Carboxyl methylation regulates phosphoprotein phosphatase 2A by controlling the association of regulatory B subunits

Phosphoprotein phosphatase 2A (PP2A) is a major phosphoserine/threonine protein phosphatase in all eukaryotes. It has been isolated as a heterotrimeric holoenzyme composed of a 65 kDa A subunit, which serves as a scaffold for the association of the 36 kDa catalytic C subunit, and a variety of B subunits that control phosphatase specificity. The C subunit is reversibly methyl esterified by specific methyltransferase and methylesterase enzymes at a completely conserved C-terminal leucine residue. Here we show that methylation plays an essential role in promoting PP2A holoenzyme assembly and that demethylation has an opposing effect. Changes in methylation indirectly regulate PP2A phosphatase activity by controlling the binding of regulatory B subunits to AC dimers.     

Tandem reporter assay for myristoylated proteins post-translationally (TRAMPP) identifies novel substrates for post-translational myristoylation: PKCε, a case study

Myristoylation, the addition of a 14-carbon fatty acid to the N-terminal glycine of a protein, is key to protein-membrane and protein-protein interactions. Typically, myristoylation occurs cotranslationally; however, post-translational myristoylation of caspase-cleaved proteins is now emerging as a well-established protein modification and as a novel regulator of apoptosis. To identify additional post-translationally myristoylated proteins, we engineered a plasmid vector encoding for a caspase-cleavable reporter protein named tandem reporter assay for myristoylation of proteins post-translationally (TRAMPP). pTRAMPP consists of tdTomato-DEVD-"test myristoylation sequence"-enhanced green fluorescent protein (EGFP). After induction of apoptosis, the reporter protein is cleaved by caspases, which frees a new N-terminal glycine residue attached to EGFP that can be myristoylated. We used pTRAMPP in appropriately transfected cells to identify 7 post-translationally myristoylated proteins. First, we confirmed the post-translational myristoylation of two previously identified putative substrates, cytoplasmic dynein intermediate chain 2A and PKCε (ctPKCε), and identified 5 more caspase-cleaved potential substrates for myristoylation that include the antiapoptotic regulator of apoptosis, Mcl-1, and the causative agent of Huntington's disease, huntingtin protein. Further investigation revealed that post-translationally myristoylated ctPKCε localized to membranes and increased Erk signaling and degradation of the proapoptotic protein Bim, which prevented a significant loss of mitochondrial potential of 17% over nonmyristoylated ctPKCε in HeLa cells in the presence of apoptotic stimuli. Taken together, these findings suggest a possible antiapoptotic role for post-translationally myristoylated caspase-cleaved ctPKCε.     

Myristoylation is important at multiple stages in poliovirus assembly.

The N-terminal glycine of the VP4 capsid subunit of poliovirus is covalently modified with myristic acid (C14 saturated fatty acid). To investigate the function of VP4 myristoylation in poliovirus replication, amino acid substitutions were placed within the myristoylation consensus sequence at the alanine residue (4003A) adjacent to the N-terminal glycine by using site-directed mutagenesis methods. Mutants which replace the alanine residue with a small hydrophobic residue such as leucine, valine, or glycine displayed normal levels of myristoylation and normal growth kinetics. Replacement with the polar amino acid histidine (4003A.H) also resulted in a level of myristoylation comparable to that of the wild type. However, replacement of the alanine residue with aspartic acid (4003A.D) caused a dramatic reduction (about 40 to 60%) in myristoylation levels of the VP4 precursors (P1 and VP0). In contrast, no differences in modification levels were found in either VP0 and VP4 proteins isolated from mature mutant virions, indicating that myristoylation is required for assembly of the infectious virion. The myristoylation levels of the VP0 proteins found in capsid assembly intermediates indicate that there is a strong but not absolute preference for myristoyl-modified subunits during pentamer formation. Complete myristoylation was observed in mature virions but not in assembly intermediates, indicating that there is a selection for myristoyl-modified subunits during stable RNA encapsidation to form the mature virus particle. In addition, even though mutant infectious virions are fully modified, the severe reduction in specific infectivity of both 4003A.D and 4003A.H purified viruses indicates that the amino acid residue adjacent to the N-terminal glycine apparently has an additional role early during viral infection and that mutations at this position induce pleiotropic effects.     

