Identification of the major regulatory phosphorylation site in sucrose-phosphate synthase

Sucrose phosphate synthase 8 is required for the remobilization of carbon reserves in rice stems during grain filling

Carbon reserve remobilization in stems is closely related to rice grain filling. Sucrose phosphate synthase (SPS) is highly associated with carbon reserve remobilization. In this study, we investigated the expression pattern of SPS genes in various rice tissues, and found that SPS8 is the major SPS isoform in rice stems during the grain-filling stage. We then constructed sps8 mutants using the CRISPR/Cas9 system. The SPS activity of the sps8 mutants was markedly reduced in the stems. In addition, the sps8 mutants exhibited significant starch accumulation in stems. 14C-labelling experiments revealed that the remobilization of non-structural carbohydrates from rice stems to grains was impaired in the sps8 mutants. In the sps8 mutants, grain filling was delayed and yield decreased by 15% due to a reduced percentage of ripened grains. RNA sequencing and quantitative PCR analyses indicated that the genes involved in starch synthesis and degradation were up-regulated in the sps8 mutant stems. In addition, the activity of the enzymes involved in starch synthesis and degradation was increased in the sps8 stems. These results demonstrate that SPS8 is required for carbon reserve remobilization from rice stems to grains, and that its absence may enhance 'futile cycles' of starch synthesis and degradation in rice stems.

The interplay of post-translational protein modifications in Arabidopsis leaves during photosynthesis induction

Diurnal dark to light transition causes profound physiological changes in plant metabolism. These changes require distinct modes of regulation as a unique feature of photosynthetic lifestyle. The activities of several key metabolic enzymes are regulated by light-dependent post-translational modifications (PTM) and have been studied at depth at the level of individual proteins. In contrast, a global picture of the light-dependent PTMome dynamics is lacking, leaving the response of a large proportion of cellular function undefined. Here, we investigated the light-dependent metabolome and proteome changes in Arabidopsis rosettes in a time resolved manner to dissect their kinetic interplay, focusing on phosphorylation, lysine acetylation, and cysteine-based redox switches. Of over 24 000 PTM sites that were detected, more than 1700 were changed during the transition from dark to light. While the first changes, as measured 5 min after onset of illumination, occurred mainly in the chloroplasts, PTM changes at proteins in other compartments coincided with the full activation of the Calvin-Benson cycle and the synthesis of sugars at later timepoints. Our data reveal connections between metabolism and PTM-based regulation throughout the cell. The comprehensive multiome profiling analysis provides unique insight into the extent by which photosynthesis reprograms global cell function and adds a powerful resource for the dissection of diverse cellular processes in the context of photosynthetic function.

Phosphorylation of aldose-6-phosphate reductase from Prunus persica leaves

Sugar-alcohols are major photosynthates in plants from the Rosaceae family. Expression of the gene encoding aldose-6-phosphate reductase (Ald6PRase), the critical enzyme for glucitol synthesis in rosaceous species, is regulated by physiological and environmental cues. Additionally, Ald6PRase is inhibited by small molecules (hexose-phosphates and inorganic orthophosphate) and oxidizing compounds. This work demonstrates that Ald6PRase from peach leaves is phosphorylated in planta at the N-terminus. We also show in vitro phosphorylation of recombinant Ald6PRase by a partially purified kinase extract from peach leaves containing Ca²⁺-dependent protein kinases (CDPKs). Moreover, phosphorylation of recombinant Ald6PRase was inhibited by hexose-phosphates, phosphoenolpyruvate and pyrophosphate. We further show that phosphorylation of recombinant Ald6PRase was maximal using recombinant CDPKs. Overall, our results suggest that phosphorylation could fine-tune the activity of Ald6PRase.

Global Profiling of Phosphorylation Reveals the Barley Roots Response to Phosphorus Starvation and Resupply

Phosphorus (P) deficiency is a major threat to the crop production, and for understanding the response mechanism of plant roots, P stress may facilitate the development of crops with increased tolerance. Phosphorylation plays a critical role in the regulation of proteins for plant responses to biotic and abiotic stress; however, its functions in P starvation/resupply are largely unknown for barley (Hordeum vulgare) growth. Here, we performed a global review of phosphorylation in barley roots treated by P starvation/resupply. We identified 7,710 phosphorylation sites on 3,373 proteins, of which 76 types of conserved motifs were extracted from 10,428 phosphorylated peptides. Most phosphorylated proteins were located in the nucleus (36%) and chloroplast (32%). Compared with the control, 186 and 131 phosphorylated proteins under P starvation condition and 156 and 111 phosphorylated proteins under P resupply condition showed significant differences at 6 and 48 h, respectively. These proteins mainly participated in carbohydrate metabolism, phytohormones, signal transduction, cell wall stress, and oxidases stress. Moreover, the pathways of the ribosome, RNA binding, protein transport, and metal binding were significantly enriched under P starvation, and only two pathways of ribosome and RNA binding were greatly enriched under Pi resupply according to the protein-protein interaction analysis. The results suggested that the phosphorylation proteins might play important roles in the metabolic processes of barley roots in response to Pi deficiency/resupply. The data not only provide unique access to phosphorylation reprogramming of plant roots under deficiency/resupply but also demonstrate the close cooperation between these phosphorylation proteins and key metabolic functions.

Combined multivariate analysis and machine learning reveals a predictive module of metabolic stress response in Arabidopsis thaliana

Abiotic stress exposure of plants induces metabolic reprogramming which is tightly regulated by signalling cascades connecting transcriptional with translational and metabolic regulation. Complexity of such interconnected metabolic networks impedes the functional understanding of molecular plant stress response compromising the design of breeding strategies and biotechnological processes. Thus, defining a molecular network to enable the prediction of a plant's stress mode will improve the understanding of stress responsive biochemical regulation and will yield novel molecular targets for technological application. Arabidopsis wild type plants and two mutant lines with deficiency in sucrose or starch metabolism were grown under ambient and combined cold/high light stress conditions. Stress-induced dynamics of the primary metabolome and the proteome were quantified by mass spectrometry. Wild type data were used to train a machine learning algorithm to classify mutant lines under control and stress conditions. Multivariate analysis and classification identified a module consisting of 23 proteins enabling the reliable prediction of combined temperature/high light stress conditions. 18 of these 23 proteins displayed putative protein-protein interactions connecting transcriptional regulation with regulation of primary and secondary metabolism. The identified stress-responsive core module supports prediction of complex biochemical regulation under changing environmental conditions.

