Molecular Characterization of the Brittle-2 Gene Effect on Maize Endosperm ADPglucose Pyrophosphorylase Subunits

Revisiting maize Brittle endosperm-2 reveals new insights in BETL development and starchy endosperm filling

Rerouting the starch biosynthesis pathway in maize can generate specialty types, like sweet corn and waxy corn, with a drastically increasing global demand. Hence, a fine-tuning of starch metabolism is relevant to create diverse maize cultivars for end-use applications. Here, we characterized a new maize brittle endosperm mutant, referred to as bt1774, which exhibited decreased starch content but a dramatic increase of soluble sugars at maturity. Both endosperm and embryo development was impaired in bt1774 relative to the wild-type (WT), with a prominently arrested basal endosperm transfer layer (BETL). Map-based cloning revealed that BRITTLE ENDOSPERM2 (Bt2), which encodes a small subunit of ADP-glucose pyrophosphorylase (AGPase), is the causal gene for bt1774. A MuA2 element was found to be inserted into intron 2 of Bt2, leading to a severe decrease of its expression, in bt1774. This is in line with the irregular and loosely packed starch granules in the mutant. Transcriptome of endosperm at grain filling stage identified 1, 013 differentially expressed genes in bt1774, which were notably enriched in the BETL compartment, including ZmMRP1, Miniature1, MEG1, and BETLs. Gene expression of the canonical starch biosynthesis pathway was marginally disturbed in Bt1774. Combined with the residual 60% of starch in this nearly null mutant of Bt2, this data strongly suggests that an AGPase-independent pathway compensates for starch synthesis in the endosperm. Consistent with the BETL defects, zein accumulation was impaired in bt1774. Co-expression network analysis revealed that Bt2 probably has a role in intracellular signal transduction, besides starch synthesis. Altogether, we propose that Bt2 is likely involved in carbohydrate flux and balance, thus regulating both the BETL development and the starchy endosperm filling.

Identification of a stable major sucrose-related QTL and diagnostic marker for flavor improvement in peanut

An InDel marker closely linked with a major and stable quantitative trait locus (QTL) on chromosome A08, qSUCA08.2, controlling sucrose content will benefit peanut flavor improvement. Sucrose is the main soluble sugar in mature peanut kernel, and its content is a key determinant of flavor. However, the genetic basis of sucrose content in peanut remains poorly understood, which limits the progress of flavor improvement. In the present study, two genomic regions (qSUCA08a and qSUCB06a) for sucrose content on chromosomes A08 and B06 were identified by QTL-seq in a RIL population derived from a cross between Zhonghua 10 and ICG 12625. In the interval of qSUCB06a, QTL qSUCB06.2 was detected through QTL mapping in a single environment. The qSUCA08a was further dissected into 3 adjacent genomic regions using linkage analysis including a major QTL qSUCA08.2 explaining 5.43-17.84% phenotypic variation across five environments. A 61-bp insertion at position 35,099,320 in the higher sucrose parent ICG 12625 was found in qSUCA08.2. An InDel marker SUC.InDel.A08 based on the insertion/deletion polymorphism was developed and validated within a natural population containing 172 peanut cultivars in two environments. The mean sucrose content of 93 cultivars with ICG 12625 allele was significantly higher than that of 79 cultivars with Zhonghua 10 allele. The qSUCA08.2 corresponding to a 2.11 Mb interval harbored 110 genes. Among these genes, a total of 19 genes were considered as candidate genes including 5 non-synonymous mutation genes and 14 differentially expressed genes during seed development. These results provide new insights into the genetic basis of sucrose regulation in peanut and benefit the breeding program for developing new varieties with excellent flavor.

A Systemic Investigation of Genetic Architecture and Gene Resources Controlling Kernel Size-Related Traits in Maize

Grain yield is the most critical and complex quantitative trait in maize. Kernel length (KL), kernel width (KW), kernel thickness (KT) and hundred-kernel weight (HKW) associated with kernel size are essential components of yield-related traits in maize. With the extensive use of quantitative trait locus (QTL) mapping and genome-wide association study (GWAS) analyses, thousands of QTLs and quantitative trait nucleotides (QTNs) have been discovered for controlling these traits. However, only some of them have been cloned and successfully utilized in breeding programs. In this study, we exhaustively collected reported genes, QTLs and QTNs associated with the four traits, performed cluster identification of QTLs and QTNs, then combined QTL and QTN clusters to detect consensus hotspot regions. In total, 31 hotspots were identified for kernel size-related traits. Their candidate genes were predicted to be related to well-known pathways regulating the kernel developmental process. The identified hotspots can be further explored for fine mapping and candidate gene validation. Finally, we provided a strategy for high yield and quality maize. This study will not only facilitate causal genes cloning, but also guide the breeding practice for maize.

Allelic variation in shrunken2 gene affecting kernel sweetness in exotic-and indigenous-maize inbreds

Sweet corn has become a popular food worldwide. It possesses six-times more sugar than field corn due to the presence of recessive shrunken2 (sh2) gene. Despite availability of diverse sweet corn germplasm, comprehensive characterization of sh2 has not been undertaken so far. Here, entire Sh2 gene (7320 bp) among five field corn-(Sh2Sh2) and six sweet corn-(sh2sh2) inbreds was sequenced. A total of 686 SNPs and 372 InDels were identified, of which three SNPs differentiated the wild-(Sh2) and mutant-(sh2) allele. Ten InDel markers were developed to assess sh2 gene-based diversity among 23 sweet corn and 25 field corn lines. Twenty-five alleles and 47 haplotypes of sh2 were identified among 48 inbreds. Among markers, MGU-InDel-2, MGU-InDel-3, MGU-InDel-5 and MGU-InDel-8 had PIC>0.5. Major allele frequency varied from 0.458–0.958. The gene sequence of these maize inbreds was compared with 25 orthologues of monocots. Sh2 gene possessed 15–18 exons with 6-225bp among maize, while it was 6–21 exons with 30-441bp among orthologues. While intron length across maize genotypes varied between 67-2069bp, the same among orthologues was 57–2713 bp. Sh2-encoded AGPase domain was more conserved than NTP transferase domain. Nucleotide and protein sequences of sh2 in maize and orthologues revealed that rice orthologue was closer to maize than other monocots. The study also provided details of motifs and domains present in sh2 gene, physicochemical properties and secondary structure of SH2 protein in maize inbreds and orthologues. This study reports detailed characterization and diversity analysis in sh2 gene of maize and related orthologues in various monocots.

easyMF: A Web Platform for Matrix Factorization-Based Gene Discovery from Large-scale Transcriptome Data

