Phosphorylation of DPE2 at S786 partially regulates starch degradation

In plants, transitory starch is synthetized during the day and degraded at night to provide the continuous carbon needed for growth and development. Starch metabolism is highly coordinated, as the starch degradation rate must be coupled to the amount of starch synthetized during the day. Maltose is one of the chloroplastic products obtained from starch degradation, and maltose is exported to the cytosol where disproportionating enzyme-2 (DPE2) is responsible for its metabolism. The amount of DPE2 remained unchanged throughout the day, but its activity notably increased at the end of the day (7 p.m.), suggesting that posttranslational modification drives the mechanism underlying the regulatory activity of this enzyme. Sucrose nonfermenting-related kinase-1 (SnRK1), a protein kinase that controls the activity of several metabolic enzymes, was able to interact and phosphorylate DPE2 at three different residues localized in the α-glucanotransferase domain. This phosphorylation acts as a positive regulator of DPE2, increasing its activity. Complementation of dpe2-deficient mutants with the wild-type (WT) and S786A forms of DPE2 showed that the nonphosphorylated form of DPE2 only partially restored starch degradation, suggesting that phosphorylation at S786 is involved in enzyme regulation.

BIIDXI, a DUF642 Cell Wall Protein That Regulates Pectin Methyl Esterase Activity, Is Involved in Thermotolerance Processes in Arabidopsis thaliana

Plant cell wall remodeling is an important process during plant responses to heat stress. Pectins, a group of cell wall polysaccharides with a great diversity of complex chemical structures, are also involved in heat stress responses. Enzymatic activity of the pectin methyl esterases, which remove methyl groups from pectins in the cell wall, is regulated by DUF642 proteins, as described in different plants, including Arabidopsis thaliana and Oryza sativa. Our results demonstrated that heat stress altered the expression of the DUF642 gene, BIIDXI. There was an important decrease in BIIDXI expression during the first hour of HS, followed by an increase at 24 h. bdx-1 seedlings had less tolerance to heat stress but presented a normal heat stress response; HSFA2 and HSP22 expressions were highly increased, as they were in WT seedlings. Thermopriming triggered changes in pectin methyl esterase activity in WT seedlings, while no increases in PME activity were detected in bdx-1 seedlings at the same conditions. Taken together, our results suggest that BIIDXI is involved in thermotolerance via PME activation.

Starch accumulation in bean fruit pericarp is mediated by the differentiation of chloroplasts into amyloplasts

The sucrose supply to bean fruits remains almost constant during seed development, and the early stages of this process are characterized by a significant amount of starch and soluble sugars (glucose, fructose and sucrose) accumulated in the pericarp. Bean fruits are photosynthetically active; however, our results indicated that starch synthesis in the pericarp was largely dependent on the photosynthetic activity of the leaves. The photosynthetic activity and the amount of the Rubisco large subunit were gradually reduced in the fruit pericarp, and a large increase in the amount of the ADP-glucose pyrophosphorylase small subunit (AGPase SS) was observed. These changes suggested differentiation of chloroplasts into amyloplasts. Pericarp chloroplasts imported glucose 1-P to support starch synthesis, and their differentiation into amyloplasts allowed the surplus sucrose to be used in the synthesis of starch, which was later degraded to meet the needs of fast-growing seeds. Starch stored in the bean fruit pericarp was not degraded in response to drought stress, but it was rapidly used under severe nutrient restriction. Together, this work indicated that starch accumulation in the pericarp of bean fruits is important to adjust the needs of developing seeds to the amount of sucrose that is provided to fruits.

Review: How do SnRK1 protein kinases truly work?

