Cloning and sequencing of a cDNA encoding alpha-glucosidase from sugar beet

Multiple forms of α-glucosidase in rice seeds (Oryza sativa L., var Nipponbare)

Two isoforms of alpha-glucosidases (ONG2-I and ONG2-II) were purified from dry rice seeds (Oryza sativa L., var Nipponbare). Both ONG2-I and ONG2-II were the gene products of ONG2 mRNA expressed in ripening seeds. Each enzyme consisted of two components of 6kDa-peptide and 88kDa-peptide encoded by this order in ONG2 cDNA (ong2), and generated by post-translational proteolysis. The 88kDa-peptide of ONG2-II had 10 additional N-terminal amino acids compared with the 88kDa-peptide of ONG2-I. The peptides between 6kDa and 88kDa components (26 amino acids for ONG2-I and 16 for ONG2-II) were removed by post-translational proteolysis. Proteolysis induced changes in adsorption and degradation of insoluble starch granules. We also obtained three alpha-glucosidase cDNAs (ong1, ong3, and ong4) from ripening seeds. The ONG1, ONG2, and ONG4 genes were situated in distinct locus of rice genome. The transcripts encoding ONG2 and ONG3 were generated by alternative splicing. Members of alpha-glucosidase multigene family are differentially expressed during ripening and germinating stages in rice.

Purification and characterization of Acremonium implicatum α-glucosidase having regioselectivity for α-1,3-glucosidic linkage

alpha-Glucosidase with a high regioselectivity for alpha-1,3-glucosidic linkages for hydrolysis and transglucosylation was purified from culture broth of Acremonium implicatum. The enzyme was a tetrameric protein (M.W. 440,000), of which the monomer (M.W. 103,000; monomeric structure was expected from cDNA sequence) was composed of two polypeptides (M.W. 51,000 and 60,000) formed possibly by posttranslational proteolysis. Nigerose and maltose were hydrolyzed by the enzyme rapidly, but slowly for kojibiose. The k(0)/K(m) value for nigerose was 2.5-fold higher than that of maltose. Isomaltose was cleaved slightly, and sucrose was not. Maltotriose, maltotetraose, p-nitrophenyl alpha-maltoside and soluble starch were good substrates. The enzyme showed high affinity for maltooligosaccharides and p-nitrophenyl alpha-maltoside. The enzyme had the alpha-1,3- and alpha-1,4-glucosyl transfer activities to synthesize oligosaccharides, but no ability to form alpha-1,2- and alpha-1,6-glucosidic linkages. Ability for the formation of alpha-1,3-glucosidic linkage was two to three times higher than that for alpha-1,4-glucosidic linkage. Eight kinds of transglucosylation products were synthesized from maltose, in which 3(2)-O-alpha-nigerosyl-maltose and 3(2)-O-alpha-maltosyl-maltose were novel saccharides.

Carboxyl group of residue Asp647 as possible proton donor in catalytic reaction of alpha-glucosidase from Schizosaccharomyces pombe

cDNA encoding Schizosaccharomyces pombe alpha-glucosidase was cloned from a library constructed from mRNA of the fission yeast, and expressed in Saccharomyces cerevisiae. The cDNA, 4176 bp in length, included a single ORF composed of 2910 bp encoding a polypeptide of 969 amino-acid residues with M(r) 106 138. The deduced amino-acid sequence showed a high homology to those of alpha-glucosidases from molds, plants and mammals. Therefore, the enzyme was categorized into the alpha-glucosidase family II. By site-directed mutagenesis, Asp481, Glu484 and Asp647 residues were confirmed to be essential in the catalytic reaction. The carboxyl group (-COOH) of the Asp647 residue was for the first time shown to be the most likely proton donor acting as the acid catalyst in the alpha-glucosidase of family II. Studies with the chemical modifier conduritol B epoxide suggested that the carboxylate group (-COO-) of the Asp481 residue was the catalytic nucleophile, although the role of the Glu484 residue remains obscure.