With the development of high-throughput experimental technologies, large-scale RNA sequencing (RNA-Seq) data have been and continue to be produced, but have led to challenges in extracting relevant biological knowledge hidden in the produced high-dimensional gene expression matrices. Here, we develop easyMF ( https://github.com/cma2015/easyMF ), a web platform that can facilitate functional gene discovery from large-scale transcriptome data using matrix factorization (MF) algorithms. Compared with existing MF-based software packages, easyMF exhibits several promising features, such as greater functionality, flexibility and ease of use. The easyMF platform is equipped using the Big-Data-supported Galaxy system with user-friendly graphic user interfaces, allowing users with little programming experience to streamline transcriptome analysis from raw reads to gene expression, carry out multiple-scenario MF analysis, and perform multiple-way MF-based gene discovery. easyMF is also powered with the advanced packing technology to enhance ease of use under different operating systems and computational environments. We illustrated the application of easyMF for seed gene discovery from temporal, spatial, and integrated RNA-Seq datasets of maize (Zea mays L.), resulting in the identification of 3,167 seed stage-specific, 1,849 seed compartment-specific, and 774 seed-specific genes, respectively. The present results also indicated that easyMF can prioritize seed-related genes with superior prediction performance over the state-of-art network-based gene prioritization system MaizeNet. As a modular, containerized and open-source platform, easyMF can be further customized to satisfy users' specific demands of functional gene discovery and deployed as a web service for broad applications.

The novel ZmTCP7 transcription factor targets AGPase-encoding gene ZmBt2 to regulate storage starch accumulation in maize

The process of starch biosynthesis is a major developmental event that affects the final grain yield and quality in maize (Zea mays L.), and transcriptional regulation plays a key role in modulating the expression of the main players in the pathway. ZmBt2, which encodes the small subunits of AGPase, is a rate-controlling gene of the pathway; however, much remains unknown about its transcriptional regulation. Our earlier study identifies a short functional fragment of ZmBt2 promoter (394-bp), and further shows it contains multiple putative cis-acting regulatory elements, demonstrating that several transcription factors may govern ZmBt2 expression. Here, we identified a novel TCP transcription factor (TF), ZmTCP7, that interacted with the functional fragment of the ZmBt2 promoter in a yeast one hybrid screening system. We further showed that ZmTCP7 is a non-autonomous TF targeted to the nucleus and predominantly expressed in maize endosperm. Using promoter deletion analyzes by transient expression in maize endosperm protoplasts combined with electrophoretic mobility shift assays, we found that ZmTCP7 bound to GAACCCCAC elements on the ZmBt2 promoter to suppress its expression. Transgenic overexpression of ZmTCP7 in maize caused a significant repression of ZmBt2 transcription by ~77.58%, resulting in a 21.51% decrease in AGPase activity and a 9.58% reduction in the endosperm starch content of transgenic maize. Moreover, the expressions of ZmBt1, ZmSSI, ZmSSIIa, and ZmSSIIIa were increased, while those of ZmSh2 and ZmSSIV reduced significantly in the endosperm of the transgenic maize. Overall, this study shows that ZmTCP7 functions as a transcriptional repressor of ZmBt2 and a negative regulator of endosperm starch accumulation, providing new insights into the regulatory networks that govern ZmBt2 expression and starch biosynthesis pathway in maize.

Genetic dissection of QTLs for starch content in four maize DH populations

Starch is the principal carbohydrate source in maize kernels. Understanding the genetic basis of starch content (SC) benefits greatly in improving maize yield and optimizing end-use quality. Here, four double haploid (DH) populations were generated and were used to identify quantitative trait loci (QTLs) associated with SC. The phenotype of SC exhibited continuous and approximate normal distribution in each population. A total of 13 QTLs for SC in maize kernels was detected in a range of 3.65-16.18% of phenotypic variation explained (PVE). Among those, only some partly overlapped with QTLs previously known to be related to SC. Meanwhile, 12 genes involved in starch synthesis and metabolism located within QTLs were identified in this study. These QTLs will lay the foundation to explore candidate genes regulating SC in maize kernel and facilitate the application of molecular marker-assisted selection for a breeding program to cultivate maize varieties with a deal of grain quality.

Maize kernel development

Maize (Zea mays) is a leading cereal crop in the world. The maize kernel is the storage organ and the harvest portion of this crop and is closely related to its yield and quality. The development of maize kernel is initiated by the double fertilization event, leading to the formation of a diploid embryo and a triploid endosperm. The embryo and endosperm are then undergone independent developmental programs, resulting in a mature maize kernel which is comprised of a persistent endosperm, a large embryo, and a maternal pericarp. Due to the well-characterized morphogenesis and powerful genetics, maize kernel has long been an excellent model for the study of cereal kernel development. In recent years, with the release of the maize reference genome and the development of new genomic technologies, there has been an explosive expansion of new knowledge for maize kernel development. In this review, we overviewed recent progress in the study of maize kernel development, with an emphasis on genetic mapping of kernel traits, transcriptome analysis during kernel development, functional gene cloning of kernel mutants, and genetic engineering of kernel traits.

Cassava AGPase genes and their encoded proteins are different from those of other plants

Cassava AGPase and AGPase genes have some unique characteristics. ADP-glucose pyrophosphorylase (AGPase) is a rate-limiting enzyme for starch synthesis. In this study, cassava AGPase genes (MeAGP) were analyzed based on six cultivars and one wild species. A total of seven MeAGPs was identified, including four encoding AGPase large subunits (MeAGPLs 1, 2, 3 and 4) and three encoding AGPase small subunits (MeAGPSs 1, 2 and 3). The copy number of MeAGPs varied in cassava germplasm materials. There were 14 introns for MeAGPLs 1, 2 and 3, 13 introns for MeAGPL4, and 8 introns for other three MeAGPSs. Multiple conservative amino acid sequence motifs were found in the MeAGPs. There were differences in amino acids at binding sites of substrates and regulators among different MeAGP subunits and between MeAGPs and a potato AGPase small subunit (1YP2:B). MeAGPs were all located in chloroplasts. MeAGP expression was not only associated with gene copy number and types/combinations, regions and levels of the DNA methylation but was also affected by environmental factors with the involvement of various transcription factors in multiple regulation networks and in various cis-elements in the gene promoter regions. The MeAGP activity also changed with environmental conditions and had potential differences among the subunits. Taken together, MeAGPs differ in number from those of Arabidopsis, potato, maize, banana, sweet potato, and tomato.

Novel Grain Weight Loci Revealed in a Cross between Cultivated and Wild Sorghum

Grain weight has increased during domestication of cereals. Together with grain number it determines yield, but the two are often negatively correlated. Understanding the genetic architecture of grain weight and its relationship with grain number is critical to enhance crop yield. Sorghum is an important food, feed, and biofuel crop well-known for its adaptation to drought and heat. This study aimed to dissect the genetic basis of thousand grain weight (TGW) in a BCF population between a domesticated sorghum accession and its wild progenitor, subsp. and investigate its relationship with grain number. Thousand grain weight, grain number, and yield were measured in field trials in two successive years. A strong negative correlation between TGW and grain number was observed in both trials. In total, 17 TGW quantitative trait loci (QTL) were identified, with 11 of them exhibiting an opposing effect on grain number, implying the correlation between TGW and grain number is due to pleiotropy. Nine grain size candidate genes were identified within 6 TGW QTL, and of these 5 showed signatures of selection during sorghum domestication. Large-effect QTL in this study that have not been identified previously in cultivated sorghum were found to contain candidate genes with domestication signal, indicating that these QTL were affected during sorghum domestication. This study sheds new light on the genetic basis of TGW, its relationship with grain number, and sorghum domestication.

