An enzymatic activity in bovine brain that catalyzes the reversal of the C-terminal methyl esterification of protein phosphatase 2A

Plant protein phosphatases: What do we know about their mechanism of action?

Protein phosphorylation is a major reversible post-translational modification. Protein phosphatases function as 'critical regulators' in signaling networks through dephosphorylation of proteins, which have been phosphorylated by protein kinases. A large understanding of their working has been sourced from animal systems rather than the plant or the prokaryotic systems. The eukaryotic protein phosphatases include phosphoprotein phosphatases (PPP), metallo-dependent protein phosphatases (PPM), protein tyrosine (Tyr) phosphatases (PTP), and aspartate (Asp)-dependent phosphatases. The PPP and PPM families are serine(Ser)/threonine(Thr)-specific phosphatases (STPs), while PTP family is Tyr specific. Dual-specificity phosphatases (DsPTPs/DSPs) dephosphorylate Ser, Thr, and Tyr residues. PTPs lack sequence homology with STPs, indicating a difference in catalytic mechanisms, while the PPP and PPM families share a similar structural fold indicating a common catalytic mechanism. The catalytic cysteine (Cys) residue in the conserved HCX₅ R active site motif of the PTPs acts as a nucleophile during hydrolysis. The PPP members require metal ions, which coordinate the phosphate group of the substrate, followed by a nucleophilic attack by a water molecule and hydrolysis. The variable holoenzyme assembly of protein phosphatase(s) and the overlap with other post-translational modifications like acetylation and ubiquitination add to their complexity. Though their functional characterization is extensively reported in plants, the mechanistic nature of their action is still being explored by researchers. In this review, we exclusively overview the plant protein phosphatases with an emphasis on their mechanistic action as well as structural characteristics.

Global loss of leucine carboxyl methyltransferase-1 causes severe defects in fetal liver hematopoiesis

Leucine carboxyl methyltransferase-1 (LCMT-1) methylates the C-terminal leucine α-carboxyl group of the catalytic subunits of the protein phosphatase 2A (PP2A) subfamily of protein phosphatases, PP2Ac, PP4c, and PP6c. LCMT-1 differentially regulates the formation and function of a subset of the heterotrimeric complexes that PP2A and PP4 form with their regulatory subunits. Global LCMT-1 knockout causes embryonic lethality in mice, but LCMT-1 function in development is unknown. In this study, we analyzed the effects of global LCMT-1 loss on embryonic development. LCMT-1 knockout causes loss of PP2Ac methylation, indicating that LCMT-1 is the sole PP2Ac methyltransferase. PP2A heterotrimers containing the Bα and Bδ B-type subunits are dramatically reduced in whole embryos, and the steady-state levels of PP2Ac and the PP2A structural A subunit are also down ∼30%. Strikingly, global loss of LCMT-1 causes severe defects in fetal hematopoiesis and usually death by embryonic day 16.5. Fetal livers of homozygous lcmt-1 knockout embryos display hypocellularity, elevated apoptosis, and greatly reduced numbers of hematopoietic stem and progenitor cell-enriched Kit+Lin-Sca1+ cells. The percent cycling cells and mitotic indices of WT and lcmt-1 knockout fetal liver cells are similar, suggesting that hypocellularity may be due to a combination of apoptosis and/or defects in specification, self-renewal, or survival of stem cells. Indicative of a possible intrinsic defect in stem cells, noncompetitive and competitive transplantation experiments reveal that lcmt-1 loss causes a severe multilineage hematopoietic repopulating defect. Therefore, this study reveals a novel role for LCMT-1 as a key player in fetal liver hematopoiesis.

Leucine Carboxyl Methyltransferase 1 (LCMT-1) Methylates Protein Phosphatase 4 (PP4) and Protein Phosphatase 6 (PP6) and Differentially Regulates the Stable Formation of Different PP4 Holoenzymes

The protein phosphatase 2A (PP2A) subfamily of phosphatases, PP2A, PP4, and PP6, are multifunctional serine/threonine protein phosphatases involved in many cellular processes. Carboxyl methylation of the PP2A catalytic subunit (PP2Ac) C-terminal leucine is regulated by the opposing activities of leucine carboxyl methyltransferase 1 (LCMT-1) and protein phosphatase methylesterase 1 (PME-1) and regulates PP2A holoenzyme formation. The site of methylation on PP2Ac is conserved in the catalytic subunits of PP4 and PP6, and PP4 is also methylated on that site, but the identities of the methyltransferase enzyme for PP4 are not known. Whether PP6 is methylated is also not known. Here we use antibodies specific for the unmethylated phosphatases to show that PP6 is carboxyl-methylated and that LCMT-1 is the major methyltransferase for PP2A, PP4, and PP6 in mouse embryonic fibroblasts (MEFs). Analysis of PP2A and PP4 complexes by blue native polyacrylamide gel electrophoresis (BN-PAGE) indicates that PP4 holoenzyme complexes, like those of PP2A, are differentially regulated by LCMT-1, with the PP4 regulatory subunit 1 (PP4R1)-containing PP4 complex being the most dramatically affected by the LCMT-1 loss. MEFs derived from LCMT-1 knock-out mouse embryos have reduced levels of PP2A B regulatory subunit and PP4R1 relative to control MEFs, indicating that LCMT-1 is important for maintaining normal levels of these subunits. Finally, LCMT-1 homozygous knock-out MEFs exhibited hyperphosphorylation of HDAC3, a reported target of the methylation-dependent PP4R1-PP4c complex. Collectively, our data suggest that LCMT-1 coordinately regulates the carboxyl methylation of PP2A-related phosphatases and, consequently, their holoenzyme assembly and function.

