Regulation of spinach-leaf nitrate reductase by reversible phosphorylation

Role of protein phosphatases in the regulation of nitrogen nutrition in plants

The reversible protein phosphorylation and dephosphorylation mediated by protein kinases and phosphatases regulate different biological processes and their response to environmental cues, including nitrogen (N) availability. Nitrate assimilation is under the strict control of phosphorylation-dephosphorylation mediated post-translational regulation. The protein phosphatase family with approximately 150 members in Arabidopsis and around 130 members in rice is a promising player in N uptake and assimilation pathways. Protein phosphatase 2A (PP2A) enhances the activation of nitrate reductase (NR) by deactivating SnRK1 and reduces the binding of inhibitory 14-3-3 proteins on NR. The functioning of nitrate transporter NPF6.3 is regulated by phosphorylation of CBL9 (Calcineurin B like protein 9) and CIPK23 (CBL interacting protein kinase 23) module. Phosphorylation by CIPK23 inhibits the activity of NPF6.3, whereas protein phosphatases (PP2C) enhance the NPF6.3-dependent nitrate sensing. PP2Cs and CIPK23 also regulate ammonium transporters (AMTs). Under either moderate ammonium supply or high N demand, CIPK23 is bound and inactivated by PP2Cs. Ammonium uptake is mediated by nonphosphorylated and active AMT1s. Whereas, under high ammonium availability, CIPK23 gets activated and phosphorylate AMT1;1 and AMT1;2 rendering them inactive. Recent reports suggest the critical role of protein phosphatases in regulating N use efficiency (NUE). In rice, PP2C9 regulates NUE by improving N uptake and assimilation. Comparative leaf proteome of wild type and PP2C9 over-expressing transgenic rice lines showed 30 differentially expressed proteins under low N level. These proteins are involved in photosynthesis, N metabolism, signalling, and defence.

Nitrate triggered phosphoproteome changes and a PIN2 phosphosite modulating root system architecture

Nitrate commands genome‐wide gene expression changes that impact metabolism, physiology, plant growth, and development. In an effort to identify new components involved in nitrate responses in plants, we analyze the Arabidopsis thaliana root phosphoproteome in response to nitrate treatments via liquid chromatography coupled to tandem mass spectrometry. 176 phosphoproteins show significant changes at 5 or 20 min after nitrate treatments. Proteins identified by 5 min include signaling components such as kinases or transcription factors. In contrast, by 20 min, proteins identified were associated with transporter activity or hormone metabolism functions, among others. The phosphorylation profile of NITRATE TRANSPORTER 1.1 (NRT1.1) mutant plants was significantly altered as compared to wild‐type plants, confirming its key role in nitrate signaling pathways that involves phosphorylation changes. Integrative bioinformatics analysis highlights auxin transport as an important mechanism modulated by nitrate signaling at the post‐translational level. We validated a new phosphorylation site in PIN2 and provide evidence that it functions in primary and lateral root growth responses to nitrate.

SnRK1 and TOR: modulating growth-defense trade-offs in plant stress responses

The evolutionarily conserved protein kinase complexes SnRK1 and TOR are central metabolic regulators essential for plant growth, development, and stress responses. They are activated by opposite signals, and the outcome of their activation is, in global terms, antagonistic. Similarly to their yeast and animal counterparts, SnRK1 is activated by the energy deficit often associated with stress to restore homeostasis, while TOR is activated in nutrient-rich conditions to promote growth. Recent evidence suggests that SnRK1 represses TOR in plants, revealing evolutionary conservation also in their crosstalk. Given their importance for integrating environmental information into growth and developmental programs, these signaling pathways hold great promise for reducing the growth penalties caused by stress. Here we review the literature connecting SnRK1 and TOR to plant stress responses. Although SnRK1 and TOR emerge mostly as positive regulators of defense and growth, respectively, the outcome of their activities in plant growth and performance is not always straightforward. Manipulation of both pathways under similar experimental setups, as well as further biochemical and genetic analyses of their molecular and functional interaction, is essential to fully understand the mechanisms through which these two metabolic pathways contribute to stress responses, growth, and development.

Light regulation of nitrate reductase by catalytic subunits of protein phosphatase 2A

PP2A catalytic subunit C2 is of special importance for light/dark regulation of nitrate reductase activity. The level of unmethylated PP2A catalytic subunits decreases in darkness. Protein phosphatase 2A (PP2A) dephosphorylates and activates nitrate reductase (NR) in photosynthetically active tissue when plants are transferred from darkness to light. In the present work, investigation of Arabidopsis thaliana PP2A mutant lines revealed that one of the five PP2A catalytic subunit genes, e.g., C2, was of special importance for NR activation. Impairment of NR activation was, especially pronounced in the c2c4 double mutant. Though weaker, NR activation was also impaired in the c2 single mutant, and c1c2 and c2c5 double mutants. On the other hand, NR activation in the c4c5 double mutant was as efficient as in WT. The c4 single mutant had low PP2A activity, whereas the c2 single mutant possessed WT levels of extractable PP2A activity. PP2A activity was low in both c2c4 and c4c5. Differences in extracted PP2A activity among mutants did not strictly correlate with differences in NR activation, but underpinned that C2 has a special function in NR activation in vivo. The terminal leucine in PP2A catalytic subunits is generally methylated to a high degree, but regulation and impact of methylation/demethylation is barely studied. In WT and PP2A mutants, the level of unmethylated PP2A catalytic subunits decreased during 45 min of darkness, but did not change much when light was switched on. In leucine carboxyl methyl transferase1 (LCMT1) knockout plants, which possess mainly unmethylated PP2A, NR was still activated, although not fully as efficient as in WT.

Nitrate signaling and early responses in Arabidopsis roots

Nitrogen (N) is an essential macronutrient that impacts many aspects of plant physiology, growth, and development. Besides its nutritional role, N nutrient and metabolites act as signaling molecules that regulate the expression of a wide range of genes and biological processes. In this review, we describe recent advances in the understanding of components of the nitrate signaling pathway. Recent evidence posits that in one nitrate signaling pathway, nitrate sensed by NRT1.1 activates a phospholipase C activity that is necessary for increased cytosolic calcium levels. The nitrate-elicited calcium increase presumably activates calcium sensors, kinases, or phosphatases, resulting in changes in expression of primary nitrate response genes. Consistent with this model, nitrate treatments elicit proteome-wide changes in phosphorylation patterns in a wide range of proteins, including transporters, metabolic enzymes, kinases, phosphatases, and other regulatory proteins. Identifying and characterizing the function of the different players involved in this and other nitrate signaling pathways and their functional relationships is the next step to understand N responses in plants.