The AMPK/SNF1/SnRK1 family of protein kinases is involved in cellular responses to energy stress. They also interact with molecules of other signaling pathways to regulate many aspects of growth and development. The biochemical, genetic and molecular knowledge of SnRK1 in plants lags behind that of AMPK and SNF1 and is freely extrapolated such that, in many cases, it is assumed that plant enzymes behave in the same way as homologs in other organisms. In this review, we present data that support the evidence that the structural characteristics of the SnRK1 subunits determine the functional properties of the complex. We also discuss results suggesting that the SnRK1 subunits participate in the assembly of different complexes and that not all combinations are equally important. The activity of SnRK1 is dependent on the phosphorylation of SnRK1α^Thr175 found in the activation loop of the catalytic domain. However, we propose that the phosphorylation of sites close to SnRK1α^Thr175 might contribute to the fine-tuned regulation of SnRK1 activity and thus requires further evaluation. Finally, we also call attention to the interaction of the SnRK1α with regulatory proteins that are not typically identified as putative substrates. The additional functions of the SnRK1 subunits, in addition to those of the active complex, may be necessary for the cell to respond to the complicated conditions presented by energy stress.

Effect of catalytic subunit phosphorylation on the properties of SnRK1 from Phaseolus vulgaris embryos

Legume seed development represents a high demand for energy and metabolic resources to support the massive synthesis of starch and proteins. However, embryo growth occurs in an environment with reduced O₂ that forces the plant to adapt its metabolic activities to maximize efficient energy use. SNF1-related protein kinase1 (SnRK1) is a master metabolic regulator needed for cells adaptation to conditions that reduce energy availability, and its activity is needed for the successful development of seeds. In bean embryo extracts, SnRK1 can be separated by anion exchange chromatography into two pools: one where the catalytic subunit is phosphorylated (SnRK1-p) and another with reduced phosphorylation (SnRK1-np). The phosphorylation of the catalytic subunit produces a large increase in SnRK1 activity but has a minor effect in determining its sensitivity to metabolic inhibitors such as trehalose 6-P (T6P), ADP-glucose (ADPG), glucose 1-P (G1P) and glucose 6-P (G6P). In Arabidopsis thaliana, upstream activating kinases (SnAK) phosphorylate the SnRK1 catalytic subunit at T175/176, promoting and enhancing its activity. Recombinant Phaseolus vulgaris homologous to SnAK proteins (PvSnAK), can phosphorylate and activate the catalytic domains of the α-subunits of Arabidopsis, as well as the SnRK1-np pool purified from bean embryos. While the phosphorylation process is extremely efficient for catalytic domains, the phosphorylation of the SnRK1-np complex was less effective but produced a significant increase in activity. The presence of SnRK1-np could contribute to a quick response to unexpected adverse conditions. However, in addition to PvSnAK kinases, other factors might contribute to regulating the activation of SnRK1.

A role for the carbohydrate-binding module (CBM) in regulatory SnRK1 subunits: the effect of maltose on SnRK1 activity

SnRK1 is a protein kinase complex that is involved in several aspects of plant growth and development. There are published data indicative of a participation of SnRK1 in the regulation of the synthesis and degradation of starch, although the molecular mechanism is not known. In this work, we performed electron microscopy to explore the in vivo localization of the regulatory and catalytic subunits that constitute the SnRK1 complex. The results indicated that all the subunits are present in the chloroplast and, in particular, the SnRK1 βγ and SnRK1 β3 subunits are associated with starch. Furthermore, the regulatory subunits bind maltose, a relevant product of starch degradation. The kinase activity of immunoprecipitated complexes containing the βγ regulatory subunit was positively regulated by maltose only in the complexes obtained from Arabidopsis leaves collected at dusk. Recombinant complexes with the SnRK1α1 catalytic subunit, SnRK1βγ and three different β subunits showed that maltose only had an effect on a complex formed with the β3 subunit. Truncation of the CBM domain form SnRK1 βγ abolished the maltose activation of the complex and the activity was significantly reduced, indicating that the CBM is a positive regulator of SnRK1. A model of the SnRK1α1/βγ/β3 complex suggests the presence of two putative maltose-binding sites, both involving ligand interactions with the βγ subunit and the α subunit.