Purification and characterization of an extracellular alpha-glucosidase protein from Trichoderma viride which degrades a phytotoxin associated with sheath blight disease in rice

AIMS

To purify and characterize an extracellular alpha-glucosidase from Trichoderma viride capable of inactivating a host-specific phytotoxin, designated RS toxin, produced by the rice sheath blight pathogen, Rhizoctonia solani Kühn.

METHODS AND RESULTS

The host-specific RS toxin was purified from both culture filtrates (culture filtrate toxin, CFTox) and R. solani-inoculated rice sheaths (sheath blight toxin, SBTox). Sodium dodecyl sulphate-polyacrylamide gel electrophoresis analyses of extracellular proteins, purified from a biocontrol fungus T. viride (TvMNT7) grown on SBTox and CFTox separately, were carried out. The antifungal activity of the purified high molecular weight protein (110 kDa) was studied against RS toxin as well as on the sclerotial germination and mycelial growth of R. solani. Enzyme assay and Western blot analysis with the antirabbit TvMNT7 110-kDa protein indicated that the protein was an alpha-glucosidase. The 110-kDa protein was highly specific to RS toxin and its Michaelis-Menten constant value was 0.40 mmol l-1 when p-nitrophenyl alpha-D-glucopyranoside was used as the substrate. The isoelectric point of the protein was 5.2. N-terminal sequencing of the alpha-glucosidase protein showed that its amino acid sequence showed no homology with other known alpha-glucosidases.

CONCLUSION

This appears to be the first report of the purification and characterization of an alpha-glucosidase capable of inactivating a host-specific toxin of fungal origin. The alpha-glucosidase is specific to RS toxin and is different from the known alpha-glucosidases.

SIGNIFICANCE AND IMPACT OF THE STUDY

As RS toxin could be inactivated by the microbial alpha-glucosidase enzyme, isolation of the gene that codes for the enzyme from T. viride and transfer of the gene to rice plants would lead to enhanced resistance against sheath blight pathogen by inactivation of RS toxin.

A potato alpha-glucosidase gene encodes a glycoprotein-processing alpha-glucosidase II-like activity. Demonstration of enzyme activity and effects of down-regulation in transgenic plants

In order to elucidate more fully the function of a potato gene (MAL1) encoding alpha-glucosidase activity, transgenic plants in which MAL1 expression was down-regulated were generated using antisense technology. In transgenic lines severely down-regulated in the expression of MAL1, total alpha-glucosidase activity was not decreased in leaves and tubers, and the contents of starch, glucose, fructose and sucrose remained unchanged in tubers. Phylogenetic analysis indicated that the MAL1 gene product was more similar to the glycoprotein-processing alpha-glucosidase II of mammalian and yeast origin than to other plant alpha-glucosidases. Using [14C-Glc]-labelled Glc2Man9GlcNAc2 as a substrate, it was demonstrated that glucosidase II activity was markedly down-regulated in microsomes isolated from tubers of four independent antisense lines studied in detail, strongly suggesting that MAL1 encodes glucosidase II activity. In field trials (but not in the glasshouse), MAL1 down-regulation produced an extremely stunted phenotype - the leaves were curled and tuber yield was decreased by 90% compared to control values. Microscopic analysis of leaves revealed significant differences between the antisense and control samples. Plants with down-regulated glucosidase II activity showed a greater degree of plasmolysis, and an increase in the size of mesophyll intracellular spaces. Analysis of cell walls also indicated changes in structure as a result of MAL1 down-regulation. In leaves from four antisense lines, the steady-state transcript level corresponding to the endoplasmic reticulum chaperone, BiP, was enhanced. This is diagnostic of stress in the endoplasmic reticulum.