Protein phosphatases PP2A, PP4 and PP6: mediators and regulators in development and responses to environmental cues

The three closely related groups of serine/threonine protein phosphatases PP2A, PP4 and PP6 are conserved throughout eukaryotes. The catalytic subunits are present in trimeric and dimeric complexes with scaffolding and regulatory subunits that control activity and confer substrate specificity to the protein phosphatases. In Arabidopsis, three scaffolding (A subunits) and 17 regulatory (B subunits) proteins form complexes with five PP2A catalytic subunits giving up to 255 possible combinations. Three SAP-domain proteins act as regulatory subunits of PP6. Based on sequence similarities with proteins in yeast and mammals, two putative PP4 regulatory subunits are recognized in Arabidopsis. Recent breakthroughs have been made concerning the functions of some of the PP2A and PP6 regulatory subunits, for example the FASS/TON2 in regulation of the cellular skeleton, B' subunits in brassinosteroid signalling and SAL proteins in regulation of auxin transport. Reverse genetics is starting to reveal also many more physiological functions of other subunits. A system with key regulatory proteins (TAP46, TIP41, PTPA, LCMT1, PME-1) is present in all eukaryotes to stabilize, activate and inactivate the catalytic subunits. In this review, we present the status of knowledge concerning physiological functions of PP2A, PP4 and PP6 in Arabidopsis, and relate these to yeast and mammals.

New insight into the strategy for nitrogen metabolism in plant cells

Nitrogen (N) is one of the most important mineral nutrients required by higher plants. Primary N absorbed by higher plants includes nitrate (NO3(-)), ammonium (NH4(+)), and organic N. Plants have developed several mechanisms for regulating their N metabolism in response to N availability and environmental conditions. Numerous transporters have been characterized and the mode of N movement within plants has been demonstrated. For further assimilation of N, various enzymes are involved in the key processes of NO3(-) or NH4(+) assimilation. N and carbon (C) metabolism are tightly coordinated in the fundamental biochemical pathway that permits plant growth. As N and C metabolism are the fundamental constituents of plant life, understanding N regulation is essential for growing plants and improving crop production. Regulation of N metabolism at the transcriptional and posttranscriptional levels provides important perceptions in the complex regulatory network of plants to adapt to changing N availability. In this chapter, recent advances in elucidating molecular mechanisms of N metabolism processes and regulation strategy, as well as interactions between C and N, are discussed. This review provides new insights into the strategy for studying N metabolism at the cellular level for optimum plant growth in different environments.

Sensing nutrient and energy status by SnRK1 and TOR kinases

The perception of nutrient and energy levels inside and outside the cell is crucial to adjust growth and metabolism to available resources. The signaling pathways centered on the conserved TOR and SnRK1/Snf1/AMPK kinases have crucial and numerous roles in nutrient and energy sensing and in translating this information into metabolic and developmental adaptations. In plants evidence is mounting that, like in other eukaryotes, these signaling pathways have pivotal and antagonistic roles in connecting external or intracellular cues to many biological processes, including ribosome biogenesis, regulation of translation, cell division, accumulation of reserves and autophagy. Data on the plant TOR pathway have been hitherto rather scarce but recent findings have shed new light on its roles in plants. Moreover, the distinctive energy metabolism of photosynthetic organisms may reveal new features of these ancestral eukaryotic signaling elements.

Molecular mechanism of 14-3-3 protein-mediated inhibition of plant nitrate reductase

14-3-3 proteins regulate key processes in eukaryotic cells including nitrogen assimilation in plants by tuning the activity of nitrate reductase (NR), the first and rate-limiting enzyme in this pathway. The homodimeric NR harbors three cofactors, each of which is bound to separate domains, thus forming an electron transfer chain. 14-3-3 proteins inhibit NR by binding to a conserved phosphorylation site localized in the linker between the heme and molybdenum cofactor-containing domains. Here, we have investigated the molecular mechanism of 14-3-3-mediated NR inhibition using a fragment of the enzyme lacking the third domain, allowing us to analyze electron transfer from the heme cofactor via the molybdenum center to nitrate. The kinetic behavior of the inhibited Mo-heme fragment indicates that the principal point at which 14-3-3 acts is the electron transfer from the heme to the molybdenum cofactor. We demonstrate that this is not due to a perturbation of the reduction potentials of either the heme or the molybdenum center and conclude that 14-3-3 most likely inhibits nitrate reductase by inducing a conformational change that significantly increases the distance between the two redox-active sites.

Modulation of plant HMG-CoA reductase by protein phosphatase 2A: positive and negative control at a key node of metabolism

The enzyme HMG-CoA reductase (HMGR) has a key regulatory role in the mevalonate pathway for isoprenoid biosynthesis, critical not only for normal plant development, but also for the adaptation to demanding environmental conditions. Consistent with this notion, plant HMGR is modulated by many diverse endogenous signals and external stimuli. Protein phosphatase 2A (PP2A) is involved in auxin, abscisic acid, ethylene and brassinosteroid signaling and now emerges as a positive and negative multilevel regulator of plant HMGR, both during normal growth and in response to a variety of stress conditions. The interaction with HMGR is mediated by B" regulatory subunits of PP2A, which are also calcium binding proteins. The new discoveries uncover the potential of PP2A to integrate developmental and calcium-mediated environmental signals in the control of plant HMGR.

Protein phosphatase 2A regulatory subunits are starting to reveal their functions in plant metabolism and development

Canonical protein phosphatase 2A (PP2A) consists of a catalytic subunit (C), a scaffolding subunit (A), and a regulatory subunit (B). The B subunits are believed to confer substrate specificity and cellular localization to the PP2A complex, and are generally divided into three non-related families in plants, i.e., B55, B' and B''. The two Arabidopsis B55 subunits (α and β) interact with nitrate reductase (NR) in the bimolecular fluorescence complementation assay in planta, and are necessary for rapid activation of NR. Interestingly, knockout of all four B55 alleles is probably lethal, because a homozygous double knockout (pp2a-b55αβ) could not be found. The B55 subunits, therefore, appear to have essential functions that cannot be replaced by other regulatory B subunits. A double mutant (pp2a-b'αβ) of two close B' homologs show severely impaired fertility, pointing to the essential role also of B' subunits in plant development.

Protein phosphatase 2A B55 and A regulatory subunits interact with nitrate reductase and are essential for nitrate reductase activation

Posttranslational activation of nitrate reductase (NR) in Arabidopsis (Arabidopsis thaliana) and other higher plants is mediated by dephosphorylation at a specific Ser residue in the hinge between the molybdenum cofactor and heme-binding domains. The activation of NR in green leaves takes place after dark/light shifts, and is dependent on photosynthesis. Previous studies using various inhibitors pointed to protein phosphatases sensitive to okadaic acid, including protein phosphatase 2A (PP2A), as candidates for activation of NR. PP2As are heterotrimeric enzymes consisting of a catalytic (C), structural (A), and regulatory (B) subunit. In Arabidopsis there are five, three, and 18 of these subunits, respectively. By using inducible artificial microRNA to simultaneously knock down the three structural subunits we show that PP2A is necessary for NR activation. The structural subunits revealed overlapping functions in the activation process of NR. Bimolecular fluorescence complementation was used to identify PP2A regulatory subunits interacting with NR, and the two B55 subunits were positive. Interactions of NR and B55 were further confirmed by the yeast two-hybrid assay. In Arabidopsis the B55 group consists of the close homologs B55α and B55β. Interestingly, the homozygous double mutant (b55α × b55β) appeared to be lethal, which shows that the B55 group has essential functions that cannot be replaced by other regulatory subunits. Mutants homozygous for mutation in Bβ and heterozygous for mutation in Bα revealed a slower activation rate for NR than wild-type plants, pointing to these subunits as part of a PP2A complex responsible for NR dephosphorylation.

