Serine/threonine protein phosphatases

Protein phosphatase beta, a putative type-2A protein phosphatase from the human malaria parasite Plasmodium falciparum

Protein phosphatases play a critical role in the regulation of the eukaryotic cell cycle and signal transduction. A putative protein serine/threonine phosphatase gene has been isolated from the human malaria parasite Plasmodium falciparum. The gene has an unusual intron that contains four repeats of 32 nucleotides and displays a high degree of size polymorphism among different strains of P. falciparum. The open reading frame reconstituted by removal of the intron encodes a protein of 466 amino acids with a predicted molecular mass of approximately 53.7 kDa. The encoded protein, termed protein phosphatase beta (PP-beta), is composed of two distinct domains. The C-terminal domain comprises 315 amino acids and exhibits a striking similarity to the catalytic subunits of the type-2A protein phosphatases. Database searches revealed that the catalytic domain has the highest similarity to Schizosaccharomyces pombe Ppa1 (58% identity and 73% similarity). However, it contains a hydrophilic insert consisting of five amino acids. The N-terminal domain comprises 151 amino acid residues and exhibits several striking features, including high levels of charged amino acids and asparagine, and multiple consensus phosphorylation sites for a number of protein kinases. An overall structural comparison of PP-beta with other members of the protein phosphatase 2A group revealed that PP-beta is more closely related to Saccharomyces cerevisiae PPH22. Southern blots of genomic DNA digests and chromosomal separations showed that PP-beta is a single-copy gene and is located on chromosome 9. A 2800-nucleotide transcript of this gene is expressed specifically in the sexual erythrocytic stage (gametocytes). The results indicate that PP-beta may be involved in sexual stage development.

B cell receptor-associated protein alpha4 displays rapamycin-sensitive binding directly to the catalytic subunit of protein phosphatase 2A

Recently, TAP42 was isolated as a high copy suppressor of sit4-, a yeast phosphatase related to protein phosphatase 2A (PP2A). TAP42 is related to the murine alpha4 protein, which was discovered independently by its association with Ig-alpha in the B cell receptor complex. Herein we show that a glutathione S-transferase (GST)-alpha4 fusion protein bound the catalytic subunit (C) of human PP2A from monomeric or multimeric preparations of PP2A in a "pull-down" assay. In an overlay assay, the GST-alpha4 protein bound to the phosphorylated and unphosphorylated forms of C that were separated in two-dimensional gels and immobilized on filters. The results show direct and exclusive binding of alpha4 to C. This is unusual because all known regulatory B subunits, or tumor virus antigens, bind stably only to the AC dimer of PP2A. The alpha4-C form of PP2A had an increased activity ratio compared with the AC form of PP2A when myelin basic protein phosphorylated by mitogen-activated protein kinase and phosphorylase a were used as substrates. Recombinant alpha4 cleaved from GST was phosphorylated by p56(lck) tyrosine kinase and protein kinase C. A FLAG-tagged alpha4 expressed in COS7 cells was recovered as a protein containing phosphoserine and coimmunoprecipitated with the C but not the A subunit of PP2A. Treatment of cells with rapamycin prevented the association of PP2A with FLAG-alpha4. The results reveal a novel heterodimer alpha4-C form of PP2A that may be involved in rapamycin-sensitive signaling pathways in mammalian cells.

The binding mode of calyculin A to protein phosphatase-1. A novel spiroketal vector model

The catalytic subunits of serine/threonine protein phosphatases 1 and 2A are subject to inhibition by various toxins such as the microcystins, the nodularins, okadaic acid, tautomycin, and the calyculins. A recent paper (Bagu, J. R., Sykes, B. D, Craig, M. M., and Holmes, C. F. B. (1997) J. Biol. Chem. 272, 5087-5097) reported the successful docking of the crystal structure of calyculin A to the crystal structure of protein phosphatase-1. Unfortunately, the model presented there is based on the structure of the unnatural enantiomer of calyculin A and must therefore be incorrect. We have developed a spiroketal vector model which appears to account for the spatial orientation of the hydrophobic and basic chains extending from the spiroketal-phosphate core of calyculin A. The model also clearly demonstrates why the unnatural enantiomer of calyculin A does not fit properly into the pocket of the active site. Based on our model, we present a possible open binding mode for calyculin A in the enzyme. This open structure is conceptually similar to the predicted binding mode of the peptide inhibitor DARPP-32 to the enzyme; the hydrophobic, metal-binding, and electrostatic interactions are all retained in this model.

Purification of a fatty acid-stimulated protein-serine/threonine phosphatase from bovine brain and its identification as a homolog of protein phosphatase 5

An arachidonic acid-stimulated Ser/Thr phosphatase activity was detected in soluble extracts prepared from rat pituitary clonal GH4C1 cells, rat or bovine brain, and bovine heart. The enzyme activity was purified to homogeneity from bovine brain as a monomer with a Mr of 63,000 and a specific activity of 32 nmol of Pi released per min/mg of protein when assayed in the presence of 10 microM phosphocasein in the absence of lipid. Arachidonic acid stimulated activity 4-14-fold, with half-maximal stimulation at 50-100 microM, when assayed in the presence of a variety of phosphosubstrates including casein, reduced carboxamidomethylated and maleylated lysozyme, myelin basic protein, and histone. Oleic acid, linoleic acid, and palmitoleic acid also stimulated activity; however, saturated fatty acids and alcohol or methyl ester derivatives of fatty acids did not significantly affect activity. The lipid-stimulated phosphatase was identified as the bovine equivalent of protein phosphatase 5 or a closely related homolog by sequence analysis of proteolytic fragments generated from the purified enzyme. When recombinant rat protein phosphatase 5 was expressed as a cleavable glutathione S-transferase fusion protein, the affinity-purified thrombin-cleaved enzyme exhibited a specific activity and sensitivity to arachidonic acid similar to those of the purified bovine brain enzyme. These results suggest that protein phosphatase 5 may be regulated in vivo by a lipid second messenger or another endogenous activator.

Inhibition of the Ser-Thr phosphatases PP1 and PP2A by naturally occurring toxins

The okadaic acid class of naturally occurring toxins is a structurally diverse group of molecules that inhibit the protein phosphatases PP1 and PP2A. Studies providing information about the mode of binding between the toxins and the phosphatases contribute to an overall understanding of the signal transduction pathways in which the phosphatases are involved.

Modulation of the enzymatic properties of protein phosphatase 2A catalytic subunit by the recombinant 65-kDa regulatory subunit PR65alpha

All protein phosphatase 2A (PP2A) holoenzymes contain a 36-kDa catalytic subunit (PP2Ac) and a regulatory subunit of 65 kDa (PR65). We have studied the interaction between PP2Ac and PR65 in an in vitro system, using PP2Ac isolated from rabbit skeletal muscle and recombinant PR65alpha expressed in bacteria or insect cells. Bacterially expressed PR65alpha exhibited identical biochemical properties to the protein expressed and isolated from the baculoviral expression system. The association of recombinant PR65 with PP2Ac was very tight (K(D)app = 85 pM) and led to a suppression of PP2A activity, which was maximal (70-80%) when phosphoproteins were used as substrates. When less-structured or smaller substrates (such as phosphopeptides) were used, this inhibition was only 30%. PR65 stimulated PP2Ac activity when the assays were performed in the presence of polycations. This indicates that the PR65 not only serves the previously predicted structural role as a molecular scaffold, but also allosterically modulates the enzymatic properties of PP2Ac. Furthermore, we identified a site of interaction between PP2Ac and PR65alpha by disruption of a stretch of basic amino acids by introduction of a glutamate at position 416. This produced an almost 100-fold reduced affinity for PP2Ac and indicated that this basic motif is an important determinant for the interaction of PR65 and PP2Ac.