Purification and Characterization of an Alkaline Phosphatase Induced by Phosphorus Starvation in Common Bean (Phaseolus vulgaris L.) Roots

Two phosphatase isoforms from roots of the common bean (<em>Phaseolus vulgaris</em> L.) showed an increase in activity in response to phosphate deficiency. One of them (APIII) was chosen for further purification through ionic exchange chromatography and preparative electrophoresis. The estimated molecular mass of APIII was 35 kDa by both SDS-PAGE and gel filtration analyses, suggesting a monomeric form of the active enzyme. The phosphatase was classified as an alkaline phosphatase based on the requirement of pH 8 for optimum catalysis. It not only exhibited broad substrate specificity, with the most activity against pyrophosphate, but also effectively catalyzed the hydrolysis of polyphosphate, glucose-1-phosphate and phosphoenolpyruvate. Activity was completely inhibited by molybdate, vanadate and phosphate but was only partially inhibited by fluoride. Although divalent cations were not essential for the pyrophosphatase activity of this enzyme, the hydrolysis of pyrophosphate increased substantially in the presence of Mg²⁺.

Oxidized Glutathione Promotes the Association of Proteins from Bean Seeds to Potato Starch

In this paper, we report the use of oxidized glutathione (GSSG) in promoting the association of bean seed proteins to starch granules purified from potato tubers. The resulting interaction was stable and only a small fraction of the bound proteins was released by washing with 5% glucose. Higher concentrations of glucose or other sugars (fructose, mannose and mannitol) failed to further increase protein release. Dithiothreitol (DTT) at concentrations up to 50 mM did not liberate the proteins associated to starch; however, applied after GSSG but before adding the proteins, it was very effective in preventing binding of proteins to starch. Lastly, starch modified by the bound proteins increased its water absorption capacity.

Expression of recombinant SnRK1 in E. coli. Characterization of adenine nucleotide binding to the SnRK1.1/AKINβγ-β3 complex

The SnRK1 complexes in plants belong to the family of AMPK/SNF1 kinases, which have been associated with the control of energy balance, in addition to being involved in the regulation of other aspects of plant growth and development. Analysis of complex formation indicates that increased activity is achieved when the catalytic subunit is phosphorylated and bound to regulatory subunits. SnRK1.1 subunit activity is higher than that of SnRK1.2, which also exhibits reduced activation due to the regulatory subunits. The catalytic phosphomimetic subunits (T175/176D) do not exhibit high activity levels, which indicate that the amino acid change does not produce the same effect as phosphorylation. Based on the mammalian AMPK X-ray structure, the plant SnRK1.1/AKINβγ-β3 was modeled by homology modeling and Molecular Dynamics simulations (MD). The model predicted an intimate and extensive contact between a hydrophobic region of AKINβγ and the β3 subunit. While the AKINβγ prediction retains the 4 CBS domain organization of the mammalian enzyme, significant differences are found in the putative nucleotide binding pockets. Docking and MD studies identified two sites between CBS 3 and 4 which may bind adenine nucleotides, but only one appears to be functional, as judging from the predicted binding energies. The recombinant AKINβγ-βs complexes were found to bind adenine nucleotides with dissociation constant (Kd) in the range of the AMP low affinity site in AMPK. The saturation binding data was consistent with a one-site model, in agreement with the in silico calculations. As has been suggested previously, the effect of AMP was found to slow down dephosphorylation but did not influence activity.

Changes in nutrient distribution are part of the mechanism that promotes seed development under severe nutrient restriction

When bean fruits are detached from a plant at 20 days after anthesis (DAA), the material accumulating in the pod is relocalized to the seeds. This mobilization is more active during the first five days after the fruits are removed, which allows some seeds to continue their development. In freshly removed fruits, (14)C-sucrose was evenly distributed among seeds; however, in fruits that were removed three days before, the labeled sugar was concentrated in 1-2 seeds. In addition, in removed pods, embryos dissected from seeds that no longer continue development can assimilate and efficiently use sucrose for protein and starch synthesis. Our results support the hypothesis that most embryos in removed fruits are forced to stop developing by removal of the nutrient supply. We also observed that SnRK1 activity increased in embryos that were subjected to nutrient deprivation, supporting the role of SnRK1 in the metabolic adaptation to stress conditions.