Structural basis of protein phosphatase 2A stable latency

The catalytic subunit of protein phosphatase 2A (PP2Ac) is stabilized in a latent form by α4, a regulatory protein essential for cell survival and biogenesis of all PP2A complexes. Here we report the structure of α4 bound to the N-terminal fragment of PP2Ac. This structure suggests that α4 binding to the full-length PP2Ac requires local unfolding near the active site, which perturbs the scaffold subunit binding site at the opposite surface via allosteric relay. These changes stabilize an inactive conformation of PP2Ac and convert oligomeric PP2A complexes to the α4 complex upon perturbation of the active site. The PP2Ac–α4 interface is essential for cell survival and sterically hinders a PP2A ubiquitination site, important for the stability of cellular PP2Ac. Our results show that α4 is a scavenger chaperone that binds to and stabilizes partially folded PP2Ac for stable latency, and reveal a mechanism by which α4 regulates cell survival, and biogenesis and surveillance of PP2A holoenzymes.

The protein α4 is essential for the formation, stability and activity of protein phosphatase 2A (PP2A) complexes. Here the authors solve the crystal structure of a truncated PP2A bound to α4 and show that α4 binds to a partially folded form of the protein, stabilizing the enzyme in an inactive state.

Methionine inhibits autophagy and promotes growth by inducing the SAM-responsive methylation of PP2A

Autophagy is a process of cellular self-digestion induced by various forms of starvation. Although nitrogen deficit is a common trigger, some yeast cells induce autophagy upon switch from a rich to minimal media without nitrogen starvation. We show that the amino acid methionine is sufficient to inhibit such non-nitrogen-starvation (NNS)-induced autophagy. Methionine boosts synthesis of the methyl donor, S-adenosylmethionine (SAM). SAM inhibits autophagy and promotes growth through the action of the methyltransferase Ppm1p, which modifies the catalytic subunit of PP2A in tune with SAM levels. Methylated PP2A promotes dephosphorylation of Npr2p, a component of a conserved complex that regulates NNS autophagy and other growth-related processes. Thus, methionine and SAM levels represent a critical gauge of amino acid availability that is sensed via the methylation of PP2A to reciprocally regulate cell growth and autophagy.

Phosphoinositide 3-kinase γ inhibits cardiac GSK-3 independently of Akt

Activation of cardiac phosphoinositide 3-kinase α (PI3Kα) by growth factors, such as insulin, or activation of PI3Kγ downstream of heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors stimulates the activity of the kinase Akt, which phosphorylates and inhibits glycogen synthase kinase-3 (GSK-3). We found that PI3Kγ inhibited GSK-3 independently of the insulin-PI3Kα-Akt axis. Although insulin treatment activated Akt in PI3Kγ knockout mice, phosphorylation of GSK-3 was decreased compared to control mice. GSK-3 is activated when dephosphorylated by the protein phosphatase 2A (PP2A), which is activated when methylated by the PP2A methyltransferase PPMT-1. PI3Kγ knockout mice showed increased activity of PPMT-1 and PP2A and enhanced nuclear export of the GSK-3 substrate NFATc3. GSK-3 inhibits cardiac hypertrophy, and the hearts of PI3Kγ knockout mice were smaller compared to those of wild-type mice. Cardiac overexpression of a catalytically inactive PI3Kγ (PI3Kγ(inact)) transgene in PI3Kγ knockout mice reduced the activities of PPMT-1 and PP2A and increased phosphorylation of GSK-3. Furthermore, PI3Kγ knockout mice expressing the PI3Kγ(inact) transgene had larger hearts than wild-type or PI3Kγ knockout mice. Our studies show that a kinase-independent function of PI3Kγ could directly inhibit GSK-3 function by preventing the PP2A-PPMT-1 interaction and that this inhibition of GSK-3 was independent of Akt.

Glycogen synthase kinase-3β regulates leucine-309 demethylation of protein phosphatase-2A via PPMT1 and PME-1

Protein phosphatase-2A (PP2A) activity is significantly suppressed in Alzheimer's disease. We have reported that glycogen synthase kinase-3β (GSK-3β) inhibits PP2A via upregulating the phosphorylation of PP2A catalytic subunit (PP2A(C)). Here we studied the effects of GSK-3β on the inhibitory demethylation of PP2A at leucine-309 (dmL309-PP2A(C)). We found that GSK-3β regulates dmL309-PP2A(C) level by regulating PME-1 and PPMT1. Knockdown of PME-1 or PPMT1 eliminated the effects of GSK-3β on PP2A(C). GSK-3 could negatively regulate PP2A regulatory subunit protein level. We conclude that GSK-3β can inhibit PP2A by increasing the inhibitory L309-demethylation involving upregulation of PME-1 and inhibition of PPMT1.