Multilevel control of Arabidopsis 3-hydroxy-3-methylglutaryl coenzyme A reductase by protein phosphatase 2A

Plants synthesize a myriad of isoprenoid products that are required both for essential constitutive processes and for adaptive responses to the environment. The enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes a key regulatory step of the mevalonate pathway for isoprenoid biosynthesis and is modulated by many endogenous and external stimuli. In spite of that, no protein factor interacting with and regulating plant HMGR in vivo has been described so far. Here, we report the identification of two B'' regulatory subunits of protein phosphatase 2A (PP2A), designated B''α and B''β, that interact with HMGR1S and HMGR1L, the major isoforms of Arabidopsis thaliana HMGR. B''α and B''β are Ca²⁺ binding proteins of the EF-hand type. We show that HMGR transcript, protein, and activity levels are modulated by PP2A in Arabidopsis. When seedlings are transferred to salt-containing medium, B''α and PP2A mediate the decrease and subsequent increase of HMGR activity, which results from a steady rise of HMGR1-encoding transcript levels and an initial sharper reduction of HMGR protein level. In unchallenged plants, PP2A is a posttranslational negative regulator of HMGR activity with the participation of B''β. Our data indicate that PP2A exerts multilevel control on HMGR through the five-member B'' protein family during normal development and in response to a variety of stress conditions.

The PP2A regulatory subunit Tap46, a component of the TOR signaling pathway, modulates growth and metabolism in plants

Tap42/α4, a regulatory subunit of protein phosphatase 2A, is a downstream effector of the target of rapamycin (TOR) protein kinase, which regulates cell growth in coordination with nutrient and environmental conditions in yeast and mammals. In this study, we characterized the functions and phosphatase regulation of plant Tap46. Depletion of Tap46 resulted in growth arrest and acute plant death with morphological markers of programmed cell death. Tap46 interacted with PP2A and PP2A-like phosphatases PP4 and PP6. Tap46 silencing modulated cellular PP2A activities in a time-dependent fashion similar to TOR silencing. Immunoprecipitated full-length and deletion forms of Arabidopsis thaliana TOR phosphorylated recombinant Tap46 protein in vitro, supporting a functional link between Tap46 and TOR. Tap46 depletion reproduced the signature phenotypes of TOR inactivation, such as dramatic repression of global translation and activation of autophagy and nitrogen mobilization, indicating that Tap46 may act as a positive effector of TOR signaling in controlling those processes. Additionally, Tap46 silencing in tobacco (Nicotiana tabacum) BY-2 cells caused chromatin bridge formation at anaphase, indicating its role in sister chromatid segregation. These findings suggest that Tap46, in conjunction with associated phosphatases, plays an essential role in plant growth and development as a component of the TOR signaling pathway.

Kinetic analysis of 14-3-3-inhibited Arabidopsis thaliana nitrate reductase

Eukaryotic assimilatory nitrate reductase (NR) is a dimeric multidomain molybdo-heme-flavo protein that catalyzes the first and rate-limiting step in the nitrate assimilation of plants, algae, and fungi. Nitrate reduction takes place at the N-terminal molybdenum cofactor-containing domain. Reducing equivalents are derived from NADH, which reduce the C-terminal FAD domain followed by single-electron transfer steps via the middle heme domain to the molybdenum center. In plants, nitrate reduction is post-translationally inhibited by phosphorylation and subsequent binding of 14-3-3 protein to a conserved phosphoserine located in the surface-exposed hinge between the catalytic and heme domain. Here we investigated Arabidopsis thaliana NR activity upon phosphorylation and 14-3-3 binding by using a fully defined in vitro system with purified proteins. We demonstrate that among different calcium-dependent protein kinases (CPKs), CPK-17 efficiently phosphorylates Ser534 in NR. Out of eight purified Arabidopsis 14-3-3 proteins, isoforms ω, κ, and λ exhibited the strongest inhibition of NR. The kinetic parameters of noninhibited, phosphorylated NR (pNR) and pNR in a complex with 14-3-3 were investigated. An 18-fold reduction in k(cat) and a decrease in the apparent K(M)(nitrate) (from 280 to 141 μM) were observed upon binding of 14-3-3 to pNR, suggesting a noncompetitive inhibition with a preferential binding to the substrate-bound state of the enzyme. Recording partial activities of NR demonstrated that the transfer of electrons to the heme is not affected by 14-3-3 binding. The Ser534Ala variant of NR was not inhibited by 14-3-3 proteins. We propose that 14-3-3 binding to Ser534 blocks the transfer of electrons from heme to nitrate by arresting the domain movement via hinge 1.

14-3-3 and FHA domains mediate phosphoprotein interactions

Many aspects of plant growth and development require specific protein interactions to carry out biochemical and cellular functions. Several proteins mediate these interactions, two of which specifically recognize phosphoproteins: 14-3-3 proteins and proteins with FHA domains. These are the only phosphobinding domains identified in plants. Both domains are present in animals and plants, and are used by plant proteins to regulate metabolic, developmental, and signaling pathways. 14-3-3s regulate sugar metabolism, proton gradients, and control transcription factor localization. FHA domains are modular domains often found in multidomain proteins that are involved in signal transduction and plant development.

Signalling cascades integrating light-enhanced nitrate metabolism

In higher plants, light is crucial for regulation of nitrate uptake, translocation and assimilation into organic compounds. Part of this metabolism is tightly coupled to photosynthesis because the enzymes involved, nitrite reductase and glutamate synthase, are localized to the chloroplasts and receive reducing power from photosynthetic electron transport. However, important enzymes in nitrate acquisition and reduction are localized to cellular compartments other than chloroplasts and are also up-regulated by light, i.e. transporters in cell and organellar membranes and nitrate reductase in the cytosol. This review describes the different light-dependent signalling cascades regulating nitrate metabolism at the transcriptional as well as post-transcriptional level, and how reactions in different compartments of the cell are co-ordinated. Essential players in this network are phytochrome and HY5 (long hypocotyls 5)/HYH (HY5 homologue)-dependent signalling pathways, the energy-related AMPK (AMP-activated protein kinase) protein kinase homologue SNRK1 (sucrose non-fermenting kinase 1-related kinase), chloroplastic thioredoxins and the prokaryotically originated PII protein. A complex light-dependent network of regulation emerges, which appears to be necessary for optimal nitrogen assimilation and for avoiding the accumulation of toxic intermediates and side products, such as nitrite and reactive oxygen compounds.