BIIDXI, the At4g32460 DUF642 gene, is involved in pectin methyl esterase regulation during Arabidopsis thaliana seed germination and plant development

BACKGROUND

DUF642 proteins constitute a highly conserved family of proteins that are associated with the cell wall and are specific to spermatophytes. Transcriptome studies have suggested that members of this family are involved in seed development and germination processes. Previous in vitro studies have revealed that At4g32460- and At5g11420-encoded proteins interact with the catalytic domain of pectin methyl esterase 3 (AtPME3, which is encoded by At3g14310). PMEs play an important role in plant development, including seed germination. The aim of this study was to evaluate the function of the DUF642 gene At4g32460 during seed germination and plant development and to determine its relation to PME activity regulation.

RESULTS

Our results indicated that the DUF642 proteins encoded by At4g32460 and At5g11420 could be positive regulators of PME activity during several developmental processes. Transgenic lines overexpressing these proteins showed increased PME activity during seed germination, and improved seed germination performance. In plants expressing At4g32460 antisense RNA, PME activity was decreased in the leaves, and the siliques were very short and contained no seeds. This phenotype was also present in the SALK_142260 and SALK_054867 lines for At4g32460.

CONCLUSIONS

Our results suggested that the DUF642 family contributes to the complexity of the methylesterification process by participating in the fine regulation of pectin status during plant development.

SnRK1 is differentially regulated in the cotyledon and embryo axe of bean (Phaseolus vulgaris L) seeds

SnRK1 activity is developmentally regulated in bean seeds and exhibits a transient increase with the highest value at 20 days after anthesis (DAA), which coincides with the beginning of protein and starch accumulation. The catalytic subunit of SnRK1 shows a consistent decrease throughout the seed development period. However, by 15 DAA a significant proportion of the catalytic subunit appears phosphorylated. The increase in activity and phosphorylation of the catalytic subunit coincides with a decrease in hexoses. However, SnRK1 activity is differentially regulated in the cotyledon and embryo axe, where a larger proportion of the catalytic subunit is phosphorylated. SnRK1 obtained from endosperm extract is inhibited by T6P and to a lesser extent by ADPG and UDPG, whereas the enzyme isolated from embryo is virtually insensitive to T6P but exhibits some inhibition by ADPG and UDPG. In cotyledon extracts, the effects of T6P and ADPG on SnRK1 activity are additive, whereas in embryo extract, T6P inhibits the enzyme only when ADPG is present. After fractionation on Sephacryl-S300, SnRK1 activity obtained from cotyledon extracts is detected as a single peak associated with a molecular weight of 250 kDa whereas that obtained form embryo axe extracts detected as 2 peaks associated with molecular weight of 250 and 180 kDa. In both cases, the catalytic subunit exhibits a wide distribution but is concentrated in the fractions with the highest activity. To analyse the composition of the complex, cotyledon and embryo extracts were treated with a reversible crosslinker (DSP). DSP induced the formation of complexes with molecular weights of 97 and 180 kDa in the cotyledon and embryo extracts, respectively. Since all the phosphorylated catalytic subunit is present in the complexes induced by DSP, it appears that the phosphorylation favors its interaction with other proteins.

Structural and functional basis for starch binding in the SnRK1 subunits AKINβ2 and AKINβγ