Highly Efficient Single-Step Enrichment of Low Abundance Phosphopeptides from Plant Membrane Preparations

Mass spectrometry (MS)-based large scale phosphoproteomics has facilitated the investigation of plant phosphorylation dynamics on a system-wide scale. However, generating large scale data sets for membrane phosphoproteins usually requires fractionation of samples and extended hands-on laboratory time. To overcome these limitations, we developed "ShortPhos," an efficient and simple phosphoproteomics protocol optimized for research on plant membrane proteins. The optimized workflow allows fast and efficient identification and quantification of phosphopeptides, even from small amounts of starting plant materials. "ShortPhos" can produce label-free datasets with a high quantitative reproducibility. In addition, the "ShortPhos" protocol recovered more phosphorylation sites from membrane proteins, especially plasma membrane and vacuolar proteins, when compared to our previous workflow and other membrane-based data in the PhosPhAt 4.0 database. We applied "ShortPhos" to study kinase-substrate relationships within a nitrate-induction experiment on Arabidopsis roots. The "ShortPhos" identified significantly more known kinase-substrate relationships compared to previous phosphoproteomics workflows, producing new insights into nitrate-induced signaling pathways.

Arabidopsis thaliana sucrose phosphate synthase (sps) genes are expressed differentially in organs and tissues, and their transcription is regulated by osmotic stress

Sucrose is synthesized from UDP-Glc and Fru-6-phosphate via the activity of sucrose-phosphate synthase (SPS) enzymes, which produce Suc-6-phosphate. Suc-6-phosphate is rapidly dephosphorylated by phosphatases to produce Suc and inorganic phosphate. Arabidopsis has four sps genes encoding SPS enzymes. Of these enzymes, AtSPS1F and AtSPS2F have been grouped with other dicotyledonous SPS enzymes, while AtSPS3F and AtSPS4F are included in groups with both dicotyledonous and monocotyledonous SPS enzymes. In this work, we generated Arabidopsis thaliana transformants containing the promoter region of each sps gene fused to gfp::uidA reporter genes. A detailed characterization of expression conferred by the sps promoters in organs and tissues was performed. We observed expression of AtSPS1F, AtSPS2F and AtSPS3F in the columella roots of the plants that support sucrose synthesis. Hence, these findings support the idea that sucrose synthesis occurs in the columella cells, and suggests that sucrose has a role in this tissue. In addition, the expression of AtSPS4F was identified in embryos and suggests its participation in this developmental stage. Quantitative transcriptional analysis of A. thaliana plants grown in media with different osmotic potential showed that AtSPS2F and AtSPS4F respond to osmotic stress.

Quantitative Circadian Phosphoproteomic Analysis of Arabidopsis Reveals Extensive Clock Control of Key Components in Physiological, Metabolic, and Signaling Pathways

The circadian clock provides adaptive advantages to an organism, resulting in increased fitness and survival. The phosphorylation events that regulate circadian-dependent signaling and the processes which post-translationally respond to clock-gated signals are largely unknown. To better elucidate post-translational events tied to the circadian system we carried out a survey of circadian-regulated protein phosphorylation events in Arabidopsis seedlings. A large-scale mass spectrometry-based quantitative phosphoproteomics approach employing TiO2-based phosphopeptide enrichment techniques identified and quantified 1586 phosphopeptides on 1080 protein groups. A total of 102 phosphopeptides displayed significant changes in abundance, enabling the identification of specific patterns of response to circadian rhythms. Our approach was sensitive enough to quantitate oscillations in the phosphorylation of low abundance clock proteins (early flowering4; ELF4 and pseudoresponse regulator3; PRR3) as well as other transcription factors and kinases. During constant light, extensive cyclic changes in phosphorylation status occurred in critical regulators, implicating direct or indirect regulation by the circadian system. These included proteins influencing transcriptional regulation, translation, metabolism, stress and phytohormones-mediated responses. We validated our analysis using the elf4-211 allele, in which an S45L transition removes the phosphorylation herein identified. We show that removal of this phosphorylatable site diminishes interaction with early flowering3 (ELF3), a key partner in a tripartite evening complex required for circadian cycling. elf4-211 lengthens period, which increases with increasing temperature, relative to the wild type, resulting in a more stable temperature compensation of circadian period over a wider temperature range.

Phosphoproteomics reveals the effect of ethylene in soybean root under flooding stress

Flooding has severe negative effects on soybean growth. To explore the flooding-responsive mechanisms in early-stage soybean, a phosphoproteomic approach was used. Two-day-old soybean plants were treated without or with flooding for 3, 6, 12, and 24 h, and root tip proteins were then extracted and analyzed at each time point. After 3 h of flooding exposure, the fresh weight of soybeans increased, whereas the ATP content of soybean root tips decreased. Using a gel-free proteomic technique, a total of 114 phosphoproteins were identified in the root tip samples, and 34 of the phosphoproteins were significantly changed with respect to phosphorylation status after 3 h of flooding stress. Among these phosphoproteins, eukaryotic translation initiation factors were dephosphorylated, whereas several protein synthesis-related proteins were phosphorylated. The mRNA expression levels of sucrose phosphate synthase 1F and eukaryotic translation initiation factor 4 G were down-regulated, whereas UDP-glucose 6-dehydrogenase mRNA expression was up-regulated during growth but down-regulated under flooding stress. Furthermore, bioinformatic protein interaction analysis of flooding-responsive proteins based on temporal phosphorylation patterns indicated that eukaryotic translation initiation factor 4 G was located in the center of the network during flooding. Soybean eukaryotic translation initiation factor 4 G has homology to programmed cell death 4 protein and is implicated in ethylene signaling. The weight of soybeans was increased with treatment by an ethylene-releasing agent under flooding condition, but it was decreased when plants were exposed to an ethylene receptor antagonist. These results suggest that the ethylene signaling pathway plays an important role, via the protein phosphorylation, in mechanisms of plant tolerance to the initial stages of flooding stress in soybean root tips.