Protein phosphatases and Alzheimer's disease

Alzheimer's Disease (AD) is characterized by progressive loss of cognitive function, linked to marked neuronal loss. Pathological hallmarks of the disease are the accumulation of the amyloid-β (Aβ) peptide in the form of amyloid plaques and the intracellular formation of neurofibrillary tangles (NFTs). Accumulating evidence supports a key role for protein phosphorylation in both the normal and pathological actions of Aβ as well as the formation of NFTs. NFTs contain hyperphosphorylated forms of the microtubule-binding protein tau, and phosphorylation of tau by several different kinases leads to its aggregation. The protein kinases involved in the generation and/or actions of tau or Aβ are viable drug targets to prevent or alleviate AD pathology. However, it has also been recognized that the protein phosphatases that reverse the actions of these protein kinases are equally important. Here, we review recent advances in our understanding of serine/threonine and tyrosine protein phosphatases in the pathology of AD.

Discovery and optimization of sulfonyl acrylonitriles as selective, covalent inhibitors of protein phosphatase methylesterase-1

The serine hydrolase protein phosphatase methylesterase-1 (PME-1) regulates the methylesterification state of protein phosphatase 2A (PP2A) and has been implicated in cancer and Alzheimer's disease. We recently reported a fluorescence polarization-activity-based protein profiling (fluopol-ABPP) high-throughput screen for PME-1 that uncovered a remarkably potent and selective class of aza-β-lactam (ABL) PME-1 inhibitors. Here, we describe a distinct set of sulfonyl acrylonitrile inhibitors that also emerged from this screen. The optimized compound, 28 (AMZ30), selectively inactivates PME-1 and reduces the demethylated form of PP2A in living cells. Considering that 28 is structurally unrelated to ABL inhibitors of PME-1, these agents, together, provide a valuable set of pharmacological probes to study the role of methylation in regulating PP2A function. We furthermore observed that several serine hydrolases were sensitive to analogues of 28, suggesting that more extensive structural exploration of the sulfonyl acrylonitrile chemotype may result in useful inhibitors for other members of this large enzyme class.

Assembly and structure of protein phosphatase 2A

Protein phosphatase 2A (PP2A) represents a conserved family of important protein serine/threonine phosphatases in species ranging from yeast to human. The PP2A core enzyme comprises a scaffold subunit and a catalytic subunit. The heterotrimeric PP2A holoenzyme consists of the core enzyme and a variable regulatory subunit. The catalytic subunit of PP2A is subject to reversible methylation, mediated by two conserved enzymes. Both the PP2A core and holoenzymes are regulated through interaction with a large number of cellular cofactors. Recent biochemical and structural investigation reveals critical insights into the assembly and function of the PP2A core enzyme as well as two families of holoenzyme. This review focuses on the molecular mechanisms revealed by these latest advances.

Structural mechanism of demethylation and inactivation of protein phosphatase 2A

Protein phosphatase 2A (PP2A) is an important serine/threonine phosphatase that plays a role in many biological processes. Reversible carboxyl methylation of the PP2A catalytic subunit is an essential regulatory mechanism for its function. Demethylation and negative regulation of PP2A is mediated by a PP2A-specific methylesterase PME-1, which is conserved from yeast to humans. However, the underlying mechanism of PME-1 function remains enigmatic. Here we report the crystal structures of PME-1 by itself and in complex with a PP2A heterodimeric core enzyme. The structures reveal that PME-1 directly binds to the active site of PP2A and that this interaction results in the activation of PME-1 by rearranging the catalytic triad into an active conformation. Strikingly, these interactions also lead to inactivation of PP2A by evicting the manganese ions that are required for the phosphatase activity of PP2A. These observations identify a dual role of PME-1 that regulates PP2A activation, methylation, and holoenzyme assembly in cells.

Leucine carboxyl methyltransferase-1 is necessary for normal progression through mitosis in mammalian cells

Protein phosphatase 2A (PP2A) is a multifunctional phosphatase that plays important roles in many cellular processes including regulation of cell cycle and apoptosis. Because PP2A is involved in so many diverse processes, it is highly regulated by both non-covalent and covalent mechanisms that are still being defined. In this study we have investigated the importance of leucine carboxyl methyltransferase-1 (LCMT-1) for PP2A methylation and cell function. We show that reduction of LCMT-1 protein levels by small hairpin RNAs causes up to a 70% reduction in PP2A methylation in HeLa cells, indicating that LCMT-1 is the major mammalian PP2A methyltransferase. In addition, LCMT-1 knockdown reduced the formation of PP2A heterotrimers containing the Balpha regulatory subunit and, in a subset of the cells, induced apoptosis, characterized by caspase activation, nuclear condensation/fragmentation, and membrane blebbing. Knockdown of the PP2A Balpha regulatory subunit induced a similar amount of apoptosis, suggesting that LCMT-1 induces apoptosis in part by disrupting the formation of PP2A(BalphaAC) heterotrimers. Treatment with a pan-caspase inhibitor partially rescued cells from apoptosis induced by LCMT-1 or Balpha knockdown. LCMT-1 knockdown cells and Balpha knockdown cells were more sensitive to the spindle-targeting drug nocodazole, suggesting that LCMT-1 and Balpha are important for spindle checkpoint. Treatment of LCMT-1 and Balpha knockdown cells with thymidine dramatically reduced cell death, presumably by blocking progression through mitosis. Consistent with these results, homozygous gene trap knock-out of LCMT-1 in mice resulted in embryonic lethality. Collectively, our results indicate that LCMT-1 is important for normal progression through mitosis and cell survival and is essential for embryonic development in mice.

