Structural and kinetic properties of high and low molecular mass phosphoenolpyruvate carboxylase isoforms from the endosperm of developing castor oilseeds

Conservation and Divergence of PEPC Gene Family in Different Ploidy Bamboos

Phosphoenolpyruvate carboxylase (PEPC), as a necessary enzyme for higher plants to participate in photosynthesis, plays a key role in photosynthetic carbon metabolism and the stress response. However, the molecular biology of the PEPC family of Bambusoideae has been poorly studied, and the function of its members in the growth and development of Bambusoideae is still unclear. Here, we identified a total of 62 PEPC family members in bamboo. All the PEPC genes in the bamboo subfamily were divided into twelve groups, each group typically containing significantly fewer PEPC members in Olyra latifolia than in Phyllostachys edulis, Dendrocalamus latiflorus and Dendrocalamus brandisii. The results of an intraspecific and interspecies collinearity analysis showed that fragment replication and whole genome replication were the main driving forces of bamboo PEPC members. Furthermore, the Ka/Ks values of collinear genes showed that bamboo PEPC experienced purification selection. In addition, the promoter region of PEPC genes contains cis-acting elements related to light response, plant hormone response and response to stress. An analysis of the expression levels of the PEPC family in different developmental stages and tissues of bamboo shoots has shown that PhePEPC7, PhePEPC9 and PhePEPC10 were highly expressed in the leaves of non-flowering plants and culms. Furthermore, PhePEPC6 was significantly upregulated in leaves after GA treatment. Further research has shown that PhePEPC6 was mainly localized in the cell membrane. This provides a solid bioinformatics foundation for further understanding the biological functions of the bamboo PEPC family.

CAM Models: Lessons and Implications for CAM Evolution

The evolution of Crassulacean acid metabolism (CAM) by plants has been one of the most successful strategies in response to aridity. On the onset of climate change, expanding the use of water efficient crops and engineering higher water use efficiency into C3 and C4 crops constitute a plausible solution for the problems of agriculture in hotter and drier environments. A firm understanding of CAM is thus crucial for the development of agricultural responses to climate change. Computational models on CAM can contribute significantly to this understanding. Two types of models have been used so far. Early CAM models based on ordinary differential equations (ODE) reproduced the typical diel CAM features with a minimal set of components and investigated endogenous day/night rhythmicity. This line of research brought to light the preponderant role of vacuolar malate accumulation in diel rhythms. A second wave of CAM models used flux balance analysis (FBA) to better understand the role of CO₂ uptake in flux distribution. They showed that flux distributions resembling CAM metabolism emerge upon constraining CO₂ uptake by the system. We discuss the evolutionary implications of this and also how CAM components from unrelated pathways could have integrated along evolution.

Autophosphorylation Inhibits RcCDPK1, a Dual-Specificity Kinase that Phosphorylates Bacterial-Type Phosphoenolpyruvate Carboxylase in Castor Oil Seeds

Phosphoenolpyruvate carboxylase (PEPC) is a tightly regulated enzyme that plays a crucial anaplerotic role in central plant metabolism. Bacterial-type PEPC (BTPC) of developing castor oil seeds (COS) is highly expressed as a catalytic and regulatory subunit of a novel Class-2 PEPC heteromeric complex. Ricinus communis Ca2+-dependent protein kinase-1 (RcCDPK1) catalyzes in vivo inhibitory phosphorylation of COS BTPC at Ser451. Autokinase activity of recombinant RcCDPK1 was detected and 42 autophosphorylated Ser, Thr or Tyr residues were mapped via liquid chromatography-tandem mass spectrometry. Prior autophosphorylation markedly attenuated the ability of RcCDPK1 to transphosphorylate its BTPC substrate at Ser451. However, fully dephosphorylated RcCDPK1 rapidly autophosphorylated during the initial stages of a BTPC transphosphorylation assay. This suggests that Ca2+-dependent binding of dephospho-RcCDPK1 to BTPC may trigger a structural change that leads to rapid autophosphorylation and subsequent substrate transphosphorylation. Tyr30 was identified as an autophosphorylation site via LC-MS/MS and immunoblotting with a phosphosite-specific antibody. Tyr30 occurs at the junction of RcCDPK1's N-terminal variable (NTVD) and catalytic domains and is widely conserved in plant and protist CDPKs. Interestingly, a reduced rate and extent of BTPC transphosphorylation occurred with a RcCDPK1Y30F mutant. Prior research demonstrated that RcCDPK1's NTVD is essential for its Ca2+-dependent autophosphorylation or BTPC transphosphorylation activities but plays no role in target recognition. We propose that Tyr30 autophosphorylation facilitates a Ca2+-dependent interaction between the NTVD and Ca2+-activation domain that primes RcCDPK1 for transphosphorylating BTPC at Ser451. Our results provide insights into links between the post-translational control of COS anaplerosis, Ca2+-dependent signaling and the biological significance of RcCDPK1 autophosphorylation.

Underwater CAM photosynthesis elucidated by Isoetes genome

To conserve water in arid environments, numerous plant lineages have independently evolved Crassulacean Acid Metabolism (CAM). Interestingly, Isoetes, an aquatic lycophyte, can also perform CAM as an adaptation to low CO2 availability underwater. However, little is known about the evolution of CAM in aquatic plants and the lack of genomic data has hindered comparison between aquatic and terrestrial CAM. Here, we investigate underwater CAM in Isoetes taiwanensis by generating a high-quality genome assembly and RNA-seq time course. Despite broad similarities between CAM in Isoetes and terrestrial angiosperms, we identify several key differences. Notably, Isoetes may have recruited the lesser-known 'bacterial-type' PEPC, along with the 'plant-type' exclusively used in other CAM and C4 plants for carboxylation of PEP. Furthermore, we find that circadian control of key CAM pathway genes has diverged considerably in Isoetes relative to flowering plants. This suggests the existence of more evolutionary paths to CAM than previously recognized.

Differential Expression, Tissue-Specific Distribution, and Posttranslational Controls of Phosphoenolpyruvate Carboxylase

Phosphoenolpyruvate carboxylase (PEPC) is a ubiquitous cytosolic enzyme, which is crucial for plant carbon metabolism. PEPC participates in photosynthesis by catalyzing the initial fixation of atmospheric CO2 and is abundant in both C4 and crassulacean acid metabolism leaves. PEPC is differentially expressed at different stages of plant development, mostly in leaves, but also in developing seeds. PEPC is known to show tissue-specific distribution in leaves and in other plant organs, such as roots, stems, and flowers. Plant PEPC undergoes reversible phosphorylation and monoubiquitination, which are posttranslational modifications playing important roles in regulatory processes and in protein localization. Phosphorylation activates the PEPC enzyme, making it more sensitive to glucose-6-phosphate and less sensitive to malate or aspartate. PEPC phosphorylation is known to be diurnally regulated and delicately changed in response to various environmental stimuli, in addition to light. PEPCs belong to a small gene family encoding several plant-type and distantly related bacterial-type PEPCs. This paper provides a minireview of the general information on PEPCs in both C4 and C3 plants.