A kinase-phosphatase signaling module with BSK8 and BSL2 involved in regulation of sucrose-phosphate synthase

External supply of sucrose to carbon-starved Arabidopsis seedlings induced changes in phosphorylation of Brassinosteroid Signaling Kinase 8 (BSK8) at two different sites. Serine S(20) lies within a phosphorylation hotspot at the N-terminal region of the protein, while S(213) is located within the kinase domain of BSK8. Upon sucrose supply phosphorylation of BSK8(S20) and BSK8(S213) showed opposite behavior with increasing phosphorylation of S(213) and decreased phosphorylation of S(20) at 5 min after sucrose supply. Here we aim to systematically analyze the effects of BSK8 mutations on downstream cellular regulatory events and characterize molecular functions of BSK8 and its phosphorylation. Comparative phosphoproteomic profiling of a bsk8 knockout mutant and wild type revealed potential targets in sucrose metabolism. Activity of sucrose-phosphate synthase (SPS) was decreased by phosphorylation at S(152), and SPS phosphorylation inversely correlated with sucrose-induced BSK8 activity. Furthermore, BSK8 was found to interact with BSL2, a Kelch-type phosphatase. On the basis of a combination of kinase activity measurements, SPS activity assays, and phosphorylation site mutations in BSK8 at S(20) and S(213), we conclude that regulation of SPS by BSK8 occurs through activation of a phosphatase that in turn may dephosphorylate SPS and thus activates the enzyme.

A novel isoform of sucrose synthase is targeted to the cell wall during secondary cell wall synthesis in cotton fiber

Sucrose (Suc) synthase (Sus) is the major enzyme of Suc breakdown for cellulose biosynthesis in cotton (Gossypium hirsutum) fiber, an important source of fiber for the textile industry. This study examines the tissue-specific expression, relative abundance, and temporal expression of various Sus transcripts and proteins present in cotton. A novel isoform of Sus (SusC) is identified that is expressed at high levels during secondary cell wall synthesis in fiber and is present in the cell wall fraction. The phylogenetic relationships of the deduced amino acid sequences indicate two ancestral groups of Sus proteins predating the divergence of monocots and dicots and that SusC sequences form a distinct branch in the phylogeny within the dicot-specific clade. The subcellular location of the Sus isoforms is determined, and it is proposed that cell wall-localized SusC may provide UDP-glucose for cellulose and callose synthesis from extracellular sugars.

Decrease in leaf sucrose synthesis leads to increased leaf starch turnover and decreased RuBP regeneration-limited photosynthesis but not Rubisco-limited photosynthesis in Arabidopsis null mutants of SPSA1

We investigated the individual effect of null mutations of each of the four sucrose-phosphate synthase (SPS) genes in Arabidopsis (SPSA1, SPSA2, SPSB and SPSC) on photosynthesis and carbon partitioning. Null mutants spsa1 and spsc led to decreases in maximum SPS activity in leaves by 80 and 13%, respectively, whereas null mutants spsa2 and spsb had no significant effect. Consistently, isoform-specific antibodies detected only the SPSA1 and SPSC proteins in leaf extracts. Leaf photosynthesis at ambient [CO₂] was not different among the genotypes but was 20% lower in spsa1 mutants when measured under saturating [CO₂] levels. Carbon partitioning at ambient [CO₂] was altered only in the spsa1 null mutant. Cold treatment of plants (4 °C for 96 h) increased leaf soluble sugars and starch and increased the leaf content of SPSA1 and SPSC proteins twofold to threefold, and of the four null mutants, only spsa1 reduced leaf non-structural carbohydrate accumulation in response to cold treatment. It is concluded that SPSA1 plays a major role in photosynthetic sucrose synthesis in Arabidopsis leaves, and decreases in leaf SPS activity lead to increased starch synthesis and starch turnover and decreased Ribulose 1,5-bisphosphate regeneration-limited photosynthesis but not ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco)-limited photosynthesis, indicating a limitation of triose-phosphate utilization (TPU).

Coupling oxidative signals to protein phosphorylation via methionine oxidation in Arabidopsis

The mechanisms involved in sensing oxidative signalling molecules, such as H₂O₂, in plant and animal cells are not completely understood. In the present study, we tested the postulate that oxidation of Met (methionine) to MetSO (Met sulfoxide) can couple oxidative signals to changes in protein phosphorylation. We demonstrate that when a Met residue functions as a hydrophobic recognition element within a phosphorylation motif, its oxidation can strongly inhibit peptide phosphorylation in vitro. This is shown to occur with recombinant soybean CDPKs (calcium-dependent protein kinases) and human AMPK (AMP-dependent protein kinase). To determine whether this effect may occur in vivo, we monitored the phosphorylation status of Arabidopsis leaf NR (nitrate reductase) on Ser⁵³⁴ using modification-specific antibodies. NR was a candidate protein for this mechanism because Met⁵³⁸, located at the P+4 position, serves as a hydrophobic recognition element for phosphorylation of Ser⁵³⁴ and its oxidation substantially inhibits phosphorylation of Ser⁵³⁴in vitro. Two lines of evidence suggest that Met oxidation may inhibit phosphorylation of NR-Ser⁵³⁴in vivo. First, phosphorylation of NR at the Ser⁵³⁴ site was sensitive to exogenous H₂O₂ and secondly, phosphorylation in normal darkened leaves was increased by overexpression of the cytosolic MetSO-repair enzyme PMSRA3 (peptide MetSO reductase A3). These results are consistent with the notion that oxidation of surface-exposed Met residues in kinase substrate proteins, such as NR, can inhibit the phosphorylation of nearby sites and thereby couple oxidative signals to changes in protein phosphorylation.

Large-scale phenotyping of transgenic tobacco plants (Nicotiana tabacum) to identify essential leaf functions

Two of the major challenges in functional genomics are to identify genes that play a key role in biological processes, and to elucidate the biological role of the large numbers of genes whose function is poorly characterized or still completely unknown. In this study, a combination of large-scale expressed sequence tag sequencing, high-throughput gene silencing and visual phenotyping was used to identify genes in which partial inhibition of expression leads to marked phenotypic changes, mostly on leaves. Three normalized tobacco (Nicotiana tabacum) cDNA libraries were prepared directly in a binary vector using different tissues of tobacco as an RNA source, randomly sequenced and clustered. The Agrobacterium-tobacco leaf disc transformation system was used to generate sets of antisense or co-suppression transgenic tobacco plants for over 20 000 randomly chosen clones, each representing an independent cluster. After transfer to the glasshouse, transgenic plants were scored visually after 10-14 days for changes in growth, leaf form and chlorosis or necrosis. Putative hits were validated by repeating the transformation. This procedure is more stringent than the analysis of knockout mutants, because it requires that even a partial decrease in expression generates a phenotype. This procedure identified 88 validated gene/phenotype relations. These included several previously characterized gene/phenotype relationships, demonstrating the validity of the approach. For about one-third, a function could be inferred, but a loss-of-function phenotype had not been described previously. Strikingly, almost one-half of the validated genes were poorly annotated, or had no known function. For 77 of these tobacco sequences, a single or small number of potential orthologues were identified in Arabidopsis. The genes for which orthologues were identified in Arabidopsis included about one-half of the genes whose function was completely unknown. Comparison with published gene/phenotype relations for Arabidopsis knockout mutants revealed surprisingly little overlap with the present study. Our results indicate that partial gene silencing identifies novel gene/phenotype relationships, which are distinct from those uncovered by knockout screens. They also show that it is possible to perform these analyses in a crop species in which full genome sequence information is lacking, and subsequently to transfer the information to a reference species in which functional studies can be performed more effectively.

