Identification by molecular cloning of two cDNA sequences from the plant Brassica napus which are very similar to mammalian protein phosphatases-1 and -2A

De Novo Mutations in PPP3CA Cause Severe Neurodevelopmental Disease with Seizures

Exome sequencing has readily enabled the discovery of the genetic mutations responsible for a wide range of diseases. This success has been particularly remarkable in the severe epilepsies and other neurodevelopmental diseases for which rare, often de novo, mutations play a significant role in disease risk. Despite significant progress, the high genetic heterogeneity of these disorders often requires large sample sizes to identify a critical mass of individuals with disease-causing mutations in a single gene. By pooling genetic findings across multiple studies, we have identified six individuals with severe developmental delay (6/6), refractory seizures (5/6), and similar dysmorphic features (3/6), each harboring a de novo mutation in PPP3CA. PPP3CA encodes the alpha isoform of a subunit of calcineurin. Calcineurin encodes a calcium- and calmodulin-dependent serine/threonine protein phosphatase that plays a role in a wide range of biological processes, including being a key regulator of synaptic vesicle recycling at nerve terminals. Five individuals with de novo PPP3CA mutations were identified among 4,760 trio probands with neurodevelopmental diseases; this is highly unlikely to occur by chance (p = 1.2 × 10^-8) given the size and mutability of the gene. Additionally, a sixth individual with a de novo mutation in PPP3CA was connected to this study through GeneMatcher. Based on these findings, we securely implicate PPP3CA in early-onset refractory epilepsy and further support the emerging role for synaptic dysregulation in epilepsy.

Importance of a Surface Hydrophobic Pocket on Protein Phosphatase-1 Catalytic Subunit in Recognizing Cellular Regulators

Cellular functions of protein phosphatase-1 (PP1), a major eukaryotic serine/threonine phosphatase, are defined by the association of PP1 catalytic subunits with endogenous protein inhibitors and regulatory subunits. Many PP1 regulators share a consensus RVXF motif, which docks within a hydrophobic pocket on the surface of the PP1 catalytic subunit. Although these regulatory proteins also possess additional PP1-binding sites, mutations of the RVXF sequence established a key role of this PP1-binding sequence in the function of PP1 regulators. WT PP1alpha, the C-terminal truncated PP1alpha-(1-306), a chimeric PP1alpha containing C-terminal sequences from PP2A, another phosphatase, PP1alpha-(1-306) with the RVXF-binding pocket substitutions L289R, M290K, and C291R, and PP2A were analyzed for their regulation by several mammalian proteins. These studies established that modifications of the RVXF-binding pocket had modest effects on the catalytic activity of PP1, as judged by recognition of substrates and sensitivity to toxins. However, the selected modifications impaired the sensitivity of PP1 to the inhibitor proteins, inhibitor-1 and inhibitor-2. In addition, they impaired the ability of PP1 to bind neurabin-I, the neuronal regulatory subunit, and G(M), the skeletal muscle glycogen-targeting subunit. These data suggested that differences in RVXF interactions with the hydrophobic pocket dictate the affinity of PP1 for cellular regulators. Substitution of a distinct RVXF sequence in inhibitor-1 that enhanced its binding and potency as a PP1 inhibitor emphasized the importance of the RVXF sequence in defining the function of this and other PP1 regulators. Our studies suggest that the diversity of RVXF sequences provides for dynamic physiological regulation of PP1 functions in eukaryotic cells.

Crystal Structure and Mutagenesis of a Protein Phosphatase-1:Calcineurin Hybrid Elucidate the Role of the β12-β13 Loop in Inhibitor Binding