OPAQUE11 Is a Central Hub of the Regulatory Network for Maize Endosperm Development and Nutrient Metabolism

Maize (Zea mays) endosperm is a primary tissue for nutrient storage and is highly differentiated during development. However, the regulatory networks of endosperm development and nutrient metabolism remain largely unknown. Maize opaque11 (o11) is a classic seed mutant with a small and opaque endosperm showing decreased starch and protein accumulation. We cloned O11 and found that it encodes an endosperm-specific bHLH transcription factor (TF). Loss of function of O11 significantly affected transcription of carbohydrate/amino acid metabolism and stress response genes. Genome-wide binding site analysis revealed 9885 O11 binding sites distributed over 6033 genes. Using chromatin immunoprecipitation sequencing (ChIP-seq) coupled with RNA sequencing (RNA-seq) assays, we identified 259 O11-modulated target genes. O11 was found to directly regulate key TFs in endosperm development (NKD2 and ZmDOF3) and nutrient metabolism (O2 and PBF). Moreover, O11 directly regulates cyPPDKs and multiple carbohydrate metabolic enzymes. O11 is an activator of ZmYoda, suggesting its regulatory function through the MAPK pathway in endosperm development. Many stress-response genes are also direct targets of O11. In addition, 11 O11-interacting proteins were identified, including ZmIce1, which coregulates stress response targets and ZmYoda with O11. Therefore, this study reveals an endosperm regulatory network centered around O11, which coordinates endosperm development, metabolism and stress responses.

The U6 Biogenesis-Like 1 Plays an Important Role in Maize Kernel and Seedling Development by Affecting the 3' End Processing of U6 snRNA

Regulation of gene expression at the post-transcriptional level is of crucial importance in the development of an organism. Here we present the characterization of a maize gene, U6 biogenesis-like 1 (UBL1), which plays an important role in kernel and seedling development by influencing pre-mRNA splicing. The ubl1 mutant, exhibiting small kernel and weak seedling, was isolated from a Mutator-tagged population. Transgenic complementation and three independent mutant alleles confirmed that UBL1, which encodes a putative RNA exonuclease belonging to the 2H phosphodiesterase superfamily, is responsible for the phenotype of ubl1. We demonstrated that UBL1 possess the RNA exonuclease activity in vitro and found that loss of UBL1 function in ubl1 causes decreased level and abnormal 3' end constitution of snRNA U6, resulting in splicing defect of mRNAs. Through the in vitro and in vivo studies replacing two histidines with alanines in the H-X-T/S-X (X is a hydrophobic residue) motifs we demonstrated that these two motifs are essential for the normal function of UBL1. We further showed that the function of UBL1 may be conserved across a wide phylogenetic distance as the heterologous expression of maize UBL1 could complement the Arabidopsis ubl1 mutant.

Comparative Analysis of AGPase Genes and Encoded Proteins in Eight Monocots and Three Dicots with Emphasis on Wheat

ADP-glucose pyrophosphorylase (AGPase) is a heterotetrameric enzyme with two large subunits (LS) and two small subunits (SS). It plays a critical role in starch biosynthesis. We are reporting here detailed structure, function and evolution of the genes encoding the LS and the SS among monocots and dicots. "True" orthologs of maize Sh2 (AGPase LS) and Bt2 (AGPase SS) were identified in seven other monocots and three dicots; structure of the enzyme at protein level was also studied. Novel findings of the current study include the following: (i) at the DNA level, the genes controlling the SS are more conserved than those controlling the LS; the variation in both is mainly due to intron number, intron length and intron phase distribution; (ii) at protein level, the SS genes are more conserved relative to those for LS; (iii) "QTCL" motif present in SS showed evolutionary differences in AGPase belonging to wheat 7BS, T. urartu, rice and sorghum, while "LGGG" motif in LS was present in all species except T. urartu and chickpea; SS provides thermostability to AGPase, while LS is involved in regulation of AGPase activity; (iv) heterotetrameric structure of AGPase was predicted and analyzed in real time environment through molecular dynamics simulation for all the species; (v) several cis-acting regulatory elements were identified in the AGPase promoters with their possible role in regulating spatial and temporal expression (endosperm and leaf tissue) and also the expression, in response to abiotic stresses; and (vi) expression analysis revealed downregulation of both subunits under conditions of heat and drought stress. The results of the present study have allowed better understanding of structure and evolution of the genes and the encoded proteins and provided clues for exploitation of variability in these genes for engineering thermostable AGPase.

AGPase: its role in crop productivity with emphasis on heat tolerance in cereals

AGPase, a key enzyme of starch biosynthetic pathway, has a significant role in crop productivity. Thermotolerant variants of AGPase in cereals may be used for developing cultivars, which may enhance productivity under heat stress. Improvement of crop productivity has always been the major goal of plant breeders to meet the global demand for food. However, crop productivity itself is influenced in a large measure by a number of abiotic stresses including heat, which causes major losses in crop productivity. In cereals, crop productivity in terms of grain yield mainly depends upon the seed starch content so that starch biosynthesis and the enzymes involved in this process have been a major area of investigation for plant physiologists and plant breeders alike. Considerable work has been done on AGPase and its role in crop productivity, particularly under heat stress, because this enzyme is one of the major enzymes, which catalyses the rate-limiting first committed key enzymatic step of starch biosynthesis. Keeping the above in view, this review focuses on the basic features of AGPase including its structure, regulatory mechanisms involving allosteric regulators, its sub-cellular localization and its genetics. Major emphasis, however, has been laid on the genetics of AGPases and its manipulation for developing high yielding cultivars that will have comparable productivity under heat stress. Some important thermotolerant variants of AGPase, which mainly involve specific amino acid substitutions, have been highlighted, and the prospects of using these thermotolerant variants of AGPase in developing cultivars for heat prone areas have been discussed. The review also includes a brief account on transgenics for AGPase, which have been developed for basic studies and crop improvement.