Specialized carbohydrate-binding domains, the Starch-Binding Domain (SBD) and the Glycogen Binding Domain (GBD), are motifs of approximately 100 amino acids directly or indirectly associated with starch or glycogen metabolism. Members of the regulatory β subunit of the heterotrimeric complex AMPK/SNF1/SnRK1 contain an SBD or GBD. In Arabidopsis thaliana, the β regulatory subunit AKINβ2 and a γ-type subunit, AKINβγ, also have an SBD. In this work, we compared the SBD of AKINβ2 and AKINβγ with the GBD present in rat AMPKβ1 and demonstrated that they conserved the same overall topology. The majority of the amino acids identified in the protein-carbohydrate interactions in the rat AMPKβ1 are conserved in the two plant proteins. In AKINβγ, there is an insertion of three amino acids that creates a loop adjacent to one of the conserved tryptophan residues. Functionally, the SBD from AKINβγ and AKINβ2 could bind starch, but there was an important difference in the association when an amylose/amylopectin (A/A) mixture was used. The physiological relevance of binding to starch was clear for AKINβγ, because immunolocalization experiments identified this protein inside the chloroplast. SnRK1 activity was not affected by the addition of A/A to the reaction mixture. However, addition of starch inhibited the activity 85%. Furthermore, proteins associated with A/A and starch in an in vitro-binding assay accounted for 10-20% of total SnRK1 kinase activity. Interestingly, the identification of the SnRK1 subunits associated to the protein-carbohydrate complex indicated that only the catalytic subunits, AKIN10 and AKIN11, and the regulatory subunit AKINβγ were present. These results suggest that a dimer formed between either catalytic subunit and AKINβγ could be associated with the A/A mixture in its active form but the same subunits are inactivated when binding to starch.

The activity of SnRK1 is increased in Phaseolus vulgaris seeds in response to a reduced nutrient supply

Phaseolus vulgaris seeds can grow and develop at the expense of the pod reserves after the fruits have been removed from the plant (Fountain etal., 1989). Because this process involves sensing the reduction of nutrients and the remobilisation of pod reserves, we investigated the effect on sucrose non-fermenting related kinase 1 (SnRK1) activity during this process. Bean fruits removed from the plant at 20 days after flowering (DAF) demonstrated active remobilisation of nutrients from the pod to the seeds. After 5 days, the pod dry weight was reduced by 50%. The process was characterized by a rapid degradation of starch, with the greatest decrease observed on day 1 after the fruits were removed. The pod nutrients were insufficient for the needs of all the seeds, and only some seeds continued their development. Those seeds exhibited a transient reduction in sucrose levels on day 1 after the fruits were removed. However, the normal level of sucrose was recovered, and the rate of starch synthesis was identical to that of a seed developed under normal conditions. Removing the fruits from the plant had no effect on the activity of SnRK1 in the pods, whereas in the seeds, the activity was increased by 35%. Simultaneously, a large reduction in seed sucrose levels was observed. The increase in SnRK1 activity was observed in both the cotyledon and embryo axes, but it was higher in the cotyledon. At 20-25 DAF, cotyledons actively accumulate storage materials. It is possible that the increase in SnRK1 activity observed in seeds developed in fruits that have been removed from the plant is part of the mechanism required for nutrient remobilisation under conditions of stress.

Wheat grain development is characterized by remarkable trehalose 6-phosphate accumulation pregrain filling: tissue distribution and relationship to SNF1-related protein kinase1 activity

Trehalose 6-phosphate (T6P) is a sugar signal that regulates metabolism, growth, and development and inhibits the central regulatory SNF1-related protein kinase1 (SnRK1; AKIN10/AKIN11). To better understand the mechanism in wheat (Triticum aestivum) grain, we analyze T6P content and SnRK1 activities. T6P levels changed 178-fold 1 to 45 d after anthesis (DAA), correlating with sucrose content. T6P ranged from 78 nmol g(-1) fresh weight (FW) pregrain filling, around 100-fold higher than previously reported in plants, to 0.4 nmol g(-1) FW during the desiccation stage. In contrast, maximum SnRK1 activity changed only 3-fold but was inhibited strongly by T6P in vitro. To assess SnRK1 activity in vivo, homologs of SnRK1 marker genes in the wheat transcriptome were identified using Wheat Estimated Transcript Server. SnRK1-induced and -repressed marker genes were expressed differently pregrain filling compared to grain filling consistent with changes in T6P. To investigate this further maternal and filial tissues were compared pre- (7 DAA) and during grain filling (17 DAA). Strikingly, in vitro SnRK1 activity was similar in all tissues in contrast to large changes in tissue distribution of T6P. At 7 DAA T6P was 49 to 119 nmol g(-1) FW in filial and maternal tissues sufficient to inhibit SnRK1; at 17 DAA T6P accumulation was almost exclusively endospermal (43 nmol g(-1) FW) with 0.6 to 0.8 nmol T6P g(-1) FW in embryo and pericarp. The data show a correlation between T6P and sucrose overall that belies a marked effect of tissue type and developmental stage on T6P content, consistent with tissue-specific regulation of SnRK1 by T6P in wheat grain.