Protein phosphatase-1 and protein phosphatase-2B (calcineurin) are eukaryotic serine/threonine phosphatases that share 40% sequence identity in their catalytic subunits. Despite the similarities in sequence, these phosphatases are widely divergent when it comes to inhibition by natural product toxins, such as microcystin-LR and okadaic acid. The most prominent region of non-conserved sequence between these phosphatases corresponds to the beta12-beta13 loop of protein phosphatase-1, and the L7 loop of toxin-resistant calcineurin. In the present study, mutagenesis of residues 273-277 of the beta12-beta13 loop of the protein phosphatase-1 catalytic subunit (PP-1c) to the corresponding residues in calcineurin (312-316), resulted in a chimeric mutant that showed a decrease in sensitivity to microcystin-LR, okadaic acid, and the endogenous PP-1c inhibitor protein inhibitor-2. A crystal structure of the chimeric mutant in complex with okadaic acid was determined to 2.0-A resolution. The beta12-beta13 loop region of the mutant superimposes closely with that of wild-type PP-1c bound to okadaic acid. Systematic mutation of each residue in the beta12-beta13 loop of PP-1c showed that a single amino acid change (C273L) was the most influential in mediating sensitivity of PP-1c to toxins. Taken together, these data indicate that it is an individual amino acid residue substitution and not a change in the overall beta12-beta13 loop conformation of protein phosphatase-1 that contributes to disrupting important interactions with inhibitors such as microcystin-LR and okadaic acid.

Protein phosphatases in plants

Phosphorylation and dephosphorylation of a protein often serve as an "on-and-off" switch in the regulation of cellular activities. Recent studies demonstrate the involvement of protein phosphorylation in almost all signaling pathways in plants. A significant portion of the sequenced Arabidopsis genome encodes protein kinases and protein phosphatases that catalyze reversible phosphorylation. For optimal regulation, kinases and phosphatases must strike a balance in any given cell. Only a very small fraction of the thousands of protein kinases and phosphatases in plants has been studied experimentally. Nevertheless, the available results have demonstrated critical functions for these enzymes in plant growth and development. While serine/threonine phosphorylation is widely accepted as a predominant modification of plant proteins, the function of tyrosine phosphorylation, desptie its overwhelming importance in animal systems, had been largely neglected until recently when tyrosine phosphatases (PTPs) were characterized from plants. This review focuses on the structure, regulation, and function of protein phosphatases in higher plants.

Cloning and expression analyses of cDNA encoding an ADP-ribosylation factor from wheat: tissue-specific expression of wheat ARF

We isolated and characterized a cDNA clone encoding a small GTP-binding protein, which has a high similarity to mammalian ADP-ribosylation factor (ARF), from a cDNA library prepared from immature spikes of wheat (Triticum aestivum L.). The cDNA contained an open reading frame that encodes a polypeptide of 181 amino acids with a calculated molecular mass of 20.7 kDa. The deduced amino acid sequence showed a quite high homology to known ARFs from other organisms. In particular, the wheat ARF was completely identical to the rice ARF1. Genomic Southern hybridization suggested that wheat ARF is encoded by at least two or three copies of ARF genes. Northern analyses showed that the accumulation of the ARF transcripts was nearly constant throughout various environmental stresses in both shoots and roots. However the RNA transcript was preferentially expressed in roots rather than in shoots. A similar expression pattern was also observed at the protein level by Western analysis. The relative abundance of the ARF proteins in root and flower tissues may indicate a high level of vesicular transporting activity in the roots and flowers of wheat plants.

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

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

Characterisation of two protein phosphatase 2A holoenzymes from maize seedlings

Two holoenzymes of protein phosphatase 2A (PP2A), designated PP2AI and PP2AII, were purified from maize seedlings. The subunit composition of maize holoenzymes generally resembled those of animal PP2A. Using SDS/PAGE and Western blots with antibodies generated against peptides derived from animal PP2A, we established the subunit composition of plant protein phosphatase 2A. In both maize holoenzymes, a 38000 catalytic (PP2Ac) and a 66000 constant regulatory subunit (A) constituting the core dimer of PP2A were present. In addition, PP2AI (180000-200000) contained a protein of 57000 which reacted with antibodies generated against the peptide (EFDYLKSLEIEE) conserved in all eukaryotic Balpha regulatory subunits. In contrast, none of the proteins visualised in PP2AII (140000-160000) by double staining reacted with these antibodies. The activity of PP2AI measured with (32)P-labelled phosphorylase a in the presence of protamine and ammonium sulfate is about two times higher than that of PP2AII. PP2AI and PP2AII displayed different patterns of activation by protamine, polylysine and histone H1 and exhibit high sensitivity toward inhibition by okadaic acid. The data obtained provide direct biochemical evidence for the existence in plants of PP2A holoenzymes composed of a catalytic subunit complexed with one or two regulatory subunits.