Distinct type-2A protein phosphatases activate HMGCoA reductase and acetyl-CoA carboxylase in liver

Acetyl-CoA carboxylase and HMGCoA reductase are inactivated by the same AMP-activated protein kinase and are activated by type-2A protein phosphatase. To determine whether the same species of protein phosphatase-2A were involved, we studied the interconversion of acetyl-CoA carboxylase and HMGCoA reductase in isolated rat hepatocytes. We show that (i) these enzymes are differently regulated in hepatocytes and (ii) the species of type-2A protein phosphatase involved in their activation are different and can be separated by anion-exchange chromatography.

Specificity of different isoforms of protein phosphatase-2A and protein phosphatase-2C studied using site-directed mutagenesis of HMG-CoA reductase

We have expressed the catalytic domain of Chinese hamster HMG-CoA reductase, and 13 point mutations involving the region around the single phosphorylation site for AMP-activated protein kinase. After phosphorylation, these were used to test the specificity of isoforms of protein phosphatase-2A [bovine PP2A(C) (catalytic subunit) and PP2A1 (ABC heterotrimer)] and protein phosphatase-2C (human alpha; mouse alpha, beta1, beta2, beta3, beta4, beta5). PP2A1 had > 50-fold higher activity for HMG-CoA reductase variants than PP2A(C), but their relative selectivity for different variants was similar. Although the specificities of PP2A and PP2C were distinct, no dramatic differences in selectivity were observed between different PP2C isoforms.

Inhibition of translation by mRNA encoding NIPP-1, a nuclear inhibitor of protein phosphatase-1

Transient transfection of COS-1 cells with an expression vector for NIPP-1, a nuclear subunit of protein phosphatase-1, did not result in an overexpression of NIPP-1 protein, although the levels of mRNA encoding NIPP-1 increased dramatically. Moreover, high concentrations of NIPP-1 mRNA inhibited the translation in reticulocyte lysates of various unrelated mRNAs. This inhibition of translation was caused by the NIPP-1 messenger and not by the translation product, since mutation of the start codon abolished NIPP-1 protein production, but had no influence on the translational inhibition. Analysis of deletion mutants showed that the inhibition was mediated by a 0.5-kb fragment in the 5'-end of the NIPP-1 mRNA. This region, when inserted in the 5'-untranslated region of the beta-galactosidase messenger, inhibited the translation of beta-galactosidase mRNA in COS-1 cells. A predicted highly stable secondary structure deltaG = -239.5 kJ/mol) is present between residues 300 and 500 of NIPP-1 mRNA. The possible importance of this structure in the translational inhibition is discussed.

Novel protein serine/threonine phosphatases: variety is the spice of life

The dephosphorylation of proteins on serine and threonine residues is a major mechanism of cellular regulation. Many novel protein serine/threonine phosphatases in the PPP family have recently been discovered and the insights that have been gained into their different functions are summarised in this review.

Inhibitors of phosphoprotein phosphatases 1 and 2A cause activation of a 53 kDa protein kinase accompanying the apoptotic response of breast cancer cells

Treatment of MCF-7 breast cancer cells with 50 nM okadaic acid triggers an apoptotic response which is accompanied by a 7-fold increase in the activity of a protein kinase with a relative molecular mass of 53 kDa. The activity of the kinase was stimulated by cell treatment with inhibitors of phosphoprotein phosphatase 1 and 2A, but not by stressing conditions. Okadaic acid-induced stimulation of the 53 kDa protein kinase was not abolished by coincubation of cells with cycloheximide. We conclude that stimulation of the 53 kDa protein kinase by inhibitors of phosphoprotein phosphatases involves pre-existing molecular components whose activity depends on the phosphorylation state of serine/threonine residues.

Regulation of calcium-induced exocytosis from gastric chief cells by protein phosphatase-2B (calcineurin)

The molecular mechanisms whereby calcium stimulates secretion are uncertain. In the present study, we used streptolysin O (SLO)-permeabilized chief cells from guinea pig stomach to investigate whether protein phosphatase-2B (calcineurin), a calcium/calmodulin-dependent, serine/threonine phosphatase plays a role in mediating calcium-induced pepsinogen secretion. Preincubation of cells with alpha-naphthylphosphate, a non-specific phosphatase inhibitor, decreased calcium-induced secretion. Likewise, specific inhibitors of protein phosphatase-2B (cyclosporin-A and FK-506) caused a dose-dependent reduction in calcium-induced pepsinogen secretion. Moreover, in intact cells, cyclosporin-A and FK-506 inhibited pepsinogen secretion caused by cholecystokinin, carbamylcholine and A23187, agonists known to increase chief cell cytosolic calcium. Okadaic acid, an inhibitor of protein phosphatase-1 and -2A, had no effect on secretion caused by these agonists. Chief cell calcium-dependent phosphatase activity, measured using radiolabeled casein as substrate, was reduced selectively by inhibitors of protein phosphatase-2B. Endogenous substrates for calcium/calmodulin-dependent phosphatase activity were identified by analyzing chief cell lysates using 2-dimensional gel electrophoresis. Increasing the cytosolic calcium concentration resulted in dephosphorylation of a 55-kDa, acidic cytoskeletal protein. FK-506 inhibited dephosphorylation of this protein. Thus, in permeabilized chief cells, specific inhibitors of protein phosphatase-2B inhibit calcium-induced pepsinogen secretion, calcium/calmodulin-dependent phosphatase activity and calcium-induced dephosphorylation of a 55-kDa, acidic cytoskeletal protein. These results support the hypothesis that protein phosphatase-2B (calcineurin) plays an important role in mediating calcium-induced exocytosis.

Constitutively active protein phosphatase 1alpha causes Rb-dependent G1 arrest in human cancer cells

BACKGROUND

The retinoblastoma protein (Rb) needs to be phosphorylated by cyclin-dependent kinases (CDKs) before mammalian cells can enter the S phase of the cell cycle. As protein phosphatase 1 (PP1) activates Rb and is itself a target for inhibitory phosphorylation by CDKs in vitro, we asked whether any effects of PP1 on cell cycle progression depend on its phosphorylation and are mediated through Rb.

RESULTS

Using electrotransfer of recombinant protein into Rb-positive and Rb-negative cells, we have compared the effects of a wild-type PP1 catalytic subunit, PP1alpha, and a constitutively active mutant of this subunit (PP1alphaT320A) on G1 progression, proliferation rates, and cell viability. In treated cells, PP1alpha levels were elevated 6-16-fold and remained stable for at least 48 hours. In Rb-positive cells, PP1alphaT320A, but not PP1alpha, caused cell cycle arrest in late G1, which was associated with a lack of Rb phosphorylation. In Rb-negative cells, neither wild-type nor mutant phosphatase caused any change in cell cycle progression. Increased cell death was observed in both Rb-positive and Rb-negative cells, however, upon introduction of excess PP1alpha.