Partial purification and biochemical characterization of a heteromeric protein phosphatase 2A holoenzyme from maize (Zea mays L.) leaves that dephosphorylates C4 phosophoenolpyruvate carboxylase

The activity and allosteric properties of plant phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) are controlled posttranslationally by specific reversible phosphorylation of a strictly conserved serine residue near the N-terminus. This up/down-regulation of PEPC is catalyzed by a dedicated and highly regulated serine/threonine (Ser/Thr) kinase (PEPC-kinase) and an opposing type-2A Ser/Thr phosphatase (PP2A). In marked contrast to PEPC-kinase, the PP2A holoenzyme from photosynthetic tissue has been virtually unstudied to date. In the present investigation, we have partially purified and characterized the native form of this PP2A from illuminated leaves of maize (Zea mays L.), a C4 plant, using maize [32P]PEPC as substrate. Various conventional chromatographic matrices, together with thiophosphorylated C4 PEPC-peptide and microcystin-LR affinity-supports, were exploited for the enrichment of this PP2A from soluble leaf extracts. Biochemical and immunological results indicate that the C4-leaf holoenzyme is analogous to other eukaryotic PP2As in being a approximately 170-kDa heteromer comprised of a core PP2Ac-A heterodimer (approximately 38- and approximately 65-kDa subunits, respectively) complexed with a putative, approximately 74-kDa B-type regulatory/targeting subunit. This heterotrimer lacks any strict substrate specificity in that it dephosphorylates C4 PEPC, mammalian phosphorylase a, and casein in vitro. This activity is independent of free Me2+, insensitive to levamisole and the Inhibitor-2 protein that targets PP1, activated by several polycations such as protamine and poly-L-lysine, and highly sensitive to inhibition by microcystin-LR and okadaic acid (IC50 approximately 30 pM), all of which are diagnostic features of yeast and mammalian PP2As. In addition, this C4-leaf PP2A holoenzyme (i) is inhibited in vitro by physiological concentrations of certain C4 PEPC-related metabolites (L-malate, PEP, glucose 6-phosphate, but not the activator glycine) when either 32P-labeled maize PEPC or rabbit muscle phosphorylase a is used as substrate, suggesting a direct effect on this Ser/Thr phosphatase; and (ii) displays, at best, only modest light/dark effects in vivo on its apparent molecular mass, component core subunits and activity against C4 PEPC, in marked contrast to the opposing activity of PEPC-kinase in C4 and Crassulacean acid metabolism leaves. This report represents one of the few studies of a heteromeric PP2A holoenzyme from photosynthetic tissue that dephosphorylates a known target enzyme in plants, such as PEPC, sucrose-phosphate synthase or nitrate reductase.     

Allosteric activation of protein phosphatase 2C by D-chiro-inositol-galactosamine, a putative mediator mimetic of insulin action

Insulin-stimulated glucose disposal in skeletal muscle proceeds predominantly through a nonoxidative pathway with glycogen synthase as a rate-limiting enzyme, yet the mechanisms for insulin activation of glycogen synthase are not understood despite years of investigation. Isolation of putative insulin second messengers from beef liver yielded a pseudo-disaccharide consisting of pinitol (3-O-methyl-d-chiro-inositol) beta-1,4 linked to galactosamine chelated with Mn(2+) (called INS2). Here we show that chemically synthesized INS2 has biological activity that significantly enhances insulin reduction of hyperglycemia in streptozotocin diabetic rats. We used computer modeling to dock INS2 onto the known three-dimensional crystal structure of protein phosphatase 2C (PP2C). Modeling and FlexX/CScore energy minimization predicted a unique favorable site on PP2C for INS2 in a surface cleft adjacent to the catalytic center. Binding of INS2 is predicted to involve formation of multiple H-bonds, including one with residue Asp163. Wild-type PP2C activity assayed with a phosphopeptide substrate was potently stimulated in a dose-dependent manner by INS2. In contrast, the D163A mutant of PP2C was not activated by INS2. The D163A mutant and wild-type PP2C in the absence of INS2 had the same Mn(2+)-dependent phosphatase activity with p-nitrophenyl phosphate as a substrate, showing that this mutation did not disrupt the catalytic site. We propose that INS2 allosterically activates PP2C, fulfilling the role of a putative mediator mimetic of insulin signaling to promote protein dephosphorylation and metabolic responses.     