The Developmental Enhancement of a C4 System With Non-Typical C4 Physiological Characteristics in Salsola ferganica (Kranz Anatomy), an Annual Desert Halophyte

Variations of photosynthetic structures in different tissues or cells are in coordination with changes in various aspects, e.g. physiology, biochemistry, gene expression, etc. Most C4 plant species undergo developmental enhancement of the photosynthetic system, which may present different modes of changes between anatomy and physiology/biochemistry. In the current study, we investigated a Kranz-type C4 species Salsola ferganica with the progressive development of photosynthetic (PS) structure, performance of PS physiology, induction of PS enzymes, and transcriptional and translational regulation of PS genes, results revealed that S. ferganica presented C3 type anatomy in cotyledons but C4 type in leaves (C3/L4), with the C4 system separation of initial carbon fixation in the palisade mesophyll (M) cells and the following incorporation into triosephosphates and sugars in the bundle sheath (BS) cells, respectively. The BS cells continuously surrounded the vascular bundles and water storage cells in leaf anatomic structure. Compared to the single-cell C4 species Suaeda aralocaspica, S. ferganica exhibited similar developmental enhancement of C4 syndrome temporally and spatially in anatomic structures, enzyme activities, and gene expression, which suggests that completion of differentiation of the photosynthetic system is necessary for a C4 assimilation pathway. Besides, S. ferganica also displayed some different characteristics compared to S. aralocaspica in photosynthetic physiology, e.g. a more flexible δ13C value, much lower phosphoenolpyruvate carboxylase (PEPC) activity, and an insensitive response to stimuli, etc., which were not typical C4 characteristics. We speculate that this may suggest a different status of these two species in the evolutionary process of the photosynthesis pathway. Our findings will contribute to further understanding of the diversity of photosynthesis systems in Kranz-type C4 species and the Salsola genus.

The plant-type phosphoenolpyruvate carboxylase Gmppc2 is developmentally induced in immature soy seeds at the late maturation stage: a potential protein biomarker for seed chemical composition

Phosphoenolpyruvate carboxylase (PEPC) is a carbon-fixing enzyme with critical roles in seed development. Previously we observed a positive correlation between PEPC activity and protein content in mature seeds among soybean cultivars and varietal differences of PEPC activity in immature seeds, which is concordant with seed protein accumulation. Here, we report a PEPC isoform (Gmppc2) which is preferentially expressed in immature soybean seeds at the late maturation stage. Gmppc2 was co-expressed with enzyme genes involved in starch degradation: α-amylase, hexokinase, and α-glucan phosphorylase. Gmppc2 was developmentally induced in the external seed coats, internal seed coats, hypocotyls, and cotyledons at the late maturation stage. The expression of Gmppc2 protein was negatively regulated by the application of a nitrogen fertilizer, which suppressed nodule formation. These results imply that Gmppc2 is involved in the metabolism of nitrogen originated from nodules into seeds, and Gmppc2 might be applicable as a biomarker of seed protein content.Abbreviations: PEP: phosphoenolpyruvate; PEPC: phosphoenolpyruvate carboxylase; RNA-Seq: RNA sequencing; PCA: principal component analysis; SE: standard error.

Diurnal changes in concerted plant protein phosphorylation and acetylation in Arabidopsis organs and seedlings

Protein phosphorylation and acetylation are the two most abundant post-translational modifications (PTMs) that regulate protein functions in eukaryotes. In plants, these PTMs have been investigated individually; however, their co-occurrence and dynamics on proteins is currently unknown. Using Arabidopsis thaliana, we quantified changes in protein phosphorylation, acetylation and protein abundance in leaf rosettes, roots, flowers, siliques and seedlings at the end of day (ED) and at the end of night (EN). This identified 2549 phosphorylated and 909 acetylated proteins, of which 1724 phosphorylated and 536 acetylated proteins were also quantified for changes in PTM abundance between ED and EN. Using a sequential dual-PTM workflow, we identified significant PTM changes and intersections in these organs and plant developmental stages. In particular, cellular process-, pathway- and protein-level analyses reveal that the phosphoproteome and acetylome predominantly intersect at the pathway- and cellular process-level at ED versus EN. We found 134 proteins involved in core plant cell processes, such as light harvesting and photosynthesis, translation, metabolism and cellular transport, that were both phosphorylated and acetylated. Our results establish connections between PTM motifs, PTM catalyzing enzymes and putative substrate networks. We also identified PTM motifs for further characterization of the regulatory mechanisms that control cellular processes during the diurnal cycle in different Arabidopsis organs and seedlings. The sequential dual-PTM analysis expands our understanding of diurnal plant cell regulation by PTMs and provides a useful resource for future analyses, while emphasizing the importance of analyzing multiple PTMs simultaneously to elucidate when, where and how they are involved in plant cell regulation.

Avoiding Proteolysis during the Extraction and Purification of Active Plant Enzymes

The aim of this article is to discuss approaches to diagnose and prevent unwanted proteolysis during extraction and isolation of active enzymes from plant tissues. Enzymes are protein catalysts that require great care during sample processing in order to ensure that they remain intact and fully active. Preventing artifactual enzyme modifications ex planta is of utmost importance in order to obtain biologically relevant data. This is particularly problematic following enzyme extraction from plant tissues, which relative to microbes or animals contain relatively low protein amounts coupled with high concentrations of vacuolar proteases. Although cytoplasmic enzymes are not directly accessible to vacuolar proteases owing their physical segregation into different subcellular compartments, this compartmentation is destroyed during cell lysis. Unwanted proteolysis by endogenous proteases is an insidious problem because in many cases the enzyme of interest is only partially degraded and retains catalytic activity. This can not only lead to erroneous conclusions about an enzyme's size, subunit structure and post-translational modifications, but can also result in striking changes to its kinetic and regulatory (i.e. allosteric) properties. Furthermore, the routine addition of class-specific protease inhibitors and/or commercially available (and expensive) protease inhibitor cocktails to extraction and purification buffers does not necessarily preclude partial proteolysis of plant enzymes by endogenous proteases. When antibodies are available, plant scientists are advised to employ immunoblotting to diagnose potential in vitro proteolytic truncation of the enzymes that they wish to characterize, as well as to test the effectiveness of specific protease inhibitors in overcoming this recurrent issue.

Transcript profiling indicates a widespread role for bacterial-type phosphoenolpyruvate carboxylase in malate-accumulating sink tissues

Phosphoenolpyruvate carboxylase (PEPC) is an important regulatory enzyme situated at a key branch point of central plant metabolism. Plant genomes encode several plant-type PEPC (PTPC) isozymes, along with a distantly related bacterial-type PEPC (BTPC). BTPC is expressed at high levels in developing castor oil seeds where it tightly interacts with co-expressed PTPC polypeptides to form unusual hetero-octameric Class-2 PEPC complexes that are desensitized to allosteric inhibition by L-malate. Analysis of RNA-Seq and microarray transcriptome datasets revealed two distinct patterns of tissue-specific BTPC expression in vascular plants. Species such as Arabidopsis thaliana, strawberry, rice, maize, and poplar mainly exhibited pollen- or floral-specific BTPC expression. By contrast, BTPC transcripts were relatively abundant in developing castor, cotton, and soybean seeds, cassava tubers, as well as immature tomato, cucumber, grape, and avocado fruit. Immunoreactive 118 kDa BTPC polypeptides were detected on immunoblots of cucumber and tomato fruit extracts. Co-immunoprecipitation established that as in castor, BTPCs physically interact with endogenous PTPCs to form Class-2 PEPC complexes in tomato and cucumber fruit. We hypothesize that Class-2 PEPCs simultaneously maintain rapid anaplerotic PEP carboxylation and respiratory CO2 refixation in diverse, biosynthetically active sinks that accumulate high malate levels.