CONCLUSIONS

The difference between the effects of wild-type and mutant forms of PP1alpha suggests that PP1alpha has the potential to arrest cell growth in G1 unless it is inactivated by periodic phosphorylation at Thr320, presumably by CDKs that regulate passage through the G1-S cell cycle transition. Together, the effects in both cell types suggest that PP1alpha requires functional Rb to induce growth arrest, and that possibly another pool of PP1alpha induces cell death. This identifies PP1 as a potential target for therapeutic anti-proliferative strategies.

Molecular genetic analysis of Rts1p, a B' regulatory subunit of Saccharomyces cerevisiae protein phosphatase 2A

The Saccharomyces cerevisiae gene RTS1 encodes a protein homologous to a variable B-type regulatory subunit of the mammalian heterotrimeric serine/threonine protein phosphatase 2A (PP2A). We present evidence showing that Rts1p assembles into similar heterotrimeric complexes in yeast. Strains in which RTS1 has been disrupted are temperature sensitive (ts) for growth, are hypersensitive to ethanol, are unable to grow with glycerol as their only carbon source, and accumulate at nonpermissive temperatures predominantly as large-budded cells with a 2N DNA content and a nondivided nucleus. This cell cycle arrest can be overcome and partial suppression of the ts phenotype of rts1-null cells occurs if the gene CLB2, encoding a Cdc28 kinase-associated B-type cyclin, is expressed on a high-copy-number plasmid. However, CLB2 overexpression has no suppressive effects on other aspects of the rts1-null phenotype. Expression of truncated forms of Rts1p can also partially suppress the ts phenotype and can fully suppress the inability of cells to grow on glycerol and the hypersensitivity of cells to ethanol. By contrast, the truncated forms do not suppress the accumulation of large-budded cells at high temperatures. Coexpression of truncated Rts1p and high levels of Clb2p fully suppresses the ts phenotype, indicating that the inhibition of growth of rts1-null cells at high temperatures is due to both stress-related and cell cycle-related defects. Genetic analyses show that the role played by Rts1p in PP2A regulation is distinctly different from that played by the other known variable B regulatory subunit, Cdc55p, a protein recently implicated in checkpoint control regulation.

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.

Acute rise in the concentration of free cytoplasmic calcium leads to dephosphorylation of the microtubule-associated protein tau

The objective of this study was to asses the response of the microtubule-associated protein tau to acute rise in the concentration of free cytoplasmic calcium ([Ca2+]i) in rat cortical neurons and mouse cerebellar granule cells in culture. One-hour exposure to glutamate (100 microM), N-methyl-D-aspartate (100 microM), KCl (50 mM), and ionomycin (5 microM) led to tau protein dephosphorylation as indicated by an appearance of additional faster moving bands on Western immunoblots with a phosphorylation-independent antibody and an increase in the tau-1 immunoreactivity associated with the appearance of an additional faster moving band. Lowering the extracellular concentration of Ca2+ to less than 1 microM fully prevented the drug-induced tau protein dephosphorylation indicating a dependence on Ca2+ influx from the extracellular environment. Administration of okadaic acid (inhibitor of phosphatase 1/2A) simultaneously with the above mentioned drugs decreased the drug-mediated dephosphorylation. Pre-incubation with okadaic acid fully prevented the dephosphorylation. Treatment with cypermethrin (inhibitor of phosphatase 2B) was without effect when administered either alone, simultaneously with the drugs, or pre-incubated. These findings indicate that, independently of the influx pathway, [Ca2+]i elevation leads to dephosphorylation of the microtubule-associated protein tau and implicate phosphatase 1 and/or 2A in the process.

Regulated phosphorylation and dephosphorylation of tau protein: effects on microtubule interaction, intracellular trafficking and neurodegeneration

This review attempts to summarize what is known about tau phosphorylation in the context of both normal cellular function and dysfunction. However, conceptions of tau function continue to evolve, and it is likely that the regulation of tau distribution and metabolism is complex. The roles of microtubule-associated kinases and phosphatases have yet to be fully described, but may afford insight into how tau phosphorylation at the distal end of the axon regulates cytoskeletal-membrane interactions. Finally, lipid and glycosaminoglycan modification of tau structure affords yet more complexity for regulation and aggregation. Continued work will help to determine what is causal and what is coincidental in Alzheimer's disease, and may lead to identification of therapeutic targets for halting the progression of paired helical filament formation.

Structural basis for the recognition of regulatory subunits by the catalytic subunit of protein phosphatase 1

The diverse forms of protein phosphatase 1 in vivo result from the association of its catalytic subunit (PP1c) with different regulatory subunits, one of which is the G-subunit (G(M)) that targets PP1c to glycogen particles in muscle. Here we report the structure, at 3.0 A resolution, of PP1c in complex with a 13 residue peptide (G(M[63-75])) of G(M). The residues in G(M[63-75]) that interact with PP1c are those in the Arg/Lys-Val/Ile-Xaa-Phe motif that is present in almost every other identified mammalian PP1-binding subunit. Disrupting this motif in the G(M[63-75]) peptide and the M(110[1-38]) peptide (which mimics the myofibrillar targeting M110 subunit in stimulating the dephosphorylation of myosin) prevents these peptides from interacting with PP1. A short peptide from the PP1-binding protein p53BP2 that contains the RVXF motif also interacts with PP1c. These findings identify a recognition site on PP1c, invariant from yeast to humans, for a critical structural motif on regulatory subunits. This explains why the binding of PP1 to its regulatory subunits is mutually exclusive, and suggests a novel approach for identifying the functions of PP1-binding proteins whose roles are unknown.

Neuronal nuclear acetyltransferases involved in the synthesis of platelet-activating factor are located in the nuclear envelope and show differential losses in activity

Neuronal nuclear fraction N1 was isolated from cerebral cortices of 15-day-old rabbits, and nuclear subfractions prepared, in order to study the location of nuclear lyso platelet-activating factor (lyso-PAF) acetyltransferase and alkylglycerophosphate (AGP) acetyltransferase, and factors that affect the loss of these two nuclear activities. Subfractionation of prelabelled N1 indicated that the nuclear envelope had the highest percentage of the radioactive acetylated products alkylacetylglycerophosphate (AAGP) and PAF, and the distribution of these phospholipids reflected phospholipid distributions in the nuclear subfractions. The majority (95%) of radioactive AAGP and PAF was also recovered in Triton X-100 extracts of prelabelled nuclei, suggesting that these acetylated lipids are located in nuclear membranes rather than in the nuclear matrix/chromatin. Of the nuclear subfractions, the envelope had the highest AGP and lyso-PAF acetyltransferase specific activities which were close to corresponding values seen in the parent N1 fraction. Thus the nuclear AGP and lyso-PAF acetyltransferases were principally localized to the nuclear membranes. Differentials in activity loss were seen for the two acetyltransferase activities. In the nuclear envelope fractions, the lyso-PAF acetyltransferase was the more susceptible to oxidation reactions which could be reversed or blocked by the use of reducing agents. In preincubations, N1 showed greater losses in lyso-PAF acetyltransferase activity than in AGP acetyltransferase activity, losses which were not attributable to oxidation. Addition of cytosolic fraction S3 to preincubations promoted losses for each acetyltransferase in N1, and gave evidence for cytosolic and endogenous nuclear contributions to the activity loss. Addition of okadaic acid to the preincubations did not prevent the decline of either acetyltransferase in intact nuclei, but did diminish the loss of nuclear lyso-PAF acetyltransferase activity promoted by S3 addition, and also blocked the loss of this acetyltransferase seen in preincubations of isolated nuclear envelopes. This suggests that nuclear lyso-PAF acetyltransferase is susceptible to okadaic acid-sensitive nuclear and cytosolic protein phosphatase activities, while AGP acetyltransferase may lose activity by the action of other phosphatases or by other mechanisms within the nucleus.

