Purification and characterization of two wheat-embryo protein phosphatases

Wheat germ lipase: isolation, purification and applications

In recent years, wheat germ lipase (WGL) is attracting considerable interest. To date, several WGL applications have already been described: (i) fats and oils modification; (ii) esterification reactions in organic media, accepting a wide range of acids and alcohols as substrates; (iii) the asymmetric resolution of various chiral racemic intermediates; (iv) more recently, the promiscuous activity of WGL has been shown in carbon-carbon bond formation. To date, no crystallographic structure of this enzyme has been published, which means its activity, catalytic potential and substrate scope is being assessed empirically. Therefore, new catalytic activities of this enzyme are constantly being discovered. Taking into account the emergency and the current interest in environmentally sustainable processes, this review aims to highlight the origin, isolation, stabilization by immobilization and applications of the wheat germ lipase.HIGHLIGHTSWheat germ as an inexpensive source of biocatalystsWheat germ lipase an efficient catalyst for various chemical transformationsWheat germ lipase in food productionIndustrial applications of wheat germ lipaseWheat germ lipase as a promiscuous biocatalystImmobilization of wheat germ lipase as a method of stabilization.

Sugar signaling of fructan metabolism: New insights on protein phosphatases in sucrose-fed wheat leaves

Protein phosphatase type 2A (PP2A) activity is required for the sucrose induction of fructan metabolism in wheat leaves, as shown in experiments with the addition of the specific inhibitor okadaic acid (OA) together with sucrose. However, a decrease in total PP2A activity has been found along sucrose treatment. Here we analyze the effect of sucrose feeding to wheat leaves on PP2A activity profiles after Deae-Sephacel and Superose separation, in comparison with those of control leaves. The results show no evidence of changes in PP2A activity profiles as a consequence of sucrose feeding. In all, our data suggest that constitutive levels of PP2A activity may be sufficient for the sucrose-mediated induction of fructan metabolism and that general decrease of PP2A activity produced by long-term treatment with sucrose may be due to a negative feedback regulation.

Sugar signaling of fructan metabolism: New insights on protein phosphatases in sucrose-fed wheat leaves

Protein phosphatase type 2A (PP2A) activity is required for the sucrose induction of fructan metabolism in wheat leaves, as shown in experiments with the addition of the specific inhibitor okadaic acid (OA) together with sucrose. However, a decrease in total PP2A activity has been found along sucrose treatment. Here we analyze the effect of sucrose feeding to wheat leaves on PP2A activity profiles after Deae-Sephacel and Superose separation, in comparison with those of control leaves. The results show no evidence of changes in PP2A activity profiles as a consequence of sucrose feeding. In all, our data suggest that constitutive levels of PP2A activity may be sufficient for the sucrose-mediated induction of fructan metabolism and that general decrease of PP2A activity produced by long-term treatment with sucrose may be due to a negative feedback regulation.

Involvement of protein kinase and extraplastidic serine/threonine protein phosphatases in signaling pathways regulating plastid transcription and the psbD blue light-responsive promoter in barley

We investigated the signaling pathways that control changes in plastid transcription in response to development and light. Plastid gene expression was analyzed in dark-grown barley (Hordeum vulgare L.) seedlings treated in vivo with an inhibitor of protein phosphatases 1 and 2A, okadaic acid (OA), or an inhibitor of protein kinases (K252a), followed by exposure of the seedlings to either red, blue, or white light. OA prevented blue light from activating the plastid pshD blue-light-responsive promoter (BLRP) and prevented red and blue light from activating the expression of the plastid-encoded rbcl and psbA and the nuclear-encoded RbcS and Lhcb genes. OA reduced total plastid transcription activity in dark- and light-grown seedlings by 77 to 80%, indicating that OA prevented light-responsive transcription by reducing total plastid transcription. In contrast, K252a activated the accumulation of mRNAs arising from the BLRP. Blue light in combination with K252a increased psbD mRNA levels in an additive manner. The results indicate that protein phosphatases 1 and/or 2A, which reside external to the organelle, are required for proper function of plastid transcription and chloroplast development, whereas a protein kinase represses the BLRP in plants grown in the dark.

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.