Virus-induced gene silencing of 14-3-3 genes abrogates dark repression of nitrate reductase activity in Nicotiana benthamiana

In order to study the effect of repression of 14-3-3 genes on actual activity of the nitrate reductase (NR) in Nicotiana benthamiana leaves, Nb14-3-3a gene was silenced by virus-induced gene silencing (VIGS) method using potato virus X (PVX). Expression of Nb14-3-3a as well as Nb14-3-3b genes was altogether repressed in the leaves of PVX-14-3a-infected plants. Furthermore, two-dimensional gel electrophoresis and immunoblot analysis with anti-14-3-3 antiserum suggested that the expressions of Nb14-3-3a and Nb14-3-3b proteins are accordingly repressed in PVX-14-3a-infected plants. It is well known that binding of 14-3-3 proteins to phosphorylated NR leads to substantial decrease in NR activity of leaves under darkness. Therefore, we studied the changes in NR activity in response to light/dark transitions in the leaves of PVX-14-3a-infected plants. NR activation state was kept at a high level under darkness in PVX-14-3a-infected plants, but not in PVX-green fluorescent protein (GFP)-infected and control plants. This result suggests that Nb14-3-3a and/or Nb14-3-3b proteins are indeed involved in the inactivation of NR activity under darkness in N. benthamiana.

Evidence for increased proton dissociation in low-activity forms of dephosphorylated squash-leaf nitrate reductase

The pH dependence of squash-leaf nitrate reductase has been studied. It has been found that high- and low-activity forms of purified nitrate reductase (both forms dephosphorylated) have different optimum pH values. A high-activity form has always a higher pH optimum compared with a low-activity form. Model computations show that the decrease in activity and the corresponding change of the pH optimum is apparently due to a conformation-dependent increase of proton dissociation of the enzyme. As previously shown, this behavior is also observed in leaf extracts during the conversion (and probably phosphorylation of nitrate reductase) from a high-active form to a low-active form when plants are transferred from light to darkness.

Arabidopsis PPP family of serine/threonine phosphatases

Serine/threonine-specific phosphoprotein phosphatases (PPPs) are ubiquitous enzymes in all eukaryotes, but their regulatory functions are largely unknown in higher plants. The Arabidopsis genome encodes 26 PPP catalytic subunits related to type 1, type 2A and so-called novel phosphatases, including four plant-specific enzymes carrying large N-terminal kelch-domains, but no apparent homologue of the PP2B family. The catalytic subunits of PPPs associate with regulatory protein partners that target them to well defined cellular locations and modulate their activity. Recent studies of phosphatase partners and their interactions have directed attention again to functional dissection of plant PPP families, and highlight their intriguing roles in the regulation of metabolism, cell cycle and development, as well as their roles in light, stress and hormonal signalling.

Salinity-induced tissue-specific diurnal changes in nitrogen assimilatory enzymes in tomato seedlings grown under high or low nitrate medium

We studied the salt stress (100 mM NaCl) effects on the diurnal changes in N metabolism enzymes in tomato seedlings (Lycopersicon esculentum Mill. cv. Chibli F1) that were grown under high nitrogen (HN, 5 mM NO(3)(-)) or low nitrogen (LN, 0.1 mM NO(3)(-)). NaCl stress led to a decrease in plant DW production and leaf surface to higher extent in HN than in LN plants. Total leaf chlorophyll (Chl) content was decreased by salinity in HN plants, but unchanged in LN plants. Soluble protein content was decreased by salt in the leaves from HN and LN plants, but increased in the stems-petioles from LN plants. Nitrate reductase (NR, EC 1.6.1.6) showed an activity peak during first part of the light period, but no diurnal changes were observed for the nitrite reductase (NiR, EC 1.7.7.1) activity. Glutamine synthetase (GS, EC 6.3.1.2) and glutamate synthase (Fd-GOGAT, EC 1.4.7.1) activities increased in HN plant leaves during the second part of the light period, probably when enough ammonium is produced by nitrate reduction. NR and NiR activities in the leaves were more decreased by NaCl in LN than in HN plants, whereas the opposite response was obtained for the GS activity. Fd-GOGAT activity was inhibited by NaCl in HN plant leaves, while salinity did not shift the peak of the NR and Fd-GOGAT activities during a diurnal cycle. The induction by NaCl stress occurred for the NR and GS activities in the roots of both HN and LN plants. Glutamate dehydrogenase (GDH, EC 1.4.1.2) activity shifted from the deaminating activity to the aminating activity in all tissues of HN plants. In LN plants, both aminating and deaminating activities were increased by salinity in the leaves and roots. The differences in the sensitivity to NaCl between HN and LN plants are discussed in relation to the N metabolism status brought on by salt stress.

Posttranslational regulation of nitrate reductase strongly affects the levels of free amino acids and nitrate, whereas transcriptional regulation has only minor influence

Diurnal variations in nitrate reductase (NR) activity and nitrogen metabolites were examined in wild-type Nicotiana plumbaginifolia and transformants with various degrees of NR deregulation. In the C1 line, NR was only deregulated at the transcriptional level by placing the NR gene under the control of the cauliflower mosaic virus 35S RNA promoter. In the Del8 and S521D lines, NR was additionally deregulated at the posttranslational level either by a deletion mutation in the N-terminal domain or by a mutation of the regulatory phosphorylation site (serine-521). Posttranslational regulation was essential for pronounced diurnal variations in NR activity. Low nitrate content was related to deregulation of NR, whereas the level of total free amino acids was much higher in plants with fully deregulated NR. Abolishing transcriptional and posttranslational regulation (S521D plants) resulted in an increase of glutamine and asparagine by a factor of 9 and 14, respectively, compared with wild type, whereas abolishing transcriptional regulation (C1 plants) only resulted in increases of glutamine and asparagine by factors <2. Among the minor amino acids, isoleucine and threonine, in particular, showed enhanced levels in S521D. Nitrate uptake rates were the same in S521D and wild type as determined with (15)N feeding. Deregulation of NR appears to set the level of certain amino acids, whereas diurnal variations were still determined by light/dark. Generally, deregulation of NR at the transcriptional level did not have much influence on metabolite levels, but additional deregulation at the posttranslational level resulted in profound changes of nitrogen metabolite levels.

Structure, evolution and expression of a second subfamily of protein phosphatase 2A catalytic subunit genes in the rice plant (Oryza sativa L.)

Protein phosphatase 2A (PP2A) is one of the major serine/threonine protein phosphatases in the cell and plays a variety of regulatory roles in metabolism and signal transduction. Previously, we described the structure and expression of two genes encoding PP2A catalytic subunits (PP2Ac)--OsPP2A-1 and OsPP2A-3--in the rice plant (Yu et al. 2003). Here, we report the isolation and characterisation of a second structurally distinguishable PP2Ac subfamily comprised of three additional isogenes, OsPP2A-2, OsPP2A-4 (each containing ten introns) and OsPP2A-5 (which contains nine introns). Northern blot analysis demonstrated that the three isogenes are ubiquitously expressed in all rice tissues during plant development, and differentially expressed in response to high salinity and the combined stresses of drought and heat. Phylogenetic analyses indicated that the two PP2Ac subfamilies are descended from two ancient lineages, which derived from gene duplications that occurred after the monocotyledon-dicotyledon split. In the second subfamily, it is proposed that two duplication events were involved; in which, the initial duplication of a ten-intron primordial gene yielded OsPP2A-2 and the progenitor of OsPP2A-4 and OsPP2A-5. The OsPP2A-4/OsPP2A-5 progenitor, in turn, underwent a second duplication event, resulting in the present day OsPP2A-4 and OsPP2A-5. It is proposed that loss of the 5'-most intron from OsPP2A-5 occurred after these two duplication events.