Calcineurin regulation of synaptic function: from ion channels to transmitter release and gene transcription

Calcineurin is a calcium (Ca2+)/calmodulin (CaM)-dependent protein phosphatase that has been shown to regulate the activity of ion channels, neurotransmitter and hormone release, synaptic plasticity and gene transcription. At glutamatergic synapses, the inhibition of calcineurin with immunosuppressant drugs has been reported to enhance both the presynaptic release of glutamate and postsynaptic responsiveness. Several other ligand- and voltage-gated ion channels are negatively regulated by calcineurin. Hormone release in insulin-secreting pancreatic beta cells and pituitary corticotrope tumour (AtT20) cells is also negatively regulated by calcineurin. In this article, Jerrel Yakel discusses the evidence that calcineurin plays a vital role in regulating neuronal excitability and hormone release.

Differential expression of three Arabidopsis genes encoding the B' regulatory subunit of protein phosphatase 2A

Numerous plant processes ranging from signal transduction to metabolism appear to be mediated, in part, by type 2A protein serine/threonine phosphatases (PP2A). In an effort to identify factors that control the activity of this enzyme in plants, we have isolated and characterized DNA sequences encoding the B' regulatory subunit of PP2A from Arabidopsis thaliana. Specifically, we used PCR to amplify a segment of Arabidopsis cDNA that encodes a conserved section of the B' polypeptide. This PCR fragment was subsequently used as a probe to screen an Arabidopsis cDNA library and cDNA clones derived from three distinct genes were identified. The AtB' alpha and AtB' beta genes encode highly similar 57-kDa B' regulatory subunits while the third gene, AtB' gamma, encodes a more divergent 59-kDa B' protein. A comparison of the three Arabidopsis B' polypeptides to those of yeast and animals shows the core region of this protein to be the most conserved while the amino and carboxy termini vary both in length and sequence. Genomic Southern blots indicate that at most the Arabidopsis genome contains five genes encoding the B' regulatory subunit. The three genes identified in this study are expressed in all Arabidopsis organs, albeit at varying levels. In addition, mRNAs derived from the three genes accumulate differentially in response to heat shock. Our results indicate that the activity of plant PP2A might be regulated by a B' type regulatory subunit similar to those found in animals and yeast, and suggest possible roles for B'-containing PP2A complexes within plant cells.

Saccharomyces cerevisiae homologs of mammalian B and B' subunits of protein phosphatase 2A direct the enzyme to distinct cellular functions

Protein phosphatase 2A (PP2A) is a major cellular serine/threonine protein phosphatase, present in the cell in a variety of heterotrimeric forms that differ in their associated regulatory B-subunit. Cloning of the mammalian B' subunit has allowed the identification of a highly homologous Saccharomyces cerevisiae gene, RTS1. Disruption of the gene results in a temperature-sensitive growth defect that can be suppressed by expression of rabbit B'alpha or B'gamma isoforms. The B'alpha subunit is much more effective in restoring normal growth at 37 degrees C than B'gamma. Immunoprecipitated Rts1p was found associated with type 2A-specific protein phosphatase activity that is sensitive to 2 nM okadaic acid, but not to 100 nM phosphatase inhibitor-2, and to be phosphorylated in vivo. However, overexpression of RTS1 was unable to suppress the cold sensitivity, defective cytokinesis, and abnormal cell morphology resulting from defects in the CDC55 gene, which encodes the yeast homolog of a different B subunit of another form of 2A phosphatase, PP2A1. These results indicate that Rts1p is a yeast homolog of the mammalian B' subunit and that the various regulatory B-subunits of PP2A are not functionally redundant but direct the enzyme to distinct cellular functions.

Purification and characterization of a smooth muscle myosin light chain kinase-phosphatase complex

We show that a myofibrillar form of smooth muscle myosin light chain phosphatase (MLCPase) forms a multienzyme complex with myosin light chain kinase (MLCKase). The stability of the complex was indicated by the copurification of MLCKase and MLCPase activities through multiple steps that included myofibril preparation, gel filtration chromatography, cation (SP-Sepharose BB) and anion (Q-Sepharose FF) exchange chromatography, and affinity purification on calmodulin and on thiophosphorylated regulatory light chain columns. In addition, the purified complex eluted as a single peak from a final gel filtration column in the presence of calmodulin (CaM). Because a similar MLCPase is present in varying amounts in standard preparations of both MLCKase and myosin filaments, we have named it a kinase- and myosin-associated protein phosphatase (KAMPPase). The KAMPPase multienzyme complex was composed of a 37-kDa catalytic (PC) subunit, a 67-kDa targeting (PT) subunit, and MLCKase with or without CaM. The approximate molar ratio of the PC and PT subunits was 1:2 with a variable and usually higher molar content of MLCKase. The targeting role of the PT subunit was directly demonstrated in binding experiments in which the PT subunit bound to both the kinase and to CaM. Its binding to CaM was, however, Ca2+-independent. MLCKase and the PT subunit potentiated activity of the PC subunit when intact myosin was used as the substrate. These data indicated that there is a Ca2+-independent interaction among the MLCPase, MLCKase, and CaM that are involved in the regulation of phosphatase activity.

The interconversion of protein phosphatase 2A between PP2A1 and PP2A0 during retinoic acid-induced granulocytic differentiation and a modification on the catalytic subunit in S phase of HL-60 cells

Alterations in protein phosphatase 2A (PP2A) during retinoic acid-induced differentiation of HL-60 cells have been investigated. PP2A activity of HL-60 cells for phosphorylated myelin basic protein showed a sharp and transient increase after 18-h treatment with 1 microM retinoic acid, which corresponded to G1/S boundary of the cell cycle. This PP2A of the 18-h treated cells was eluted from a DEAE-Sepharose column with 0.13 M NaCl, while PP2A from control cells was eluted with 0.23 M NaCl. The phosphorylase phosphatase activity of PP2A in the 0.13 M eluate was greatly enhanced in the presence of protamine compared with that of the later eluting PP2A. Immunoblot analyses with antisera against B' and B alpha subunits showed that the PP2A in the 0.13 M NaCl eluate from 18-h retinoic acid-treated cells was PP2A0 (AC-B'), whereas the PP2A eluted with 0.23 M NaCl from 24-h retinoic acid-treated cells and 0-, 18-, and 24-h control cells was PP2A1 (AC-B alpha). These results strongly suggest that PP2A undergoes a transient and reversible interconversion of holoenzyme forms during the initial stage of retinoic acid-induced granulocytic differentiation. PP2A activity assayed after dissociation of the catalytic subunit, for phosphorylase as substrate, showed a sharp and transient decrease in S phase of HL-60 cells irrespective of the presence or absence of retinoic acid. Immunoblot analyses with antisera against C-terminus and N-terminus of the catalytic subunit of PP2A suggested that a modification at the C-terminus is responsible for the decrease in PP2A activity. Immunoreactivity to the C-terminal antibody was restored after treatments of the S-phase extract with alkali or ethanol, the conditions which remove the methyl group from the C-terminus. These results suggest that the C-terminus of PP2A catalytic subunit is transiently methylated in S phase of HL-60 cells.