Phosphorylation of bacterial-type phosphoenolpyruvate carboxylase by a Ca2+-dependent protein kinase suggests a link between Ca2+ signalling and anaplerotic pathway control in developing castor oil seeds

The aim of the present study was to characterize the native protein kinase [BTPC (bacterial-type phosphoenolpyruvate carboxylase)-K (BTPC Ser451 kinase)] that in vivo phosphorylates Ser451 of the BTPC subunits of an unusual Class-2 PEP (phosphoenolpyruvate) carboxylase hetero-octameric complex of developing COS (castor oil seeds). COS BTPC-K was highly purified by PEG fractionation and hydrophobic size-exclusion anion-exchange and affinity chromatographies. BTPC-K phosphorylated BTPC strictly at Ser451 (Km=1.0 μM; pH optimum=7.3), a conserved target residue occurring within an intrinsically disordered region, as well as the protein histone III-S (Km=1.7 μM), but not a COS plant-type PEP carboxylase or sucrose synthase or α-casein. Its activity was Ca2+- (K0.5=2.7 μM) and ATP- (Km=6.6 μM) dependent, and markedly inhibited by trifluoperazine, 3-phosphoglycerate and PEP, but insensitive to calmodulin or 14-3-3 proteins. BTPC-K exhibited a native molecular mass of ~63 kDa and was soluble rather than membrane-bound. Inactivation and reactivation occurred upon BTPC-K's incubation with GSSG and then DTT respectively. Ser451 phosphorylation by BTPC-K inhibited BTPC activity by ~50% when assayed under suboptimal conditions (pH 7.3, 1 mM PEP and 10 mM L-malate). Our collective results indicate a possible link between cytosolic Ca2+ signalling and anaplerotic flux control in developing COS.

Comparative proteomic analysis of early somatic and zygotic embryogenesis in Theobroma cacao L

Somatic embryogenesis can efficiently foster the propagation of Theobroma cacao, but the poor quality of resulted plantlet hinders the use of this technique in the commercial scale. The current study has been initiated to systematically compare the physiological mechanisms underlying somatic and zygotic embryogenesis in T. cacao on the proteome level. About 1000 protein spots per fraction could be separated by two-dimensional isoelectric focusing/SDS PAGE. More than 50 of the protein spots clearly differed in abundance between zygotic and somatic embryos: 33 proteins spots were at least 3-fold higher in abundance in zygotic embryos and 20 in somatic embryos. Analyses of these protein spots differing in volume by mass spectrometry resulted in the identification of 68 distinct proteins. Many of the identified proteins are involved in genetic information processing (21 proteins), carbohydrate metabolism (11 proteins) and stress response (7 proteins). Somatic embryos especially displayed many stress related proteins, few enzymes involved in storage compound synthesis and an exceptional high abundance of endopeptidase inhibitors. Phosphoenolpyruvate carboxylase, which was accumulated more than 3-fold higher in zygotic embryos, represents a prominent enzyme in the storage compound metabolism in cacao seeds. Implications on the improvement of somatic embryogenesis in cacao are discussed.

Bacterial- and plant-type phosphoenolpyruvate carboxylase isozymes from developing castor oil seeds interact in vivo and associate with the surface of mitochondria

Phosphoenolpyruvate carboxylase (PEPC) from developing castor oil seeds (COS) exists as two distinct oligomeric isoforms. The typical class-1 PEPC homotetramer consists of 107-kDa plant-type PEPC (PTPC) subunits, whereas the allosterically desensitized 910-kDa class-2 PEPC hetero-octamer arises from the association of class-1 PEPC with 118-kDa bacterial-type PEPC (BTPC) subunits. The in vivo interaction and subcellular location of COS BTPC and PTPC were assessed by imaging fluorescent protein (FP)-tagged PEPCs in tobacco suspension-cultured cells. The BTPC-FP mainly localized to cytoplasmic punctate/globular structures, identified as mitochondria by co-immunostaining of endogenous cytochrome oxidase. Inhibition of respiration with KCN resulted in proportional decreases and increases in mitochondrial versus cytosolic BTPC-FP, respectively. The FP-PTPC and NLS-FP-PTPC (containing an appended nuclear localization signal, NLS) localized to the cytosol and nucleus, respectively, but both co-localized with mitochondrial-associated BTPC when co-expressed with BTPC-FP. Transmission electron microscopy of immunogold-labeled developing COS revealed that BTPC and PTPC are localized at the mitochondrial (outer) envelope, as well as the cytosol. Moreover, thermolysin-sensitive BTPC and PTPC polypeptides were detected on immunoblots of purified COS mitochondria. Overall, our results demonstrate that: (i) COS BTPC and PTPC interact in vivo as a class-2 PEPC complex that associates with the surface of mitochondria, (ii) BTPC's unique and divergent intrinsically disordered region mediates its interaction with PTPC, whereas (iii) the PTPC-containing class-1 PEPC is entirely cytosolic. We hypothesize that mitochondrial-associated class-2 PEPC facilitates rapid refixation of respiratory CO(2) while sustaining a large anaplerotic flux to replenish tricarboxylic acid cycle C-skeletons withdrawn for biosynthesis.

The bacterial-type phosphoenolpyruvate carboxylase isozyme from developing castor oil seeds is subject to in vivo regulatory phosphorylation at serine-451

Phosphoenolpyruvate carboxylase (PEPC) is a tightly controlled anaplerotic enzyme situated at a pivotal branch point of plant carbohydrate-metabolism. In developing castor oil seeds (COS) a novel allosterically-densensitized 910-kDa Class-2 PEPC hetero-octameric complex arises from a tight interaction between 107-kDa plant-type PEPC and 118-kDa bacterial-type PEPC (BTPC) subunits. Mass spectrometry and immunoblotting with anti-phosphoSer451 specific antibodies established that COS BTPC is in vivo phosphorylated at Ser451, a highly conserved target residue that occurs within an intrinsically disordered region. This phosphorylation was enhanced during COS development or in response to depodding. Kinetic characterization of a phosphomimetic (S451D) mutant indicated that Ser451 phosphorylation inhibits the catalytic activity of BTPC subunits within the Class-2 PEPC complex.