Mechanism and importance of post-translational regulation of nitrate reductase

In higher plants, nitrate reductase (NR) is inactivated by the phosphorylation of a conserved Ser residue and binding of 14-3-3 proteins in the presence of divalent cations or polyamines. A transgenic Nicotiana plumbaginifolia line (S521) has been constructed where the regulatory, conserved Ser 521 of tobacco NR (corresponding to Ser 534 in Arabidopsis) was mutated into Asp. This mutation resulted in the complete abolition of activation/inactivation in response to light/dark transitions or other treatments known to regulate the activation state of NR. Analysis of the transgenic plants showed that, under certain conditions, when whole plants or cut tissues are exposed to high nitrate supply, post-translational regulation is necessary to avoid nitrite accumulation. Abolition of the post-translational regulation of NR also results in an increased flux of nitric oxide from the leaves and roots. In view of the results obtained from examining the different transgenic N. plumbaginifolia lines, compartmentation of nitrate into an active metabolic pool and a large storage pool appears to be an important factor for regulating nitrate reduction. The complex regulation of nitrate reduction is likely to have evolved not only to optimize nitrogen assimilation, but also to prevent and control the formation of toxic, and possibly regulatory, products of NR activities. Phos phorylation of NR has previously been found to influence the degradation of NR in spinach leaves and Arabidopsis cell cultures. However, experiments with whole plants of N. plumbaginifolia, Arabidopsis, or squash are in favour of NR degradation being the same in light and darkness and independent of phosphorylation at the regulatory Ser.

Cloning of a nitrate reductase inactivator (NRI) cDNA from Spinacia oleracea L. and expression of mRNA and protein of NRI in cultured spinach cells

The spinach ( Spinacia oleracea L. (cv. Hoyo) nitrate reductase inactivator (NRI) is a novel protein that irreversibly inactivates NR. Using degenerate primers based on an N-terminal amino acid sequence of NRI purified from spinach leaves and a cDNA library, we isolated a full-length NRI cDNA from spinach that contains an open reading frame encoding 479 amino acid residues. This protein shares 67.4% and 51.1-68.3% amino acid sequence similarities with a nucleotide pyrophosphatase (EC 3.6.1.9) from rice and three types of the nucleotide pyrophosphatase-like protein from Arabidopsis thaliana, respectively. Immunoblot analysis revealed that NRI was constitutively expressed in suspension-cultured spinach cells; however, its expression level is quite low in 1-day-subcultured cells. Moreover, northern blot analysis indicated that this expression was regulated at the mRNA level. These results suggest that NRI functions in mature cells.

Function and specificity of 14-3-3 proteins in the regulation of carbohydrate and nitrogen metabolism

Protein phosphorylation is key to the regulation of many proteins. Altered protein activity often requires the interaction of the phosphorylated protein with a class of "adapters" known as 14-3-3 proteins. This review will cover aspects of 14-3-3 interaction with key proteins of carbon and nitrogen metabolism such as nitrate reductase, glutamine synthetase and sucrose-phosphate synthase. It will also address 14-3-3 involvement in signal transduction pathways with emphasis on the regulation of plant metabolism. To date, 14-3-3 proteins have been identified and studied in many diverse systems, yielding a plethora of data, requiring careful analysis and interpretation. Problems such as these are not uncommon when dealing with multigene families. The number of isoforms makes the question of redundancy versus specificity of 14-3-3 proteins a crucial one. This issue is discussed in relation to structure, function and expression of 14-3-3 proteins.

A protein kinase activated by darkness phosphorylates nitrate reductase in Komatsuna (Brassica campestris) leaves

Although it has been shown that leaf nitrate reductase (NR: EC 1.6.6.1) is phosphorylated by subjecting plants to darkness, there is no evidence for the existence of dark-activated or dark-induced NR kinase. This study was undertaken to investigate the occurrence of a protein kinase phosphorylating NR in response to dark treatments. Immediately after transferring Komatsuna (Brassica campestris L.) plants to darkness, we observed rapid increases in the phosphorylating activity of the synthetic peptide, which is designed for the amino acid sequence surrounding the regulatory serine residue of the hinge 1 region of Komatsuna NR, in crude extracts from leaves. The activity reached a maximum after 10 min of darkness. Inactivation states of NR estimated from relative activities with or without Mg2+ were correlated to activities of the putative dark-activated protein kinase. Using the synthetic peptide as a substrate, we purified a protein kinase from dark-treated leaves by means of successive chromatographies on Q-Sepharose, Blue Sepharose, FPLC Q-Sepharose, and ATP-gamma-Sepharose columns. The purified kinase had an apparent molecular mass of 150 kDa with a catalytic subunit of 55 kDa, and it was Ca2+-independent. The purified kinase phosphorylated a recombinant cytochrome c reductase protein, a partial protein of NR, and holo NR, and inactivated NR in the presence of both 14-3-3 protein and Mg2+. The kinase also phosphorylated synthetic peptide substrates designed for sucrose phosphate synthase and 3-hydroxy-3-methylglutaryl-Coenzyme A reductase. Among inhibitors tested, only K252a, a potent and specific serine/threonine kinase inhibitor, completely inhibited the activity of the dark-activated kinase. The activity of the purified kinase was also specifically inhibited by K252a. Taken together with these findings, results obtained suggest that the putative dark-activated protein kinase may be the purified kinase itself, and may be responsible for in vivo phosphorylation of NR and its inactivation during darkness.

Nitrate reductase from winter wheat leaves is activated at low temperature via protein dephosphorylation

The effect of short-term low temperature treatment on nitrate reductase (NR, EC 1.6.6.1) activity, NR protein and NR transcript levels in excised leaves of winter wheat (Triticum aestivum L. cv. Sadovo-1) was investigated. NR activity, measured in the presence of Mg2+ (NRact), doubled within 2 h at 4 degrees C, whereas NR activity, measured in the presence of EDTA (NRmax), did not respond to the cold treatment. Such an activation of NR occurred only if leaves were exposed to low temperature in the light but not in the dark. It was not affected by feeding cytoplasmic protein synthesis inhibitor, cycloheximide, or protein kinase inhibitor, staurosporin, but was completely prevented by okadaic acid, an inhibitor of protein phosphatases of the type 1 and 2 A. This inhibitory effect decreased gradually when okadaic acid-concentration in the nutrient solution was lowered below 1 &mgr;M and tended to disappear when leaves were fed with 10 nM okadaic acid. It was demonstrated that the cold-induced NR activation was dependent neither on cold-triggered calcium influx nor on high endogenous abscisic acid levels. The increased NRact in cold-exposed leaves was found to correlate with a higher level of NR transcript but not with an increased NR protein level. Feeding okadaic acid to these leaves prevented the cold-induced accumulation of NR mRNA. These data point to protein phosphatases of the type 2 A being involved in NR protein dephosphorylation and NR transcript accumulation as targets of activation by low temperature treatment.