Heterologous expression of yeast Hxt2 in Arabidopsis thaliana alters sugar uptake, carbon metabolism and gene expression leading to glucose tolerance of germinating seedlings

The hexose transporter 2 gene (Hxt2) from Saccharomyces cerevisiae was expressed in Arabidopsis thaliana under control of the 35S promoter. Several independent transgenic lines were selected after confirming single gene insertion by southern blot analysis in the T4 generation. Northern blots revealed the presence of heterologous transcript. Radiolabeling experiments revealed an increased rate of incorporation of the non-metabolizable analog 3-O-methyl-[U-14C]-glucose. This confirmed that the yeast Hxt2 transporter was functional in Arabidopsis. No phenotypic changes at the vegetative and reproductive stages could be detected in the transgenic lines when compared to wild type plants. Shortly after germination some differences in development and glucose signaling were observed. Transgenic seedlings cultivated in liquid medium or on solid agar plates were able to grow with 3% glucose (producing bigger plants and longer roots), while development of wild type plants was delayed under those conditions. Metabolite analysis revealed that the Hxt2 transgenic lines had higher rates of sugar utilization. Transcriptional profiling showed that particular genes were significantly up- or down-regulated. Some transcription factors like At1g27000 were repressed, while others, such as At3g58780, were induced. The mRNA from classical sugar signaling genes such as STP1, Hxk1, and ApL3 behaved similarly in transgenic lines and wild type lines. Results suggest that the Hxt2 transgene altered some developmental processes related to the perception of high carbon availability after the germination stage. We conclude that the developmental arrest of wild type plants at 3% glucose not only depends on Hxk1 as the only sugar sensor but might also be influenced by the route of hexose transport across the plasma membrane.

SnRK1 isoforms AKIN10 and AKIN11 are differentially regulated in Arabidopsis plants under phosphate starvation

During phosphate starvation, Snf1-related kinase 1 (SnRK1) activity significantly decreases compared with plants growing under normal nutritional conditions. An analysis of the expression of the genes encoding for the catalytic subunits of SnRK1 showed that these subunits were not affected by phosphate starvation. Transgenic Arabidopsis (Arabidopsis thaliana) plants overexpressing the AKIN10 and AKIN11 catalytic subunits fused with green fluorescent protein (GFP) were produced, and their localizations were mainly chloroplastic with low but detectable signals in the cytoplasm. These data were corroborated with an immunocytochemistry analysis using leaf and root sections with an anti-AKIN10/AKIN11 antibody. The SnRK1 activity in transgenic plants overexpressing AKIN11-GFP was reduced by 35% to 40% in phosphate starvation, in contrast with the results observed in plants overexpressing AKIN10-GFP, which increased the activity by 100%. No differences in activity were observed in plants growing in phosphate-sufficient conditions. Biochemical analysis of the proteins indicated that AKIN11 is specifically degraded under these limited conditions and that the increase in AKIN10-GFP activity was not due to the phosphorylation of threonine-175. These results are consistent with an important role of AKIN10 in signaling during phosphate starvation. Moreover, akin10 mutant plants were deficient in starch mobilization at night during inorganic phosphate starvation, and under this condition several genes were up-regulated and down-regulated, indicating their important roles in the control of general transcription. This finding reveals novel roles for the different catalytic subunits during phosphate starvation.