Tissue-specific expression and post-translational modifications of plant- and bacterial-type phosphoenolpyruvate carboxylase isozymes of the castor oil plant, Ricinus communis L

This study employs transcript profiling together with immunoblotting and co-immunopurification to assess the tissue-specific expression, protein:protein interactions, and post-translational modifications (PTMs) of plant- and bacterial-type phosphoenolpyruvate carboxylase (PEPC) isozymes (PTPC and BTPC, respectively) in the castor plant, Ricinus communis. Previous studies established that the Class-1 PEPC (PTPC homotetramer) of castor oil seeds (COS) is activated by phosphorylation at Ser-11 and inhibited by monoubiquitination at Lys-628 during endosperm development and germination, respectively. Elimination of photosynthate supply to developing COS by depodding caused the PTPC of the endosperm and cotyledon to be dephosphorylated, and then subsequently monoubiquitinated in vivo. PTPC monoubiquitination rather than phosphorylation is widespread throughout the castor plant and appears to be the predominant PTM of Class-1 PEPC that occurs in planta. The distinctive developmental patterns of PTPC phosphorylation versus monoubiquitination indicates that these two PTMs are mutually exclusive. By contrast, the BTPC: (i) is abundant in the inner integument, cotyledon, and endosperm of developing COS, but occurs at low levels in roots and cotyledons of germinated COS, (ii) shows a unique developmental pattern in leaves such that it is present in leaf buds and young expanding leaves, but undetectable in fully expanded leaves, and (iii) tightly interacts with co-expressed PTPC to form the novel and allosterically-desensitized Class-2 PEPC heteromeric complex. BTPC and thus Class-2 PEPC up-regulation appears to be a distinctive feature of rapidly growing and/or biosynthetically active tissues that require a large anaplerotic flux from phosphoenolpyruvate to replenish tricarboxylic acid cycle C-skeletons being withdrawn for anabolism.

The chloroplast proteome: a survey from the Chlamydomonas reinhardtii perspective with a focus on distinctive features

The unicellular green alga Chlamydomonas reinhardtii has emerged to be an important model organism for the study of oxygenic eukaryotic photosynthesis as well as other processes occurring in the chloroplast. However, the chloroplast proteome in C. reinhardtii has only recently been comprehensively characterized, made possible by proteomics emerging as an accessible and powerful tool over the last decade. In this review, we introduce a compiled list of 996 experimentally chloroplast-localized proteins for C. reinhardtii, stemming largely from our previous proteomic dataset comparing chloroplasts and mitochondria samples to localize proteins. In order to get a taste of some cellular functions taking place in the C. reinhardtii chloroplast, we will focus this review particularly on metabolic differences between chloroplasts of C. reinhardtii and higher plants. Areas that will be covered are photosynthesis, chlorophyll biosynthesis, carbon metabolism, fermentative metabolism, ferredoxins and ferredoxin-interacting proteins.

The remarkable diversity of plant PEPC (phosphoenolpyruvate carboxylase): recent insights into the physiological functions and post-translational controls of non-photosynthetic PEPCs

PEPC [PEP (phosphoenolpyruvate) carboxylase] is a tightly controlled enzyme located at the core of plant C-metabolism that catalyses the irreversible β-carboxylation of PEP to form oxaloacetate and Pi. The critical role of PEPC in assimilating atmospheric CO2 during C4 and Crassulacean acid metabolism photosynthesis has been studied extensively. PEPC also fulfils a broad spectrum of non-photosynthetic functions, particularly the anaplerotic replenishment of tricarboxylic acid cycle intermediates consumed during biosynthesis and nitrogen assimilation. An impressive array of strategies has evolved to co-ordinate in vivo PEPC activity with cellular demands for C4–C6 carboxylic acids. To achieve its diverse roles and complex regulation, PEPC belongs to a small multigene family encoding several closely related PTPCs (plant-type PEPCs), along with a distantly related BTPC (bacterial-type PEPC). PTPC genes encode ~110-kDa polypeptides containing conserved serine-phosphorylation and lysine-mono-ubiquitination sites, and typically exist as homotetrameric Class-1 PEPCs. In contrast, BTPC genes encode larger ~117-kDa polypeptides owing to a unique intrinsically disordered domain that mediates BTPC's tight interaction with co-expressed PTPC subunits. This association results in the formation of unusual ~900-kDa Class-2 PEPC hetero-octameric complexes that are desensitized to allosteric effectors. BTPC is a catalytic and regulatory subunit of Class-2 PEPC that is subject to multi-site regulatory phosphorylation in vivo. The interaction between divergent PEPC polypeptides within Class-2 PEPCs adds another layer of complexity to the evolution, physiological functions and metabolic control of this essential CO2-fixing plant enzyme. The present review summarizes exciting developments concerning the functions, post-translational controls and subcellular location of plant PTPC and BTPC isoenzymes.

Phosphorylation of bacterial-type phosphoenolpyruvate carboxylase at Ser425 provides a further tier of enzyme control in developing castor oil seeds

PEPC [PEP (phosphoenolpyruvate) carboxylase] is a tightly controlled anaplerotic enzyme situated at a pivotal branch point of plant carbohydrate metabolism. Two distinct oligomeric PEPC classes were discovered in developing COS (castor oil seeds). Class-1 PEPC is a typical homotetramer of 107 kDa PTPC (plant-type PEPC) subunits, whereas the novel 910-kDa Class-2 PEPC hetero-octamer arises from a tight interaction between Class-1 PEPC and 118 kDa BTPC (bacterial-type PEPC) subunits. Mass spectrometric analysis of immunopurified COS BTPC indicated that it is subject to in vivo proline-directed phosphorylation at Ser425. We show that immunoblots probed with phosphorylation site-specific antibodies demonstrated that Ser425 phosphorylation is promoted during COS development, becoming maximal at stage IX (maturation phase) or in response to depodding. Kinetic analyses of a recombinant, chimaeric Class-2 PEPC containing phosphomimetic BTPC mutant subunits (S425D) indicated that Ser425 phosphorylation results in significant BTPC inhibition by: (i) increasing its Km(PEP) 3-fold, (ii) reducing its I50 (L-malate and L-aspartate) values by 4.5- and 2.5-fold respectively, while (iii) decreasing its activity within the physiological pH range. The developmental pattern and kinetic influence of Ser425 BTPC phosphorylation is very distinct from the in vivo phosphorylation/activation of COS Class-1 PEPC's PTPC subunits at Ser11. Collectively, the results establish that BTPC's phospho-Ser425 content depends upon COS developmental and physiological status and that Ser425 phosphorylation attenuates the catalytic activity of BTPC subunits within a Class-2 PEPC complex. To the best of our knowledge, this study provides the first evidence for protein phosphorylation as a mechanism for the in vivo control of vascular plant BTPC activity.

Cloning, expression, purification and physical and kinetic characterization of the phosphoenolpyruvate carboxylase from orange (Citrus sinensis osbeck var. Valencia) fruit juice sacs

Phosphoenolpyruvate (PEP) carboxylase (PEPCase) from orange fruit juice sacs has been cloned and heterogously expressed in high yield. The purified recombinant enzyme displays properties typical of plant PEPCase, including activation by sugar phosphates and inhibition by malate and citrate. Malate inhibition is weak in the physiological pH range, and the enzyme is also poorly affected by Glu and Asp, known inhibitors of C(3) plants PEPCases. However, it is strongly inhibited by citrate. Orange fruit PEPCase phosphorylation by mammalian protein kinase A decreased inhibition by malate. The enzyme presents an unusual high molecular mass in the absence of PEP, while in its presence it displays a more common tetrameric arrangement. The overall properties of the enzyme suggest that it is suited for organic acid synthesis and NADH reoxidation in the mature fruit. The present study provides the first analysis of a recombinant fruit PEPCase.

