Evolution and differential expression of the (1-->3)-beta-glucan endohydrolase-encoding gene family in barley, Hordeum vulgare

Identification of novel QTLs for late leaf spot resistance and validation of a major rust QTL in peanut (Arachis hypogaea L.)

Co-occurrence of two devastating foliar-fungal diseases of peanut, viz., late leaf spot (LLS), and rust may cause heavy yield loss besides adversely affecting the quality of kernel and fodder. This study reports the mapping of seven novel stress-related candidate EST-SSRs in a region having major QTLs for LLS and rust diseases using an F₂ mapping population (GJG17 × GPBD4) consisting of 328 individuals. The parental polymorphism using 1311 SSRs revealed 84 SSRs (6.4%) as polymorphic and of these 70 SSRs could be mapped on 14 linkage groups (LG). QTL analysis has identified a common QTL (LLS_QTL1/Rust_QTL) for LLS and rust diseases in the map interval of 1.41 cM on A03 chromosome, explaining 47.45% and 70.52% phenotypic variations, respectively. Another major QTL for LLS (LLS_QTL1), explaining a 29.06% phenotypic variation was also found on LG_A03. A major rust QTL has been validated which was found harboring R-gene and resistance-related genes having a role in inducing hypersensitive response (HR). Further, 23 linked SSRs including seven novel EST-SSRs were also validated in 177 diverse Indian groundnut genotypes. Twelve genotypes resistant to both LLS and rust were found carrying the common (rust and LLS) QTL region, LLS QTL region, and surrounding regions. These identified and validated candidate EST-SSR markers would be of great use for the peanut breeding groups working for the improvement of foliar-fungal disease resistance.

Mapping of a dominant rust resistance gene revealed two R genes around the major Rust_QTL in cultivated peanut (Arachis hypogaea L.)

A consensus rust QTL was identified within a 1.25 cM map interval of A03 chromosome in cultivated peanut. This map interval contains a TIR-NB-LRR R gene and four pathogenesis-related genes. Disease resistance in plants is manifested due to the specific interaction between the R gene product and its cognate avirulence gene product (AVR) in the pathogen. Puccinia arachidis Speg. causes rust disease and inflicts economic damages to peanut. Till now, no experimental evidence is known for the action of R gene in peanut for rust resistance. A fine mapping approach towards the development of closely linked markers for rust resistance gene was undertaken in this study. Phenotyping of an RIL population at five environments for field rust score and subsequent QTL analysis has identified a 1.25 cM map interval that harbored a consensus major Rust_QTL in A03 chromosome. This Rust_QTL is flanked by two SSR markers: FRS72 and SSR_GO340445. Both the markers clearly identified strong association of the mapped region with rust reaction in both resistant and susceptible genotypes from a collection of 95 cultivated peanut germplasm. This 1.25 cM map interval contained 331.7 kb in the physical map of A. duranensis and had a TIR-NB-LRR category R gene (Aradu.Z87JB) and four glucan endo-1,3 β glucosidase genes (Aradu.RKA6 M, Aradu.T44NR, Aradu.IWV86 and Aradu.VG51Q). Another resistance gene analog was also found in the vicinity of mapped Rust_QTL. The sequence between SSR markers, FRS72 and FRS49, contains an LRR-PK (Aradu.JG217) which is equivalent to RHG4 in soybean. Probably, the protein kinase domain in AhRHG4 acts as an integrated decoy for the cognate AVR from Puccinia arachidis and helps the TIR-NB-LRR R-protein to initiate a controlled program cell death in resistant peanut plants.

Morphology, Carbohydrate Distribution, Gene Expression, and Enzymatic Activities Related to Cell Wall Hydrolysis in Four Barley Varieties during Simulated Malting

Many biological processes, such as cell wall hydrolysis and the mobilisation of nutrient reserves from the starchy endosperm, require stringent regulation to successfully malt barley (Hordeum vulgare) grain in an industrial context. Much of the accumulated knowledge defining these events has been collected from individual, unrelated experiments, and data have often been extrapolated from Petri dish germination, rather than malting, experiments. Here, we present comprehensive morphological, biochemical, and transcript data from a simulated malt batch of the three elite malting cultivars Admiral, Navigator, and Flagship, and the feed cultivar Keel. Activities of lytic enzymes implicated in cell wall and starch depolymerisation in germinated grain have been measured, and transcript data for published cell wall hydrolytic genes have been provided. It was notable that Flagship and Keel exhibited generally similar patterns of enzyme and transcript expression, but exhibited a few key differences that may partially explain Flagship's superior malting qualities. Admiral and Navigator also showed matching expression patterns for these genes and enzymes, but the patterns differed from those of Flagship and Keel, despite Admiral and Navigator having Keel as a common ancestor. Overall (1,3;1,4)-β-glucanase activity differed between cultivars, with lower enzyme levels and concomitantly higher amounts of (1,3;1,4)-β-glucan in the feed variety, Keel, at the end of malting. Transcript levels of the gene encoding (1,3;1,4)-β-glucanase isoenzyme EI were almost three times higher than those encoding isoenzyme EII, suggesting a previously unrecognised importance for isoenzyme EI during malting. Careful morphological examination showed that scutellum epithelial cells in mature dry grain are elongated but expand no further as malting progresses, in contrast to equivalent cells in other cereals, perhaps demonstrating a morphological change in this critical organ over generations of breeding selection. Fluorescent immuno-histochemical labelling revealed the presence of pectin in the nucellus and, for the first time, significant amounts of callose throughout the starchy endosperm of mature grain.