High molecular weight protein phosphatase type 1 dephosphorylates the retinoblastoma protein

pRb controls cell proliferation by restricting inappropriate entry of cells into the cell division cycle. As dephosphorylation of pRb during mitotic exit activates its growth suppressive function, identification of the protein phosphatase that dephosphorylates pRb, and characterization of the mechanism of its regulation, are essential to elucidating the mechanisms of cell growth control. By fractionating mitotic CV-1P cell extracts, we identify the protein phosphatase which dephosphorylates pRb as a type 1 serine/threonine phosphoprotein phosphatase (PP1). Molecular sizing analyses indicate that the catalytic enzyme (PP1c) is present in a high molecular weight complex, with a predicted molecular mass of 166 kDa. PP1-interacting proteins in the mitotic cell extracts are identified. Two PP1-interacting proteins (41 and 110 kDa) are shown to form distinct complexes with PP1c from fractions of separated mitotic cell extracts containing phosphorylase phosphatase activity. However, only the 110-kDa PP1-interacting protein is present in fractions containing pRb-directed phosphatase activity, identifying this protein as a putative activator of PP1 function toward pRb during mitosis.

Negative modulation of membrane localization of the Raf-1 protein kinase by hyperphosphorylation

The serine/threonine-specific protein kinase Raf-1 plays a key role in mitogenic signal transduction by coupling Ras to the mitogen-activated protein (MAP) kinase cascade. Ras-mediated translocation to the plasma membrane represents a crucial step in the process of serum-stimulated Raf-1 kinase activation. The exact role of the multisite phosphorylation in Raf regulation, however, is not clear. We have previously reported that the mobility shift-associated hyperphosphorylation of Raf correlates with a reduction of serum-stimulated Raf kinase activity (Wartmann, M., and Davis, R. J. (1994) J. Biol. Chem. 269, 6695-6701). Here we show that incubation of serum-starved CHO cells with D609, a purported inhibitor of phosphatidylcholine-specific phospholipase C, also results in a mobility shift of Raf-1 that is due to hyperphosphorylation on sites identical to those observed following mitogen stimulation. Subcellular fractionation analyses revealed that D609-induced mobility shift-associated hyperphosphorylation was paralleled by a decreased membrane association of Raf-1. Similar results were obtained in an in vitro reconstitution system. Furthermore, PD98059, a specific inhibitor of activation of the MAP kinase kinase MEK, prevented D609-induced Raf hyperphosphorylation and restored the amount of membrane-bound Raf to control levels. Taken together, these data suggest that mobility shift-associated hyperphosphorylation of Raf-1, by virtue of reducing the amount of plasma membrane-bound Raf-1, represents a negative feedback mechanism contributing to the desensitization of the MAP kinase signaling cascade.

Cdc55p, the B-type regulatory subunit of protein phosphatase 2A, has multiple functions in mitosis and is required for the kinetochore/spindle checkpoint in Saccharomyces cerevisiae

Saccharomyces cerevisiae, like most eucaryotic cells, can prevent the onset of anaphase until chromosomes are properly aligned on the mitotic spindle. We determined that Cdc55p (regulatory B subunit of protein phosphatase 2A [PP2A]) is required for the kinetochore/spindle checkpoint regulatory pathway in yeast. ctf13 cdc55 double mutants could not maintain a ctf13-induced mitotic delay, as determined by antitubulin staining and levels of histone H1 kinase activity. In addition, cdc55::LEU2 mutants and tpd3::LEU2 mutants (regulatory A subunit of PP2A) were nocodazole sensitive and exhibited the phenotypes of previously identified kinetochore/spindle checkpoint mutants. Inactivating CDC55 did not simply bypass the arrest that results from inhibiting ubiquitin-dependent proteolysis because cdc16-1 cdc55::LEU2 and cdc23-1 cdc55::LEU2 double mutants arrested normally at elevated temperatures. CDC55 is specific for the kinetochore/spindle checkpoint because cdc55 mutants showed normal sensitivity to gamma radiation and hydroxyurea. The conditional lethality and the abnormal cellular morphogenesis of cdc55::LEU2 were suppressed by cdc28F19, suggesting that the cdc55 phenotypes are dependent on the phosphorylation state of Cdc28p. In contrast, the nocodazole sensitivity of cdc55::LEU2 was not suppressed by cdc28F19. Therefore, the mitotic checkpoint activity of CDC55 (and TPD3) is independent of regulated phosphorylation of Cdc28p. Finally, cdc55::LEU2 suppresses the temperature sensitivity of cdc20-1, suggesting additional roles for CDC55 in mitosis.

Combinatorial synthesis and biological evaluation of library of small-molecule Ser/Thr-protein phosphatase inhibitors

In eukaryotes, phosphorylation of serine, threonine, and tyrosine residues on proteins is a fundamental posttranslational regulatory process for such functions as signal transduction, gene transcription, RNA splicing, cellular adhesion, apoptosis, and cell cycle control. Based on functional groups present in natural product serine/threonine protein phosphatase (PSTPase) inhibitors, we have designed pharmacophore model 1 and demonstrated the feasibility of a combinatorial chemistry approach for the preparation of functional analogues of 1. Preliminary biological testing of 18 structural variants of 1 has identified two compounds with growth inhibitory activity against cultured human breast cancer cells. In vitro inhibition of the PSTPase PP2A was demonstrated with compound 1d. Using flow cytometry we observed that compound f1 caused prominent inhibition in the G1 phase of the cell cycle. Thus, the combinatorial modifications of the minimal pharmacophore 1 can generate biologically interesting antiproliterative agents.

Purification of a protein phosphatase from chloroplast stroma capable of dephosphorylating the light-harvesting complex-II

A protein phosphatase was purified from the stroma of Pea (Pisum sativum L.) chloroplasts that is capable of dephosphorylating synthetic phosphopeptides. Following chromatographic purification of greater than 400-fold, two-dimensional electrophoresis indicated that the stromal protein phosphatase is a 29-kD protein. A similar molecular size was determined for the protein-phosphatase activity using gel-permeation chromatography, indicating that the stromal protein phosphatase is probably a monomer. The purified enzyme was able to dephosphorylate synthetic phosphopeptides, which mimic the thylakoid light-harvesting complex II (LHC-II) N terminus, as well as LHC-II in thylakoid membranes, but did not dephosphorylate the major 64-kD phosphoprotein in the stroma. The stromal protein phosphatase did not discriminate between dephosphorylation of phosphothreonine and phosphoserine residues in synthetic peptide substrates, providing further evidence that this enzyme is distinct from the protein phosphatase localized in thylakoid membranes. The exact physiological role of the stromal protein phosphatase has yet to be determined, but it may function in the dephosphorylation of LHC-II.