N myristoylation of the spleen necrosis virus matrix protein is required for correct association of the Gag polyprotein with intracellular membranes and for particle formation.

To determine whether myristoylation is required for spleen necrosis virus replication, we constructed a substitution mutation in the gag gene that alters the putative myristate acceptor glycine residue. This single amino acid change was lethal for virus replication, resulted in aberrant proteolytic processing, and interrupted virion assembly and the release of virus from cells. Immunofluorescence analysis indicated that the amount of Gag polyprotein at the cell periphery and in Golgi-associated vesicles is severely reduced in the myristoylation mutant, indicating that correct intracellular targeting is affected by a lack of myristoylation. Coexpression of wild-type Gag polyprotein did not complement and rescue the replication-defective phenotype of the myristoylation mutant. Thus, it appears that the nonmyristoylated polyproteins are incapable of interacting with their myristoylated counterparts to form biologically active particles.     

Molecular cloning of PP2Ceta, a novel member of the protein phosphatase 2C family

We have cloned a novel member of the mouse protein phosphatase 2C family, PP2Ceta. Sequence analysis suggests that PP2Ceta, PP2Czeta and NERPP-2C constitute a unique subgroup of the PP2C family. PP2Ceta had extremely low activity against alpha-casein compared with PP2Calpha and was localized mainly in cell nuclei, suggesting that PP2Ceta dephosphorylates a unique nuclear protein(s) in the cells.     

Requirement of Calcium Binding,Myristoylation, and Protein-Protein Interaction for the Copine BON1 Function in Arabidopsis

Copines are highly conserved proteins with lipid-binding activities found in animals, plants, and protists. They contain two calcium-dependent phospholipid binding C2 domains at the amino terminus and a VWA domain at the carboxyl terminus. The biological roles of most copines are not understood and the biochemical properties required for their functions are largely unknown. The Arabidopsis copine gene BON1/CPN1 is a negative regulator of cell death and defense responses. Here we probed the potential biochemical activities of BON1 through mutagenic studies. We found that mutations of aspartates in the C2 domains did not alter plasma membrane localization but compromised BON1 activity. Mutation at putative myristoylation residue glycine 2 altered plasma membrane localization of BON1 and rendered BON1 inactive. Mass spectrometry analysis of BON1 further suggests that the N-peptide of BON1 is modified. Furthermore, mutations that affect the interaction between BON1 and its functional partner BAP1 abolished BON1 function. This analysis reveals an unanticipated regulation of copine protein localization and function by calcium and lipid modification and suggests an important role in protein-protein interaction for the VWA domain of copines.     

Purification of porcine brain protein phosphatase 2A leucine carboxyl methyltransferase and cloning of the human homologue

The carboxyl methyltransferase, which is claimed to exclusively methylate the carboxyl group of the C-terminal leucine residue of the catalytic subunit of protein phosphatase 2A (Leu(309)), was purified from porcine brain. On the basis of tryptic peptides, the cDNA encoding the human homologue was cloned. The cDNA of this gene encodes for a protein of 334 amino acids with a calculated M(r) of 38 305 and a predicted pI of 5.72. Database screening reveals the presence of this protein in diverse phyla. Sequence analysis shows that the novel methyltransferase is distinct from other known protein methyltransferases, sharing only sequence motifs supposedly involved in the binding of adenosylmethionine. The recombinant protein expressed in bacteria is soluble and the biophysical, catalytic, and immunological properties are indistinguishable from the native enzyme. The methylation of PP2A by the recombinant protein is restricted to Leu(309) of PP2A(C). No direct effects on phosphatase activity changes were observed upon methylation of the dimeric or trimeric forms of PP2A.     