Characterization of bacterial-type phosphoenolpyruvate carboxylase expressed in male gametophyte of higher plants

BACKGROUND

Phosphoenolpyruvate carboxylase (PEPC) is a critical enzyme catalyzing the β-carboxylation of phosphoenolpyruvate (PEP) to oxaloacetate, a tricarboxylic acid (TCA) cycle intermediate. PEPC typically exists as a Class-1 PEPC homotetramer composed of plant-type PEPC (PTPC) polypeptides, and two of the subunits were reported to be monoubiquitinated in germinating castor oil seeds. By the large-scale purification of ubiquitin (Ub)-related proteins from lily anther, two types of PEPCs, bacterial-type PEPC (BTPC) and plant-type PEPC (PTPC), were identified in our study as candidate Ub-related proteins. Until now, there has been no information about the properties of the PEPCs expressed in male reproductive tissues of higher plants.

RESULTS

Expression analyses showed that lily BTPC (LlBTPC) and Arabidopsis BTPC (AtBTPC) were significantly expressed in pollen. The fusion protein AtBTPC-Venus localized in the cytoplasm of the vegetative cell (VC). Both LlBTPC and AtBTPC expression initiated after the last mitosis before pollen germination. Lily PTPC (LlPTPC) and monoubiquitinated LlPTPC (Ub-LlPTPC) remained at constant levels during pollen development. In late bicellular pollen of lily, LlBTPC forms a hetero-octameric Class-2 PEPC complex with LlPTPC to express PEPC activity.

CONCLUSION

Our results suggest that an LlBTPC:Ub-LlPTPC:LlPTPC complex is formed in the VC cytoplasm during late pollen development. Both LlBTPC and AtBTPC expression patterns are similar to the patterns of the appearance of storage organelles during pollen development in lily and Arabidopsis, respectively. Therefore, BTPC is thought to accelerate the metabolic flow for the synthesis of storage substances during pollen maturation. Our study provides the first characterization of BTPC in pollen, the male gametophyte of higher plants.

Bacterial-type phosphoenolpyruvate carboxylase (PEPC) functions as a catalytic and regulatory subunit of the novel class-2 PEPC complex of vascular plants

Phosphoenolpyruvate carboxylase (PEPC) is a tightly regulated anaplerotic enzyme situated at a major branch point of the plant C metabolism. Two distinct oligomeric classes of PEPC occur in the triglyceride-rich endosperm of developing castor oil seeds (COS). Class-1 PEPC is a typical homotetramer composed of identical 107-kDa plant-type PEPC (PTPC) subunits (encoded by RcPpc3), whereas the novel Class-2 PEPC 910-kDa hetero-octameric complex arises from a tight interaction between Class-1 PEPC and distantly related 118-kDa bacterial-type PEPC (BTPC) polypeptides (encoded by RcPpc4). Here, COS BTPC was expressed from full-length RcPpc4 cDNA in Escherichia coli as an active PEPC that exhibited unusual properties relative to PTPCs, including a tendency to form large aggregates, enhanced thermal stability, a high K(m)((PEP)), and insensitivity to metabolite effectors. A chimeric 900-kDa Class-2 PEPC hetero-octamer having a 1:1 stoichiometry of BTPC:PTPC subunits was isolated from a mixture of clarified extracts containing recombinant RcPPC4 and an Arabidopsis thaliana Class-1 PEPC (the PTPC, AtPPC3). The purified Class-2 PEPC exhibited biphasic PEP saturation kinetics with high and low affinity sites attributed to its AtPPC3 and RcPPC4 subunits, respectively. The RcPPC4 subunits: (i) catalyzed the majority of the Class-2 PEPC V(max), particularly in the presence of the inhibitor l-malate, and (ii) also functioned as Class-2 PEPC regulatory subunits by modulating PEP binding and catalytic potential of its AtPPC3 subunits. BTPCs appear to associate with PTPCs to form stable Class-2 PEPC complexes in vivo that are hypothesized to maintain high flux from PEP under physiological conditions that would otherwise inhibit Class-1 PEPCs.

Regulation of phosphoenolpyruvate carboxylase in PVY(NTN)-infected tobacco plants

The effect of viral infection on the regulation of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) in Nicotiana tabacum L. leaves was studied. PEPC activity was 3 times higher in infected plant leaves compared to healthy plants. Activity of plant PEPC can be regulated, e.g., by de novo synthesis or reversible phosphorylation. The reason for the increase of PEPC activity as a consequence of PVY(NTN) infection was studied. The amount of PEPC determined by Western blot analysis or by relative estimation of PEPC mRNA by real-time PCR did not differ in control and PVY(NTN)-infected plants. Changes in posttranslational modification of PEPC by phosphorylation were evaluated by comparing activity of the native and the dephosphorylated enzyme. The infected plants were characterized by a higher decrease of the enzyme activity after its dephosphorylation, which indicated a higher phosphorylation level. Immunochemical detection of phosphoproteins by Western blot analysis showed a more intensive band corresponding to PEPC from the infected material. This strengthens the hypothesis of an infection-related phosphorylation, which could be part of the plant's response to pathogen attack. The physiological implications of the increase in PEPC activity during PVY(NTN) infection are discussed.

Arabidopsis UDP-sugar pyrophosphorylase: evidence for two isoforms

Arabidopsis UDP-sugar pyrophosphorylase (AtUSP, EC 2.7.7.64) is a broad substrate pyrophosphorylase that exhibits activity with GlcA-1-P, Gal-1-P and Glc-1-P. Immunoblots using polyclonal antibodies raised to recombinant AtUSP demonstrated the presence of two USP isoforms of approximately 70 kDa (USP1) and 66 kDa (USP2) in crude extracts of Arabidopsis tissues. The 66 kDa isoform was not the result of proteolytic cleavage of USP1 during extraction. Trypsin digestion of bands on SDS gels corresponding to the location of the two isoforms followed by tandem mass spectrometry confirmed that USP peptides were present in both bands. Both USP isoforms were detected in the cytosol as determined by immunoblots of cellular fractions obtained by differential centrifugation. However, some USP1 was also detected in the microsomal fraction. Immunoprecipitation assays demonstrated that AtUSP antibodies removed USP activity (UDP-GlcA-->GlcA-1-P) measured in floret extracts. These results indicate that USP is the only pyrophosphorylase that utilizes UDP-GlcA as a substrate and suggest that it serves as the terminal enzyme of the myo-inositol oxidation pathway.

Regulation of phosphoenolpyruvate carboxylase phosphorylation by metabolites and abscisic acid during the development and germination of barley seeds

During barley (Hordeum vulgare) seed development, phosphoenolpyruvate carboxylase (PEPC) activity increased and PEPC-specific antibodies revealed housekeeping (103-kD) and inducible (108-kD) subunits. Bacterial-type PEPC fragments were immunologically detected in denatured protein extracts from dry and imbibed conditions; however, on nondenaturing gels, the activity of the recently reported octameric PEPC (in castor [Ricinus communis] oil seeds) was not detected. The phosphorylation state of the PEPC, as judged by l-malate 50% inhibition of initial activity values, phosphoprotein chromatography, and immunodetection of the phosphorylated N terminus, was found to be high between 8 and 18 d postanthesis (DPA) and during imbibition. In contrast, the enzyme appeared to be in a low phosphorylation state from 20 DPA up to dry seed. The time course of 32/36-kD, Ca(2+)-independent PEPC kinase activity exhibited a substantial increase after 30 DPA that did not coincide with the PEPC phosphorylation profile. This kinase was found to be inhibited by l-malate and not by putative protein inhibitors, and the PEPC phosphorylation status correlated with high glucose-6-phosphate to malate ratios, thereby suggesting an in vivo metabolic control of the kinase. PEPC phosphorylation was also regulated by photosynthate supply at 11 DPA. In addition, when fed exogenously to imbibing seeds, abscisic acid significantly increased PEPC kinase activity. This was further enhanced by the cytosolic protein synthesis inhibitor cycloheximide but blocked by protease inhibitors, thereby suggesting that the phytohormone acts on the stability of the kinase. We propose that a similar abscisic acid-dependent effect may contribute to produce the increase in PEPC kinase activity during desiccation stages.