Methylation of the C-terminal leucine residue of the PP2A catalytic subunit is unnecessary for the catalytic activity and the binding of regulatory subunit (PR55/B)

Protein phosphatase 2A (PP2A) is composed of structural (A), catalytic (C), and regulatory (B) subunits. The catalytic subunit (PP2A(C)) undergoes reversible carboxyl-methylation and -demethylation at its C-terminal leucine residue (Leu309), catalyzed by PP2A-methyltransferase (PMT) and PP2A methylesterase (PME-1), respectively. In this study, we observed that the activity of PP2A was largely unaffected by the addition of PME-1, and that the regulatory subunit (PR55/B) could bind demethylated PP2A(D). Furthermore, to study the precise effect of Leu309 demethylation on PP2A activity, we generated two His(8)-tagged mutant versions of PP2A(C) containing an alanine residue in place of Leu309, and a deletion of Leu309. Both recombinant mutants exhibited phosphatase activity. In addition, we demonstrated that both mutants could constitute a holoenzyme with the regulatory A and B subunits. Our collective results indicate that methylation of Leu309 of PP2A(C) is unnecessary for the PP2A activity and the binding of PR55/B.

The protein phosphatase 2A phosphatase activator is a novel peptidyl-prolyl cis/trans-isomerase

The protein phosphatase 2A (PP2A) phosphatase activator (PTPA) is an essential protein involved in the regulation of PP2A and the PP2A-like enzymes. In this study we demonstrate that PTPA and its yeast homologues Ypa1 and Ypa2 can induce a conformational change in some model substrates. Using these model substrates in different assays with and without helper proteases, this isomerase activity is similar to the isomerase activity of FKBP12, the human cyclophilin A, and one of its yeast homologs Cpr7 but dissimilar to the isomerase activity of Pin1. However, neither FKBP12 nor Cpr7 can reactivate the inactive form of PP2A. Therefore, PTPA belongs to a novel peptidyl-prolyl cis/trans-isomerase (PPIase) family. The PPIase activity of PTPA correlates with its activating activity since both are stimulated by the presence of Mg2+ATP, and a PTPA mutant (Delta208-213) with 400-fold less activity in the activation reaction of PP2A also showed almost no PPIase activity. The point mutant Asp205 --> Gly (in Ypa1) identified this amino acid as essential for both activities. Moreover, PTPA dissociates the inactive form from the complex with the PP2A methylesterase. Finally, Pro190 in the catalytic subunit of PP2A (PP2AC) could be identified as the target Pro isomerized by PTPA/Mg2+ATP since among the 14 Pro residues present in 12 synthesized peptides representing the microenvironments of these prolines in PP2AC, only Pro190 could be isomerized by PTPA/Mg2+ATP. This Pro190 is present in a predicted loop structure near the catalytic center of PP2AC and, if mutated into a Phe, the phosphatase is inactive and can no longer be activated by PTPA/Mg2+ATP.

Regulation by glucose and calcium of the carboxylmethylation of the catalytic subunit of protein phosphatase 2A in insulin-secreting INS-1 cells

Previously, we reported that the catalytic subunit of protein phosphatase 2A (PP2Ac) undergoes carboxylmethylation (CML) at its COOH-terminal leucine, and that inhibitors of such a posttranslational modification markedly attenuate nutrient-induced insulin secretion from isolated beta-cells. More recent studies have suggested direct inhibitory effects of glucose metabolites on PP2A activity in isolated beta-cells, implying that inhibition of PP2A leads to stimulation of insulin secretion. Because the CML of PP2Ac has been shown to facilitate the holoenzyme assembly and subsequent functional activation of PP2A, we investigated putative regulation by glucose of the CML of PP2Ac in insulin-secreting (INS)-1 cells. Our data indicated a marked inhibition by specific intermediates of glucose metabolism (e.g., citrate and phosphoenolpyruvate) of the CML of PP2Ac in INS-1 cell lysates. Such inhibitory effects were also demonstrable in intact cells by glucose. Mannoheptulose, an inhibitor of glucose metabolism, completely prevented inhibitory effects of glucose on the CML of PP2Ac. Moreover, glucose-mediated inhibition of the CML of PP2Ac was resistant to diazoxide, suggesting that glucose metabolism and the generation of glucose metabolites might control inhibition of the CML of PP2Ac. A membrane-depolarizing concentration of KCl also induced inhibition of the CML of PP2Ac in intact INS cells. On the basis of these data, we propose that glucose metabolism and increase in intracellular calcium facilitate inhibition of the CML of PP2Ac, resulting in functional inactivation of PP2A. This, in turn, might retain the key signaling proteins of the insulin exocytotic cascade in their phosphorylated state, leading to stimulated insulin secretion.

