSBP1 interferes with the release of merozoites, the invasive blood stage of the parasite, by increasing the red cell membrane stability. Moreover, the parasite PP1 phosphatase is the first enzyme essential for the parasite development detected in the Maurer's clefts.

Identification of a novel phosphatidic acid binding domain in protein phosphatase-1

Phosphatidic acid (PA) has been recognized as a lipid second messenger, yet few cellular targets for PA have been identified. Previous work demonstrated PA as a potent and noncompetitive tight-binding inhibitor of the catalytic subunit (gamma isoform) of protein phosphatase-1 (PP1c gamma) in vitro. The high potency of inhibition, coupled with high specificity for PA over other phospholipids, suggested the presence of a high-affinity PA binding domain on PP1c gamma. In the current study, quantification of the binding interaction and identification of the binding domain were pursued. Surface plasmon resonance was employed to quantitate the interaction between PP1c gamma and immobilized mixed lipid vesicles of PA/phosphatidylcholine (PC) or PC alone. The data disclosed a high-affinity interaction with a KD measured in the low (1-40) nanomolar range, consistent with the range of Ki previously obtained from in vitro enzymatic assays. Next, identification of the segment of PP1 necessary for PA binding was determined using a deletion mutagenesis strategy. Binding assays revealed that PP1c gamma residues between 274 and 299 were required for the interaction with the lipid. When fusions of PP1c gamma fragments with green fluorescent protein (GFP) were generated, it was then determined that PP1c gamma residues 286-296 were sufficient to confer PA binding to GFP, a protein that does not interact with PA. The minimal PA binding domain of PP1c gamma lacked similarity to the previously described PA binding segments of Raf-1 kinase and cyclic-AMP phosphodiesterase 4A1. When these results were taken together with the known crystallographic structure of PP1, they identified a novel PA binding region on PP1c gamma that contains a unique loop-strand structural fold responsible for the interaction with PA.

CAT 53: a protein phosphatase 1 nuclear targeting subunit encoded in the MHC Class I region strongly expressed in regions of the brain involved in memory, learning, and Alzheimer's disease

We identified CAT 53 by cDNA hybridization selection as an expressed sequence tag (EST), located in the vicinity of HLA-C and designated as CAT (for HLA-C associated transcript) 53. CAT 53 encodes a protein described by others and commonly known as phosphatase 1 nuclear targeting subunit (PNUTS). PNUTS is a potent inhibitor of nuclear serine/threonine protein phosphatase 1 (PP1). We present the genomic organization of CAT 53, localize specific sites of mRNA transcription in thin sections of mouse brain by in-situ hybridization, and perform a structural analysis of the peptide domains. We also characterize the protein expression pattern for PNUTS by Western blotting and immunohistochemistry with PNUTS antibody in Alzheimer's disease (AD) brains and age-matched control brains. In-situ hybridization and immunohistochemistry analysis of human and mouse brain show high CAT 53 expression in the olfactory cortex, piriform cortex, and hippocampus. Very high expression of CAT 53 was found mainly in the hippocampus, frontal, and entorhinal cortex of control brains and in the neurofibrillary tangles of AD brain. In the hippocampus, CAT 53 is expressed in CA1 and CA3 cell layers and in the dentate gyrus. The hippocampus is known to play a fundamental role in learning and episodic memories and has been implicated in a number of neurological and psychiatric disorders, including AD, epilepsy, and schizophrenia. Our findings suggest that PNUTS, encoded by CAT 53 on 6p21.3, may have a role in the progression of AD.

Protein phosphatase-1 inhibitor-3 is co-localized to the nucleoli and centrosomes with PP1γ1 and PP1α, respectively

In this study, we show that protein phosphatase-1 (PP1) inhibitor-3 (Inh3) is localized to the nucleoli and centrosomes in interphase HEK 293 cells. Inh3 exhibited a specific co-localization to the nucleoli with PP1gamma1, and to the centrosomes with PP1alpha. These findings indicate that Inh3 may act as a modulator of PP1 functions in the processes of cytokinesis, as well as of nucleolar events. The specificity of the interaction of Inh3 with the PP1 isoforms was also demonstrated in vitro, where Inh3 co-immunoprecipitated with PP1alpha and PP1gamma1, but not with PP1beta. The nuclear localization signal of Inh3 was identified as a N-terminal basic cluster (33RKRK36), while nucleolar localization was shown to be dependent on a C-terminal basic cluster (94HRKGRRR100). The importance of the individual basic residues was quantitatively assessed by site-directed mutagenesis and a novel use of laser scanning cytometry.

The Bud14p-Glc7p complex functions as a cortical regulator of dynein in budding yeast

Regulated interactions between microtubules (MTs) and the cell cortex control MT dynamics and position the mitotic spindle. In eukaryotic cells, the adenomatous polyposis coli/Kar9p and dynein/dynactin pathways are involved in guiding MT plus ends and MT sliding along the cortex, respectively. Here we identify Bud14p as a novel cortical activator of the dynein/dynactin complex in budding yeast. Bud14p accumulates at sites of polarized growth and the mother-bud neck during cytokinesis. The localization to bud and shmoo tips requires an intact actin cytoskeleton and the kelch-domain-containing proteins Kel1p and Kel2p. While cells lacking Bud14p function fail to stabilize the pre-anaphase spindle at the mother-bud neck, overexpression of Bud14p is toxic and leads to elongated astral MTs and increased dynein-dependent sliding along the cell cortex. Bud14p physically interacts with the type-I phosphatase Glc7p, and localizes Glc7p to the bud cortex. Importantly, the formation of Bud14p-Glc7p complexes is necessary to regulate MT dynamics at the cortex. Taken together, our results suggest that Bud14p functions as a regulatory subunit of the Glc7p type-I phosphatase to stabilize MT interactions specifically at sites of polarized growth.