Molecular cloning and characterization of two phosphatase 2A catalytic subunit genes from Arabidopsis thaliana

The plant Arabidopsis thaliana contains five isoforms of the catalytic subunit of protein phosphatase 2A (PP2A) that can be grouped into two families, one composed by isoforms PP2A-1, -2 and -5 and the other composed by isoforms PP2A-3 and PP2A-4. An Arabidopsis genomic library was screened and several clones corresponding to genes PP2A-3 and PP2A-4 were isolated and analysed. Both genes span over approximately 4.5kbp and are composed of 11 exons and 10 introns that show identical organization. Their untranslated regions are also highly conserved, suggesting that the two genes derive from a common ancestral gene. However, the position of intron/exon junctions completely differs from that of the human PP2A genes. Two transcription start sites have been found in the PP2A-3 gene, the major one mapping at nucleotide position -188 from the translation start codon, whereas only one is observed in PP2A-4 (-145). Functional gene promoter analysis reveals that elements required for transient expression of PP2A-3 and PP2A-4 on a protoplast system are contained within a region of about 600bp upstream from the transcription start sites. This is the first report on the cloning and characterization of genes encoding catalytic subunits of Ser/Thr protein phosphatases 2A in higher plants.

A protein phosphatase-1-binding motif identified by the panning of a random peptide display library

An unusually large number of regulatory or targeting proteins that bind to the catalytic subunit of protein phosphatase-1 have been recently reported. This can be explained by their possession of a common protein motif that interacts with a binding site on protein phosphatase-1. The existence of such a motif was established by the panning of a random peptide library in which peptide sequences are displayed on the Escherichia coli bacterial flagellin protein for bacteria that bound to protein phosphatase-1. There were 79 isolates containing 46 unique sequences with the conserved motif VXF or VXW, where X was most frequently His or Arg. In addition, this sequence was commonly preceded by 2-5 basic residues and followed by 1 acidic residue. This study demonstrates that binding to protein phosphatase-1 can be conferred to a protein by the presentation of a peptide motif on a surface loop. This binding motif is found in a number of protein phosphatase-1-binding proteins.

[Protein phosphatases and protein kinases in higher plants]

The recent gain in knowledge concerning enzymes involved in signal transduction pathways is a direct consequence of the considerable advances made in molecular biology. Protein kinases and protein phosphatases, the two major enzymes implicated in post-translational modifications, have been studied in particular. The number of characterized plant genes and/or cDNAs encoding these enzymes is increasing everyday. Since 1991, 26 genes and cDNAs coding for plant protein phosphatases have been isolated and characterized. The huge number of protein kinases (estimated at several thousands) makes it impossible to give an exhaustive list of the genes already identified, but a classification of these enzymes, based on phylogenetic criteria, allows us to appreciate the range of functions this protein family may play in plants.

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.

Reconstitution of calyculin-inhibited K-Cl cotransport in dog erythrocyte ghosts by exogenous PP-1

Osmotic swelling of dog and other mammalian erythrocytes activates Cl-dependent K transport, K-Cl cotransport. This activation can be abolished by nanomolar concentrations of calyculin, a potent inhibitor of serine-threonine protein phosphatases. Therefore, K-Cl cotransport is probably activated by dephosphorylation by a type 1 and/or type 2A protein phosphatase (PP-1 and PP-2A, respectively). This was tested directly by incorporating exogenous protein phosphatases into resealed ghosts made from dog erythrocytes previously exposed to calyculin. K-Cl cotransport was nearly completely inhibited in the ghosts. Incorporation of PP-1 reconstituted K-Cl cotransport. Maximal reconstitution was up to 90% of the control flux in the ghosts and 0.1 U PP-1/ml lysate gave half-maximal reconstitution of cotransport. In contrast, PP-2A had no effect. This result with PP-1 provides direct evidence that K-Cl cotransport is activated by PP-1 in dog erythrocytes. Half-maximal activation of K-Cl cotransport required approximately 180 molecules of PP-1 per ghost.