Phylogenetic and expression analysis of sucrose phosphate synthase isozymes in plants

In plants and microbes, sucrose phosphate synthase (SPS) is an important enzyme in sucrose biosynthesis. Several different isozymes of SPS exist in plants. Genomic and EST sequence data from Arabidopsis, rice and maize has been analyzed. This analysis has revealed that the Arabidopsis genome contains four unique SPS genes. The rice databases (Monsanto proprietary, and public databases) contain five unique full-length SPS genes. Using the Monsanto maize EST and genomic sequence databases, we have identified five full length and two partial SPS sequences, bringing the total number of presently known maize SPS genes to at least seven. Phylogenetic analysis of all known SPS sequences revealed several putative evolutionary branches of SPS. We have classified SPS genes into three major groups in higher plants, all with distinct features from the known microbial SPS genes. Furthermore, this analysis suggests evolutionary divergence of monocotyledonous (monocot) and dicotyledonous (dicot) SPS sequences. The evidence suggests that several gene duplication events occurred at various points during evolution, both before and after the monocot/dicot split. It appears that at least one of the major forms of SPS genes may have evolved after the divergence of monocots and dicots. In addition, several more recent gene duplication events may have occurred after maize/rice speciation, giving rise to additional SPS genes in maize. Some of the variants lack one or more of the presently known regulatory sites, implying that this evolutionary divergence may have given rise to enzymes with functional differences. We present evidence from transcript distribution studies using cDNA libraries as well as transcriptional profiling experiments and propose that specific SPS genes have diverse patterns of expression that are sometimes responsive to environmental signals. Our data suggests that higher plant SPS isozymes differ with respect to their patterns of expression and regulation and that our proposed phylogenetic classification reflects specific functional categories for higher plant SPS isozymes.

Large Scale Identification and Quantitative Profiling of Phosphoproteins Expressed during Seed Filling in Oilseed Rape

Seed filling is a dynamic, temporally regulated phase of seed development that determines the composition of storage reserves in mature seeds. Although the metabolic pathways responsible for storage reserve synthesis such as carbohydrates, oils, and proteins are known, little is known about their regulation. Protein phosphorylation is a ubiquitous form of regulation that influences many aspects of dynamic cellular behavior in plant biology. Here a systematic study has been conducted on five sequential stages (2, 3, 4, 5, and 6 weeks after flowering) of seed development in oilseed rape (Brassica napus L. Reston) to survey the presence and dynamics of phosphoproteins. High resolution two-dimensional gel electrophoresis in combination with a phosphoprotein-specific Pro-Q Diamond phosphoprotein fluorescence stain revealed approximately 300 phosphoprotein spots. Of these, quantitative expression profiles for 234 high quality spots were established, and hierarchical cluster analyses revealed the occurrence of six principal expression trends during seed filling. The identity of 103 spots was determined using LC-MS/MS. The identified spots represented 70 non-redundant phosphoproteins belonging to 10 major functional categories including energy, metabolism, protein destination, and signal transduction. Furthermore phosphorylation within 16 non-redundant phosphoproteins was verified by mapping the phosphorylation sites by LC-MS/MS. Although one of these sites was postulated previously, the remaining sites have not yet been reported in plants. Phosphoprotein data were assembled into a web database. Together this study provides evidence for the presence of a large number of functionally diverse phosphoproteins, including global regulatory factors like 14-3-3 proteins, within developing B. napus seed.

An integrated strategy for identification and relative quantification of site-specific protein phosphorylation using liquid chromatography coupled to MS2/MS3

Reversible and differential multisite protein phosphorylation is an important mechanism controlling the activity of cellular proteins. Here we describe a robust and highly selective approach for the identification and relative quantification of site-specific phosphorylation events. This integrated strategy has three major parts: visualisation of phosphorylated proteins using fluorescently stained polyacrylamide gels, determination of the phosphorylation site(s) using automatic MS3 triggered by the loss of phosphoric acid, and relative quantification of phosphorylation by integrating MS2- and MS3-extracted ion traces using a fast-scanning, linear ion trap mass spectrometer. As a test case, recombinant sucrose-phosphate synthase (SPS) from Arabidopsis thaliana (At5g1110) was used for identification and quantification of site-specific phosphorylation. The identified phosphorylation site of the actively expressed protein coincides with the major regulatory in vivo phosphorylation site in spinach SPS. Site-specific differential in vitro phosphorylation of native protein was demonstrated after incubation of the recombinant protein with cold-adapted plant leaf extracts from A. thaliana, suggesting regulatory phosphorylation events of this key enzyme under stress response.

Identification of a new motif for CDPK phosphorylation in vitro that suggests ACC synthase may be a CDPK substrate

1-Amino-cyclopropane-1-carboxylate synthase (ACS) catalyzes the rate-determining step in the biosynthesis of the plant hormone ethylene, and there is evidence for regulation of stability of the protein by reversible protein phosphorylation. The site of phosphorylation of the tomato enzyme, LeACS2, was recently reported to be Ser460, but the requisite protein kinase has not been identified. In the present study, a synthetic peptide based on the known regulatory phosphorylation site (KKNNLRLS460FSKRMY) in LeACS2 was found to be readily phosphorylated in vitro by several calcium-dependent protein kinases (CDPKs), but not a plant SNF1-related protein kinase or the kinase domain of the receptor-like kinase, BRI1, involved in brassinosteroid signaling. Studies with variants of the LeACS2-Ser460 peptide establish a fundamentally new phosphorylation motif that is broadly targeted by CDPKs: phi -1-[ST]0- phi +1-X-Basic+3-Basic+4, where phi is a hydrophobic residue. Database analysis using the new motif predicts a number of novel phosphorylation sites in plant proteins. Finally, we also demonstrate that CDPKs and SnRK1s do not recognize motifs presented in the reverse order, indicating that side chain interactions alone are not sufficient for substrate recognition.