The catalytic subunit of protein phosphatase 2A associates with the translation termination factor eRF1

By a number of criteria, we have demonstrated that the translation termination factor eRF1 (eukaryotic release factor 1) associates with protein phosphatase 2A (PP2A). Trimeric PP2A1 was purified from rabbit skeletal muscle using an affinity purification step. In addition to the 36 kDa catalytic subunit (PP2Ac) and established regulatory subunits of 65 kDa (PR65) and 55 kDa (PR55), purified preparations contained two proteins with apparent Mrs of 54 and 55 kDa. Protein microsequencing revealed that the 55 kDa component is a novel protein, whereas the 54 kDa protein was identified as eRF1, a protein that functions in translational termination as a polypeptide chain release factor. Using the yeast two-hybrid system, human eRF1 was shown to interact specifically with PP2Ac, but not with the PR65 or PR55 subunits. By deletion analysis, the binding domains were found to be located within the 50 N-terminal amino acids of PP2Ac, and between amino acid residues 338 and 381 in the C-terminal part of human eRF1. This association also occurs in vivo, since PP2A can be co-immunoprecipitated with eRF1 from mammalian cells. We observed a significant increase in the amount of PP2A associated with the polysomes when eRF1 was transiently expressed in COS1 cells, and eRF1 immunoprecipitated from those fractions contained associated PP2A. Since we did not observe any dramatic effects of PP2A on the polypeptide chain release activity of eRF1 (or vice versa), we postulate that eRF1 also functions to recruit PP2A into polysomes, thus bringing the phosphatase into contact with putative targets among the components of the translational apparatus.

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.

Biochemical characterization and deletion analysis of recombinant human protein phosphatase 2C alpha

The use of protein phosphatase inhibitors has been instrumental in defining the intracellular roles of protein phosphatase 1 (PP1), PP2A and PP2B. Identification of the role of PP2C in vivo has been hampered, in part, by the unavailability of specific inhibitors. In order to facilitate the identification of novel and specific inhibitors of PP2C by random screening of compounds, and to further characterize this enzyme at the molecular level by site-directed mutagenesis and X-ray crystallography, we have expressed active recombinant human PP2C alpha (rPP2C alpha) in Escherichia coli. Biochemical characterization of rPP2C alpha showed that it could hydrolyse p-nitrophenyl phosphate (pNPP) although, in contrast with native PP2C, this was not stimulated by Mg2+. As with native PP2C, okadaic acid failed to inhibit rPP2C alpha, whereas 50 mM NaF dramatically inhibited its activity. An alignment of the amino acid sequence of AMP-activated protein kinase (AMPK) with those of other serine/threonine protein kinases around the regulatory phosphorylation site (subdomains VII-VIII) revealed a high degree of conservation. Phosphopeptides derived from this region of AMPK and containing the almost invariant threonine (Thr172 in AMPK) were found to be good substrates for rPP2C alpha. We also showed that rPP2C alpha can inactivate AMPK, but only in the presence of Mg2+. To define the regions of PP2C alpha important for catalytic activity, we expressed a number of truncated proteins based on the sequence and proposed domain structure of the PP2C alpha homologue from Paramecium tetraurelia. Deletion of 75 residues (9 kDa) from the C-terminus appeared to have little effect on the catalytic activity using pNPP, phosphopeptides or AMPK as substrates. This suggests that the residues important in catalysis lie elsewhere in the protein. A further deletion of the C-terminus led to a completely inactive and very poorly soluble protein.

Localization of the calcium/calmodulin-dependent protein phosphatase, calcineurin, in the hindbrain and spinal cord of the rat

The calcium/calmodulin-dependent protein phosphatase calcineurin was localized at the light microscopic level in the rat hindbrain and spinal cord by using an antibody against the alpha-isoform of the catalytic subunit. Calcineurin was highly concentrated in axons, dendrites, and cell bodies of a subpopulation of alpha-motoneurons in hindbrain motor nuclei and the lateral motor column along the length of the spinal cord. These calcineurin-positive alpha-motoneurons appeared to be randomly distributed and represented approximately 25% of the total alpha-motoneuron pool in the motor trigeminal nucleus and the spinal cord lateral motor column. Within the facial nucleus, calcineurin-containing motoneurons were present in the medial and dorsal subdivision but not in the lateral and intermediate subdivision. In addition to the enrichment in motoneurons, calcineurin was enriched in cells of the superficial laminae of the spinal cord dorsal horn and its extension into the medulla, the caudal spinal trigeminal nucleus. Axonal staining in the white matter of the spinal cord was generally weak, except in the dorsolateral funiculus, where strongly calcineurin-positive axons formed a putative ascending tract that appeared to terminate uncrossed in the caudal lateral reticular nucleus of the medulla. This tract may originate from calcineurin-positive cells in the dorsolateral funiculus. We also compared the distribution of calcineurin with calcium/calmodulin-dependent kinase II in the spinal cord and found that the kinase is more widely expressed. Thus, calcineurin is highly restricted to a few locations in the hindbrain and spinal cord. Selective staining in facial subnuclei that innervate phasically active muscles suggests that calcineurin-positive motoneurons represent a subset of alpha-motoneurons innervating a metabolic subtype of muscle fibers, possibly fast-twitch fibers.

Molecular mechanisms of the protein serine/threonine phosphatases

The dephosphorylation of proteins on their serine, threonine and tyrosine residues is catalysed by three families of protein phosphatases that regulate numerous intracellular processes. Diversity of structure within a family is generated by targeting and regulatory subunits and domains. Structural studies of these enzymes have revealed that although the two families of protein Ser/Thr phosphatases are unrelated in sequence, the architecture of their catalytic domains is remarkably similar and distinct from the protein tyrosine phosphatases. Insights into the molecular mechanisms of catalysis and regulation of these enzymes have been obtained.

Partial purification and characterization of a type 1 protein phosphatase in purified nuclei of pea plumules

We report the isolation and characterization of a protein Ser/Thr phosphatase from highly purified pea nuclei. In subnuclear fractions, more than 75% of Ser/Thr protein phosphatase activity was associated with the chromatin fraction, whereas the other 25% was in the nuclear membrane/nucleoplasmic fraction when phosphorylase a was used as a substrate. The enzyme was purified approx. 2750-fold to a specific activity of approx. 4000 nmol/min per mg. The molecular mass of the enzyme was 34 kDa as estimated by molecular sieve chromatography, and approx. 40 kDa as estimated by SDS/PAGE. The phosphatase was inhibited by okadaic acid with an IC50 of approx. 15 nM, by rabbit muscle inhibitor 2 with an IC50 of approx. 10 nM, and by microcystin-LR with an IC50 of approx. 0.05 nM. The enzyme did not require Ca2+, Mg2+ or Mn2+ for its activity; instead, these cations showed some inhibitory effects. It was inhibited by NaF or citrate but not by tartrate, molybdate or vanadate under the conditions tested. Its sensitivities towards the various phosphatase inhibitors and its substrate specificity were very similar to those characteristic of the type I Ser/Thr protein phosphatases well studied in animal systems. The enzyme was able to selectively dephosphorylate a 92 kDa nuclear protein that had been phosphorylated by one or more endogenous protein kinases.