Molecular cloning of the cDNA encoding a stress-inducible protein phosphatase 1 (PP1) catalytic subunit from French bean (Phaseolus vulgaris L.)

A cDNA showing high sequence similarity (> 70%) to plant protein phosphatase 1 catalytic subunit variants from other species has been isolated from a cDNA library derived from mRNAs expressed in elicitor-treated suspension-cultured cells. The clone appears to be a near full-length 1431 bp with a 172 bp 5'-untranslated region and a 317 bp 3'-untranslated region. The open reading frame, determined by sequence similarity, codes for a protein with predicted M(r) of 35,552. Alternatively an ATG situated to the 5' end of the putative start site would increase the protein size by 6 amino acids. The mRNA for Pvpp1 was shown to be rapidly induced by elicitor treatment of suspension-cultured cells of French bean. The cloned cDNA represents one of the few examples of a gene product that is probably involved in dephosphorylation events arising after the initial responses to biotic stress.

Classification and expression of a family of cyclin gene homologues in Brassica napus

In order to investigate the role of cell division in plant development, we isolated several plant genes which encode homologues of animal and yeast cell cycle regulators known as cyclins. Through the use of degenerate primers and the polymerase chain reaction (PCR) we isolated a Brassica sequence which showed homology to the 'cyclin box' functional domain found within cyclin proteins. Southern blot analysis indicated that Brassica napus has a large number of genes containing cyclin box-related sequences. This was further supported by the isolation of cyclin box sequences from six different genomic clones. In addition, we have isolated two different cyclin cDNA clones, BnCYC1 and BnCYC2, from a Brassica napus shoot apical cDNA library. Both of the cDNA clones contain a 'destruction box' regulatory domain similar to animal mitotic cyclins. Northern blot analysis using BnCYC2 shows mRNA levels which correlate well with the level of cell division in various tissues. Messenger RNA abundance was highest in 1-3 mm leaves, root tips and shoot apices. The mRNA detected using BnCYC1 was restricted to young leaves and the shoot apex, suggesting divergent, organ-specific roles for cyclin family members. The results demonstrate that the plant cyclin gene family is more extensive than previously demonstrated and consists of genes expressed in all dividing tissues as well as a subset of developmentally specific members.

Characterisation of cDNA and genomic clones encoding homologues of the 65 kDa regulatory subunit of protein phosphatase 2A in Arabidopsis thaliana

Two cDNA species encoding sequences homologous to the 65 kDa regulatory subunit (PR 65) of protein phosphatase 2A (PP2A) have been isolated from an Arabidopsis thaliana cDNA library. These were designated pDF1 and pDF2. pDF1 is 1795 bp long and by comparison with the human and porcine PP2A regulatory subunit sequences represents a full-length clone. It encodes a predicted polypeptide of 587 amino acid residues. pDF2 is truncated at the 5' end by 237 bp. The complete nucleotide sequences have been determined for both cDNA species. Comparison of the nucleotide and the deduced amino acid sequences showed that the two sequences were homologous but not identical and therefore must be derived from two different genes. Northern blot analysis was performed on total RNA and poly(A)+ RNA isolated from seed at various stages of development and from young leaf material of Brassica napus L. (oilseed rape). Both cDNA probes hybridised to a single major mRNA species of ca. 2.2 kb. The highest level of expression was observed in the total RNA from developing rape seed at about 33 days after flowering, and the transcript level in the poly(A)+ RNA of the seed was higher than in young leaf of oilseed rape. Southern blot analysis was performed on two varieties of A. thaliana and B. napus genomic DNA; this identified a small family of genes in A. thaliana consisting of at least 2 or 3 members and a larger multigene family in B. napus of at least 5 or 6 members. Two independent genomic clones were isolated from an A. thaliana genomic library. Sequencing of a fragment common to both revealed that the sequence was identical in both clones and, therefore, they were assumed to contain the same genomic sequence. The genomic sequence selected, designated regA, is 3639 bp long and the coding sequence contains eleven introns. The gene encodes a predicted polypeptide of 590 amino acid residues. The sequence comparison with both cDNA sequences showed that it is homologous but not identical to the two, confirming that at least three different genes exist in A. thaliana which encode PR65 of PP2A.