Purification, enzymatic characterization, and nucleotide sequence of a high-isoelectric-point alpha-glucosidase from barley malt

High-isoelectric-point (pI) alpha-glucosidase was purified 7, 300-fold from an extract of barley (Hordeum vulgare) malt by ammonium sulfate fractionation, ion-exchange, and butyl-Sepharose chromatography. The enzyme had high activity toward maltose (k(cat) = 25 s(-1)), with an optimum at pH 4.5, and catalyzed the hydrolysis by a retaining mechanism, as shown by nuclear magnetic resonance. Acarbose was a strong inhibitor (K(i) = 1.5 microM). Molecular recognition revealed that all OH-groups in the non-reducing ring and OH-3 in the reducing ring of maltose formed important hydrogen bonds to the enzyme in the transition state complex. Mass spectrometry of tryptic fragments assigned the 92-kD protein to a barley cDNA (GenBank accession no. U22450) that appears to encode an alpha-glucosidase. A corresponding sequence (HvAgl97; GenBank accession no. AF118226) was isolated from a genomic phage library using a cDNA fragment from a barley cDNA library. HvAgl97 encodes a putative 96.6-kD protein of 879 amino acids with 93.8% identity to the protein deduced from U22450. The sequence contains two active site motifs of glycoside hydrolase family 31. Three introns of 86 to 4,286 bp interrupt the coding region. The four exons vary from 218 to 1,529 bp. Gene expression analysis showed that transcription reached a maximum 48 h after the start of germination.

alpha-1,4-glucan lyases producing 1,5-anhydro-D-fructose from starch and glycogen have sequence similarity to alpha-glucosidases

In the past few years a novel enzyme alpha-1,4-glucan lyase (EC 4.2. 2.13), which releases 1,5-anhydrofructose from starch and glycogen, has been cloned and characterized from red algae and fungi. Accumulated evidence indicates that the lytic degradation of starch and glycogen also occurs in other organisms. The present review focuses on the biochemical and molecular aspects of eight known alpha-1,4-glucan lyases and their genes from red algae and fungi. While the amino acid sequence identity is 75-80% among the alpha-1, 4-glucan lyases from each of the taxonomic groups, the identity between the algal and fungal alpha-1,4-glucan lyases is only 25-28%. Notably database searches disclosed that the alpha-1,4-glucan lyases have a clear identity of 23-28% with alpha-glucosidases of glycoside hydrolase family 31, thus for the first time linking enzymes from the class of hydrolases with that of lyases. The alignment of lyases and alpha-glucosidases revealed seven well-conserved regions, three of which have been reported to be involved in catalysis and substrate binding in alpha-glucosidases. The shared substrate and inhibitor specificity and sequence similarity of alpha-1,4-glucan lyases with alpha-glucosidases suggest that related structural elements are involved in the two different catalytic mechanisms.

Structure, properties, and tissue localization of apoplastic alpha-glucosidase in crucifers

Apoplastic alpha-glucosidases occur widely in plants but their function is unknown because appropriate substrates in the apoplast have not been identified. Arabidopsis contains at least three alpha-glucosidase genes; Aglu-1 and Aglu-3 are sequenced and Aglu-2 is known from six expressed sequence tags. Antibodies raised to a portion of Aglu-1 expressed in Escherichia coli recognize two proteins of 96 and 81 kD, respectively, in vegetative tissues of Arabidopsis, broccoli (Brassica oleracea L.), and mustard (Brassica napus L.). The acidic alpha-glucosidase activity from broccoli flower buds was purified using concanavalin A and ion-exchange chromatography. Two active fractions were resolved and both contained a 96-kD immunoreactive polypeptide. The N-terminal sequence from the 96-kD broccoli alpha-glucosidase indicated that it corresponds to the Arabidopsis Aglu-2 gene and that approximately 15 kD of the predicted N terminus was cleaved. The 81-kD protein was more abundant than the 96-kD protein, but it was not active with 4-methylumbelliferyl-alpha-D-glucopyranoside as the substrate and it did not bind to concanavalin A. In situ activity staining using 5-bromo-4-chloro-3-indolyl-alpha-D-glucopyranoside revealed that the acidic alpha-glucosidase activity is predominantly located in the outer cortex of broccoli stems and in vascular tissue, especially in leaf traces.