Novel phosphorylation-dependent regulation in an unstructured protein

This work explores how phosphorylation of an unstructured protein region in inhibitor-2 (I2) regulates protein phosphatase-1 (PP1) enzyme activity using molecular dynamics (MD). Free I2 is largely unstructured; however, when bound to PP1, three segments adopt a stable structure. In particular, an I2 helix (i-helix) blocks the PP1 active site and inhibits phosphatase activity. I2 phosphorylation in the PP1-I2 complex activates phosphatase activity without I2 dissociation. The I2 Thr74 regulatory phosphorylation site is in an unstructured domain in PP1-I2. PP1-I2 MD demonstrated that I2 phosphorylation promotes early steps of PP1-I2 activation in explicit solvent models. Moreover, phosphorylation-dependent activation occurred in PP1-I2 complexes derived from I2 orthologs with diverse sequences from human, yeast, worm, and protozoa. This system allowed exploration of features of the 73-residue unstructured human I2 domain critical for phosphorylation-dependent activation. These studies revealed that components of I2 unstructured domain are strategically positioned for phosphorylation responsiveness including a transient α-helix. There was no evidence that electrostatic interactions of I2 phosphothreonine74 influenced PP1-I2 activation. Instead, phosphorylation altered the conformation of residues around Thr74. Phosphorylation uncurled the distance between I2 residues Glu71 to Tyr76 to promote PP1-I2 activation, whereas reduced distances reduced activation. This I2 residue Glu71 to Tyr76 distance distribution, independently from Thr74 phosphorylation, controls I2 i-helix displacement from the PP1 active site leading to PP1-I2 activation.     

ABA inhibits myristoylation and induces shuttling of the RGLG1 E3 ligase to promote nuclear degradation of PP2CA

Hormone‐ and stress‐induced shuttling of signaling or regulatory proteins is an important cellular mechanism to modulate hormone signaling and cope with abiotic stress. Hormone‐induced ubiquitination plays a crucial role to determine the half‐life of key negative regulators of hormone signaling. For ABA signaling, the degradation of clade‐A PP 2Cs, such as PP 2 CA or ABI 1, is a complementary mechanism to PYR / PYL / RCAR ‐mediated inhibition of PP 2C activity. ABA promotes the degradation of PP 2 CA through the RGLG 1 E3 ligase, although it is not known how ABA enhances the interaction of RGLG 1 with PP 2 CA given that they are predominantly found in the plasma membrane and the nucleus, respectively. We demonstrate that ABA modifies the subcellular localization of RGLG 1 and promotes nuclear interaction with PP 2 CA . We found RGLG 1 is myristoylated in vivo , which facilitates its attachment to the plasma membrane. ABA inhibits the myristoylation of RGLG 1 through the downregulation of N‐myristoyltransferase 1 ( NMT 1 ) and promotes nuclear translocation of RGLG 1 in a cycloheximide‐insensitive manner. Enhanced nuclear recruitment of the E3 ligase was also promoted by increasing PP 2 CA protein levels and the formation of RGLG 1–receptor–phosphatase complexes. We show that RGLG 1 ^Gly2Ala mutated at the N‐terminal myristoylation site shows constitutive nuclear localization and causes an enhanced response to ABA and salt or osmotic stress. RGLG 1/5 can interact with certain monomeric ABA receptors, which facilitates the formation of nuclear complexes such as RGLG 1– PP 2 CA – PYL 8. In summary, we provide evidence that an E3 ligase can dynamically relocalize in response to both ABA and increased levels of its target, which reveals a mechanism to explain how ABA enhances RGLG 1– PP 2 CA interaction and hence PP 2 CA degradation.     

Mutation of a cysteine residue in polyomavirus middle T antigen abolishes interactions with protein phosphatase 2A, pp60c-src, and phosphatidylinositol-3 kinase, activation of c-fos expression, and cellular transformation.