A possible tyrosine phosphorylation of phytochrome

Red/far-red light signal transduction by the phytochrome family of photoreceptors regulates plant growth and development. We investigated the possibility that tyrosine kinases and/or phosphatases are involved in phytochrome-mediated signal transduction using crude extracts of oat seedlings that are grown in the dark. We found that a 124 kDa protein was tyrosine-phosphorylated as determined by Western blotting with a phosphotyrosine-specific monoclonal antibody. The 124 kDa protein was recognized by the anti-phosphotyrosine antibody in anti-phytochrome A immunoprecipitates. The level of anti-phosphotyrosine antibody binding to the 124 kDa protein(s) in phytochrome immunoprecipitates that had been treated with red light prior to immunoprecipitation decreased relative to dark controls. These results suggest that either phytochrome from dark-grown seedlings is tyrosine phosphorylated or that it co-immunoprecipitates with a phosphotyrosine-containing protein of the same molecular weight. The implications of these results in the regulation of (a) the putative Ser/Thr kinase activity of the photoreceptor and (b) the binding of signaling molecules, such as phospholipase C to phytochrome, are discussed.

PLANT PROTEIN PHOSPHATASES

Posttranslational modification of proteins by phosphorylation is a universal mechanism for regulating diverse biological functions. Recognition that many cellular proteins are reversibly phosphorylated in response to external stimuli or intracellular signals has generated an ongoing interest in identifying and characterizing plant protein kinases and protein phosphatases that modulate the phosphorylation status of proteins. This review discusses recent advances in our understanding of the structure, regulation, and function of plant protein phosphatases. Three major classes of enzymes have been reported in plants that are homologues of the mammalian type-1, -2A, and -2C protein serine/threonine phosphatases. Molecular genetic and biochemical studies reveal a role for some of these enzymes in signal transduction, cell cycle progression, and hormonal regulation. Studies also point to the presence of additional phosphatases in plants that are unrelated to these major classes.

Evidence for protein phosphatase 1 and 2A regulation of K+ channels in two types of leaf cells

Ion channels control ion fluxes across membranes, membrane potential, and signal transduction between and within cells. Protein kinases and phosphatases are important regulators involved in stimulus-response coupling in eukaryotic organisms. We have identified in extracts of Vicia faba leaf cells protein phosphatase activities inhibited by okadaic acid (OA) and calyculin A (CA), two inhibitors of protein phosphatases 1 and 2A. Using whole-cell patch-clamp techniques, we have demonstrated that inward K+ currents in guard cells are inhibited by nanomolar concentrations of OA or CA, whereas outward K+ currents are not affected. However, the same inhibitors enhance the magnitude of outward K+ currents in mesophyll cells. A phosphatase antagonist, adenosine-5'-O-(3-thiotriphosphate), has an effect similar to OA and CA on outward K+ currents in mesophyll cells. Our findings suggest that protein phosphatases 1 and/or 2A play different physiological roles in modulating the activity of K+ channels in mesophyll cells and guard cells.

Inhibition of wheat embryo calcium-dependent protein kinase by acridines and azaacridines

The inhibition of wheat Ca(2+)-dependent protein kinase (CDPK) by substituted acridines and substituted 5-, 6-, 7- and 8-azaacridines (5-AA, 6-AA, 7-AA and 8-AA) was examined. Of a total of 71 substituted acridines and azaacridines examined, only 20 have IC50 values for wheat CDPK of less than 200 microM. Of these, effective compounds all have neutral or basic 4-substituents, except for 2,7-dibromo-4-carboxy-5-AAO (IC50 73 microM), the carboxymethyl ester of which is a much better inhibitor (IC50 20 microM). There is a large aza position effect so that various 4-substituted azaacridines can be either very active or very poor inhibitors depending upon the azaacridine nucleus substituted. One of the most potent inhibitors found is the 8-AA 4-N-(2-dimethylaminoethyl)carboxamide (4-P) derivative (IC50 1.5 microM), but the corresponding 4-substituted acridine analogue is a very poor inhibitor. Other potent inhibitors found include 1-nitro-4-P-8-AA (IC50 4 microM) and 7-bromo-4-methyl-5-AA (IC50 0.7 microM). These potent and relatively specific CDPK inhibitors may be useful in obtaining evidence for CDPK involvement in plant cell responses to specific signals.