Regulatory monoubiquitination of phosphoenolpyruvate carboxylase in germinating castor oil seeds

Phosphoenolpyruvate carboxylase (PEPC) is a tightly regulated enzyme situated at the core of plant C-metabolism. Although its anaplerotic role and control by allosteric effectors, reversible phosphorylation, and oligomerization have been well documented in the endosperm of developing castor oil seeds (COS), relatively little is known about PEPC in germinating COS. The initial phase of COS germination was accompanied by elevated PEPC activity and accumulation of comparable amounts of pre-existing 107-kDa and inducible 110-kDa immunoreactive PEPC polypeptides (p107 and p110, respectively). A 440-kDa PEPC heterotetramer composed of an equivalent ratio of non-phosphorylated p110 and p107 subunits was purified from germinated COS. N-terminal microsequencing, mass spectrometry, and immunoblotting revealed that both subunits arose from the same gene (RcPpc3) that encodes the p107 subunit of a phosphorylated 410-kDa PEPC homotetramer in developing COS but that p110 is a monoubiquitinated form of p107. Tandem mass spectrometry sequencing of a diglycinated tryptic peptide identified Lys-628 as p110's monoubiquitination site. This residue is conserved in vascular plant PEPCs and is proximal to a PEP-binding/catalytic domain. Incubation with a human deubiquitinating enzyme (USP-2 core) converted the p110:p107 PEPC heterotetramer into a p107 homotetramer while significantly reducing the enzyme's K(m)(PEP) and sensitivity to allosteric activators (hexose-Ps, glycerol-3-P) and inhibitors (malate, aspartate). Monoubiquitination is a non-destructive and reversible post-translational modification involved in the control of diverse processes such as transcription, endocytosis, and signal transduction. The current study demonstrates that tissue-specific monoubiquitination of a metabolic enzyme can also occur and that this modification influences its kinetic and regulatory properties.

Arabidopsis thaliana NADP-malic enzyme isoforms: high degree of identity but clearly distinct properties

The Arabidopsis thaliana genome contains four NADP-malic enzymes genes (NADP-ME1-4). NADP-ME4 is localized to plastids whereas the other isoforms are cytosolic. NADP-ME2 and 4 are constitutively expressed, while NADP-ME1 is restricted to secondary roots and NADP-ME3 to trichomes and pollen. Although the four isoforms share remarkably high degree of identity (75-90%), recombinant NADP-ME1 through 4 show distinct kinetic properties, both in the forward (malate oxidative decarboxylation) and reverse (pyruvate reductive carboxylation) reactions. The four isoforms behave differently in terms of reversibility, with NADP-ME2 presenting the highest reverse catalytic efficiency. When analyzing the activity of each isoform in the presence of metabolic effectors, NADP-ME2 was the most highly regulated isoform, especially in its activation by certain effectors. Several metabolites modulate both the forward and reverse reactions, exhibiting dual effects in some cases. Therefore, pyruvate reductive carboxylation may be relevant in vivo, especially in some cellular compartments and conditions. In order to identify residues or segments of the NADP-ME primary structure that could be involved in the differences among the isoforms, NADP-ME2 mutants and deletions were analysed. The results obtained show that Arg115 is involved in fumarate activation, while the amino-terminal part is critical for aspartate and CoA activation, as well as for the reverse reaction. As a whole, these studies show that minimal changes in the primary structure are responsible for the different kinetic behaviour of each AtNADP-ME isoform. In this way, the co-expression of some isoforms in the same cellular compartment would not imply redundancy but represents specificity of function.

Coimmunopurification of phosphorylated bacterial- and plant-type phosphoenolpyruvate carboxylases with the plastidial pyruvate dehydrogenase complex from developing castor oil seeds

The phosphoenolpyruvate carboxylase (PEPC) interactome of developing castor oil seed (COS; Ricinus communis) endosperm was assessed using coimmunopurification (co-IP) followed by proteomic analysis. Earlier studies suggested that immunologically unrelated 107-kD plant-type PEPCs (p107/PTPC) and 118-kD bacterial-type PEPCs (p118/BTPC) are subunits of an unusual 910-kD hetero-octameric class 2 PEPC complex of developing COS. The current results confirm that a tight physical interaction occurs between p118 and p107 because p118 quantitatively coimmunopurified with p107 following elution of COS extracts through an anti-p107-IgG immunoaffinity column. No PEPC activity or immunoreactive PEPC polypeptides were detected in the corresponding flow-through fractions. Although BTPCs lack the N-terminal phosphorylation motif characteristic of PTPCs, Pro-Q Diamond phosphoprotein staining, immunoblotting with phospho-serine (Ser)/threonine Akt substrate IgG, and phosphate-affinity PAGE established that coimmunopurified p118 was multiphosphorylated at unique Ser and/or threonine residues. Tandem mass spectrometric analysis of an endoproteinase Lys-C p118 peptide digest demonstrated that Ser-425 is subject to in vivo proline-directed phosphorylation. The co-IP of p118 with p107 did not appear to be influenced by their phosphorylation status. Because p118 phosphorylation was unchanged 48 h following elimination of photosynthate supply due to COS depodding, the signaling mechanisms responsible for photosynthate-dependent p107 phosphorylation differ from those controlling p118's in vivo phosphorylation. A 110-kD PTPC coimmunopurified with p118 and p107 when depodded COS was used. The plastidial pyruvate dehydrogenase complex (PDC(pl)) was identified as a novel PEPC interactor. Thus, a putative metabolon involving PEPC and PDC(pl) could function to channel carbon from phosphoenolpyruvate to acetyl-coenzyme A and/or to recycle CO(2) from PDC(pl) to PEPC.