Light-regulated differential expression of pea chloroplast and cytosolic fructose-1,6-bisphosphatases

The light-regulated differential expression of pea chloroplast and cytosolic fructose-1,6-bisphosphatases (FBPase) was investigated using enzyme activity assay, immunoblot, and Northern blot analyses. The enzyme activities of both chloroplast and cytosolic FBPases gradually increased under continuous white light illumination, although the increase in chloroplast FBPase was more drastic. Northern and immunoblot analyses also indicated that light stimulated the expression of both enzymes. Enzyme activity and the transcript levels of both enzymes gradually decreased under the dark treatment, although protein levels were unchanged for up to 24 h following the initiation of culture in the dark, indicating that reversible modifications of the enzymes may occur during the transition from light to dark (or the reverse). Light pulse experiments using blue (420 nm) and red/far-red (660/730 nm) light were carried out to analyze the photoreceptors related to the light-mediated expression of both enzymes. Expression of the chloroplast enzyme was very sensitive to red or far-red light pulses-it was induced by red light, but suppressed by far-red light pulses, as determined by enzyme activity, immunoblot, and Northern blot analyses, suggesting that red light signaling is involved in the control of chloroplast FBPase expression. However, cytosolic FBPase was virtually insensitive to blue or red/far-red light pulses in terms of enzyme activity, as determined by protein and transcript levels, indicating that cytosolic enzyme expression is not directly regulated by light signals. Instead, the expression of the cytosolic enzyme may be closely related to photosynthetic energy conversion accompanied by continuous white light irradiation.

Brassinosteroids. Plant counterparts to animal steroid hormones?

Brassinosteroids are polyhydroxylated derivatives of common plant membrane sterols such as campesterol. They occur throughout the plant kingdom and have been shown by genetic and biochemical analyses to be essential for normal plant growth and development. Numerous reviews have detailed the recent progress in our understanding of the biosynthesis, physiological responses, and molecular modes of action of brassinosteroids. It is clear that like their animal steroid counterparts, brassinosteroids have a defined receptor, can regulate the expression of specific genes, and can orchestrate complex physiological responses involved in growth. This review summarizes the current status of BR research, pointing out where appropriate the similarities and differences between the mechanism of action of brassinosteroids and the more thoroughly studied animal steroid hormones.

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.

Calcium signaling through protein kinases. The Arabidopsis calcium-dependent protein kinase gene family

In plants, numerous Ca(2+)-stimulated protein kinase activities occur through calcium-dependent protein kinases (CDPKs). These novel calcium sensors are likely to be crucial mediators of responses to diverse endogenous and environmental cues. However, the precise biological function(s) of most CDPKs remains elusive. The Arabidopsis genome is predicted to encode 34 different CDPKs. In this Update, we analyze the Arabidopsis CDPK gene family and review the expression, regulation, and possible functions of plant CDPKs. By combining emerging cellular and genomic technologies with genetic and biochemical approaches, the characterization of Arabidopsis CDPKs provides a valuable opportunity to understand the plant calcium-signaling network.

Phosphorylation of synthetic peptides by a CDPK and plant SNF1-related protein kinase. Influence of proline and basic amino acid residues at selected positions

Spinach (Spinacia oleracea L.) leaf sucrose-phosphate synthase (SPS) can be inactivated by phosphorylation of Ser-158 by calmodulin-like domain protein kinases (CDPKs) or SNF1-related protein kinases (SnRK1) in vitro. While the phosphorylation site sequence is relatively conserved, most of the deduced sequences of SPS from dicot species surrounding the Ser-158 regulatory phosphorylation site contain a Pro residue at P-4 (where P is the phosphorylated Ser); spinach is the exception and contains an Arg at P-4. We show that a Pro at P-4 selectively inhibits phosphorylation of the peptide by a CDPK relative to a SnRK1. The presence of a Pro at P-4, by allowing a tight turn in the peptide substrate, may interfere with proper binding of residues at P-5 and beyond. Both kinases had greater activity with peptides having basic residues at P-6 and P+5 (in addition to the known requirement for an Arg at P-3/P-4), and when the residue at P-6 was a His, the pH optimum for phosphorylation of the peptide was acid shifted. The results are used to predict proteins that may be selectively phosphorylated by SnRK1s (as opposed to CDPKs), such as SPS in dicot species, or may be phosphorylated in a pH-dependent manner.

Regulation of sucrose metabolism in higher plants: localization and regulation of activity of key enzymes

Sucrose (Suc) plays a central role in plant growth and development. It is a major end product of photosynthesis and functions as a primary transport sugar and in some cases as a direct or indirect regulator of gene expression. Research during the last 2 decades has identified the pathways involved and which enzymes contribute to the control of flux. Availability of metabolites for Suc synthesis and 'demand' for products of sucrose degradation are important factors, but this review specifically focuses on the biosynthetic enzyme sucrose-phosphate synthase (SPS), and the degradative enzymes, sucrose synthase (SuSy), and the invertases. Recent progress has included the cloning of genes encoding these enzymes and the elucidation of posttranslational regulatory mechanisms. Protein phosphorylation is emerging as an important mechanism controlling SPS activity in response to various environmental and endogenous signals. In terms of Suc degradation, invertase-catalyzed hydrolysis generally has been associated with cell expansion, whereas SuSy-catalyzed metabolism has been linked with biosynthetic processes (e.g., cell wall or storage products). Recent results indicate that SuSy may be localized in multiple cellular compartments: (1) as a soluble enzyme in the cytosol (as traditionally assumed); (2) associated with the plasma membrane; and (3) associated with the actin cytoskeleton. Phosphorylation of SuSy has been shown to occur and may be one of the factors controlling localization of the enzyme. The purpose of this review is to summarize some of the recent developments relating to regulation of activity and localization of key enzymes involved in sucrose metabolism in plants.

Recombinant brassinosteroid insensitive 1 receptor-like kinase autophosphorylates on serine and threonine residues and phosphorylates a conserved peptide motif in vitro

BRASSINOSTEROID-INSENSITIVE 1 (BRI1) encodes a putative Leucine-rich repeat receptor kinase in Arabidopsis that has been shown by genetic and molecular analysis to be a critical component of brassinosteroid signal transduction. In this study we examined some of the biochemical properties of the BRI1 kinase domain (BRI1-KD) in vitro, which might be important predictors of in vivo function. Recombinant BRI1-KD autophosphorylated on serine (Ser) and threonine (Thr) residues with p-Ser predominating. Matrix-assisted laser desorption/ionization mass spectrometry identified a minimum of 12 sites of autophosphorylation in the cytoplasmic domain of BRI1, including five in the juxtamembrane region (N-terminal to the catalytic KD), five in the KD (one each in sub-domains I and VIa and three in sub-domain VIII), and two in the carboxy terminal region. Five of the sites were uniquely identified (Ser-838, Thr-842, Thr-846, Ser-858, and Thr-872), whereas seven were localized on short peptides but remain ambiguous due to multiple Ser and/or Thr residues within these peptides. The inability of an active BRI1-KD to transphosphorylate an inactive mutant KD suggests that the mechanism of autophosphorylation is intramolecular. It is interesting that recombinant BRI1-KD was also found to phosphorylate certain synthetic peptides in vitro. To identify possible structural elements required for substrate recognition by BRI1-KD, a series of synthetic peptides were evaluated, indicating that optimum phosphorylation of the peptide required R or K residues at P - 3, P - 4, and P + 5 (relative to the phosphorylated Ser at P = 0).