AGPase: its role in crop productivity with emphasis on heat tolerance in cereals

AGPase, a key enzyme of starch biosynthetic pathway, has a significant role in crop productivity. Thermotolerant variants of AGPase in cereals may be used for developing cultivars, which may enhance productivity under heat stress. Improvement of crop productivity has always been the major goal of plant breeders to meet the global demand for food. However, crop productivity itself is influenced in a large measure by a number of abiotic stresses including heat, which causes major losses in crop productivity. In cereals, crop productivity in terms of grain yield mainly depends upon the seed starch content so that starch biosynthesis and the enzymes involved in this process have been a major area of investigation for plant physiologists and plant breeders alike. Considerable work has been done on AGPase and its role in crop productivity, particularly under heat stress, because this enzyme is one of the major enzymes, which catalyses the rate-limiting first committed key enzymatic step of starch biosynthesis. Keeping the above in view, this review focuses on the basic features of AGPase including its structure, regulatory mechanisms involving allosteric regulators, its sub-cellular localization and its genetics. Major emphasis, however, has been laid on the genetics of AGPases and its manipulation for developing high yielding cultivars that will have comparable productivity under heat stress. Some important thermotolerant variants of AGPase, which mainly involve specific amino acid substitutions, have been highlighted, and the prospects of using these thermotolerant variants of AGPase in developing cultivars for heat prone areas have been discussed. The review also includes a brief account on transgenics for AGPase, which have been developed for basic studies and crop improvement.

AGPase: its role in crop productivity with emphasis on heat tolerance in cereals

AGPase, a key enzyme of starch biosynthetic pathway, has a significant role in crop productivity. Thermotolerant variants of AGPase in cereals may be used for developing cultivars, which may enhance productivity under heat stress. Improvement of crop productivity has always been the major goal of plant breeders to meet the global demand for food. However, crop productivity itself is influenced in a large measure by a number of abiotic stresses including heat, which causes major losses in crop productivity. In cereals, crop productivity in terms of grain yield mainly depends upon the seed starch content so that starch biosynthesis and the enzymes involved in this process have been a major area of investigation for plant physiologists and plant breeders alike. Considerable work has been done on AGPase and its role in crop productivity, particularly under heat stress, because this enzyme is one of the major enzymes, which catalyses the rate-limiting first committed key enzymatic step of starch biosynthesis. Keeping the above in view, this review focuses on the basic features of AGPase including its structure, regulatory mechanisms involving allosteric regulators, its sub-cellular localization and its genetics. Major emphasis, however, has been laid on the genetics of AGPases and its manipulation for developing high yielding cultivars that will have comparable productivity under heat stress. Some important thermotolerant variants of AGPase, which mainly involve specific amino acid substitutions, have been highlighted, and the prospects of using these thermotolerant variants of AGPase in developing cultivars for heat prone areas have been discussed. The review also includes a brief account on transgenics for AGPase, which have been developed for basic studies and crop improvement.

Functions of multiple genes encoding ADP-glucose pyrophosphorylase subunits in maize endosperm, embryo, and leaf

ADP-glucose pyrophosphorylase (AGPase) provides the nucleotide sugar ADP-glucose and thus constitutes the first step in starch biosynthesis. The majority of cereal endosperm AGPase is located in the cytosol with a minor portion in amyloplasts, in contrast to its strictly plastidial location in other species and tissues. To investigate the potential functions of plastidial AGPase in maize (Zea mays) endosperm, six genes encoding AGPase large or small subunits were characterized for gene expression as well as subcellular location and biochemical activity of the encoded proteins. Seven transcripts from these genes accumulate in endosperm, including those from shrunken2 and brittle2 that encode cytosolic AGPase and five candidates that could encode subunits of the plastidial enzyme. The amino termini of these five polypeptides directed the transport of a reporter protein into chloroplasts of leaf protoplasts. All seven proteins exhibited AGPase activity when coexpressed in Escherichia coli with partner subunits. Null mutations were identified in the genes agpsemzm and agpllzm and shown to cause reduced AGPase activity in specific tissues. The functioning of these two genes was necessary for the accumulation of normal starch levels in embryo and leaf, respectively. Remnant starch was observed in both instances, indicating that additional genes encode AGPase large and small subunits in embryo and leaf. Endosperm starch was decreased by approximately 7% in agpsemzm- or agpllzm- mutants, demonstrating that plastidial AGPase activity contributes to starch production in this tissue even when the major cytosolic activity is present.

Chloroplast-localized 6-phosphogluconate dehydrogenase is critical for maize endosperm starch accumulation

Plants have duplicate versions of the oxidative pentose phosphate pathway (oxPPP) enzymes with a subset localized to the chloroplast. The chloroplast oxPPP provides NADPH and pentose sugars for multiple metabolic pathways. This study identified two loss-of-function alleles of the Zea mays (maize) chloroplast-localized oxPPP enzyme 6-phosphogluconate dehydrogenase (6PGDH). These mutations caused a rough endosperm seed phenotype with reduced embryo oil and endosperm starch. Genetic translocation experiments showed that pgd3 has separate, essential roles in both endosperm and embryo development. Endosperm metabolite profiling experiments indicated that pgd3 shifts redox-related metabolites and increases reducing sugars similar to starch-biosynthetis mutants. Heavy isotope-labelling experiments indicates that carbon flux into starch is altered in pgd3 mutants. Labelling experiments with a loss of cytosolic 6PGDH did not affect flux into starch. These results support the known role for plastid-localized oxPPP in oil synthesis and argue that amyloplast-localized oxPPP reactions are integral to endosperm starch accumulation in maize kernels.

Molecular and biochemical analysis of the plastidic ADP-glucose transporter (ZmBT1) from Zea mays

Physiological studies on the Brittle1 maize mutant have provided circumstantial evidence that ZmBT1 (Zea mays Brittle1 protein) is involved in the ADP-Glc transport into maize endosperm plastids, but up to now, no direct ADP-Glc transport mediated by ZmBT1 has ever been shown. The heterologous synthesis of ZmBT1 in Escherichia coli cells leads to the functional integration of ZmBT1 into the bacterial cytoplasmic membrane. ZmBT1 transports ADP-Glc in counterexchange with ADP with apparent affinities of about 850 and 465 mum, respectively. Recently, a complete ferredoxin/thioredoxin system has been identified in cereal amyloplasts and BT1 has been proposed as a potential Trx target protein (Balmer, Y., Vensel, W. H., Cai, N., Manieri, W., Schurmann, P., Hurkman, W. J., and Buchanan, B. B. (2006) Proc. Natl. Acad. Sci. U. S. A. 103, 2988-2993). Interestingly, we revealed that the transport activity of ZmBT1 is reversibly regulated by redox reagents such as diamide and dithiothreitol. The expression of ZmBT1 is restricted to endosperm tissues during starch synthesis, whereas a recently identified BT1 maize homologue, the ZmBT1-2, exhibits a ubiquitous expression pattern in hetero- and autotrophic tissues indicating different physiological roles for both maize BT1 isoforms. BT1 homologues are present in both mono- and dicotyledonous plants. Phylogenetic analyses classify the BT1 family into two phylogenetically and biochemically distinct groups. The first group comprises BT1 orthologues restricted to cereals where they mediate the ADP-Glc transport into cereal endosperm storage plastids during starch synthesis. The second group occurs in mono- and dicotyledonous plants and is most probably involved in the export of adenine nucleotides synthesized inside plastids.