Protein phosphatase 2A and its [3H]cantharidin/[3H]endothall thioanhydride binding site. Inhibitor specificity of cantharidin and ATP analogues

The target site for cantharidin (CA) and its analogues was isolated recently from mouse liver and identified as protein phosphatase (PP2A) in the heterodimeric form known as PP2A2. The most toxic CA analogue, endothall thioanhydride (ETA) (mouse i.p. LD50 0.3 mg/kg), appears to have the same binding site in mouse liver and brain based on studies comparing [3H]ETA and [3H]CA. ATP and its nonhydrolyzable analogues and pyrophosphate and related compounds including phosphonoformic acid inhibited both [3H]CA and [3H]ETA binding with IC50 values ranging from 2 to 81 microM. As with CA itself, the most potent inhibitors have two negatively charged groups in close proximity to each other. Inhibition of [3H]CA binding by 5,5'-dithiobis(2-nitrobenzoic acid) and stimulation by N-ethylmaleimide indicated the involvement of a thiol site in the CA-binding domain. CA and three analogues (cantharidic acid, palasonin and endothall) inhibited PP2A and protein phosphatase 1 (PP1) but not PP2B or PP2C. The catalytic subunit of PP2A was 5- to 12-fold more sensitive to these CA analogues than the catalytic subunit of PP1. CA and the herbicide endothall also inhibited spinach leaf PP1 and PP2A and, at 50 microM, decreased the PP2A-mediated light-induced activation of nitrate reductase in intact spinach leaves by 62 and 56%, respectively. This is consistent with PP2A as their site of action in plants, and indicates the potential use of CA analogues as pharmacological probes to investigate cellular processes that are regulated by reversible protein phosphorylation in vivo.

Protein phosphatases in higher plants: multiplicity of type 2A phosphatases in Arabidopsis thaliana

Two DNA fragments, AP-1 and AP-2, encoding amino acid sequences closely related to Ser/Thr protein phosphatases were amplified from Arabidopsis thaliana genomic DNA. Fragment AP-1 was used to screen A. thaliana cDNA libraries and several positive clones were isolated. Clones EP8a and EP14a were sequenced and found to encode almost identical proteins (97% identity). Both proteins are 306 amino acids in length and are very similar (79-80% identity) to the mammalian isotypes of the catalytic subunit of protein phosphatase 2A. Therefore, they have been designated PP2A-1 and PP2A-2. A third cDNA clone, EP7, was isolated and sequenced. The polypeptide encoded (308 amino acids, lacking the initial Met codon) is 80% identical with human phosphatases 2A and was named PP2A-3. The PP2A-3 protein is extremely similar (95% identity) to the predicted protein from a cDNA clone previously found in Brassica napus. Southern blot analysis of genomic DNA using AP-1 and AP-2 probes, as well as probes derived from clones EP7, EP8a and EP14a strongly indicates that at least 6 genes closely related to type 2A phosphatases are present in the genome of A. thaliana. Northern blot analysis using the same set of probes demonstrates that, at the seedling stage, the mRNA levels for PP2A-1, PP2A-3 and the gene containing the AP-1 sequence are much higher than those of PP2A-2 and AP-2. These results demonstrate that a multiplicity of type 2A phosphatases might be differentially expressed in higher plants.

Rapid purification and N-terminal sequencing of a potato tuber cyclic nucleotide binding phosphatase

A high affinity cyclic nucleotide binding phosphatase was purified to homogeneity from potato tubers by a rapid procedure involving batchwise elution from carboxymethylcellulose and gel filtration. The phosphatase has a molecular weight of 28,000 as estimated from both SDS-PAGE and gel filtration. The phosphatase binds to Con A-agarose and is eluted by 0.5 M alpha-methylglucoside. The phosphatase catalyses the hydrolysis of nucleoside monophosphates, p-nitrophenylphosphate and O-phospho-L-tyrosine, but not of O-phospho-L-serine or O-phospho-L-threonine. N-terminal sequencing of the phosphatase has revealed significant homology with two similar-size soybean leaf and stem storage glycoproteins.