Bacterial- and plant-type phosphoenolpyruvate carboxylase polypeptides interact in the hetero-oligomeric Class-2 PEPC complex of developing castor oil seeds

Two classes of phosphoenolpyruvate carboxylase (PEPC) sharing the same 107-kDa catalytic subunit (p107) were previously purified from developing castor oil seed (COS) endosperm. The association of p107 with an immunologically unrelated 64-kDa polypeptide (p64) causes pronounced physical and kinetic differences between the Class-1 PEPC p107 homotetramer and Class-2 PEPC p107/p64 hetero-octamer. Tryptic peptide sequencing matched p64 to the deduced C-terminal half of several bacterial-type PEPCs (BTPCs) of vascular plants. Immunoblots probed with anti-(COS p64 peptide or p107)-IgG established that: (i) BTPC exists in vivo as an approximately 118-kDa polypeptide (p118) that is rapidly truncated to p64 by an endogenous cysteine endopeptidase during incubation of COS extracts on ice, and (ii) mature and germinated COS contain Class-1 PEPC and p107, but no detectable Class-2 PEPC nor p118. Non-denaturing PAGE, in-gel PEPC activity staining and immunoblotting of developing COS extracts demonstrated that p118 and p107 are subunits of the non-proteolysed approximately 910-kDa Class-2 PEPC complex. As total PEPC activity of clarified COS extracts was unaffected following p118 truncation to p64, the BTPC p118 may function as a regulatory rather than catalytic subunit of the Class-2 PEPC. Moreover, recombinant AtPPC3 and AtPPC4 (Arabidopsis orthologs of COS p107 and p118) expressed as active and inactive PEPCs, respectively. Cloning of cDNAs encoding p118 (RcPpc4) and p107 (RcPpc3) confirmed their respective designation as bacterial- and plant-type PEPCs. Levels of RcPpc3 and RcPpc4 transcripts generally mirrored the respective amounts of p107 and p118. The collective findings provide insights into the molecular features and functional significance of vascular plant BTPCs.

Phosphoenolpyruvate carboxylase protein kinase from developing castor oil seeds: partial purification, characterization, and reversible control by photosynthate supply

Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) protein kinase (PPCK) was purified approximately 1,500-fold from developing castor oil seeds (COS). Gel filtration and immunoblotting with anti-(rice PPCK2)-immune serum indicated that this Ca2+-insensitive PPCK exists as a 31-kDa monomer. COS PPCK-mediated rephosphorylation of the 107-kDa subunit (p107) of COS PEPC1 (Km = 2.2 microM) activated PEPC1 by approximately 80% when assayed under suboptimal conditions (pH 7.3, 0.2 mM PEP, and 0.125 mM malate). COS PPCK displayed remarkable selectivity for phosphorylating COS PEPC1 (relative to tobacco, sorghum, or maize PEPCs), exhibited a broad pH-activity optima of approximately pH 8.5, and at pH 7.3 was activated 40-65% by 1 mM PEP, or 10 mM Gln or Asn, but inhibited 65% by 10 mM L-malate. The possible control of COS PPCK by disulfide-dithiol interconversion was suggested by its rapid inactivation and subsequent reactivation when incubated with oxidized glutathione and then dithiothreitol. In vitro PPCK activity correlated with in vivo p107 phosphorylation status, with both peaking in mid-cotyledon to full-cotyledon developing COS. Notably, PPCK activity and p107 phosphorylation of developing COS were eliminated following pod excision or prolonged darkness of intact plants. Both effects were fully reversed 12 h following reillumination of darkened plants. These results implicate a direct relationship between the up-regulation of COS PPCK and p107 phosphorylation during the recommencement of photosynthate delivery from illuminated leaves to the non-photosynthetic COS. Overall, the results support the hypothesis that PEPC and PPCK participate in the control of photosynthate partitioning into C-skeletons needed as precursors for key biosynthetic pathways of developing COS.

The two divergent PEP-carboxylase catalytic subunits in the green microalga Chlamydomonas reinhardtii respond reversibly to inorganic-N supply and co-exist in the high-molecular-mass, hetero-oligomeric Class-2 PEPC complex

Our recent molecular studies revealed two divergent PEP-carboxylase (PEPC [Ppc]) encoding genes in the green microalga Chlamydomonas reinhardtii, CrPpc1 and CrPpc2, which are coordinately responsive to changes in inorganic-N and -C supply at the transcript level [Mamedov, T.G., Moellering, E.R. and Chollet, R. (2005) Identification and expression analysis of two inorganic C- and N-responsive genes encoding novel and distinct molecular forms of eukaryotic phosphoenolpyruvate carboxylase in the green microalga C. reinhardtii, Plant J. 42, 832-843]. Here, we report the distribution of these two encoded catalytic subunits in the minor Class-1 and predominant Class-2 PEPC enzyme-forms, the latter of which is a novel high-molecular-mass, hetero-oligomeric complex containing both CrPpc1 (p109) and CrPpc2 (p131) polypeptides. The Class-1 enzyme, however, is a typical PEPC homotetramer comprised solely of p109. We also document that the amount of both CrPpc1/2 catalytic subunits is up-/down-regulated by varying levels of NH(4)(+) supplied to the culture medium.

Arabidopsis phosphoenolpyruvate carboxylase genes encode immunologically unrelated polypeptides and are differentially expressed in response to drought and salt stress

The phosphoenolpyruvate carboxylase (PEPC) gene family of Arabidopsis is composed of four genes. Based on sequence analysis it was deduced that Atppc1, Atppc2 and Atppc3 genes encode plant-type PEPCs, whereas Atppc4 encodes a PEPC without phosphorylation motif, but no data at the protein level have been reported. Here, we describe the analysis of the four Arabidopsis PEPC polypeptides, which were expressed in Escherichia coli. Immunological characterization with anti plant-type PEPC and an anti-AtPPC4 antibody, raised in this work, showed that the bacterial-type PEPC is unrelated with plant-type PEPCs. Western-blot analysis of different Arabidopsis organs probed with anti plant-type PEPC antibodies detected a double band, the one with low molecular weight corresponding to the three plant-type PEPCs. The high molecular weight subunit is not encoded by any of the Arabidopsis PEPC genes. No bands were detected with the anti-AtPPC4 antibody. PEPC genes show differential expression in Arabidopsis organs and in response to environmental stress. Atppc2 transcripts were found in all Arabidopsis organs suggesting that it is a housekeeping gene. In contrast, Atppc3 gene was expressed in roots and Atppc1 in roots and flowers, as Atppc4. Highest PEPC activity was found in roots, which showed expression of the four PEPC genes. Salt and drought exerted a differential induction of PEPC gene expression in roots, Atppc4 showing the highest induction in response to both stresses. These results show that PEPC is part of the adaptation of the plant to salt and drought and suggest that this is the function of the new bacterial-type PEPC.

Cytosolic pyruvate kinase: subunit composition, activity, and amount in developing castor and soybean seeds, and biochemical characterization of the purified castor seed enzyme

Antibodies against Brassica napus cytosolic pyruvate kinase (PKc) (EC 2.7.1.40) were employed to examine PKc subunit composition and developmental profiles in castor and soybean seeds. A 56-kDa immunoreactive polypeptide was uniformly detected on immunoblots of clarified extracts from developing castor endosperm or soybean embryos. Maximal PKc activities occurred early in castor oil seed (COS) and soybean development (7.1 and 5.5 (micromol of pyruvate produced/min) g(-1) FW, respectively) and were up to 25-fold greater than those of fully mature seeds. Time-course studies revealed a close correlation between extractable PKc activity and the relative amount of the immunoreactive 56-kDa PKc polypeptide. PKc from developing COS was purified 1,874-fold to homogeneity and a final specific activity of 73.1 (micromol of pyruvate produced/min) mg(-1) protein. Gel filtration and SDS-PAGE indicated that this PKc exists as a 230-kDa homotetramer composed of 56-kDa subunits. The mass fingerprint of tryptic peptides of the 56-kDa COS PKc subunit best matched three putative PK(c)s from Arabidopsis thaliana. The purified enzyme was relatively heat-stable and displayed a broad pH optimum of 6.4. However, more efficient substrate utilization (in terms of Vmax /Km for phosphoenolpyruvate or ADP) was observed at pH 7.4. Glutamate was the most effective inhibitor, whereas aspartate functioned as an activator by partially relieving glutamate inhibition. Together with our previous studies, the results: (1) allow a model to be formulated regarding the coordinate allosteric control of PKc and phosphoenolpyruvate carboxylase by aspartate and glutamate in developing COS, and (2) provide further biochemical evidence that castor plant PKc exists as tissue-specific isozymes that exhibit substantial differences in their respective physical and regulatory properties.