Characterization of a type A response regulator in the common bean (Phaseolus vulgaris) in response to phosphate starvation

Type A response regulators are a family of genes in Arabidopsis thaliana involved primarily in cytokinin signal transduction. A member of this family was isolated from a cDNA library constructed from bean plants (Phaseolus vulgaris) grown under conditions of phosphate starvation. The complete cDNA sequence showed the presence of the DDK domain, which is the hallmark of the response regulator family. Expression of the P. vulgaris response regulator 1 (PvRR1) showed clear regulation based on phosphate availability because transcript levels increased during phosphate starvation and returned to basal levels after resupplementation with phosphorus. Nitrogen and potassium starvation also upregulated PvRR1, indicating that cross talk with other nutrient signaling pathways might occur. Addition of cytokinins to plants growing under phosphate-sufficient conditions stimulated PvRR1 transcript levels both in detached leaves and in roots. However, cytokinins strongly inhibited PvRR1 expression in phosphate-starved plants after 24 h of incubation. At the protein level, subcellular localization of PvRR1 indicated that it is a nuclear protein and that phosphate starvation modified protein levels but not the localization.

Possible role played by R1 protein in starch accumulation in bean (Phaseolus vulgaris) seedlings under phosphate deficiency

The effect of phosphate (Pi) deficiency on starch accumulation was studied in bean (Phaseolus vulgaris). After 3 weeks of Pi deprivation total Pi concentration in root and shoot was reduced by 68% and 42%, respectively; however, only shoot growth was affected. In leaves, Pi deprivation induced glucose, fructose and starch accumulation. Pi deficiency did not affect starch synthesis, but it reduced its mobilization during the dark period. At the same time, starch produced by Pi deficient plants have fewer Pi bound and was also less susceptible to beta-amylase hydrolysis. R1 protein is the protein responsible of phosphorylating C3 and C6 glucosyl residues of the polyglucan, increasing the hydration capacity and the interaction with amylolytic enzymes. Pi deprivation did not change the amount of R1 protein detected in total extracts but decreased its association with starch granules.

Rubisco activase, a possible new member of the molecular chaperone family

The present research addresses the question of whether Rubisco activase (R-A), the enzyme reported to activate Rubisco, is actually a molecular chaperone rather than a conventional enzyme. Several biochemical properties known to be characteristics of molecular chaperones were tested for R-A with positive results. The experiments were performed either in vitro with purified spinach Rubisco and Rubisco activase or in vivo in maize seedling leaves. Our results confirmed that activation of Rubisco by R-A is an ATP hydrolysis-dependent process and further demonstrated that (a) R-A binds preferably to non-native Rubisco protein, than to the native form, and dissociates from this complex after addition of ATP, (b) R-A increases during heat shock treatment in maize seedling leaves, and (c) a large recovery of Rubisco activity is achieved from heat-inactivated Rubisco by addition of R-A and an energy source. We conclude that R-A characteristics strongly suggest that this protein belongs to the molecular chaperone group. The possible role of R-A on maintaining Rubisco activity in vivo is discussed.

Regulation of ribulose-1,5-bisphosphate carboxylase expression in second leaves of maize seedlings from low and high yield populations

Ribulose-1,5-bisphosphate carboxylase oxygenase (EC 4.1.1.39) (Rubisco) activity, Rubisco-protein, and Rubisco large and small subunit gene (rbcL and rbcS) transcripts were measured at seven stages of development in the second leaf of maize (Zea mays L.) seedlings belonging to low and high yield populations. During the three early stages of development, when the leaf has not yet expanded, it was determined that increments in Rubisco-activity were caused by increases in Rubisco-protein and its mRNAs. Afterward, the rbcS level decreased sharply down to nondetectable levels at the seventh stage, when the leaf was at the beginning of senescence. As a contrast, rbcL transcript decreased slowly and Rubisco-protein accumulated up to the fifth stage, when the leaf reached its maximum expansion. A slight decrease in Rubisco-protein was then observed. These results suggest that at early stages of development Rubisco-activity and Rubisco-protein are regulated mainly at the transcriptional level. At the later phase the regulation seems to be at other biochemical levels. Neither Rubisco activity nor Rubisco-protein showed correlation with yield for both maize populations at this stage of development. Slightly higher levels of both transcripts were observed in the high yield population.