Ser162-Dependent inactivation of overproduced sucrose-phosphate synthase protein of maize leaf in transgenic rice plants

To investigate the role of Ser162 in phosphorylation-dependent regulation of maize sucrose-phosphate synthase (SPS) activities in rice, transgenic rice plants expressing wild-type or mutagenized maize SPS were produced. Our results indicate that Ser162 was responsible for overproduction-induced inactivation of SPS protein and for light/ dark modulation of this protein in vivo.

Regulation of a plant SNF1-related protein kinase by glucose-6-phosphate

One of the major protein kinases (PK(III)) that phosphorylates serine-158 of spinach sucrose-phosphate synthase (SPS), which is responsible for light/dark modulation of activity, is known to be a member of the SNF1-related family of protein kinases. In the present study, we have developed a fluorescence-based continuous assay for measurement of PK(III) activity. Using the continuous assay, along with the fixed-time-point (32)P-incorporation assay, we demonstrate that PK(III) activity is inhibited by glucose-6-phosphate (Glc-6-P). Relative inhibition by Glc-6-P was increased by decreasing pH from 8. 5 to 5.5 and by reducing the concentration of Mg(2+) in the assay from 10 to 2 mM. Under likely physiological conditions (pH 7.0 and 2 mM Mg(2+)), 10 mM Glc-6-P inhibited kinase activity approximately 70%. Inhibition by Glc-6-P could not be ascribed to contaminants in the commercial preparations. Other metabolites inhibited PK(III) in the following order: Glc-6-P > mannose-6-P, fructose-1,6P(2) > ribose-5-P, 3-PGA, fructose-6-P. Inorganic phosphate, Glc, and AMP were not inhibitory, and free Glc did not reverse the inhibition by Glc-6-P. Because SNF1-related protein kinases are thought to function broadly in the regulation of enzyme activity and gene expression, Glc-6-P inhibition of PK(III) activity potentially provides a mechanism for metabolic regulation of the reactions catalyzed by these important protein kinases.

Calcium and phospholipid activation of a recombinant calcium-dependent protein kinase (DcCPK1) from carrot (Daucus carota L.)

A calmodulin-like domain protein kinase (DcCPK1, previously designated CDPK431) cloned from carrot (Daucus carota L.) was expressed at high levels in Escherichia coli and partially purified. Ca(2+)-induced gel mobility shift and (45)Ca(2+) ligand binding assays confirmed that recombinant DcCPK1 binds Ca(2+) through its calmodulin-like domain and undergoes a significant conformational change. Ca(2+) activated the kinase activity of recombinant DcCPK1 (K(0.5)=1.7 microM) up to 20-fold. Ca(2+) combined with certain lipids, including phosphatidic acid, phosphatidylserine and phosphatidylinositol, but not diolein or lysophosphatidylcholine, provided even greater Ca(2+)-dependent protein kinase activity. DcCPK1 phosphorylated casein and histone III-S, and a variety of peptide substrates containing a hydrophobic and a basic residue situated P-5 and P-3 amino acids N-terminal to a Ser or Thr residue. The calmodulin and protein kinase inhibitors, W-7 and staurosporine, inhibited CDPK activity. The similarities between DcCPK1 and mammalian protein kinase C (PKC) in substrate specificity, sensitivity to inhibitors, and activation by Ca(2+) and phospholipid suggest that various CDPK isoforms may be responsible for some PKC-like activities in plant cells.

Purification of tomato sucrose synthase phosphorylated isoforms by Fe(III)-immobilized metal affinity chromatography

The major phosphorylation site of maize sucrose synthase (SuSy) is well conserved among plant species but absent in the deduced peptide sequence of the tomato SuSy cDNA (TOMSSF). In this study, we report the in vitro phosphorylation of 25-day-old tomato fruits SuSy on seryl residue(s) by an endogenous Ca2+-dependent protein kinase activity. Two distinct 32P-labeled peptides detected in the tryptic peptide map of in vitro 32P-radiolabeled tomato fruit SuSy were purified. Amino acid sequencing and phosphoamino acid analysis of the major 32P-labeled peptide revealed the presence of a SuSy isozyme in young tomato fruit having the N-terminus phosphorylation site present in other plant species. By using Fe(III)-immobilized metal affinity chromatography [Fe(III)-IMAC] as a final purification step of tomato fruit SuSy, two 32P-labeled tomato SuSy isoforms were separated from a nonradiolabeled SuSy fraction by using a pH gradient. The major 32P-SuSy isoform was phosphorylated exclusively at the seryl residue related to the phosphorylation site of maize SuSy. The multiphosphorylated state of the second radiolabeled SuSy fraction was indicated by a higher retention during Fe(III)-IMAC and by tryptic peptide mapping analysis. Kinetic analyses of SuSy isoforms purified by Fe(III)-IMAC have revealed that phosphorylation of the major phosphorylation site of tomato fruit SuSy was not sufficient by itself to modulate tomato SuSy activity, whereas the affinity for UDP increased about threefold for the multiphosphorylated SuSy isoform.