ADP-Glucose Pyrophosphorylase: A Regulatory Enzyme for Plant Starch Synthesis

In plants, the synthesis of starch occurs by utilizing ADP-glucose as the glucosyl donor for the elongation of alpha-1,4-glucosidic chains. In photosynthetic bacteria the synthesis of glycogen follows a similar pathway. The first committed step in these pathways is the synthesis of ADP-glucose in a reaction catalyzed by ADP-glucose pyrophosphorylase (ADPGlc PPase). Generally, this enzyme is allosterically regulated by intermediates of the major carbon assimilatory pathway in the respective organism. In oxygenic photosynthesizers, ADPGlc PPase is mainly regulated by 3-phosphoglycerate (activator) and inorganic orthophosphate (inhibitor), interacting in four different patterns. Recent reports have shown that in higher plants, some of the enzymes could also be redox regulated. In eukaryotes, the enzyme is a heterotetramer comprised of two distinct subunits, a catalytic and a modulatory subunit. The latter has been proposed as related to variations in regulation of the enzyme in different plant tissues. Random and site-directed mutagenesis experiments of conserved amino acids revealed important residues for catalysis and regulation. Prediction of the ADPGlc PPase secondary structure suggests that it shares a common folding pattern to other sugar-nucleotide pyrophosphorylases, and they evolved from a common ancestor.

ADP-Glucose Pyrophosphorylase: A Regulatory Enzyme for Plant Starch Synthesis

In plants, the synthesis of starch occurs by utilizing ADP-glucose as the glucosyl donor for the elongation of alpha-1,4-glucosidic chains. In photosynthetic bacteria the synthesis of glycogen follows a similar pathway. The first committed step in these pathways is the synthesis of ADP-glucose in a reaction catalyzed by ADP-glucose pyrophosphorylase (ADPGlc PPase). Generally, this enzyme is allosterically regulated by intermediates of the major carbon assimilatory pathway in the respective organism. In oxygenic photosynthesizers, ADPGlc PPase is mainly regulated by 3-phosphoglycerate (activator) and inorganic orthophosphate (inhibitor), interacting in four different patterns. Recent reports have shown that in higher plants, some of the enzymes could also be redox regulated. In eukaryotes, the enzyme is a heterotetramer comprised of two distinct subunits, a catalytic and a modulatory subunit. The latter has been proposed as related to variations in regulation of the enzyme in different plant tissues. Random and site-directed mutagenesis experiments of conserved amino acids revealed important residues for catalysis and regulation. Prediction of the ADPGlc PPase secondary structure suggests that it shares a common folding pattern to other sugar-nucleotide pyrophosphorylases, and they evolved from a common ancestor.

ADP-glucose pyrophosphorylase from potato tuber: site-directed mutagenesis of homologous aspartic acid residues in the small and large subunits

Asp142 in the homotetrameric ADP-glucose pyrophosphorylase (ADP-Glc PPase) enzyme from Escherichia coli was demonstrated to be involved in catalysis of this enzyme [Frueauf, J.B., Ballicora, M.A. and Preiss J. (2001) J. Biol. Chem., 276, 46319-46325]. The residue is highly conserved throughout the family of ADP-Glc PPases, as well as throughout the super-family of sugar-nucleotide pyrophosphorylases. In the heterotetrameric ADP-Glc PPase from potato (Solanum tuberosum L.) tuber, the homologous residue is present in both the small (Asp145) and the large (Asp160) subunits. It has been proposed that the small subunit of plant ADP-Glc PPases is catalytic, while the large subunit is modulatory; however, no catalytic residues have been identified. To investigate the function of these conserved Asp residues in the ADP-Glc PPase from potato tuber, we used site-directed mutagenesis to introduce either an Asn or a Glu. Kinetic analysis in the direction of synthesis or pyrophosphorolysis of ADP-Glc showed a significant decrease (more than four orders of magnitude) in the specific activity of the SD145NLwt, SD145NLD160N, and SD145NLD160E mutants, while the effect was smaller (approximately two orders of magnitude) with the SD145ELwt, SD145ELD160N, and SD145ELD160E mutants. By contrast, mutation of the large subunit alone did not affect the specific activity but did alter the apparent affinity for the activator 3-phosphoglycerate, showing two types of apparent roles for this residue in the different subunits. These results show that mutation of Asp160 of the large subunit does not affect catalysis, thus the large subunit is not catalytic, and that the negative charge of Asp145 in the small subunit is necessary for enzyme catalysis.

Starch granules: structure and biosynthesis

The emphasis of this review is on starch structure and its biosynthesis. Improvements in understanding have been brought about during the last decade through the development of new physicochemical and biological techniques, leading to real scientific progress. All this literature needs to be kept inside the general literature about biopolymers, despite some confusing results or discrepancies arising from the biological variability of starch. However, a coherent picture of starch over all the different structural levels can be presented, in order to obtain some generalizations about its structure. In this review we will focus first on our present understanding of the structures of amylose and amylopectin and their organization within the granule, and we will then give insights on the biosynthetic mechanisms explaining the biogenesis of starch in plants.

Characterization of ADG1, an Arabidopsis locus encoding for ADPG pyrophosphorylase small subunit, demonstrates that the presence of the small subunit is required for large subunit stability

Two mutants of Arabidopsis have been isolated that affect ADPG pyrophosphorylase (ADGase) activity. Previously, it has been shown that ADG2 encodes the large subunit of ADGase. This study characterizes the adg1 mutant phenotype and ADG1 gene structure. RNA blot analyses indicate that the adg1-1 mutant accumulates transcripts encoding both the large and small subunits of ADGase, while the adg1-2 mutant accumulates only large subunit transcripts. RFLP analysis and complementation of adg1 mutants with the ADGase small subunit gene demonstrate that ADG1 encodes the small subunit. Sequence analysis indicates that adg1-1 represents a missense mutation within the gene. Western blot analysis confirms that adg1 mutants contain neither the large nor the small subunit proteins, suggesting that the presence of functional small subunits is required for large subunit stability.

Molecular cloning and characterization of ADP-glucose pyrophosphorylase cDNA clones isolated from pea cotyledons

Three ADP-glucose pyrophosphorylase (ADPG-PPase) cDNA clones have been isolated and characterized from a pea cotyledon cDNA library. Two of these clones (Psagps1 and Psagps2) encode the small subunit of ADPG-PPase. The deduced amino acid sequences for these two clones are 95% identical. Expression of these two genes differs in that the Psagps2 gene shows comparatively higher expression in seeds relative to its expression in other tissues. Psagps2 expression also peaks midway through seed development at a time in which Psagps1 transcripts are still accumulating. The third cDNA isolated (Psagp11) encodes the large subunit of ADPG-PPase. It shows greater selectivity in expression than either of the small subunit clones. It is highly expressed in sink organs (seed, pod, and seed coat) and undetectable in leaves.