Post-translational regulation of nitrate reductase: mechanism, physiological relevance and environmental triggers

Assimilatory nitrate reductase (NR) of higher plants is a most interesting enzyme, both from its central function in plant primary metabolism and from the complex regulation of its expression and control of catalytic activity and degradation. Here, present knowledge about the mechanism of post-translational regulation of NR is summarized and the properties of the regulatory enzymes involved (protein kinases, protein phosphatases and 14-3-3-binding proteins) are described. It is shown that light and oxygen availability are the major external triggers for the rapid and reversible modulation of NR activity, and that sugars and/or sugar phosphates are the internal signals which regulate the protein kinase(s) and phosphatase. It is also demonstrated that stress factors like nitrate deficiency and salinity have remarkably little direct influence on the NR activation state. Further, changes in NR activity measured in vitro are not always associated with changes in nitrate reduction rates in vivo, suggesting that NR can be under strong substrate limitation. The degradation and half-life of the NR protein also appear to be affected by NR phosphorylation and 14-3-3 binding, as NR activation always correlates positively with its stability. However, it is not known whether the molecular form of NR in vivo affects its susceptibility to proteolytic degradation, or whether factors that affect the NR activation state also independently affect the activity or induction of the NR protease(s). A second and potentially important function of NR, the production of nitric oxide (NO) from nitrite is briefly described, but it remains to be determined whether NR produces NO for pathogen/stress signalling in vivo.

Modulation of nitrate reductase activity in cucumber (Cucumis sativus) roots

Nitrate reductase (NR) (EC 1.6.6.1) activity and NR activation state, i.e. activity in the presence of Mg(2+) relative to activity in the absence of Mg(2+), in cucumber (Cucumis sativus) leaves increased in the light and decreased in the dark. In contrast to leaves, NR activation state in the roots did not show light/dark-dependent changes. Root NR was activated by anoxia or by addition of uncoupler (CCCP) or mannose. These treatments decreased ATP levels in root tissue. On the contrary, high oxygen supply promoted some NR inactivation. When an extract from anoxic roots was preincubated with ATP, NR was gradually inactivated. Subsequent addition of 5'-AMP resulted in a remarkable reactivation of the enzyme. NR extracted from hyperoxygenated roots was activated by preincubation with 5'-AMP, and the process was reversed by ATP. These results suggest the participation of adenine nucleotides on the in vivo modulation of NR activity in cucumber roots. NR was activated in vivo by cellular acidification and inactivated by alkalinisation. The acid-induced activation of NR was greatly prevented by okadaic acid, a protein phosphatase inhibitor. Our data indicate that, as in barley roots, anoxia, uncouplers, and mannose feeding activate cucumber root NR, at least partly, by enhancing NR dephosphorylation via a decrease in the internal level of ATP and a concomitant cellular acidification.

Protein phosphorylation in pea root plastids

Protein phosphorylation has been investigated in non-photosynthetic plastids of pea roots. Intact and lysed preparations of plastids were incubated with [gamma-(32)P]ATP and three stromal proteins of sizes 41, 58 and 62 kDa were phosphorylated on a serine residue. No other proteins were significantly labelled under the conditions used. The 62 kDa protein is probably phosphoglucomutase and represents a phosphoenzyme catalytic intermediate. The protein kinase(s) and phosphatase(s) acting on the other proteins were not sensitive to exogenous calcium but were sensitive to magnesium. The protein phosphatase which acts on the 41 kDa protein is possibly of type 2C, whereas that acting on the 58 kDa phosphoprotein did not fall into any class defined by mammalian systems. Metabolism of exogenous glucose 6-phosphate by the oxidative pentose phosphate pathway in intact plastids abolished the phosphorylation of the 58 kDa protein. Dihydroxyacetone phosphate, phosphoenolpyruvate and 3-phosphoglycerate also inhibited phosphorylation of the 58 kDa protein and had a time-dependent effect on the phosphorylation of the 41 kDa protein. The significance of these results in relation to a possible role for protein phosphorylation in these plastids is considered.

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.

Deletion of the nitrate reductase N-terminal domain still allows binding of 14-3-3 proteins but affects their inhibitory properties

Nitrate reductase (NR) is post-translationally regulated by phosphorylation and binding of 14-3-3 proteins. Deletion of 56 amino acids in the amino-terminal domain of NR was previously shown to impair this type of regulation in tobacco (Nicotiana plumbaginifolia) (L. Nussaume, M. Vincentez, C. Meyer, J.-P. Boutin, M. Caboche [1995] Plant Cell 7: 611-621), although both full-length NR and deleted NR (DeltaNR) appeared to be phosphorylated in darkness (C. Lillo, S. Kazazaic, P. Ruoff, C. Meyer [1997] Plant Physiol 114: 1377-1383). We show here that in the presence of Mg(2+) and phosphatase inhibitors, NR and endogenous 14-3-3 proteins copurify through affinity chromatography. Assay of NR activity and western blots showed that endogenous 14-3-3 proteins copurified with both NR and DeltaNR. Electron transport in the heme-binding domain of DeltaNR was inhibited by Mg(2+)/14-3-3, whereas this was not the case for NR. This may indicate a different way of binding for 14-3-3 in the DeltaNR compared with NR. The DeltaNR was more labile than NR, in vitro. Lability was ascribed to the molybdopterin binding domain, and apparently an important function of the 56 amino acids is stabilization of this domain.

NITRATE REDUCTASE STRUCTURE, FUNCTION AND REGULATION: Bridging the Gap between Biochemistry and Physiology

Nitrate reductase (NR; EC 1.6.6.1-3) catalyzes NAD(P)H reduction of nitrate to nitrite. NR serves plants, algae, and fungi as a central point for integration of metabolism by governing flux of reduced nitrogen by several regulatory mechanisms. The NR monomer is composed of a ~100-kD polypeptide and one each of FAD, heme-iron, and molybdenum-molybdopterin (Mo-MPT). NR has eight sequence segments: (a) N-terminal "acidic" region; (b) Mo-MPT domain with nitrate-reducing active site; (c) interface domain; (d) Hinge 1 containing serine phosphorylated in reversible activity regulation with inhibition by 14-3-3 binding protein; (e) cytochrome b domain; (f) Hinge 2; (g) FAD domain; and (h) NAD(P)H domain. The cytochrome b reductase fragment contains the active site where NAD(P)H transfers electrons to FAD. A complete three-dimensional dimeric NR structure model was built from structures of sulfite oxidase and cytochrome b reductase. Key active site residues have been investigated. NR structure, function, and regulation are now becoming understood.

Arabidopsis thaliana proteins related to the yeast SIP and SNF4 interact with AKINalpha1, an SNF1-like protein kinase

AKINalpha1, a Ser/Thr kinase from Arabidopsis thaliana belongs to the highly conserved SNF1 family of protein kinases in eukaryotes. Recent data suggest that the plant SNF1-related kinases (SnRK1 family) are key enzymes implicated in the regulation of carbohydrate and lipid metabolism. In Saccharomyces cerevisiae and mammals, the SNF1 and AMPKalpha protein kinases interact with two other families of proteins, namely SNF4/AMPKgamma and SIP1/SIP2/GAL83/AMPKbeta, to form active heterotrimeric complexes. In this paper, we describe the characterisation of three novel cDNAs. AKINbeta1 and AKINbeta2 encode proteins similar to SIP1, SIP2 and GAL83 and AKINgamma codes for a protein showing similarity with SNF4. Using the two-hybrid system, specific interactions have been shown between A. thaliana AKINbeta1/beta2, AKINgamma and AKINgamma as well as between the A. thaliana and S. cerevisiae subunits. Interestingly, AKINbeta1, AKINbeta2 and AKINgamma mRNAs accumulate differentially in A. thaliana tissues and are modulated during development and under different growth conditions. These data suggest the presence in higher plants of a conserved heterotrimeric complex. Moreover, the differential transcription of different non-catalytic subunits can constitute a first level of regulation of the SNF1-like complex in plants.