Polyomavirus middle T antigen (MT) interacts with several cellular proteins involved in cell proliferation. MT forms complexes with protein phosphatase 2A (PP2A), pp60c-src (and the related kinases c-fyn and c-yes), and phosphatidylinositol-3 kinase. We made a single point mutation in MT, changing a conserved cysteine residue at position 120 to tryptophan, and characterized the biochemical and biological properties of the mutant (C120W) protein. The mutant MT protein does not associate with PP2A, pp60c-src, or phosphatidylinositol-3 kinase as judged by coimmunoprecipitation and associated phosphatase or kinase activity. The C120W mutant is defective in activation of c-fos expression and in morphological transformation of NIH 3T3 cells.     

Post-translational myristoylation: Fat matters in cellular life and death

Myristoylation corresponds to the irreversible covalent linkage of the 14-carbon saturated fatty acid, myristic acid, to the N-terminal glycine of many eukaryotic and viral proteins. It is catalyzed by N-myristoyltransferase. Typically, the myristate moiety participates in protein subcellular localization by facilitating protein-membrane interactions as well as protein-protein interactions. Myristoylated proteins are crucial components of a wide variety of functions, which include many signalling pathways, oncogenesis or viral replication. Initially, myristoylation was described as a co-translational reaction that occurs after the removal of the initiator methionine residue. However, it is now well established that myristoylation can also occur post-translationally in apoptotic cells. Indeed, during apoptosis hundreds of proteins are cleaved by caspases and in many cases this cleavage exposes an N-terminal glycine within a cryptic myristoylation consensus sequence, which can be myristoylated. The principal objective of this review is to provide an overview on the implication of myristoylation in health and disease with a special emphasis on post-translational myristoylation. In addition, new advancements in the detection and identification of myristoylated proteins are also briefly reviewed.     

Protein phosphatase methyltransferase 1 (Ppm1p) is the sole activity responsible for modification of the major forms of protein phosphatase 2A in yeast.

Protein phosphatase 2A (PP2A) is a major threonine/serine phosphatase that is involved in regulating a variety of cellular processes. It has been shown in both yeast and mammals that the PP2A catalytic subunit (PP2Ac) is methyl-esterified at the conserved C-terminal Leu residue. The recent characterization of a mammalian PP2A carboxyl methyltransferase has led to the identification of two ORFs in Saccharomyces cerevisiae as potential orthologues of the mammalian PP2A methyltransferase: protein phosphatase methyltransferase 1 (PPM1) and protein phosphatase methyltransferase 2 (PPM2). To experimentally identify the PP2A methyltransferase in yeast, we obtained deletion mutants of PPM1 and PPM2 and then constructed double mutants. Using in vivo-labeling techniques, we demonstrate that only the PPM1 gene is required for PP2Ac methylation at the C-terminus. Because yeast has at least three homologues of PP2Ac (PPH21, PPH22, and PPH3), we then asked whether all of these catalytic subunits are methylated by the PPM1 and/or PPM2 putative methyltransferases. We modified the segment corresponding to the N-terminal coding region of all three PP2Ac genomic genes with a hemagglutinin (HA) tag in the parent, ppm1, ppm2, and ppm1ppm2 mutant genetic backgrounds. Using immuoprecipitation with anti-HA antibodies followed by methyl ester analysis, we showed that only in the ppm1 mutant were both Pph21p and Pph22p not methylated. We did not detect any methylesterification of Pph3p under our conditions. Our results indicate that PPM1 is the sole methyltransferase responsible for methylating the two major homologues of PP2Ac in yeast. The function of the PPM2 gene product remains unclear.     

SMG-5, required for C.elegans nonsense-mediated mRNA decay,associates with SMG-2 and protein phosphatase 2A

mRNAs that contain premature stop codons are degraded selectively and rapidly in eukaryotes, a phenomenon termed 'nonsense-mediated mRNA decay' (NMD). We report here molecular analysis of smg-5, which encodes a novel protein required for NMD in Caenorhabditis elegans. Using a combination of immunoprecipitation and yeast two-hybrid assays, we identified a series of protein-protein interactions involving SMG-5. SMG-5 interacts with at least four proteins: (i) SMG-7, a previously identified protein required for NMD; (ii) SMG-2, a phosphorylated protein required for NMD in worms, yeasts and mammals; (iii) PR65, the structural subunit of protein phosphatase 2A (PP2A); and (iv) PP2A(C), the catalytic subunit of PP2A. Previous work demonstrated that both SMG-5 and SMG-7 are required for efficient dephosphorylation of SMG-2. Our results suggest that PP2A is the SMG-2 phosphatase, and the role of SMG-5 is to direct PP2A to its SMG-2 substrate. We discuss cycles of SMG-2 phosphorylation and their roles in NMD.     