Isolation and characterization of an alkaline phosphatase from pea thylakoids

Endogenous dephosphorylation of the light-harvesting chlorophyll-protein complex of photosystem II in pea (Pisum sativum, L. cv Progress 9) thylakoids drives the state 2 to state 1 transition; the responsible enzyme is a thylakoid-bound, fluoride-sensitive phosphatase with a pH optimum of 8.0 (Bennett J [1980] Eur J Biochem 104: 85-89). An enzyme with these characteristics was isolated from well-washed thylakoids. Its molecular mass was estimated at 51.5 kD, and this monomer was catalytically active, although the activity was labile. The active site could be labeled with orthophosphate at pH 5.0. High levels of alkaline phosphatase activity were obtained with the assay substrate, 4-methylumbelliferyl phosphate (350 micromoles per minute per milligram purified enzyme). The isolated enzyme functioned as a phosphoprotein phosphatase toward phosphorylated histone III-S and phosphorylated, photosystem II-enriched particles from pea, with typical activities in the range of 200 to 600 picomoles per minute per milligram enzyme. These activities all had a pH optimum of 8.0 and were fluoride sensitive. The enzyme required magnesium ion for maximal activity but was not dependent on this ion. Evidence supporting a putative function for this phosphatase in dephosphorylation of thylakoid proteins came from the inhibition of this process by a polyclonal antibody preparation raised against the partially purified enzyme.

Purification and characterization of wheat and pine small basic protein substrates for plant calcium-dependent protein kinase

A wheat basic protein (WBP) was purified to homogeneity from wheat germ by a protocol involving extraction, centrifugation, batchwise elution from carboxymethylcellulose (CM-52), acidification with trifluoroacetic acid, neutralization and HPLC on a SP5PW cation exchange column. WBP is a 10 kDa protein and is phosphorylated on serine residues by wheat germ Ca(2+)-dependent protein kinase (CDPK). [32P]phosphoWBP exactly comigrates with WBP on SDS-PAGE. WBP does not inhibit either wheat germ CDPK or calmodulin-dependent myosin light chain kinase. Apart from histone H1, WBP is the best endogenous substrate yet found for wheat embryo CDPK. A 12 kDa pine basic protein (PBP) was purified to homogeneity from seeds of stone pine (Pinus pinea L.) by a simple procedure involving batchwise elution from carboxymethylcellulose and cation exchange HPLC. PBP is also a good substrate for CDPK and is phosphorylated on Ser residues. N-terminal sequencing of WBP and PBP revealed that these proteins are homologous to a family of small basic plant proteins having a phospholipid transfer function.

Identification of protein phosphatase activities in maize seedlings

Three phosphatases active on phosphocasein (PhosphoCasein Phosphatases) termed PCP-I, PCP-II and PCP-III were isolated from maize seedlings by DEAE-cellulose chromatography and were shown to display a different specificity toward a variety of phosphorylated substrates including pNPP, phosphohistones, phosphorylase a and several phosphopeptides containing either phosphoserine or phosphothreonine. PCP-I and PCP-II bind to heparin-Sepharose, retain a remarkable pNPP activity, are uncapable to dephosphorylate phosphorylase a, and display striking activity toward the acidic phosphopeptide AS[32P]EEEEE. They also by far prefer phosphoseryl peptide RRAS[32P]VA over its phosphothreonyl derivative and are unsensitive to okadaic acid up to 1 microM. These properties are not consistent with the belonging of PCP-I and -II to any of the known classes of protein phosphatases and suggest that they are acidic phosphatases. Conversely, PCP-III is essentially free of pNPP activity; it readily dephosphorylates phosphohistone H1 and phosphorylase a and it displays a striking preference toward the phosphothreonyl peptides (RRAT[32P]VA and RRREEET[32P]EEEAA), while the phosphoseryl peptides (RRAS[32P]VA and AS[32P]EEEEE) are very poor substrates of the enzyme. These properties together with the findings that PCP-III does not bind to heparin-Sepharose and is highly sensitive to okadaic acid (IC50 = 0.2 nM) allow to identify PCP-III with a protein phosphatase of the PP-2A class.