Isolation and characterization of phosphoprotein phosphatase 1 from alfalfa

Protein phosphatases are central regulatory components of diverse processes in eukaryotes and are among the most highly conserved proteins known. In this paper, we report the cloning and sequencing of a type 1 protein phosphatase (pp1Ms) cDNA from alfalfa. Southern analysis indicates the presence of a gene family of PP1 proteins in alfalfa. The pp1Ms open reading frame is very similar to one of five predicted Arabidopsis type 1 protein phosphatases, indicating that different subtypes are individually conserved. Expression of the alfalfa pp1Ms in a temperature-sensitive Schizosaccharomyces pombe PP1 mutant, dis2-11, revealed no complementation, suggesting that PP1Ms is not involved in mitotic regulation. In different plant organs, different pp1Ms transcript levels were observed; in contrast, mRNA levels remained constant in all phases of the cell cycle and in logarithmically growing cells. However, when cells entered stationary phase pp1Ms transcript levels decreased considerably.

Conservation analysis and structure prediction of the protein serine/threonine phosphatases. Sequence similarity with diadenosine tetraphosphatase from Escherichia coli suggests homology to the protein phosphatases

A multiple sequence alignment of 44 serine/threonine-specific protein phosphatases has been performed. This reveals the position of a common conserved catalytic core, the location of invariant residues, insertions and deletions. The multiple alignment has been used to guide and improve a consensus secondary-structure prediction for the common catalytic core. The location of insertions and deletions has aided in defining the positions of surface loops and turns. The prediction suggests that the core protein phosphatase structure comprises two domains: the first has a single, beta sheet flanked by alpha helices, while the second is predominantly alpha helical. Knowledge of the core secondary structures provides a guide for the design of site-directed-mutagenesis experiments that will not disrupt the native phosphatase fold. A sequence similarity between eukaryotic serine/threonine protein phosphatases and the Escherichia coli diadenosine tetraphosphatase has been identified. This extends over the N-terminal 100 residues of bacteriophage phosphatases and E. coli diadenosine tetraphosphatase. Residues which are invariant amongst these classes are likely to be important in catalysis and protein folding. These include Arg92, Asn138, Asp59, Asp88, Gly58, Gly62, Gly87, Gly93, Gly137, His61, His139 and Val90 and fall into three clusters with the consensus sequences GD(IVTL)HG, GD(LYF)V(DA)RG and GNH, where brackets surround alternative amino acids. The first two consensus sequences are predicted to fall in the beta-alpha and beta-beta loops of a beta-alpha-beta-beta secondary-structure motif. This places the predicted phosphate-binding site at the N-terminus of the alpha helix, where phosphate binding may be stabilised by the alpha-helix dipole.

A homologue of the 65 kDa regulatory subunit of protein phosphatase 2A in early pea (Pisum sativum L.) embryos

A partial cDNA, isolated from an early developing pea (Pisum sativum L.) embryo library, was found to encode a plant homologue of the regulatory subunit (PR65) of protein phosphatase 2A (PP2A). Comparison of the deduced amino acid sequence with a human PR65 sequence showed that the regulatory subunit of PP2A has been highly conserved during evolution. Southern analysis demonstrated that in pea and rape the catalytic and regulatory subunits of PP2A are encoded by multigene families. The levels of the transcripts encoding each subunit are developmentally regulated during pea embryogenesis and expression of the regulatory subunit is not solely restricted to the embryo.