A conserved acidic motif in the N-terminal domain of nitrate reductase is necessary for the inactivation of the enzyme in the dark by phosphorylation and 14-3-3 binding

It has previously been shown that the N-terminal domain of tobacco (Nicotiana tabacum) nitrate reductase (NR) is involved in the inactivation of the enzyme by phosphorylation, which occurs in the dark (L. Nussaume, M. Vincentz, C. Meyer, J.P. Boutin, and M. Caboche [1995] Plant Cell 7: 611-621). The activity of a mutant NR protein lacking this N-terminal domain was no longer regulated by light-dark transitions. In this study smaller deletions were performed in the N-terminal domain of tobacco NR that removed protein motifs conserved among higher plant NRs. The resulting truncated NR-coding sequences were then fused to the cauliflower mosaic virus 35S RNA promoter and introduced in NR-deficient mutants of the closely related species Nicotiana plumbaginifolia. We found that the deletion of a conserved stretch of acidic residues led to an active NR protein that was more thermosensitive than the wild-type enzyme, but it was relatively insensitive to the inactivation by phosphorylation in the dark. Therefore, the removal of this acidic stretch seems to have the same effects on NR activation state as the deletion of the N-terminal domain. A hypothetical explanation for these observations is that a specific factor that impedes inactivation remains bound to the truncated enzyme. A synthetic peptide derived from this acidic protein motif was also found to be a good substrate for casein kinase II.

Chilling delays circadian pattern of sucrose phosphate synthase and nitrate reductase activity in tomato

Overnight low-temperature exposure inhibits photosynthesis in chilling-sensitive species such as tomato (Lycopersicon esculentum) and cucumber by as much as 60%. In an earlier study we showed that one intriguing effect of low temperature on chilling-sensitive plants is to stall the endogenous rhythm controlling transcription of certain nuclear-encoded genes, causing the synthesis of the corresponding transcripts and proteins to be mistimed when the plant is rewarmed. Here we show that the circadian rhythm controlling the activity of sucrose phosphate synthase (SPS) and nitrate reductase (NR), key control points of carbon and nitrogen metabolism in plant cells, is delayed in tomato by chilling treatments. Using specific protein kinase and phosphatase inhibitors, we further demonstrate that the chilling-induced delay in the circadian control of SPS and NR activity is associated with the activity of critical protein phosphatases. The sensitivity of the pattern of SPS activity to specific inhibitors of transcription and translation indicates that there is a chilling-induced delay in SPS phosphorylation status that is caused by an effect of low temperature on the expression of a gene coding for a phosphoprotein phosphatase, perhaps the SPS phosphatase. In contrast, the chilling-induced delay in NR activity does not appear to arise from effects on NR phosphorylation status, but rather from direct effects on NR expression. It is likely that the mistiming in the regulation of SPS and NR, and perhaps other key metabolic enzymes under circadian regulation, underlies the chilling sensitivity of photosynthesis in these plant species.

A novel autophosphorylation mediated regulation of nitrite reductase in Candida utilis

The assimilatory nitrite reductase catalyses the conversion of nitrite to ammonia. The enzyme from Candida utilis has been previously purified to homogeneity and shown to be a heterodimer consisting of 58 kDa and 66 kDa subunits. The enzyme has also been shown to be induced by nitrate and repressed by ammonium ions. The levels of nitrite reductase mRNA, its protein and the enzyme activity were modulated together indicating that the primary level of regulation of this enzyme existed at the transcriptional level. Here we report that the 58 kDa and 66 kDa subunits of the enzyme were differentially phosphorylated under the induced and repressed conditions, indicating a second level of regulation. The highly phosphorylated 66 kDa subunit was shown to be dephosphorylated by calf intestinal alkaline phosphatase. The enzymatic activity associated with the native enzyme also decreased due to the dephosphorylation. Each of the subunits could undergo autophosphorylation at serine/threonine residues as demonstrated by thin layer chromatography and recognition by antibodies to phosphoamino acids. The presence of similar phosphorylated subunits under in vivo conditions has also been demonstrated. A model has been proposed to explain the post-translational regulation of the enzyme.

Three spinach leaf nitrate reductase-3-hydroxy-3-methylglutaryl-CoA reductase kinases that are regulated by reversible phosphorylation and/or Ca2+ ions

In spinach (Spinacea oleracea L.) leaf extracts, three protein kinases (PKI, PKII and PKIII) were identified each of which phosphorylated spinach nitrate reductase on serine-543, and inactivated the enzyme in the presence of nitrate reductase inhibitor, 14-3-3. PKIII was also very active in phosphorylating and inactivating Arabidopsis (Landsberg erecta) 3-hydroxy-3-methylglutaryl-coenzyme A reductase 1 (HMGR1). PKI and PKII phosphorylated HMGR1 more slowly than PKIII, compared with their relative rates of phosphorylation of nitrate reductase. HMGR1 identical with those that are seen after phosphorylation of serine-577 by the sucrose non-fermenting (SNF1)-like PK, 3-hydroxy-3-methylglutaryl-Co A reductase kinase A (HRK-A), from cauliflower [Dale, Arró, Becerra, Morrice, Boronat, Hardie and Ferrer (1995) Eur. J. Biochem. 233, 506-513]. PKI was Ca2+-dependent when prepared in the absence of protein phosphatase (PP) inhibitors, and largely Ca2+-dependent when prepared in the presence of PP inhibitors (NaF and EGTA). The Ca2+-independent portion of PKI was inactivated by either PP2A or PP2C, while the Ca2+-dependent portion of PKI became increasingly activated during storage, which we presume was mimicking the effect of an unidentified PP. These findings indicate that PK1 is regulated by two functionally distinct phosphorylations. PKI had a molecular mass of 45 kDa on gel filtration and was active towards substrate peptides that terminated at the +2 residue from the phosphorylation site, whereas PKIII was inactive towards these peptides. PKII was Ca2+-stimulated under all conditions tested. PKIII was Ca2+-indepdented, inactivated by PP2A or PP2C, had a requirement for a hydrophobic residue in the +4 position of peptide substrates, had a molecular mass by gel filtration of approximately 140 kDa, and an antibody against the rye SNF1-related PK (RKIN1) recognized a 58 kDa subunit in fractions containing PKIII. These properties of PKIII are identical with those reported previously for the SNF1-like enzyme, HRK-A. Our results indicate a considerable complexity of kinase cascades mediating the regulation of assimilatory and biosynthetic pathways in response to environmental stimuli in plants.