Isolation and expression of a maize type 1 protein phosphatase

The dephosphorylation of phosphoproteins by protein phosphatases represents an important mechanism for regulating specific cellular processes in eukaryotic cells. The aim of the present study was to examine the structural and biochemical characteristics of a specific class of protein Ser/Thr phosphatases (type 1 protein phosphatases) which have received very little attention in higher plants. A cDNA clone (ZmPP1) was isolated from a maize (Zea mays L.) cDNA library. The deduced amino acid sequence is 80% identical with a 292-amino acid core region of rabbit and yeast type 1 protein phosphatase catalytic subunit. Southern blot analysis indicates that ZmPP1 may belong to a family of related genes in maize. ZmPP1 RNA was present in all maize tissues examined, indicating that it may play a fundamental role in cellular homeostasis. To demonstrate that ZmPP1 encodes an active protein phosphatase and, in an effort to characterize this gene product biochemically, high levels of ZmPP1 were expressed in Escherichia coli. Active ZmPP1 enzyme dephosphorylates rabbit phosphorylase a and is strongly inhibited by okadaic acid and by the mammalian inhibitor-2. These data show that ZmPP1 is structurally and biochemically very similar to the corresponding enzyme in animal cells. These results also suggest that the function and regulation of the higher plant type 1 protein phosphatases may be similar to the mammalian protein phosphatases.

Inactivation of highly activated spinach leaf sucrose-phosphate synthase by dephosphorylation

Spinach (Spinacia oleracea L.) leaf sucrose-phosphate synthase (SPS) can be phosphorylated and inactivated in vitro with [gamma-(32)P]ATP (JLA Huber, SC Huber, TH Nielsen [1989] Arch Biochem Biophys 270: 681-690). Thus, it was surprising to find that SPS, extracted from leaves fed mannose in the light to highly activate the enzyme, could be inactivated in an ATP-independent manner when desalted crude extracts were preincubated at 25 degrees C before assay. The "spontaneous" inactivation involved a loss in activity measured with limiting substrate concentrations in the presence of the inhibitor, Pi, without affecting maximum catalytic activity. The spontaneous inactivation was unaffected by exogenous carrier proteins and protease inhibitors, but was inhibited by inorganic phosphate, fluoride, and molybdate, suggesting that a phosphatase may be involved. Okadaic acid, a potent inhibitor of mammalian type 1 and 2A protein phosphatases, had no effect up to 5 micromolar. Inactivation was stimulated about twofold by exogenous Mg(2+) and was relatively insensitive to Ca(2+) and to pH over the range pH 6.5 to 8.5. Radioactive phosphate incorporated into SPS during labeling of excised leaves with [(32)P]Pi (initially in the dark and then in the light with mannose) was lost with time when desalted crude extracts were incubated at 25 degrees C, and the loss in radiolabel was substantially reduced by fluoride. These results provide direct evidence for action of an endogenous phosphatase(s) using SPS as substrate. We postulate that highly activated SPS contains phosphorylated residue(s) that increase activation state, and that spontaneous inactivation occurs by removal of these phosphate group(s). Inactivation of SPS in vivo caused by feeding uncouplers to darkened leaf tissue that had previously been fed mannose in the dark, may occur by this mechanism. However, there is no evidence that this mechanism is involved in light-dark regulation of SPS in vivo.

Identification of high levels of type 1 and type 2A protein phosphatases in higher plants

Extracts of Brassica napus (oilseed rape) seeds contain type 1 and type 2A protein phosphatases whose properties are indistinguishable from the corresponding enzymes in mammalian tissues. The type 1 activity dephosphorylated the beta-subunit of phosphorylase kinase selectively and was inhibited by the same concentrations of okadaic acid [IC50 (concentration causing 50% inhibition) approximately 10 nM], mammalian inhibitor 1 (IC50 = 0.6 nM) and mammalian inhibitor 2 (IC50 = 2.0 nM) as the rabbit muscle type 1 phosphatase. The plant type 2A activity dephosphorylated the alpha-subunit of phosphorylase kinase preferentially, was exquisitely sensitive to okadaic acid (IC50 approximately 0.1 nM), and was unaffected by inhibitors 1 and 2. As in mammalian tissues, a substantial proportion of plant type 1 phosphatase activity (40%) was particulate, whereas plant type 2A phosphatase was cytosolic. The specific activities of the plant type 1 and type 2A phosphatases were as high as in mammalian tissue extracts, but no type 2B or type 2C phosphatase activity was detected. The results demonstrate that the improved procedure for identifying and quantifying protein phosphatases in animal cells is applicable to higher plants, and suggests that okadaic acid may provide a new method for identifying plant enzymes that are regulated by reversible phosphorylation.