Biochemical characterization of two forms of 3-hydroxy-3-methylglutaryl-CoA reductase kinase from cauliflower (Brassica oleracia)

We recently reported the existence of a protein kinase cascade in higher plants, of which the central component is a 3-hydroxy-3-methylglutaryl(HMG-)-CoA reductase kinase functionally related to mammalian AMP-activated protein kinase [MacKintosh, R. W., Davies, S. P., Clarke P. R., Weekes, J., Gillespie, S. G., Gibb, B. J. & Hardie, D. G. (1992) Eur. J. Biochem. 209, 923-931]. We have now purified this protein kinase 9000-fold from cauliflower inflorescences. During the course of this work we noticed a second minor form (form B) which separated from the major form (A) on ion exchange and gel filtration. Both forms phosphorylate the catalytic fragment of mammalian HMG-CoA reductase. Both forms are markedly inactivated by incubation with the reactive ATP analogue p-fluorosulphonylbenzoyl adenosine (FSO2PhCOAdo), and also by mammalian protein phosphatase 2C, indicating that form B, like form A, is activated by phosphorylation. Form A has an apparent native molecular mass of 200 kDa by gel filtration and, after labelling with [14C]FSO2PhCOAdo, of 150 kDa by electrophoresis in non-denaturing gels. The catalytic subunit was identified as a polypeptide of 58 kDa after labelling with [14C]FSO2PhCOAdo. Form B has an apparent native molecular mass of 45 kDa by gel filtration, and was identified as a polypeptide of 45 kDa after labelling with [14C]FSO2PhCOAdo and [gamma-32P]ATP. Using a series of variants of the synthetic peptide substrate, the substrate specificities of the two forms are similar but not identical. Form B does not appear to be a proteolytic fragment of form A, and we therefore propose that it represents a closely related member of the same protein kinase sub-family.

Identification and molecular cloning of two homologues of protein phosphatase X from Arabidopsis thaliana

In a recent paper [Ariño et al., Plant Mol Biol 21: 475-485 (1993)] we reported the amplification of a DNA fragment (AP-2) from the genome of Arabidopsis thaliana encoding an amino acid sequence corresponding to a Ser/Thr protein phosphatase distantly related to type 2A protein phosphatases. In this paper we report the use of the AP-2 fragment to isolate several cDNA clones from a leaf cDNA library. Two of these (EP124 and EP129) largely overlap and contain the AP-2 sequence, whereas a third clone (EP128) is different although very related in sequence (86% of identity). Clones EP124/EP129 and EP128 were found to encode two highly related polypeptides (93% identity) of 305 residues, showing a very high identity (83%) to the catalytic subunit of protein phosphatase X (PPX) from rabbit. Therefore, they have been named PPX-1 (EP124/EP129) and PPX-2 (EP128). Southern blot analysis of genomic DNA indicates that only these two genes encoding phosphatases closely related to PPX are present in the genome of A. thaliana. Both PPX-1 and PPX-2 are expressed at very low levels in A. thaliana flowers, leaves, stems and roots. The expression levels of four previously identified type 2A phosphatases are higher than those of PPX genes. PP2A-1 appears to be the major mRNA species detected in all the tissues analyzed.

Sucrose-phosphate synthase phosphatase, a type 2A protein phosphatase, changes its sensitivity towards inhibition by inorganic phosphate in spinach leaves

The activity of a type 2A protein phosphatase from spinach leaves was monitored using phosphorylated sucrose-phosphate synthase (SPS) as a substrate. After partial purification the overall activities of sucrose-phosphate synthase phosphatase (SPS-P) recovered from leaves harvested in the dark and in the light did not vary. However, SPS-P preparations from darkened leaves were more strongly inhibited by inorganic phosphate and certain phosphorylated compounds than preparations from illuminated or mannose fed leaves. We conclude, that activation of SPS involves an interconversion of multiple forms of SPS-P activity.

Isolation and characterization of a phosphoprotein phosphatase type 2A gene from alfalfa

Phosphoprotein phosphatases are central regulatory components of the cell cycle in eukaryotes. We report the cloning and sequencing of an alfalfa phosphoprotein phosphatase type 2A (pp2aMs) cDNA. The predicted protein sequence shows high similarity to PP2A from Brassica napus, rabbit and Drosophila. No changes in pp2aMs mRNA abundance during the cell cycle were found. During growth of a batch cell culture, mRNA levels decreased gradually. In planta, all organs contained pp2a transcripts but maximal mRNA levels were detected in stems. Since Southern analysis indicated the presence of a small pp2a gene family in alfalfa, it appears that different subtypes may have specialized roles in various tissues and developmental situations which await characterization.