In vitro proteolysis of phosphoenolpyruvate carboxylase from developing castor oil seeds by an endogenous thiol endopeptidase

Two novel phosphoenolpyruvate carboxylase (PEPC) isoforms have been biochemically characterized from endosperm of developing castor oil seeds (COS). The association of a 107 kDa PEPC subunit (p107) with an immunologically unrelated bacterial PEPC-type 64 kDa polypeptide leads to marked physical and kinetic differences between the PEPC1 p107 homotetramer and PEPC2 p107/p64 heterooctamer. COS p107 is quite susceptible to limited proteolysis during PEPC purification. An endogenous asparaginyl endopeptidase appears to catalyze the in vitro cleavage of an approximately 120 amino acid polypeptide from the N-terminal end of p107, producing a truncated 98 kDa polypeptide (p98). Immunoblotting was used to estimate proteolytic activity by following the disappearance of p107 and concomitant appearance of p98 during incubation of clarified COS extracts at 4 degrees C. The in vitro proteolysis of p107 to p98 only occurred in the combined presence of 2 mM dithiothreitol and high salt concentrations (particularly SO(4) (2-) and PO(4) (2-) salts). Although p107-degrading activity was present throughout COS development, it was most pronounced in endosperm extracts from older beans. Several protease inhibitors, including two commercially available protease inhibitor cocktails, were tested for their ability to prevent p107 proteolysis. All of the inhibitors were ineffective except for 2,2'-dipyridyl disulfide (DPDS), a relatively inexpensive and underutilized active site inhibitor of plant thiol proteases. Asparaginyl endopeptidase activity of COS extracts was unaffected by 20% (NH(4))(2)SO(4) when determined in the presence or absence of 2 mM dithiothreitol using a spectrophotometric assay based upon the hydrolysis of benzoyl-L-Asn-p-nitroanilide. Thus, we propose that the combined presence of 2 mM dithiothreitol and 20% (NH(4))(2)SO(4) promotes a p107 conformational change that exposes the N-terminal region asparaginyl residue where p107 hydrolysis is believed to occur.

In vivo regulatory phosphorylation of novel phosphoenolpyruvate carboxylase isoforms in endosperm of developing castor oil seeds

Our previous research characterized two phosphoenolpyruvate (PEP) carboxylase (PEPC) isoforms (PEPC1 and PEPC2) from developing castor oil seeds (COS). The association of a shared 107-kD subunit (p107) with an immunologically unrelated bacterial PEPC-type 64-kD polypeptide (p64) leads to marked physical and kinetic differences between the PEPC1 p107 homotetramer and PEPC2 p107/p64 heterooctamer. Here, we describe the production of antiphosphorylation site-specific antibodies to the conserved p107 N-terminal serine-6 phosphorylation site. Immunoblotting established that the serine-6 of p107 is phosphorylated in COS PEPC1 and PEPC2. This phosphorylation was reversed in vitro following incubation of clarified COS extracts or purified PEPC1 or PEPC2 with mammalian protein phosphatase type 2A and is not involved in a potential PEPC1 and PEPC2 interconversion. Similar to other plant PEPCs examined to date, p107 phosphorylation increased PEPC1 activity at pH 7.3 by decreasing its K(m)(PEP) and sensitivity to L-malate inhibition, while enhancing glucose-6-P activation. By contrast, p107 phosphorylation increased PEPC2's K(m)(PEP) and sensitivity to malate, glutamic acid, and aspartic acid inhibition. Phosphorylation of p107 was promoted during COS development (coincident with a >5-fold increase in the I(50) [malate] value for total PEPC activity in desalted extracts) but disappeared during COS desiccation. The p107 of stage VII COS became fully dephosphorylated in planta 48 h following excision of COS pods or following 72 h of dark treatment of intact plants. The in vivo phosphorylation status of p107 appears to be modulated by photosynthate recently translocated from source leaves into developing COS.

Identification and expression analysis of two inorganic C- and N-responsive genes encoding novel and distinct molecular forms of eukaryotic phosphoenolpyruvate carboxylase in the green microalga Chlamydomonas reinhardtii

Phosphoenolpyruvate carboxylase (PEPC [Ppc]) has been previously purified and characterized in biochemical and immunological terms from two green microalgae, Chlamydomonas reinhardtii and Selenastrum minutum. The findings indicate that these algae possess at least two distinct PEPC enzyme-forms, homotetrameric Class-1 and heteromeric Class-2, that differ significantly from each other and their plant and prokaryotic counterparts. Surprisingly, however, green-algal PEPC has been unexplored to date in molecular terms. This study reports the molecular cloning of the two Ppc genes in C. reinhardtii (CrPpc1, CrPpc2), each of which is transcribed in vivo and encodes a fully active, recombinant PEPC that lacks the regulatory, N-terminal seryl-phosphorylation domain that typifies the vascular-plant enzyme. These distinct catalytic subunit-types differ with respect to their (i) predicted molecular mass ( approximately 108.9 [CrPpc1] versus approximately 131.2 kDa [CrPpc2]) and critical C-terminal tetrapeptide; and (ii) immunoreactivity with antisera against the p102 and p130 polypeptides of S. minutum PEPC1/PEPC2 and PEPC2, respectively. Only the Ppc1 transcript encodes the p102 catalytic subunits common to both Class-1 and Class-2 enzyme-forms in C. reinhardtii. The steady-state transcript levels of both CrPpc1/2 are coordinately up-/down-regulated by changes in [CO2] or [NH] during growth, and generally mirror the response of cytoplasmic glutamine synthetase (Gs1) transcript abundance to changes in inorganic [N] at 5% CO2. These collective findings provide key molecular insight into the Ppc genes and corresponding PEPC catalytic subunits in the eukaryotic algae.

Phosphoenolpyruvate carboxylase: a new era of structural biology

There have been remarkable advances in our knowledge of this important enzyme in the last decade. This review focuses on three recent topics: the three-dimensional structure of the protein, molecular mechanisms of catalytic and regulatory functions, and the molecular cloning and characterization of PEPC kinases, which are Ser/Thr kinases involved specifically in regulatory phosphorylation of vascular plant PEPC. Analysis by X-ray crystallography and site-directed mutagenesis for E. coli and maize PEPC identified the catalytic site and allosteric effector binding sites, and revealed the functional importance of mobile loops. We present the reaction mechanism of PEPC in which we assign the roles of individual amino acid residues. We discuss the unique molecular property of PEPC kinase and its possible regulation at the post-translational level.