Two SNF1-related protein kinases from spinach leaf phosphorylate and inactivate 3-hydroxy-3-methylglutaryl-coenzyme A reductase, nitrate reductase, and sucrose phosphate synthase in vitro

We resolved from spinach (Spinacia oleracea) leaf extracts four Ca2+-independent protein kinase activities that phosphorylate the AMARAASAAALARRR (AMARA) and HMRSAMSGLHLVKRR (SAMS) peptides, originally designed as specific substrates for mammalian AMP-activated protein kinase and its yeast homolog, SNF1. The two major activities, HRK-A and HRK-C (3-hydroxy-3-methylglutaryl-coenzyme A reductase kinase A and C) were extensively purified and shown to be members of the plant SnRK1 (SNF1-related protein kinase 1) family using the following criteria: (a) They contain 58-kD polypeptides that cross-react with an antibody against a peptide sequence characteristic of the SnRK1 family; (b) they have similar native molecular masses and specificity for peptide substrates to mammalian AMP-activated protein kinase and the cauliflower homolog; (c) they are inactivated by homogeneous protein phosphatases and can be reactivated using the mammalian upstream kinase; and (d) they phosphorylate 3-hydroxy-3-methylglutaryl-coenzyme A reductase from Arabidopsis at the inactivating site, serine (Ser)-577. We propose that HRK-A and HRK-C represent either distinct SnRK1 isoforms or the same catalytic subunit complexed with different regulatory subunits. Both kinases also rapidly phosphorylate nitrate reductase purified from spinach, which is associated with inactivation of the enzyme that is observed only in the presence of 14-3-3 protein, a characteristic of phosphorylation at Ser-543. Both kinases also inactivate spinach sucrose phosphate synthase via phosphorylation at Ser-158. The SNF1-related kinases therefore potentially regulate several major biosynthetic pathways in plants: isoprenoid synthesis, sucrose synthesis, and nitrogen assimilation for the synthesis of amino acids and nucleotides.

Site-directed mutagenesis of serine 158 demonstrates its role in spinach leaf sucrose-phosphate synthase modulation

Site-directed mutagenesis of spinach sucrose-phosphate synthase (SPS) was performed to investigate the role of Ser158 in the modulation of spinach leaf SPS. Tobacco plants expressing the spinach wild-type (WT), S158A, S158T and S157F/S158E SPS transgenes were produced. Expression of transgenes appeared not to reduce expression of the tobacco host SPS. SPS activity in the WT and the S158T SPS transgenics showed light/dark modulation, whereas the S158A and S157F/S158E mutants were not similarly light/dark modulated: the S158A mutant enzyme was not inactivated in the dark, and the S157F/S158E was not activated in the light. The inability to modulate the activity of the S158A mutant enzyme by protein phosphorylation was demonstrated in vitro. The WT spinach enzyme immunopurified from dark transgenic tobacco leaves had a low initial activation state, and could be activated by PP2A and subsequently inactivated by SPS-kinase plus ATP. Rapid purification of the S158A mutant enzyme from dark leaves of transgenic plants using spinach-specific monoclonal antibodies yielded enzyme that had a high initial activation state, and pre-incubation with leaf PP2A or ATP plus SPS-kinase (the PKIII enzyme) caused little modulation of activity. The results demonstrate the regulatory significance of Ser158 as the major site responsible for dark inactivation of spinach SPS in vivo, and indicate that the significance of phosphorylation is the introduction of a negative charge at the Ser158 position.

Sucrose-phosphate synthase from Synechocystis sp. strain PCC 6803: identification of the spsA gene and characterization of the enzyme expressed in Escherichia coli

The first identification and characterization of a prokaryotic gene (spsA) encoding sucrose-phosphate synthase (SPS) is reported for Synechocystis sp. strain PCC 6803, a unicellular non-nitrogen-fixing cyanobacterium. Comparisons of the deduced amino acid sequence and some relevant biochemical properties of the enzyme with those of plant SPSs revealed important differences in the N-terminal and UDP-glucose binding site regions, substrate specificities, molecular masses, subunit compositions, and regulatory properties.

The AMP-activated/SNF1 protein kinase subfamily: metabolic sensors of the eukaryotic cell?

Mammalian AMP-activated protein kinase and yeast SNF1 protein kinase are the central components of kinase cascades that are highly conserved between animals, fungi, and plants. The AMP-activated protein kinase cascade acts as a metabolic sensor or "fuel gauge" that monitors cellular AMP and ATP levels because it is activated by increases in the AMP:ATP ratio. Once activated, the enzyme switches off ATP-consuming anabolic pathways and switches on ATP-producing catabolic pathways, such as fatty acid oxidation. The SNF1 complex in yeast is activated in response to the stress of glucose deprivation. In this case the intracellular signal or signals have not been identified; however, SNF1 activation is associated with depletion of ATP and elevation of AMP. The SNF1 complex acts primarily by inducing expression of genes required for catabolic pathways that generate glucose, probably by triggering phosphorylation of transcription factors. SNF1-related protein kinases in higher plants are likely to be involved in the response of plant cells to environmental and/or nutritional stress.

Site-specific regulatory interaction between spinach leaf sucrose-phosphate synthase and 14-3-3 proteins

We report an Mg2+-dependent interaction between spinach leaf sucrose-phosphate synthase (SPS) and endogenous 14-3-3 proteins, as evidenced by co-elution during gel filtration and co-immunoprecipitation. The content of 14-3-3s associated with an SPS immunoprecipitate was inversely related to activity, and was specifically reduced when tissue was pretreated with 5-aminoimidazole-4-carboxamide riboside, suggesting metabolite control in vivo. A synthetic phosphopeptide based on Ser-229 was shown by surface plasmon resonance to bind a recombinant plant 14-3-3, and addition of the phosphorylated SPS-229 peptide was found to stimulate the SPS activity of an SPS:14-3-3 complex. Taken together, the results suggest a regulatory interaction of 14-3-3 proteins with Ser-229 of SPS.

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.

3-Hydroxy-3-methylglutaryl-coenzyme A reductase kinase and sucrose-phosphate synthase kinase activities in cauliflower florets: Ca2+ dependence and substrate specificities

Plant 3-hydroxy-3-methylglutaryl-CoA reductase(HMGR; EC 1.1.1.34) and sucrose-phosphate synthase (SPS; EC 2.4.1.14) and synthetic peptides designed from the known phosphorylation sites of plant HMGR (SAMS*: KSHMKYNRSTKDVK), rat acetyl-CoA carboxylase (SAMS: HMRSAMSGLHLVKRR), spinach SPS (SP2: GRRJRRISSVEJJDKK), and spinach NADH:nitrate reductase (NR6: GPTLKRTASTPFJNTTSK) were used to characterize kinase activities from cauliflower (Brassica oleracea L. ) inflorescences. The three major peaks of protein kinase activity resolved by anion-exchange FPLC are homologs of those observed previously in spinach leaves and thus are designated PKI, PKIV, and PKIII, listed in order of elution. PKIV was the most active in terms of phosphorylation and inactivation of recombinant Nicotiana HMGR and was also strictly Ca2+ dependent. The novel aspects are that PKIII has not been detected in previous cauliflower studies, that SAMS* is a more specific peptide substrate to identify potential HMGR kinases, and that the major HMGR kinase in cauliflower is Ca2+ dependent. Of the three major kinases that phosphorylated the SP2 peptide only PKI (partially Ca2+ sensitive) and PKIII (Ca2+ insensitive) inactivated native spinach leaf SPS. Cauliflower extracts contained endogenous SPS that was inactivated by endogenous kinase(s) in an ATP-dependent manner and this may be one of the substrate target proteins for PKI and/or PKIII. The substrate specificity of the three kinase peaks was studied using synthetic peptide variants of the SP2 sequence. All three kinases had a strong preference for peptides with a basic residue at P-6 (as in SP2 and SAMS*; SAMS has a free amino terminus at this position) or a Pro at P-7 (as in NR6). This requirement for certain residues at P-6 or P-7 was not recognized in earlier studies but appears to be a general requirement. In plant HMGR, a conserved His residue at P-6 is involved directly in catalysis and this may explain why substrates reduced HMGR phosphorylation in vitro.