Multiple forms of ADP-glucose pyrophosphorylase from tomato fruit

ADP-glucose pyrophosphorylase (AGP) was purified from tomato (Lycopersicon esculentum Mill.) fruit to apparent homogeneity. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis the enzyme migrated as two close bands with molecular weights of 50,000 and 51,000. Two-dimensional polyacrylamide gel electrophoresis analysis of the purified enzyme, however, revealed at least five major protein spots that could be distinguished by their slight differences in net charge and molecular weight. Whereas all of the spots were recognized by the antiserum raised against tomato fruit AGP holoenzyme, only three of them reacted strongly with antiserum raised against the potato tuber AGP large subunit, and the other two spots (with lower molecular weights) reacted specifically with antisera raised against spinach leaf AGP holoenzyme and the potato tuber AGP small subunit. The results suggest the existence of at least three isoforms of the AGP large subunit and two isoforms of the small subunit in tomato fruit in vivo. The native molecular mass of the enzyme determined by gel filtration was 220 +/- 10 kD, indicating a tetrameric structure for AGP from tomato fruit. The purified enzyme is very sensitive to 3-phosphoglycerate/inorganic phosphate regulation.

Purification and characterization of a thermostable ADP-glucose pyrophosphorylase from Thermus caldophilus GK-24

An extremely thermostable ADP-glucose pyrophosphorylase (AGPase) has been purified from Thermus caldophilus GK-24 to homogeneity by chromatographic methods, including gel filtration and ion-exchange and affinity chromatography. The specific activity of the enzyme was enriched 134.8-fold with a recovery of 10.5%. The purified enzyme was a single band by SDS/PAGE with a molecular mass of 52 kDa. The homotetrameric structure of the native enzyme was determined by gel filtration analysis, which showed a molecular mass of 230 kDa on a Superose-12 column, indicating that the structure of the enzyme is different from the heterotetrameric structures of higher-plant AGPases. The enzyme was most active at pH 6.0. The activity was maximal at 73-78 degrees C and its half-life was 30 min at 95 degrees C. Kinetic and regulatory properties were characterized. It was found that AGPase activity could be stimulated by a number of glycolytic intermediates. Fructose 6-phosphate, fructose 1,6-bisphosphate, phenylglyoxal and glucose 6-phosphate were effective activators, of which fructose 1,6-bisphosphate was the most effective. The enzyme was inhibited by phosphate, AMP or ADP. ATP and glucose 1-phosphate gave hyperbolic-shaped rate-concentration curves in the presence or absence of activator. A remarkable aspect of the amino acid composition was the existence of the hydrophobic and Ala+Gly residues. The N-terminal and internal peptide sequences were determined and compared with known sequences of various sources. It was apparently similar to those of AGPases from other bacterial and plant sources, suggesting that the enzymes are structurally related.

The maize brittle 1 gene encodes amyloplast membrane polypeptides

A chimeric protein, formed of 56 amino acids from the carboxy terminus of the maize (Zea mays L.) wild-type Brittle1 (Bt1) protein fused to the glutathione-S-transferase gene, was synthesized in Escherichia coli, and used to raise antibodies. Following affinity purification, the antibodies recognized a set of 38- to 42-kDa proteins in endosperm from wild-type Bt1 plants, as well as from brittle2, shrunken2 and sugary1 plants, but not in mutant bt1 endosperm. Bt1 proteins were not detected with the preimmune antibodies. A low level of Bt1-specific proteins was detected at 10 d after pollination (DAP) and increased to a plateau at 16 DAP. At the same time, the ratio of slow- to fast-migrating forms of the protein decreased. During endosperm fractionation by differential centrifugation and membrane sedimentation in sucrose gradients, the Bt1 proteins co-purified with the carotenoid-containing plastid membranes. They were localized to amyloplasts by electron-microscopic immunocytochemistry; most of the signal was detected at the plastid periphery. These results are consistent with predictions made from the deduced amino-acid sequence and previous in-vitro experiments that the bt1 locus encodes amyloplast membrane proteins.

ADP-glucose pyrophosphorylase in shrunken-2 and brittle-2 mutants of maize

The Shrunken-2 (Sh2) and Brittle-2 (Bt2) genes of maize encode subunits of the tetrameric maize endosperm ADPglucose pyrophosphorylase. However, in all sh2 and bt2 mutants so far examined, measurable ADPglucose pyrophosphorylase activity remains. We have investigated the origin of the residual activity found in various sh2 and bt2 mutants as well as tissue specific expression and post-translational modification of the Sh2 and Bt2 proteins. Sh2 and Bt2 cDNAs were expressed in Escherichia coli and antibodies were prepared against the resulting proteins SH2 and BT2 specific antibodies were used to demonstrate that SH2 and BT2 are endosperm specific, are altered or missing in various sh2 or bt2 mutants, and have a mol. wt. of 54 and 51 kDa respectively in the wild type. The Sh2 and Bt2 transcripts are also endosperm specific. Ten sh2 and eight bt2 mutants show varying severity of phenotypes expressed at transcript, protein subunit and kernel level. Synthesis of multiple transcripts and proteins commonly occurs as a result of sh2 or bt2 mutation. While all mutants produce detectable enzymic activity, not all produce detectable transcripts and proteins. To examine the origin of the apparent non-SH2/BT2 endosperm enzymic activity, homologs of Sh2 and Bt2, designated Agp1 and Agp2 respectively, were isolated from an embryo cDNA library and found to hybridize to endosperm transcripts distinct from those of Sh2 and Bt2. Thus Agp1 and Agp2 or closely related genes may be responsible for the residual activity in some sh2 and bt2 mutants. Surprisingly, no evidence of post-translational modification of the SH2 and BT2 protein subunits was detected.

Characterization of the kinetic, regulatory, and structural properties of ADP-glucose pyrophosphorylase from Chlamydomonas reinhardtii

ADP-glucose pyrophosphorylase (ADP-Glc PPase) from Chlamydomonas reinhardtii cells was purified over 2000-fold to a specific activity of 81 units/mg protein, and its kinetic and regulatory properties were characterized. Inorganic orthophosphate and 3-phosphoglycerate were the most potent inhibitor and activator, respectively. Rabbit antiserum raised against the spinach leaf ADP-Glc PPase (but not the one raised against the enzyme from Escherichia coli) inhibited the activity of the purified algal enzyme, which migrated as a single protein band in native polyacrylamide gel electrophoresis. Two-dimensional and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicate that the enzyme from C. reinhardtii is composed of two subunits with molecular masses of 50 and 53 kD, respectively. The molecular mass of the native enzyme is estimated to be 210 kD. Antisera raised against the spinach leaf holoenzyme and against the 51-kD spinach subunit cross-reacted with both subunits of the algal ADP-Glc PPase in immunoblot hybridization, but the cross-reaction was stronger for the 50-kD algal subunit than for the 53-kD subunit. No cross-reaction was observed when antiserum raised against the spinach leaf pyrophosphorylase 54-kD subunit was used. These results suggest that the ADP-Glc PPase from C. reinhardtii is a heterotetrameric protein, since the enzyme from higher plants and its two subunits are structurally more related to the small subunit of the spinach leaf enzyme than to its large subunit. This information is discussed in the context of the possible evolutionary changes leading from the bacterial ADP-Glc PPase to the cyanobacterial and higher plant enzymes.