The role of serine/threonine protein phosphatases in exocytosis

Modulation of exocytosis is integral to the regulation of cellular signalling, and a variety of disorders (such as epilepsy, hypertension, diabetes and asthma) are closely associated with pathological modulation of exocytosis. Emerging evidence points to protein phosphatases as key regulators of exocytosis in many cells and, therefore, as potential targets for the design of novel therapies to treat these diseases. Diverse yet exquisite regulatory mechanisms have evolved to direct the specificity of these enzymes in controlling particular cell processes, and functionally driven studies have demonstrated differential regulation of exocytosis by individual protein phosphatases. This Review discusses the evidence for the regulation of exocytosis by protein phosphatases in three major secretory systems, (1) mast cells, in which the regulation of exocytosis of inflammatory mediators plays a major role in the respiratory response to antigens, (2) insulin-secreting cells in which regulation of exocytosis is essential for metabolic control, and (3) neurons, in which regulation of exocytosis is perhaps the most complex and is essential for effective neurotransmission.

Identification and characterization of B"-subunits of protein phosphatase 2 A in Xenopus laevis oocytes and adult tissues

Protein phosphatase 2A is a phosphoserine/threonine phosphatase implicated in many cellular processes. The core enzyme comprises a catalytic and a PR65/A-subunit. The substrate specificity and subcellular localization are determined by a third regulatory B-subunit (PR55/B, PR61/B' and PR72/130/B"). To identify the proteins of the B" family in Xenopus laevis oocytes, a prophase Xenopus oocyte cDNA library was screened using human PR130 cDNA as a probe. Three different classes of cDNAs were isolated. One class is very similar to human PR130 and is probably the Xenopus orthologue of PR130 (XPR130). A second class of clones (XN73) is identical to the N-terminal part of XPR130 but ends a few amino acids downstream of the putative splicing site of PR130. To investigate how this occurs, the genomic structure of the human PR130 gene was determined. This novel protein does not act as a PP2A subunit but might compete with the function of PR130. The third set of clones (XPR70) is very similar to human PR48 but has an N-terminal extension. Further analysis of the human EST-database and the human PR48 gene structure, revealed that the human PR48 clone published is incomplete. The Xenopus orthologue of PR48 encodes a protein of 70 kDa which like the XPR130, interacts with the A-subunit in GST pull-down assays. XPR70 is ubiquitously expressed in adult tissues and oocytes whereas expression of XPR130 is very low in brain and oocytes. Expression of XN73 mainly parallels XPR130 with the exception of the brain.

Protein phosphatase 2A: a highly regulated family of serine/threonine phosphatases implicated in cell growth and signalling

Protein phosphatase 2A (PP2A) comprises a family of serine/threonine phosphatases, minimally containing a well conserved catalytic subunit, the activity of which is highly regulated. Regulation is accomplished mainly by members of a family of regulatory subunits, which determine the substrate specificity, (sub)cellular localization and catalytic activity of the PP2A holoenzymes. Moreover, the catalytic subunit is subject to two types of post-translational modification, phosphorylation and methylation, which are also thought to be important regulatory devices. The regulatory ability of PTPA (PTPase activator), originally identified as a protein stimulating the phosphotyrosine phosphatase activity of PP2A, will also be discussed, alongside the other regulatory inputs. The use of specific PP2A inhibitors and molecular genetics in yeast, Drosophila and mice has revealed roles for PP2A in cell cycle regulation, cell morphology and development. PP2A also plays a prominent role in the regulation of specific signal transduction cascades, as witnessed by its presence in a number of macromolecular signalling modules, where it is often found in association with other phosphatases and kinases. Additionally, PP2A interacts with a substantial number of other cellular and viral proteins, which are PP2A substrates, target PP2A to different subcellular compartments or affect enzyme activity. Finally, the de-regulation of PP2A in some specific pathologies will be touched upon.

Important role for phylogenetically invariant PP2Acalpha active site and C-terminal residues revealed by mutational analysis in Saccharomyces cerevisiae

PP2A is a central regulator of eukaryotic signal transduction. The human catalytic subunit PP2Acalpha functionally replaces the endogenous yeast enzyme, Pph22p, indicating a conservation of function in vivo. Therefore, yeast cells were employed to explore the role of invariant PP2Ac residues. The PP2Acalpha Y127N substitution abolished essential PP2Ac function in vivo and impaired catalysis severely in vitro, consistent with the prediction from structural studies that Tyr-127 mediates substrate binding and its side chain interacts with the key active site residues His-118 and Asp-88. The V159E substitution similarly impaired PP2Acalpha catalysis profoundly and may cause global disruption of the active site. Two conditional mutations in the yeast Pph22p protein, F232S and P240H, were found to cause temperature-sensitive impairment of PP2Ac catalytic function in vitro. Thus, the mitotic and cell lysis defects conferred by these mutations result from a loss of PP2Ac enzyme activity. Substitution of the PP2Acalpha C-terminal Tyr-307 residue by phenylalanine impaired protein function, whereas the Y307D and T304D substitutions abolished essential function in vivo. Nevertheless, Y307D did not reduce PP2Acalpha catalytic activity significantly in vitro, consistent with an important role for the C terminus in mediating essential protein-protein interactions. Our results identify key residues important for PP2Ac function and characterize new reagents for the study of PP2A in vivo.