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.

Expression of multiple type 1 phosphoprotein phosphatases in Arabidopsis thaliana

Type 1 phosphoprotein Ser/Thr phosphatases (PP1) are highly conserved enzymes found in all eukaryotes. These enzymes have multiple functions in fungal and animal cells but little is known of their function and regulation in plants. Previous studies in our laboratory indicated that maize and Arabidopsis contain a family of PP1 genes and/or pseudogenes. In this study, we report the isolation of five distinct Arabidopsis cDNA clones (TOPP1, TOPP2, TOPP3, TOPP4 and TOPP5) which encode the catalytic subunit (PP1c) of type 1 protein phosphatases. Genomic Southern blot analyses indicate that these clones are the products of five distinct genes and that an additional 2-3 PP1c genes and/or pseudogenes may be present in the Arabidopsis genome. The derived amino acid sequences of the TOPP clones are very similar to published sequences of PP1c from animals, fungi and plants. Four of the TOPP amino acid sequences show unique structural features not observed in other PP1c sequences from fungi or animals. All of the TOPP genes are expressed in Arabidopsis roots, rosettes and flowers, although TOPP1, TOPP2 and TOPP3 appear to be expressed at higher levels in these tissues than TOPP4 and TOPP5.

Cantharidin-binding protein: identification as protein phosphatase 2A

The toxic effects of cantharidin from blister beetles and its analogs, including the herbicide endothall, are attributable to their high affinity and specificity for a cantharidin-binding protein (CBP). An ammonium sulfate precipitate of mouse liver cytosol was purified by five chromatographic steps to isolate CBP in 14% yield and > 99% purity as monitored by [3H]cantharidin-binding activity. The purification factor of 2230-fold corresponds to a CBP content of 0.045% of the liver cytosolic protein. CBP is a heterodimer consisting of a 61-kDa alpha subunit and a 39-kDa beta subunit. Amino acid sequences of four peptides from CBP-alpha and three peptides from CBP-beta are identical with deduced amino acid sequences for the A alpha regulatory and C beta catalytic subunits, respectively, of protein phosphatase 2A (PP2A). This assignment of CBP as PP2A-AC from structural evidence is supported by biochemical studies with selective substrates and inhibitors. CBP dephosphorylation of phosphorylase alpha is sensitive not only to okadaic acid, as with PP2A, but also to cantharidin and its analogs, consistent with their potency in blocking the radioligand binding site of CBP. Okadaic acid is a potent inhibitor of [3H]cantharidin binding to CBP. PP2A is present in many mammalian tissues and in plants and is involved in regulatory phosphorylation-dephosphorylation events which modulate multiple cellular functions. Inhibition of PP2A activity may account for the diverse effects and toxicity of cantharidin and its analogs, including the herbicide endothall, in mammals and possibly plants.

Evidence for a protein kinase cascade in higher plants. 3-Hydroxy-3-methylglutaryl-CoA reductase kinase

Protein phosphorylation is well established as a regulatory mechanism in higher plants, but only a handful of plant enzymes are known to be regulated in this manner, and relatively few plant protein kinases have been characterized. AMP-activated protein kinase regulates key enzymes of mammalian fatty acid, sterol and isoprenoid metabolism, including 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase. We now show that there is an activity in higher plants which, by functional criteria, is a homologue of the AMP-activated protein kinase, although it is not regulated by AMP. The plant kinase inactivates mammalian HMG-CoA reductase and acetyl-CoA carboxylase, and peptide mapping suggests that it phosphorylates the same sites on these proteins as the mammalian kinase. However, with the target enzymes purified from plant sources, it inactivates HMG-CoA reductase but not acetyl-CoA carboxylase. The kinase is located in the soluble, and not the chloroplast, fraction of leaf cells, consistent with the idea that it regulates HMG-CoA reductase, and hence isoprenoid biosynthesis, in vivo. The plant kinase also appears to be part of a protein kinase cascade which has been highly conserved during evolution, since the kinase is inactivated and reactivated by mammalian protein phosphatases (2A or 2C) and mammalian kinase kinase, respectively. This contrasts with the situation for many other mammalian protein kinases involved in signal transduction, which appear to have no close homologue in higher plants. To our knowledge, this represents the first direct evidence for a protein kinase cascade in higher plants.