Expression of ADP-glucose pyrophosphorylase in maize (Zea mays L.) grain and source leaf during grain filling

The time course of ADP-glucose pyrophosphorylase activity and of starch accumulation rate measured in grain, from pollination to maturity, in Zea mays L. plants grown outdoors, was coincident for 2 years. No such correlation was observed in the adjacent leaf, which, furthermore, presented large year-to-year differences in starch accumulation pattern. Analysis of the expression of ADP-glucose synthase at the protein levels, using antibodies directed against the Bt2 or Sh2 subunits, established that the variation of activity in the grain was explained by parallel changes in the content of both subunits. The cDNA for Bt2 and Sh2 subunits were used as probes to quantify the corresponding messenger. In grain, the time course of Bt2 and Sh2 mRNA accumulation anticipated, with a similar pattern, the specific peptide variations, which suggests a transcriptional control of expression. By contrast, the control of leaf activity by protein content was less obvious than in the grain, and changes in leaf enzyme specific activity were suggested during the first 20 d after pollination. A clone homologous to the grain Bt2 subunit cDNA was isolated from a maize leaf cDNA library, and a sequence comparison showed that the leaf clone (L2) was a partial cDNA representing one-third of the mature peptide. A 97% homology was observed between Bt2 and L2 in their coding region, but homology was poor in the 3' noncoding border. This result demonstrates that Bt2 and L2 arise from different genes presenting a tissue-specific expression pattern and provides an explanation for the earlier reported differences between leaf and grain in the size of peptide and mRNA for the Bt2-homologous subunit.

Insensitivity of barley endosperm ADP-glucose pyrophosphorylase to 3-phosphoglycerate and orthophosphate regulation

Crude extracts of starchy endosperm from barley (Hordeum vulgare cv Bomi) contained high pyrophosphorolytic activity (up to 0.5 mumol of glucose-1-P formed min-1 mg-1 of protein) of ADP-glucose pyrophosphorylase (AGP) when assayed in the absence of 3-phosphoglycerate (3-PGA). This high activity was observed regardless of whether AGP had been extracted in the presence or absence of various protease inhibitors or other protectants. Western blot analysis using antibodies specific for either the small or large subunit of the enzyme demonstrated that the large, 60-kD subunit was prone to proteolysis in crude extracts, with a half-time of degradation at 4 degrees C (from 60 to 53 to 51 kD) on the order of minutes. The presence of high concentrations of protease inhibitors decreased, but did not prevent this proteolysis. The small, 51-kD subunit of barley endosperm AGP was relatively resistant to proteolysis, both in the presence or absence of protease inhibitors. For the crude, nonproteolyzed enzyme, 3-PGA acted as a weak activator of the ADP-glucose synthetic reaction (about 25% activation), whereas in the reverse reaction (pyrophosphorolysis) it served as an inhibitor rather than an activator. For both the synthetic and pyrophosphorolytic reactions, inorganic phosphate (Pi) acted as a weak competitive or mixed inhibitor of AGP. The relative insensitivity to 3-PGA/Pi regulation has been observed with both the nonproteolyzed crude enzyme and partially purified (over 60-fold) AGP, the latter characterized by two bands for the large subunit (molecular masses of 53 and 51 kD) and one band for the small subunit (51 kD). Addition of 3-PGA to assays of the partially purified, proteolyzed enzyme had little or no effect on the Km values of all substrates of AGP, but it reduced the Hill coefficient for ATP (from 2.1 to 1.0). These findings are discussed with respect to previous reports on the structure and regulation of higher plant AGP.

Reconstitution and Characterization of a Calmodulin-Stimulated Ca-Pumping ATPase Purified from Brassica oleracea L

Purification and functional reconstitution of a calmodulin-stimulated Ca(2+)-ATPase from cauliflower (Brassica oleracea L.) is described. Activity was purified about 120-fold from a microsomal fraction using calmodulin-affinity chromatography. The purified fraction showed a polypeptide at 115 kD, which formed a phosphorylated intermediate in the presence of Ca(2+), together with a few polypeptides with lower molecular masses that were not phosphorylated. The ATPase was reconstituted into liposomes by 3-([cholamidopropyl]-dimethylammonio-)1-propanesulfonate (CHAPS) dialysis. The proteoliposomes showed ATP-dependent Ca(2+) uptake and ATPase activity, both of which were stimulated about 4-fold by calmodulin. Specific ATPase activity was about 5 mumol min(-1) (mg protein)(-1), and the Ca(2+)/ATP ratio was 0.1 to 0.5 when the ATPase was reconstituted with entrapped oxalate. The purified, reconstituted Ca(2+)-ATPase was inhibited by vanadate and erythrosin B, but not by cyclopiazonic acid and thapsigargin. Activity was supported by ATP (100%) and GTP (50%) and had a pH optimum of about 7.0. The effect of monovalent and divalent cations (including Ca(2+)) on activity is described. Assay of membranes purified by two-phase partitioning indicated that approximately 95% of the activity was associated with intracellular membranes, but only about 5% with plasma membranes. Sucrose gradient centrifugation suggests that the endoplasmic reticulum is the major cellular location of calmodulin-stimulated Ca(2+)-pumping ATPase in Brassica oleracea inflorescences.

The rb Mutation of Peas Causes Structural and Regulatory Changes in ADP Glucose Pyrophosphorylase from Developing Embryos

A mutation at the rb locus of pea (Pisum sativum L.) alters the shape, reduces the starch content, and increases the lipid and sucrose contents of the seed. These effects are probably all consequences of a reduction of up to 40-fold in the maximum catalytic activity of ADP glucose pyrophosphorylase in the developing embryo of the mutant relative to the wild type. We have investigated how the mutation brings about this reduction in activity. The purified enzyme from mutant embryos has a specific activity about 10-fold lower than that from wild-type embryos, and it is much more sensitive to the effectors inorganic phosphate and 3-phosphoglycerate than the wild-type enzyme. Both wild-type and mutant enzymes consist of polypeptides of around 50 kilodaltons. One of the polypeptides of the purified wild-type enzyme is missing from the mutant enzyme. We deduce that in the wild-type embryo this protein may interact with other subunits to confer a high specific activity and a low susceptibility to effectors on the enzyme.