Protein phosphatase type-1 and glycogen bind to a domain in the skeletal muscle regulatory subunit containing conserved hydrophobic sequence motif

This study identifies a 100-residue domain within the rabbit skeletal muscle regulatory subunit (PP1G) that binds both type-1 protein phosphatase (PP1C) and glycogen. An N-terminal portion of PP1G was cloned by RT-PCR, and different sized fragments were expressed in bacteria as glutathione S-transferase (GST) fusion proteins. A GST-PP1G fusion containing residues 51-240 bound both PPIC and glycogen, whereas GST alone or fusions containing residues 51-140 or 241-360 bound neither PP1C nor glycogen. The PPIC in whole cell lysates or partially purified PP1C from skeletal muscle, or a complex of PP1C-MCLR-biotin, all bound more effectively than Mn(2+)-activated, recombinant PP1C purified from bacteria. Binding was enhanced by increasing the ionic strength and was disrupted by ethylene glycol, consistent with hydrophobic interactions being critical for stable association. Phosphorylation of the GST-PP1G fusion by cAMP-dependent protein kinase prevented completely association of PP1C. This domain of PP1G, from residues 141-240, contains two sequence motifs of hydrophobic residues: Gx8FEKx10W and DxFxFxIxL, that are conserved among the known glycogen-binding PP1 regulatory subunits. These segments are predicted to form an alpha helix and a beta sheet, and we propose that they are the sites for association with PP1C and glycogen, respectively.

The NH2-terminal extension of protein phosphatase PPZ1 has an essential functional role

Deletion of the yeast Ser/Thr protein phosphatase PPZ1 results in increased tolerance to sodium and lithium. PPZ1 is also important for cell integrity, as ppz1Delta cells undergo lysis under caffeine stress and PPZ1 overexpression overrides the lytic defect of mutants in the protein kinase C/mitogen-activated protein (MAP) kinase pathway. The PPZ1 protein can be dissected in two halves. The COOH-terminal half is related to type 1 phosphatases, whereas the NH2-terminal half is unrelated to phosphatases and contains a consensus site for N-myristoylation. Several mutated versions of PPZ1 have been constructed and tested for complementation of ppz1Delta mutants. We show that PPZ1 can be myristoylated in vivo and that change of Gly-2 to Ala results in lack of myristoylation and loss of complementation of salt tolerance. Removal of the entire NH2-terminal half results in complete loss of function, although it does not abolish the phosphatase activity of the protein expressed in Escherichia coli. The deletion of a large region of the NH2-terminal half (residues 17-193) does not affect the ability to complement the salt tolerance phenotype but abolish complementation of caffeine sensitivity, whereas the opposite behavior is observed upon removal of residues from 241 to 318. Mutation of Arg-451 to Leu results in both complete loss of function and of phosphatase activity. These results indicates that the NH2-terminal half of the protein contains structural determinants that are specific for certain functions and that the phosphatase activity is required but not sufficient for full PPZ1 function.

Interactions between a minimal protein serine/threonine phosphatase and its phosphopeptide substrate sequence

The protein phosphatase encoded by coliphage lambda (PPlambda) was found to be the equivalent of the minimal catalytic core of serine/threonine protein phosphatases (PP) by biochemical and mutational criteria. Bacterially expressed truncated versions of PP1 and PP5 phosphatases, representing the catalytic cores homologous to PPlambda, exhibited potent phosphatase activity. Unlike full-length PP1, but like PPlambda, the recombinant cores could use casein, p-nitrophenyl phosphate, and a wide variety of peptides as substrates and were resistant to okadaic acid, microcystin-LR, and trypsin. Mutations of His173, Asp208, or Arg221 had little effect on the activity of the PP1 core protein, indicating its closer identity with PPlambda than with full-length PP1. Terminal deletions of a few amino acids of the cores destroyed their activity, supporting their minimal nature. Analysis of PPlambda mutants suggested an influence of the substrate on metal ion binding. The minimal length of a phosphopeptide substrate of PPlambda appeared to be a phosphorylated serine/threonine flanked by 1 or 2 amino acid residues on either side, the N-terminal ones being more effective.

Signal-transducing protein phosphorylation cascades mediated by Ras/Rho proteins in the mammalian cell: the potential for multiplex signalling

The features of three distinct protein phosphorylation cascades in mammalian cells are becoming clear. These signalling pathways link receptor-mediated events at the cell surface or intracellular perturbations such as DNA damage to changes in cytoskeletal structure, vesicle transport and altered transcription factor activity. The best known pathway, the Ras-->Raf-->MEK-->ERK cascade [where ERK is extracellular-signal-regulated kinase and MEK is mitogen-activated protein (MAP) kinase/ERK kinase], is typically stimulated strongly by mitogens and growth factors. The other two pathways, stimulated primarily by assorted cytokines, hormones and various forms of stress, predominantly utilize p21 proteins of the Rho family (Rho, Rac and CDC42), although Ras can also participate. Diagnostic of each pathway is the MAP kinase component, which is phosphorylated by a unique dual-specificity kinase on both tyrosine and threonine in one of three motifs (Thr-Glu-Tyr, Thr-Phe-Tyr or Thr-Gly-Tyr), depending upon the pathway. In addition to activating one or more protein phosphorylation cascades, the initiating stimulus may also mobilize a variety of other signalling molecules (e.g. protein kinase C isoforms, phospholipid kinases, G-protein alpha and beta gamma subunits, phospholipases, intracellular Ca2+). These various signals impact to a greater or lesser extent on multiple downstream effectors. Important concepts are that signal transmission often entails the targeted relocation of specific proteins in the cell, and the reversible formation of protein complexes by means of regulated protein phosphorylation. The signalling circuits may be completed by the phosphorylation of upstream effectors by downstream kinases, resulting in a modulation of the signal. Signalling is terminated and the components returned to the ground state largely by dephosphorylation. There is an indeterminant amount of cross-talk among the pathways, and many of the proteins in the pathways belong to families of closely related proteins. The potential for more than one signal to be conveyed down a pathway simultaneously (multiplex signalling) is discussed. The net effect of a given stimulus on the cell is the result of a complex intracellular integration of the intensity and duration of activation of the individual pathways. The specific outcome depends on the particular signalling molecules expressed by the target cells and on the dynamic balance among the pathways.

The B56 family of protein phosphatase 2A (PP2A) regulatory subunits encodes differentiation-induced phosphoproteins that target PP2A to both nucleus and cytoplasm

Protein phosphatase 2A is a heterotrimeric protein serine/threonine phosphatase consisting of a 36-kDa catalytic C subunit, a 65-kDa structural A subunit, and a variable regulatory B subunit. The B subunits determine the substrate specificity of the enzyme. There have been three families of cellular B subunits identified to date: B55, B56 (B'), and PR72/130. We have now cloned five genes encoding human B56 isoforms. Polypeptides encoded by all but one splice variant (B56gamma1) are phosphoproteins, as shown by mobility shift after treatment with alkaline phosphatase and metabolic labeling with [32P]phosphate. All labeled isoforms contain solely phosphoserine. Indirect immunofluorescence microscopy demonstrates distinct patterns of intracellular targeting by different B56 isoforms. Specifically, B56alpha, B56beta, and B56epsilon complexed with the protein phosphatase 2A A and C subunits localize to the cytoplasm, whereas B56delta, B56gamma1, and B56gamma3 are concentrated in the nucleus. Two isoforms (B56beta and B56delta) are highly expressed in adult brain; here we show that mRNA for these isoforms increases severalfold when neuroblastoma cell lines are induced to differentiate by retinoic acid treatment. These studies demonstrate an increasing diversity of regulatory mechanisms to control the activity of this key intracellular protein phosphatase and suggest distinct functions for isoforms targeted to different intracellular locations.