Methylation increases the open probability of the epithelial sodium channel in A6 epithelia

We used single channel methods on A6 renal cells to study the regulation by methylation reactions of epithelial sodium channels. 3-Deazaadenosine (3-DZA), a methyltransferase blocker, produced a 5-fold decrease in sodium transport and a 6-fold decrease in apical sodium channel activity by decreasing channel open probability (P(o)). 3-Deazaadenosine also blocked the increase in channel open probability associated with addition of aldosterone. Sodium channel activity in excised "inside-out" patches usually decreased within 1-2 min; in the presence of S-adenosyl-l-methionine (AdoMet), activity persisted for 5-8 min. Sodium channel mean time open (t(open)) before and after patch excision was higher in the presence of AdoMet than in untreated excised patches but less than t(open) in cell-attached patches. Sodium channel activity in excised patches exposed to both AdoMet and GTP usually remained stable for more than 10 min, and P(o) and the number of active channels per patch were close to values in cell-attached patches from untreated cells. These findings suggest that a methylation reaction contributes to the activity of epithelial sodium channels in A6 cells and is directed to some regulatory element closely connected with the channel, whose activity also depends on the presence of intracellular GTP.

Effects of serine/threonine protein phosphatases on ion channels in excitable membranes

This review deals with the influence of serine/threonine-specific protein phosphatases on the function of ion channels in the plasma membrane of excitable tissues. Particular focus is given to developments of the past decade. Most of the electrophysiological experiments have been performed with protein phosphatase inhibitors. Therefore, a synopsis is required incorporating issues from biochemistry, pharmacology, and electrophysiology. First, we summarize the structural and biochemical properties of protein phosphatase (types 1, 2A, 2B, 2C, and 3-7) catalytic subunits and their regulatory subunits. Then the available pharmacological tools (protein inhibitors, nonprotein inhibitors, and activators) are introduced. The use of these inhibitors is discussed based on their biochemical selectivity and a number of methodological caveats. The next section reviews the effects of these tools on various classes of ion channels (i.e., voltage-gated Ca(2+) and Na(+) channels, various K(+) channels, ligand-gated channels, and anion channels). We delineate in which cases a direct interaction between a protein phosphatase and a given channel has been proven and where a more complex regulation is likely involved. Finally, we present ideas for future research and possible pathophysiological implications.

Functional expression of human PP2Ac in yeast permits the identification of novel C-terminal and dominant-negative mutant forms

The protein phosphatase 2A (PP2A) holoenzyme is structurally conserved among eukaryotes. This reflects a conservation of function in vivo because the human catalytic subunit (PP2Ac) functionally replaced the endogenous PP2Ac of Saccharomyces cerevisiae and bound the yeast regulatory PR65/A subunit (Tpd3p) forming a dimer. Yeast was employed as a novel system for mutagenesis and functional analysis of human PP2Ac, revealing that the invariant C-terminal leucine residue, a site of regulatory methylation, is apparently dispensable for protein function. However, truncated forms of human PP2Ac lacking larger portions of the C terminus exerted a dominant interfering effect, as did several mutant forms containing a substitution mutation. Computer modeling of PP2Ac structure revealed that interfering amino acid substitutions clustered to the active site, and consistently, the PP2Ac-L199P mutant protein was catalytically impaired despite binding Tpd3p. Thus, interfering forms of PP2Ac titrate regulatory subunits and/or substrates into non-productive complexes and will serve as useful tools for studying PP2A function in mammalian cells. The transgenic approach employed here, involving a simple screen for interfering mutants, may be applicable generally to the analysis of structure-function relationships within protein phosphatases and other conserved proteins and demonstrates further the utility of yeast for analyzing gene function.

A protein phosphatase methylesterase (PME-1) is one of several novel proteins stably associating with two inactive mutants of protein phosphatase 2A

Carboxymethylation of proteins is a highly conserved means of regulation in eukaryotic cells. The protein phosphatase 2A (PP2A) catalytic (C) subunit is reversibly methylated at its carboxyl terminus by specific methyltransferase and methylesterase enzymes which have been purified, but not cloned. Carboxymethylation affects PP2A activity and varies during the cell cycle. Here, we report that substitution of glutamine for either of two putative active site histidines in the PP2A C subunit results in inactivation of PP2A and formation of stable complexes between PP2A and several cellular proteins. One of these cellular proteins, herein named protein phosphatase methylesterase-1 (PME-1), was purified and microsequenced, and its cDNA was cloned. PME-1 is conserved from yeast to human and contains a motif found in lipases having a catalytic triad-activated serine as their active site nucleophile. Bacterially expressed PME-1 demethylated PP2A C subunit in vitro, and okadaic acid, a known inhibitor of the PP2A methylesterase, inhibited this reaction. To our knowledge, PME-1 represents the first mammalian protein methylesterase to be cloned. Several lines of evidence indicate that, although there appears to be a role for C subunit carboxyl-terminal amino acids in PME-1 binding, amino acids other than those at the extreme carboxyl terminus of the C subunit also play an important role in PME-1 binding to a catalytically inactive mutant.