Comparison of proteins of ADP-glucose pyrophosphorylase from diverse sources

The primary structures of 11 proteins of ADP-glucose pyrophosphorylase are aligned and compared for relationships among them. These comparisons indicate that many domains are retained in the proteins from both the enteric bacteria and the proteins from angiosperm plants. The proteins from angiosperm plants show two main groups, with one of the main groups demonstrating two subgroups. The two main groups of angiosperm plant proteins are based upon the two subunits of the enzyme, whereas the subgroups of the large subunit group are based upon the tissue in which the particular gene had been expressed. Additionally, the small subunit group shows a slight but distinct division into a grouping based upon whether the protein is from a monocot or dicot source. Previous structure-function studies with the Escherichia coli enzyme have identified regions of the primary structure associated with the substrate binding site, the allosteric activator binding site, and the allosteric inhibitor binding site. There is conservation of the primary structure of the polypeptides for the substrate binding site and the allosteric activator binding site. The nucleotide sequences of the coding regions of the genes of 11 of these proteins are compared for relationships among them. This analysis indicates that the protein for the small subunit has been subject to greater selective pressure to retain a particular primary structure. Also, the coding region of the precursor gene for the small subunit diverged from the coding region of the precursor gene for the large subunits slightly prior to the divergence of the two coding regions of the genes for the two tissue-specific large subunit genes.

Analysis of maize brittle-1 alleles and a defective Suppressor-mutator-induced mutable allele

A mutant allele of the maize brittle-1 (bt1) locus, brittle-1-mutable (bt1-m), was shown genetically and molecularly to result from the insertion of a defective Suppressor-mutator (dSpm) transposable element. An Spm-hybridizing restriction enzyme fragment, which cosegregates with the bt1-m allele and is absent from wild-type revertants of bt1-m, was identified and cloned. Non-Spm portions of it were used as probes to identify wild-type (Bt1) cDNAs in an endosperm library. The 4.3-kb bt1-m genomic clone contains a 3.3-kb dSpm, which is inserted in an exon and is composed of Spm termini flanking non-Spm sequences. RNA gel blot analyses, using a cloned Bt1 cDNA probe, indicated that Bt1 mRNA is present in the endosperm of developing kernels and is absent from embryo or leaf tissues. Several transcripts are produced by bt1-m. The deduced translation product from a 1.7-kb Bt1 cDNA clone has an apparent plastid transit peptide at its amino terminus and sequence similarity to several mitochondrial inner-envelope translocator proteins, suggesting a possible role in amyloplast membrane transport.

Regulatory and Structural Properties of the Cyanobacterial ADPglucose Pyrophosphorylases

ADPglucose pyrophosphorylase (EC 2.7.7.27) has been purified from two cyanobacteria: the filamentous, heterocystic, Anabaena PCC 7120 and the unicellular Synechocystis PCC 6803. The purification procedure gave highly purified enzymes from both cynobacteria with specific activities of 134 (Synechocystis) and 111 (Anabaena) units per milligram protein. The purified enzymes migrated as a single protein band in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with molecular mass corresponding to 53 (Synechocystis) and 50 (Anabaena) kilodaltons. Tetrameric structures were determined for the native enzymes by analysis of gel filtrations. Kinetic and regulatory properties were characterized for the cyanobacterial ADPglucose pyrophosphorylases. Inorganic phosphate and 3-phosphoglycerate were the most potent inhibitor and activator, respectively. The Synechocystis enzyme was activated 126-fold by 3-phosphoglycerate, with saturation curves exhibiting sigmoidicity (A(0.5) = 0.81 millimolar; n(H) = 2.0). Activation by 3-phosphoglycerate of the enzyme from Anabaena demonstrated hyperbolic kinetics (A(0.5) = 0.12 millimolar; n(H) = 1.0), having a maximal stimulation of 17-fold. I(0.5) values of 95 and 44 micromolar were calculated for the inhibition by inorganic phosphate of the Synechocystis and Anabaena enzyme, respectively. Pyridoxal-phosphate behaved as an activator of the cyanobacterial enzyme. It activated the enzyme from Synechocystis nearly 10-fold with high apparent affinity (A(0.5) = 10 micromolar; n(H) = 1.8). Phenylglyoxal modified the cyanobacterial enzyme by inactivating the activity in the presence of 3-phosphoglycerate. Antibody neutralization experiments showed that anti-spinach leaf (but not anti-Escherichia coli) ADPglucose pyrophosphorylase serum inactivated the enzyme from cyanobacteria. When the cyanobacterial enzymes were resolved on sodium dodecyl sulfate- and two-dimensional polyacrylamide gel electrophoresis and probed with Western blots, only one protein band was recognized by the anti-spinach leaf serum. The same polypeptide strongly reacted with antiserum prepared against the smaller spinach leaf 51 kilodalton subunit, whereas the anti-54 kilodalton antibody raised against the spinach subunit reacted weakly to the cyanobacterial subunit. Regulatory and immunological properties of the cyanobacterial enzyme are more related to the higher plant than the bacterial enzyme. Despite this, results suggest that the ADPglucose pyrophosphorylase from cyanobacteria is homotetrameric in structure, in contrast to the reported heterotetrameric structures of the higher plant ADPglucose pyrophosphorylase.

Transmembrane Electron Transport in Plasma Membrane Vesicles Loaded with an NADH-Generating System or Ascorbate

Sugar beet (Beta vulgaris L.) leaf plasma membrane vesicles were loaded with an NADH-generating system (or with ascorbate) and were tested spectrophotometrically for their ability to reduce external, membrane-impermeable electron acceptors. Either alcohol dehydrogenase plus NAD(+) or 100 millimolar ascorbate was included in the homogenization medium, and right-side-out (apoplastic side-out) plasma membrane vesicles were subsequently prepared using two-phase partitioning. Addition of ethanol to plasma membrane vesicles loaded with the NADH-generating system led to a production of NADH inside the vesicles which could be recorded at 340 nanometers. This system was able to reduce 2,6-dichlorophenolindophenol-3'-sulfonate (DCIP-sulfonate), a strongly hydrophilic electron acceptor. The reduction of DCIP-sulfonate was stimulated severalfold by the K(+) ionophore valinomycin, included to abolish membrane potential (outside negative) generated by electrogenic transmembrane electron flow. Fe(3+)-chelates, such as ferricyanide and ferric citrate, as well as cytochrome c, were not reduced by vesicles loaded with the NADH-generating system. In contrast, right-side-out plasma membrane vesicles loaded with ascorbate supported the reduction of both ferric citrate and DCIP-sulfonate, suggesting that ascorbate also may serve as electron donor for transplasma membrane electron transport. Differences in substrate specificity and inhibitor sensitivity indicate that the electrons from ascorbate and NADH were channelled to external acceptors via different electron transport chains. Transplasma membrane electron transport constituted only about 10% of total plasma membrane electron transport activity, but should still be sufficient to be of physiological significance in, e.g. reduction of Fe(3+) to Fe(2+) for uptake.