Calcineurin mediates calcium-induced potentiation of adenylyl cyclase activity in dispersed chief cells from guinea pig stomach. Further evidence for cross-talk between signal transduction pathways that regulate pepsinogen secretion

In cholera toxin-treated gastric chief cells, incubation with a cholinergic agonist (carbamylcholine), a regulatory peptide (cholecystokinin), or a calcium ionophore (A23187) causes a dose- and time-dependent potentiation of cAMP levels. Because this augmented response is calcium/calmodulin-dependent, we hypothesized that it was mediated by calcineurin (protein phosphatase 2B). To test this hypothesis, we examined the actions of calcineurin inhibitors on secretagogue-induced potentiation of cAMP levels in guinea pig chief cells. Preincubation of cells with 0.1 microM FK-506 completely prevented carbachol-induced augmentation of cAMP levels and pepsinogen secretion from cholera toxin-treated cells. Cyclosporin-A, another calcineurin inhibitor, also prevented the augmented cAMP response. FK-506 and cyclosporin inhibited augmentation of cAMP levels following treatment with cholecystokinin(26-33) and A23187, but not the smaller increase in cAMP following treatment with a phorbol ester that activates protein kinase C. Hence, the actions of calcineurin inhibitors were limited to secretagogues that increase cellular calcium. Rapamycin, an agent that competes with FK-506 for the immunophilin, FK binding protein 12, does not inhibit calcineurin. In the present study, preincubation with rapamycin did not prevent carbachol-induced augmentation of cAMP levels in cholera toxin-treated chief cells. However, a molar excess of rapamycin reversed the inhibitory actions of FK-506. These experiments provide further evidence that the actions of FK-506 on cholera toxin-treated gastric chief cells are caused by its inhibitory actions on calcineurin. FK-506 also inhibited potentiation of cAMP levels when carbachol was added to cells that were preincubated with forskolin, an agent that directly activates adenylyl cyclase. We conclude that, in gastric chief cells, calcineurin mediates cross-talk between the calcium/calmodulin and adenylyl cyclase signaling pathways.

Protein phosphatase 2A is essential for the activation of Ca2+-activated K+ currents by cGMP-dependent protein kinase in tracheal smooth muscle and Chinese hamster ovary cells

The regulation of Ca2+-activated K+ channels (KCa channels) by cGMP-dependent protein kinase (cGMP kinase) and its molecular mechanism were investigated in Chinese hamster ovary (CHO) and tracheal smooth muscle cells. In CHO wild-type cells (CHO-WT cells) and in CHO cells stably transfected with cGMP kinase Ialpha (CHO-cGK cells), KCa channels with intermediate conductance (approximately 50 picosiemens) were identified. Due to the basal activity of cGMP kinase, Ca2+-activated K+ currents had a higher sensitivity toward the cytosolic Ca2+ concentration in CHO-cGK cells than in CHO-WT cells. Dialysis of the active fragment of cGMP kinase (300 n) into CHO-WT cells or of cGMP into CHO-cGK cells increased the Ca2+-activated K+ current, while the catalytic subunit of cAMP-dependent protein kinase (cAMP kinase) was without effect. In cell-attached patches obtained from freshly isolated bovine tracheal smooth muscle cells, the open state probability (NPo) of maxi-KCa channels (conductance of approximately 260 picosiemens) was enhanced by 300 microM 8-(4-chlorophenylthio)-cGMP, a specific and potent activator of cGMP kinase. In contrast, 1 microM isoprenaline, 20 microM forskolin, and 3 mM 8-bromo-cAMP failed to enhance KCa channel activity. In excised inside-out patches, only the active fragment of cGMP kinase (but not that of cAMP kinase) increased NPo when applied to the cytosolic side of the patch. The enhancement of NPo by cGMP kinase was inhibited in CHO cells as well as in tracheal smooth muscle cells by the cGMP kinase inhibitor KT 5823 (1 microM) and the protein phosphatase (PP) inhibitors microcystin (5 microM) and okadaic acid (10 nM). The catalytic subunit of PP2A (but not that of PP1) mimicked the effect of cGMP kinase on NPo in excised inside-out patches. The results show that cGMP kinase regulates two different KCa channels in two unrelated cell types by the same indirect mechanism, which requires the activity of PP2A. The regulation of the KCa channel is specific for cGMP kinase and is not mimicked by cAMP kinase.

Characterization of a ribosomal inhibitory polypeptide of protein phosphatase-1 from rat liver

About 4% of the spontaneous phosphorylase phosphatase activity in a rat liver extract was associated with the ribosomal fraction and stemmed from both protein phosphatase-1 (PP-1) and protein phosphatase-2A (PP-2A). However, after repeated washing, only PP-1 remained bound to the ribosomes. The activity of ribosome-associated PP-1 (PP-1R) was partially latent and could be increased 2-3-fold by incubation with trypsin and an additional 50% by incubation with low concentrations of exogenous type-1 catalytic subunit. In contrast, incubation of the ribosomal fraction with MgATP resulted in a 50% drop in the activity of PP-1R. We have purified from a ribosomal extract a basic polypeptide (pI > or = 10.5) of 23 kDa that potently inhibited PP-1. This ribosomal inhibitor of PP-1, termed RIPP-1, was at least 30-times less efficient in inhibiting other major Ser/Thr protein phosphatases (PP-2A, PP-2B and PP-2C). RIPP-1 was identified as a non-competitive inhibitor of PP-1 with a substrate-dependent potency. The lowest Ki (approximately 20 nM) was obtained with phosphorylase and myelin basic protein as substrates. Besides instantaneously inhibiting the type-1 catalytic subunit, RIPP-1 also converted the catalytic subunit in a time-dependent manner (t 1/2 = 45 min at 25 degrees C) into a less active conformation. Unlike the inhibition, this slow inactivation was not reversed by the removal of RIPP-1. We propose that RIPP-1 accounts, at least in part, for the latency of PP-1R.

Dephosphorylation of Sp1 by protein phosphatase 1 is involved in the glucose-mediated activation of the acetyl-CoA carboxylase gene

When mouse 30A5 preadipocytes are exposed to high glucose concentrations, acetyl-CoA carboxylase is induced through glucose activation of promoter II of the acetyl-CoA carboxylase gene. Glucose treatment of the cells increases Sp1 binding to two GC-rich glucose response elements in promoter II. We have investigated the mechanism by which glucose increases Sp1 binding and transactivation of promoter II in 30A5 cells. DNA mobility shift assays have shown that nuclear extracts from glucose-treated cells exhibit increased Sp1 binding activity. This increase in the binding activity is not due to glucose-mediated changes in the amount of Sp1 in the nucleus but to an increase in the activity that modifies Sp1 so that it binds more effectively to the promoter sequence. This Sp1 modifying activity is inhibited by okadaic acid and phosphatase inhibitor 2, and has a molecular mass of 38-42 kDa. The catalytic subunit of type 1 protein phosphatase, whose molecular mass is 38 kDa, also increased the ability of Sp1 to bind to promoter II. Treatment of nuclear extract with antibodies against the catalytic subunit partially suppressed the nuclear activity for Sp1 activation. From these results, we conclude that the Sp1 transcription factor exhibits enhanced binding to promoter II and transcriptional activation is the result of glucose-induced dephosphorylation by type 1 phosphatase.

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.