Crystal structure of the protein serine/threonine phosphatase 2C at 2.0 A resolution.

Protein phosphatase 2C (PP2C) is a Mn2+- or Mg2+-dependent protein Ser/Thr phosphatase that is essential for regulating cellular stress responses in eukaryotes. The crystal structure of human PP2C reveals a novel protein fold with a catalytic domain composed of a central beta-sandwich that binds two manganese ions, which is surrounded by alpha-helices. Mn2+-bound water molecules at the binuclear metal centre coordinate the phosphate group of the substrate and provide a nucleophile and general acid in the dephosphorylation reaction. Our model presents a framework for understanding not only the classical Mn2+/Mg2+-dependent protein phosphatases but also the sequence-related domains of mitochondrial pyruvate dehydrogenase phosphatase, the Bacillus subtilus phosphatase SpoIIE and a 300-residue domain within yeast adenyl cyclase. The protein architecture and deduced catalytic mechanism are strikingly similar to the PP1, PP2A, PP2B family of protein Ser/Thr phosphatases, with which PP2C shares no sequence similarity, suggestive of convergent evolution of protein Ser/Thr phosphatases.     

Protein Phosphatases 2C Regulate the Activation of the Snf1-Related Kinase OST1 by Abscisic Acid in Arabidopsis

The plant hormone abscisic acid (ABA) orchestrates plant adaptive responses to a variety of stresses, including drought. This signaling pathway is regulated by reversible protein phosphorylation, and genetic evidence demonstrated that several related protein phosphatases 2C (PP2Cs) are negative regulators of this pathway in Arabidopsis thaliana. Here, we developed a protein phosphatase profiling strategy to define the substrate preferences of the HAB1 PP2C implicated in ABA signaling and used these data to screen for putative substrates. Interestingly, this analysis designated the activation loop of the ABA activated kinase OST1, related to Snf1 and AMPK kinases, as a putative HAB1 substrate. We experimentally demonstrated that HAB1 dephosphorylates and deactivates OST1 in vitro. Furthermore, HAB1 and the related PP2Cs ABI1 and ABI2 interact with OST1 in vivo, and mutations in the corresponding genes strongly affect OST1 activation by ABA. Our results provide evidence that PP2Cs are directly implicated in the ABA-dependent activation of OST1 and further suggest that the activation mechanism of AMPK/Snf1-related kinases through the inhibition of regulating PP2Cs is conserved from plants to human.     

Probing protein phosphatase substrate binding: affinity pull-down of ILKAP phosphatase 2C with phosphopeptides

Proteomics and high throughput analysis for systems biology can benefit significantly from solid-phase chemical tools for affinity pull-down of proteins from complex mixtures. Here we report the application of solid-phase synthesis of phosphopeptides for pull-down and analysis of the affinity profile of the integrin-linked kinase associated phosphatase (ILKAP), a member of the protein phosphatase 2C (PP2C) family. Phosphatases can potentially dephosphorylate these phosphopeptide substrates but, interestingly, performing the binding studies at 4 °C allowed efficient binding to phosphopeptides, without the need for phosphopeptide mimics or phosphatase inhibitors. As no proven ILKAP substrates were available, we selected phosphopeptide substrates among known PP2Cδ substrates including the protein kinases: p38, ATM, Chk1, Chk2 and RSK2 and synthesized directly on PEGA solid supports through a BAL type handle. The results show that phosphopeptides tethered to a flexible solid support bind with high affinity and specificity to ILKAP, which is pulled down from lysates of cells transfected with ILKAP cDNA. Phosphorylation on Ser or Thr residues is important for binding of ILKAP, but sequences around the phosphorylated residue are important for the binding affinity of ILKAP. We conclude that solid-phase affinity pull-down of proteins from complex mixtures can be applied in phosphoproteomics and systems biology.     