Cloning and characterization of full-length cDNA encoding sucrose phosphate synthase from faba bean

A 3419-bp cDNA clone encoding sucrose phosphate synthase was isolated from a seed coat-specific cDNA library of Vicia faba L. and conceptually translated into a protein of 1059 amino acid residues. The corresponding 3.4-kb mRNA was found in both source and sink tissues. Southern hybridization data are consistent with a single SPS gene.

Cloning and molecular analysis of cDNAs encoding three sucrose phosphate synthase isoforms from a citrus fruit (Citrus unshiu Marc.)

Three partial cDNA clones (pSPS1, pSPS2 and pSPS3) encoding sucrose phosphate synthases (SPS) were isolated by Reverse Transcription (RT)-PCR using first-strand cDNA prepared from the leaf or fruit of citrus (Citrus unshiu Marc.). The nucleotide and deduced amino acid sequences of the three clones showed significant similarities to SPS previously isolated in other plants. A full-length, cDNA clone, CitSPS1, was isolated from a fruit (juice sacs and pulp segment) cDNA library using one (pSPS1) of the three partial clones as a probe. The 3539-bp CitSPS1 clone coded for a 1057-amino acid polypeptide with a predicted molecular mass of 117.8 kDa. The amino acid sequence deduced from the CitSPS1 clone showed homology with SPS from maize (55.8% identity) and spinach (74.0% identity). Genomic Southern blot analysis suggested that CitSPS1 clone represents a lowcopy-number gene. RNA blot analysis of leaf, flower and fruit showed that CitSPS1 and pSPS2 were expressed in all organs. However, the levels of expression of CitSPS1 in young leaves, flowers and immature fruits were low, but high in mature leaves, and fruit. pSPS2 transcripts were barely detectable in young leaves and immature fruits, low in mature leaves, and high in flowers and mature fruits. pSPS3 transcripts were only detected in young and in mature leaves.

ROLE AND REGULATION OF SUCROSE-PHOSPHATE SYNTHASE IN HIGHER PLANTS

Sucrose-phosphate synthase (SPS; E.C. 2.4.1.14) is the plant enzyme thought to play a major role in sucrose biosynthesis. In photosynthetic and nonphotosynthetic tissues, SPS is regulated by metabolites and by reversible protein phosphorylation. In leaves, phosphorylation modulates SPS activity in response to light/dark signals and end-product accumulation. SPS is phosphorylated on multiple seryl residues in vivo, and the major regulatory phosphorylation site involved is Ser158 in spinach leaves and Ser162 in maize leaves. Regulation of the enzymatic activity of SPS appears to involve calcium, metabolites, and novel "coarse" control of the protein phosphatase that activates SPS. Activation of SPS also occurs during osmotic stress of leaf tissue in darkness, which may function to facilitate sucrose formation for osmoregulation. Manipulation of SPS expression in vivo confirms the role of this enzyme in the control of sucrose biosynthesis.

Bacterial expression of the catalytic domain of 3-hydroxy-3-methylglutaryl-CoA reductase (isoform HMGR1) from Arabidopsis thaliana, and its inactivation by phosphorylation at Ser577 by Brassica oleracea 3-hydroxy-3-methylglutaryl-CoA reductase kinase

The catalytic domain of 3-hydroxy-3-methylglutaryl-CoA reductase isoform 1 (HMGR1cd) from Arabidopsis thaliana has been expressed in Escherichia coli in a catalytically active form and purified. The high efficiency of the bacterial expression system together with the simplicity of the purification procedure used in this study resulted in the attainment of large quantities of pure enzyme (about 5 mg/l culture) with a final specific activity of up to 17 U/mg. This specific activity is higher than that reported to date for any 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) purified from a plant source. HMGR1cd activity was completely blocked by the HMGR inhibitor mevinolin (IC50 = 12.5 nM). No significant differences were observed between the Km values of HMGR1cd for NADPH (71 +/- 7 microM) and (S)-3-hydroxy-3-methylglutaryl-CoA (8.3 +/- 1.5 microM) and those of pure HMGR preparations obtained from different plant sources. The purified HMGR1cd was reversibly inactivated by phosphorylation at a single site by Brassica oleracea HMGR kinase A, which is functionally related to the mammalian AMP-activated protein kinase. The site of phosphorylation is Ser577 in the complete sequence of A. thaliana HMGR1. The results in this paper represent the first evidence that a higher plant HMGR is regulated by direct phosphorylation, at least in a cell-free system. Our results also reinforce the view that the AMP-activated protein kinase/SNF1 family is an ancient and highly conserved protein kinase system.

Cloning and expression analysis of sucrose-phosphate synthase from sugar beet (Beta vulgaris L.)

A cDNA clone encoding a sucrose-phosphate synthase from sugar beet (BvSPS 1) has been isolated by screening a tap root-specific cDNA library using a heterologous SPS cDNA from spinach. The 3635 bp sugar beet cDNA codes for a 1045 amino acid polypeptide with a predicted molecular mass of 118 kDa. The deduced amino acid sequence of sugar beet SPS shows homologies with SPS from maize (71% identity) and spinach (77% identity). Genomic Southern blot analysis suggests that BvSPS 1 is a low-copy-number gene. RNA blot analysis of sink and source leaves, root and tap root tissue shows that SPS 1 is expressed in an organ-specific manner, being predominantly active in tap root. Incubation of detached leaves of sugar beet in light in glucose-containing media leads to an accumulation of the SPS transcript, while sucrose feeding reduces the steady-state level of the mRNA.