Regulation by GTPgammaS of protein carboxylmethyltransferase activity in kidney brush border membranes

The increase in carboxyl methylation induced by guanosine 5',3-O-(thio)triphosphate (GTPgammaS) in brush border membranes from rat kidney cortex was studied, and the methyltransferase activities affected by this nucleotide analog were identified. Addition of GTPgammaS to brush border membranes stimulated the carboxyl methylation in a time-dependent manner while adenosine and guanine nucleotides were ineffective. The GTPgammaS-dependent carboxyl methylation was inhibited by the chelating agents EDTA (63%) and 1,10-phenanthroline (68%), suggesting that this activity required divalent cations. The methyl ester groups induced by the addition of GTPgammaS to brush border membranes were unstable, with about 80% of them hydrolyzed following 1 h incubation at 37 degrees C. The GTPgammaS stimulation of the carboxyl methylation in brush border membranes was unaffected by the detergent 3-[(3cholamido)-dimethylammonio]-1-propanesulfonic acid up to a concentration of 0.4% (w/v). At this latter detergent concentration, the activity of prenylated protein methyltransferase (PPMT) was strongly inhibited and that of l-isoaspartyl/d-aspartylmethyltransferase (PIMT) was increased twofold, as measured with their respective exogenous substrates, N-acetyl-S-farnesyl cysteine and ovalbumin. GTPgammaS increased the methylation of several substrates in brush border membranes. The induced methylation in substrates migrating between 20 and 36 kDa was strongly decreased by the competitive inhibitor farnesylthioacetic acid, a synthetic farnesylated substrate for PPMT, while a delta-sleep-inducing peptide containing an L-isoaspartyl residue inhibited that of substrates with molecular weights above 36 kDa, suggesting that PIMT activity was also involved. This interpretation was strengthened by the observation that the increased methylation induced by GTPgammaS in these membrane substrates was completely lost following their analysis by gel electrophoresis under alkaline conditions. Taken together, these results indicate that both PPMT and PIMT activities are regulated by guanine nucleotides in brush border membranes of rat kidney.

The possible role of protein phosphatase 2A in the sodium sensitivity of the receptor binding of opiate antagonists naloxone and naltrindole

In striatal membrane preparation used for receptor binding experiments high levels of protein phosphatase 1 and 2A activities were detected using [32P]phosphorylase a as substrate. Sodium chloride decreased the activity of protein phosphatase 2A and increased the activity of protein phosphatase 1 in a concentration-dependent manner. Sodium chloride facilitated the saturation binding of naloxone and naltrindole in rat striatal membrane preparation preincubated with ATP (50 microM) and MgCl2 (5 mM). Preincubation with calyculin A (1 nM) further increased the binding of naloxone. Addition of okadaic acid in a concentration of 2 nM, which is specific for the inhibition of protein phosphatase 2A, augmented the number of binding sites of naloxone or naltrindole. The results suggest a protein phosphatase-dependent regulation of the binding of opiate ligands in the striatum.

Differential inhibition and posttranslational modification of protein phosphatase 1 and 2A in MCF7 cells treated with calyculin-A, okadaic acid, and tautomycin

Calyculin-A (CA), okadaic acid (OA), and tautomycin (TAU) are potent inhibitors of protein phosphatases 1 (PP1) and 2A (PP2A) and are widely used on cells in culture. Despite their well characterized selectivity in vitro, their exact intracellular effects on PP1 and PP2A cannot be directly deduced from their extracellular concentration because their cell permeation properties are not known. Here we demonstrate that, due to the tight binding of the inhibitors to PP1 and/or PP2A, their cell penetration could be monitored by measuring PP1 and PP2A activities in cell-free extracts. Treatment of MCF7 cells with 10 nM CA for 2 h simultaneously inhibited PP1 and PP2A activities by more than 50%. A concentration of 1 microM OA was required to obtain a similar time course of PP2A inhibition in MCF7 cells to that observed with 10 nM CA, whereas PP1 activity was unaffected. PP1 was predominantly inhibited in MCF7 cells treated with TAU but even at 10 microM TAU PP1 inhibition was much slower than that observed with 10 nM CA. Furthermore, binding of inhibitors to PP2Ac and/or PP1c in MCF7 cells led to differential posttranslational modifications of the carboxyl termini of the proteins as demonstrated by Western blotting. OA and CA, in contrast to TAU, induced demethylation of the carboxyl-terminal Leu309 residue of PP2Ac. On the other hand, CA and TAU, in contrast to OA, elicited a marked decrease in immunoreactivity of the carboxyl terminus of the alpha-isoform of PP1c, probably reflecting proteolysis of the protein. These results suggest that in MCF7 cells OA selectively inhibits PP2A and TAU predominantly affects PP1, a conclusion supported by their differential effects on cytokeratins in this cell line.