AtPP2CG1, a protein phosphatase 2C, positively regulates salt tolerance of Arabidopsis in abscisic acid-dependent manner

AtPP2CG1 (Arabidopsis thaliana protein phosphatase 2C G Group 1) was predicted as an abiotic stress candidate gene by bioinformatic analysis in our previous study. The gene encodes a putative protein phosphatase 2C that belongs to Group G of PP2C. There is no report of Group G genes involved in abiotic stress so far. Real-time RT-PCR analysis showed that AtPP2CG1 expression was induced by salt, drought, and abscisic acid (ABA) treatment. The expression levels of AtPP2CG1 in the ABA synthesis-deficient mutant abi2-3 were much lower than that in WT plants under salt stress suggesting that the expression of AtPP2CG1 acts in an ABA-dependent manner. Over-expression of AtPP2CG1 led to enhanced salt tolerance, whereas its loss of function caused decreased salt tolerance. These results indicate that AtPP2CG1 positively regulates salt stress in an ABA-dependent manner. Under salt treatment, AtPP2CG1 up-regulated the expression levels of stress-responsive genes, including RD29A, RD29B, DREB2A and KIN1. GUS activity was detected in roots, leaves, stems, flower, and trichomes of AtPP2CG1 promoter-GUS transgenic plants. AtPP2CG1 protein was localized in nucleus and cytoplasm via AtPP2CG1:eGFP and YFP:AtPP2CG1 fusion approaches.     

Identification of the major protein phosphatases in mammalian cardiac muscle which dephosphorylate phospholamban.

The protein phosphatases which dephosphorylate native, sarcoplasmic reticulum (SR)-associated phospholamban were studied in cardiac muscle extracts and in a Triton fraction prepared by detergent extraction of myofibrils, the latter fraction containing 70-80% of the SR-associated proteins present in the tissue. At physiological concentrations of free Mg2+ (1 mM), protein phosphatase 1 (PP1) accounted for approximately 70% of the total phospholamban phosphatase activity in these fractions towards either Ser-16 (the residue labelled by cAMP-dependent protein kinase, PK-A) or Thr-17 (the residue phosphorylated by an SR-associated Ca2+/calmodulin-dependent protein kinase). Protein phosphatase 2A (PP2A) and protein phosphatase 2C (PP2C) accounted for the remainder of the activity. A major form of cardiac PP1, present in comparable amounts in both the extract and Triton fraction, was similar, if not identical, to skeletal muscle protein phosphatase 1G (PP1G), which is composed of the PP1 catalytic (C) subunit complexed to a G subunit of approximately 160 kDa, responsible for targeting PP1 to both the SR and glycogen particles of skeletal muscle. This conclusion was based on immunoblotting experiments using antibody to the G subunit, ability to bind to glycogen and the release of PP1 activity from glycogen upon incubation with PK-A and MgATP. PP1 accounted for approximately 90% of the phospholamban (Ser-16 or Thr-17) phosphatase activity in the material sedimented by centrifugation at 45,000 x g, a fraction prepared from cardiac extracts which is enriched in SR membranes. The G subunit in this fraction could be solubilised by Triton X-100, but not with 0.5 M NaCl or digestion with alpha-amylase, indicating that it is bound to membranes and not to glycogen. By analogy with the situation in skeletal muscle, the PK-A catalysed phosphorylation of the G subunit, with ensuing release of the C subunit from the SR, may prevent PP1 from dephosphorylating SR-bound substrates and represent one of the mechanisms by which adrenalin increases the phosphorylation of cardiac phospholamban (Ser-16 and Thr-17) in vivo. Hearts left in situ post mortem lose 85-95% of their PP1 activity within 20-30 min. This remarkable disappearance of PP1 may partly explain why the importance of this enzyme in cardiac muscle metabolism has not been recognized previously.