14-3-3 proteins associate with the regulatory phosphorylation site of spinach leaf nitrate reductase in an isoform-specific manner and reduce dephosphorylation of Ser-543 by endogenous protein phosphatases

Three lines of evidence indicate that the 14-3-3 proteins that inactivate the phosphorylated form of spinach leaf NADH:nitrate reductase (NR) bind to the enzyme at the regulatory phosphorylation site (Ser-543). First, a phosphorylated synthetic peptide based on the regulatory site can prevent and also reverse the inactivation of phospho-NR caused by 14-3-3 proteins. Second, sequence-specific and phosphorylation-dependent binding of the aforementioned synthetic peptide to the 14-3-3 proteins was demonstrated in vitro. Third, 14-3-3 proteins were required for the ATP-dependent phosphorylation of NR (as assessed by activity measurements) in the presence of NR-kinase and leaf protein phosphatases. Lastly, we demonstrate specificity of recombinant Arabidopsis 14-3-3 isoforms in the interaction with phospho-NR: omega> chi> upsilon>>> phi, psi.

The inhibitor protein of phosphorylated nitrate reductase from spinach (Spinacia oleracea) leaves is a 14-3-3 protein

The inhibitor protein (IP) that inactivates spinach leaf NADH:nitrate reductase (NR) has been identified for the first time as a member of the eukaryotic 14-3-3 protein family based on three lines of evidence. First, the sequence of an eight amino acid tryptic peptide, obtained from immunopurified IP, matched that of a highly conserved region of the 14-3-3 proteins. Second, an authentic member of the 14-3-3 family, recombinant Arabidopsis GF14omega, caused inactivation of phospho-NR in a magnesium-dependent manner identical to IP. Third, an anti-GF14 monoclonal antibody cross-reacted with IP and anti-IP monoclonal antibodies cross-reacted with GF14omega.

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.

Identification of a regulatory phosphorylation site in the hinge 1 region of nitrate reductase from spinach (Spinacea oleracea) leaves

Purified nitrate reductase (NR) from spinach leaves was phosphorylated in vitro by NR-inactivating kinase on Ser-543 which is located in the hinge 1 region between the molybdenum-cofactor and haem-binding domains. Phosphorylation of Ser-543 allowed NR to be inhibited by the inhibitor, NIP. Degraded NR preparations in which a proportion of the subunits had lost 45 amino acids from the N-terminus during purification could be phosphorylated by NR kinase on Ser-543, but could not subsequently be fully inhibited by NIP, suggesting a role for the N-terminal tail of NR in NIP binding.

Integration of photosynthetic carbon and nitrogen metabolism in higher plants

Concomitant assimilation of C and N in illuminated leaves requires the regulated partitioning of reductant and photosynthate to sustain the demands of amino acid and carbohydrate biosynthesis. The short-term responses of photosynthesis and photosynthate partitioning to N enrichment in wheat (Triticum aestivum, L.) and maize (Zea mays L.) leaves were studied in order to understand the regulatory strategy employed in higher plants. Transgenic tobacco plants (Tobacco plumbaginifolia) over-expressing NR or with poor NR expression were used to compare plants differing in their capacities for NO3 (-) assimilation. Similar regulatory responses to NO3 (-) were observed in leaves having C4- and C3-type photosynthesis. It was shown that the extra- C needed in the short-term to sustain amino acid synthesis was not provided by an increase in photosynthetic CO2 fixation but rather by a rapid shift in the partitioning of photosynthetic C to amino acid at the expense of sucrose biosynthesis. The modulation of three enzymes was shown to be important in this C and N interaction, namely PEPCase (EC 4.1.1.31), SPS (EC 2.4.1.14) and NADH/NR (EC 1.6.6.1). The first two enzymes were shown to share the common feature of regulatory post-transcriptional NO3 (-)-dependent phosphorylation of their proteins on a seryl-residue. While PEPCase is activated, SPS activity is decreased. In contrast the NR phosphorylation state is unchanged and all N-dependent control of NR activity is regulated at the protein level. A number of arguments support the hypothesis that Gln, the primary product of NO3 (-) assimilation, is the metabolite effector for short-term modulation of PEPCase, and SPS in response to N enrichment. Since a major effect of NO3 (-) on the PEPCase-protein kinase activity in concentrated wheat leaf extracts was demonstrated, the hypothesis is put forward that protein phosphorylation is the primary event allowing the short-term adaptation of leaf C metabolism to changes in N supply.

Characterization of a cDNA encoding the 55 kDa B regulatory subunit of Arabidopsis protein phosphatase 2A

Type 2A serine/threonine protein phosphatases (PP2A) are key components in the regulation of signal transduction and control of cell metabolism. The activity of these protein phosphatases is modulated by regulatory subunits. While PP2A activity has been characterized in plants, little is known about its regulation. We used the polymerase chain reaction to amplify a segment of a cDNA encoding the B regulatory subunit of PP2A from Arabidopsis. The amplified DNA fragment of 372 nucleotides was used as a probe to screen an Arabidopsis cDNA library and a full-length clone (AtB alpha) of 2.1 kbp was isolated. The predicted protein encoded by AtB alpha is 43 to 46% identical and 53 to 56% similar to its yeast and mammalian counterparts, and contains three unique regions of amino acid insertions not present in the animal B regulatory subunit. Genomic Southern blots indicate the Arabidopsis genome contains at least two genes encoding the B regulatory subunit. In addition, other plant species also contain DNA sequences homologous to the B regulatory subunit, indicating that regulation of PP2A activity by the 55 kDa B regulatory subunit is probably ubiquitous in plants. Northern blots indicate the AtB alpha mRNA accumulates in all Arabidopsis tissues examined, suggesting the protein product of the AtB alpha gene performs a basic housekeeping function in plant cells.

The protein phosphatase inhibitor calyculin A mimics elicitor action in plant cells and induces rapid hyperphosphorylation of specific proteins as revealed by pulse labeling with [33P]phosphate

Suspension-cultured tomato cells react to microbial signals, so-called elicitors, with rapid alkalinization of the growth medium and increased biosynthesis of the stress hormone ethylene. These responses to elicitors can be blocked by staurosporine and K-252a, two specific inhibitors of protein kinases. Here we show that calyculin A, a potent inhibitor of protein phosphatases, mimics the action of elicitors and, at nanomolar concentrations, induces medium alkalinization as well as a strong increase in the activity of 1-aminocyclopropane-1-carboxylate synthase, the key enzyme of ethylene biosynthesis. Both responses were strongly inhibited by K-252a, and calyculin A induced both responses more rapidly than did a fungal elicitor, xylanase. For example, the lag phase for medium alkalinization was only 0.2-0.4 min for calyculin A, compared with 2 min for xylanase. To study changes in the dynamics of protein phosphorylation, cells were labeled with 30-sec pulses of [33P]orthophosphate. Calyculin A strongly increased phosphorylation of several polypeptide bands within 40 sec of treatment. The same phosphorylated bands also appeared in response to xylanase, but only after a lag phase of 2-3 min. These results show that the protein phosphatase inhibitor calyculin A leads to rapid hyperphosphorylation of specific proteins in cultured cells and indicate that elicitor action could be based on inhibition of a protein phosphatase as well as on activation of a protein kinase.