Detection and characterization of a protein isoaspartyl methyltransferase which becomes trapped in the extracellular space during blood vessel injury

Injury to rat blood vessels in vivo was found to release intracellular pools of protein D-aspartyl/L-isoaspartyl carboxyl methyltransferase (PIMT) into the extracellular milieu, where it becomes trapped. This trapped cohort of PIMT is able to utilize radiolabeled S-adenosyl-L-methionine (AdoMet) introduced into the circulation to methylate blood vessel proteins containing altered aspartyl residues. As further shown in this study, methylated substrates are detected only at the specific site of injury. In vitro studies more fully characterized this endogenous PIMT activity in thoracic aorta and inferior vena cava. Methylation kinetics, immunoblotting, and the lability of methylated substrates at mild alkaline pH were used to demonstrate that both types of blood vessel contain an endogenous protein D-aspartyl/L-isoaspartyl carboxyl methyltransferase (PIMT). At least 50% of the PIMT activity is resistant to nonionic detergent extraction, suggesting that the enzyme activity becomes trapped within or behind the extracellular matrix (ECM). Quantities of lactate dehydrogenase (LDH), another soluble enzyme of presumed intracellular origin, were found to be similarly trapped in the extracellular space of blood vessels.

The mammalian immediate-early TIS21 protein and the leukemia-associated BTG1 protein interact with a protein-arginine N-methyltransferase

The TIS21 immediate-early gene and leukemia-associated BTG1 gene encode proteins with similar sequences. Two-hybrid analysis identified a protein that interacts with TIS21 and BTG1. Sequence motifs associated with S-adenosyl-L-methionine binding suggested this protein might have methyltransferase activity. A glutathione S-transferase (GST) fusion of the putative methyltransferase modifies arginine residues, in appropriate protein substrates, to form NG-monomethyl and NG,NG-dimethylarginine (asymmetric). We term the protein- arginine N-methyltransferase (EC 2.1.1.23) gene "PRMT1, " for protein-arginine methyltransferase 1. GST-TIS21 and GST-BTG1 fusion proteins qualitatively and quantitatively modulate endogenous PRMT1 activity, using control and hypomethylated RAT1 cell extracts as methyl-accepting substrates. PRMT1 message appears ubiquitous, and is constitutive in mitogen-stimulated cells. Modulation of PRMT1 activity by transiently expressed regulatory subunits may be an additional mode of signal transduction following ligand stimulation.

A specific protein carboxyl methylesterase that demethylates phosphoprotein phosphatase 2A in bovine brain

Phosphoprotein phosphatase 2A (PP2A) is one of the four major protein serine/threonine phosphatases found in all eukaryotic cells. We have shown that the 36-kDa catalytic subunit of PP2A is carboxyl methylated in eukaryotic cells, and we have previously identified and purified a novel methyltransferase (MTase) that is responsible for this modification. Here, we describe a novel protein carboxyl methyl-esterase (MEase) from bovine brain that demethylates PP2A. The enzyme has been purified to homogeneity as a monomeric 46-kDa soluble protein. The MEase is highly specific for PP2A. It does not catalyze the demethylation of other protein or peptide methylesters. Moreover, MEase activity is dramatically inhibited by nanomolar concentrations of okadaic acid, a specific inhibitor of PP2A. From these results, we conclude that PP2A methylation is controlled by two specific enzymes, a MTase and a MEase. Since PP2A methylation is highly conserved in eukaryotes ranging from human to yeast, it is likely that this system plays an important role in phosphatase regulation.

Differential methylation and altered conformation of cytoplasmic and nuclear forms of protein phosphatase 2A during cell cycle progression

Protein phosphatase 2A (PP2A) appears to be involved in the regulation of many cellular processes. Control mechanisms that lead to the activation (and deactivation) of the various holoenzymes to initiate appropriate dephosphorylation events remain obscure. The core components of all PP2A holoenzymes are the catalytic (PP2Ac) and 63-65-kD regulatory (PR65) subunits. Monospecific and affinity-purified antibodies against both PP2Ac and PR65 show that these proteins are ubiquitously localized in the cytoplasm and the nucleus in nontransformed fibroblasts. As determined by quantitative immunofluorescence the core subunits of PP2A are twofold more concentrated in the nucleus than in the cytoplasm. Detailed analysis of synchronized cells reveals striking changes in the nuclear to cytoplasmic ratio of PP2Ac-specific immunoreactivity albeit the total amounts of neither PP2Ac nor PR65 in each compartment alters significantly during the cell cycle. Our results imply that differential methylation of PP2Ac occurs at the G0/G1 and G1/S boundaries. Specifically a demethylated form of PP2Ac is found in the cytoplasm of G1 cells, and in the nucleus of S and G2 cells. In addition nuclear PP2A holoenzymes appear to undergo conformational changes at the G0/G1 and G1/S boundaries. During mitosis PP2A is lost from the nuclear compartment, and unlike protein phosphatase 1 shows no specific association with the condensed chromatin.