Purification, characterization and structure of protein phosphatase 1 from the cilia of Paramecium tetraurelia

A type 1 serine/threonine protein phosphatase (PP1) which is mostly localized in the excitable ciliary membranes from the protozoan Paramecium, was purified to homogeneity. Approximately 4 micrograms enzyme of 37 kDa was isolated from 100 l axenic culture. The enzymic properties were characterized using phosphorylase a from rabbit skeletal muscle as a substrate and several known effectors of mammalian PP1. The protozoan PP1 was enzymically indistinguishable from its mammalian congener. The amino acid sequence of the Paramecium PP1 was deduced from its cDNA. The full-length clone was obtained in several steps starting with a pair of degenerate primers made according to the two most conserved peptides of rabbit PP1 and PP2A. The gene encodes a protein of 36,392 Da. The identity of the cloned gene and the isolated ciliary PP1 was unequivocally established by microsequencing of four tryptic and cyanogen-bromide peptides which were generated from the purified protein. Paramecium PP1 shows 75% amino-acid-sequence identity with rabbit PP1 alpha. Areas of major differences are the C-termini and N-termini and a sequence between residues 219-242.

Characterization of the major phosphofructokinase-dephosphorylating protein phosphatases from Ascaris suum muscle

In contrast to the mammalian enzyme, PFK from the nematode Ascaris suum is activated following phosphorylation (Daum et al. (1986) Biochem. Biophys. Res. Commun. 139, 215-221) catalyzed by a cAMP-dependent protein kinase (Thalhofer et al. (1988) J. Biol. Chem. 263, 952-957). In the present report, we describe the characterization of the major PFK dephosphorylating phosphatases from Ascaris muscle. Two of these phosphatases exhibit apparent M(r) values of 174,000 and 126,000, respectively, and are dissociated to active 33 kDa proteins by ethanol precipitation. Denaturing electrophoresis of each of the enzyme preparations showed two bands of M(r) 33,000 and 63,000. The enzymes are classified as type 2A phosphatases according to their inhibition by subnanomolar concentrations of okadaic acid, the lack of inhibition by heat-stable phosphatase inhibitors 1 and 2, and their preference for the alpha- rather than for the beta-subunit of phosphorylase kinase. Like other type 2A phosphatases, they exhibit broad substrate specificities, are activated by divalent cations and polycations, and inhibited by fluoride, inorganic phosphate and adenine nucleotides. In addition, we have found that PFK is also dephosphorylated by an unusual protein phosphatase. This exhibits kinetic properties similar to type 2A protein phosphatases, but has a distinctly lower sensitivity towards inhibition by okadaic acid (IC50 approx. 20 nM). Partial purification of the enzyme provided evidence that it is composed of a 30 kDa catalytic subunit and probably two other subunits (molecular masses 66 and 72 kDa). The dephosphorylation of PFK by protein phosphatases is strongly inhibited by heparin. This effect, however, is substrate-specific and does not occur with Ascaris phosphorylase a.

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.

Protein phosphatase 2Bw and protein phosphatase Z are Saccharomyces cerevisiae enzymes

cDNAs encoding three protein phosphatases, termed PP2Bw (Da Cruz e Silva, E.F. and Cohen, P.T.W. (1989) Biochim. Biophys. Acta 1009, 293-296), PPZ1 and PPZ2 that have been isolated from a Clontech 'rabbit brain' library are shown to be Saccharomyces cerevisiae clones. PPZ1 and PPZ2 are two novel yeast phosphatases showing 93% amino acid sequence identity to one another. PPZ1 shows approx. 60% sequence identity to S. cerevisiae or mammalian PP1 and approx. 40% identity to S. cerevisiae or mammalian PP2A. These and other observations suggest that the two isoforms of PPZ have functions distinct from those of PP1.