Cereal grass pulvini: agronomically significant models for studying gravitropism signaling and tissue polarity

Cereal grass pulvini have emerged as model systems that are not only valuable for the study of gravitropism, but are also of agricultural and economic significance. The pulvini are regions of tissue that are apical to each node and collectively return a reoriented stem to a more vertical position. They have proven to be useful for the study of gravisensing and response and are also providing clues about the establishment of polarity across tissues. This review will first highlight the agronomic significance of these stem regions and their benefits for use as model systems and provide a brief historical overview. A detailed discussion of the literature focusing on cell signaling and early changes in gene expression will follow, culminating in a temporal framework outlining events in the signaling and early growth phases of gravitropism in this tissue. Changes in cell wall composition and gene expression that occur well into the growth phase will be touched upon briefly. Finally, some ongoing research involving both maize and wheat pulvini will be introduced along with prospects for future investigations.

Barley leaf transcriptome and metabolite analysis reveals new aspects of compatibility and Piriformospora indica-mediated systemic induced resistance to powdery mildew

Colonization of barley roots with the basidiomycete fungus Piriformospora indica (Sebacinales) induces systemic resistance against the biotrophic leaf pathogen Blumeria graminis f. sp. hordei (B. graminis). To identify genes involved in this mycorrhiza-induced systemic resistance, we compared the leaf transcriptome of P. indica-colonized and noncolonized barley plants 12, 24, and 96 h after challenge with a virulent race of B. graminis. The leaf pathogen induced specific gene sets (e.g., LRR receptor kinases and WRKY transcription factors) at 12 h postinoculation (hpi) (prepenetration phase) and vesicle-localized gene products 24 hpi (haustorium establishment). Metabolic analysis revealed a progressing shift of steady state contents of the intermediates glucose-1-phosphate, uridinediphosphate-glucose, and phosphoenolpyruvate 24 and 96 hpi, indicating that B. graminis shifts central carbohydrate metabolism in favor of sucrose biosynthesis. Both B. graminis and P. indica increased glutamine and alanine contents, whereas substrates for starch and nitrogen assimilation (adenosinediphosphate- glucose and oxoglutarate) decreased. In plants that were more B. graminis resistant due to P. indica root colonization, 22 transcripts, including those of pathogenesis-related genes and genes encoding heat-shock proteins, were differentially expressed ?twofold in leaves after B. graminis inoculation compared with non-mycorrhized plants. Detailed expression analysis revealed a faster induction after B. graminis inoculation between 8 and 16 hpi, suggesting that priming of these genes is an important mechanism of P. indica-induced systemic disease resistance.

Cell wall modifications in maize pulvini in response to gravitational stress

Changes in cell wall polysaccharides, transcript abundance, metabolite profiles, and hormone concentrations were monitored in the upper and lower regions of maize (Zea mays) pulvini in response to gravistimulation, during which maize plants placed in a horizontal position returned to the vertical orientation. Heteroxylan levels increased in the lower regions of the pulvini, together with lignin, but xyloglucans and heteromannan contents decreased. The degree of substitution of heteroxylan with arabinofuranosyl residues decreased in the lower pulvini, which exhibited increased mechanical strength as the plants returned to the vertical position. Few or no changes in noncellulosic wall polysaccharides could be detected on the upper side of the pulvinus, and crystalline cellulose content remained essentially constant in both the upper and lower pulvinus. Microarray analyses showed that spatial and temporal changes in transcript profiles were consistent with the changes in wall composition that were observed in the lower regions of the pulvinus. In addition, the microarray analyses indicated that metabolic pathways leading to the biosynthesis of phytohormones were differentially activated in the upper and lower regions of the pulvinus in response to gravistimulation. Metabolite profiles and measured hormone concentrations were consistent with the microarray data, insofar as auxin, physiologically active gibberellic acid, and metabolites potentially involved in lignin biosynthesis increased in the elongating cells of the lower pulvinus.

Stronger induction of callose deposition in barley by Russian wheat aphid than bird cherry-oat aphid is not associated with differences in callose synthase or beta-1,3-glucanase transcript abundance

The effects of infestation by the bird cherry-oat aphid (BCA), (Rhopalosiphum padi L) and the Russian wheat aphid (RWA) (Diuraphis noxia Mordvilko) on callose deposition and transcription of genes related to callose accumulation were investigated in barley (Hordeum vulgare L. cv. Clipper). The BCA, which gives no visible symptoms, induced very limited callose deposition, even after 14 days of infestation. In contrast, RWA, which causes chlorosis, white and yellow streaking and leaf rolling, induced callose accumulation already after 24 h in longitudinal leaf veins. The deposition was pronounced after 72 h, progressing during 7 and 14 days of infestation. In RWA-infested source leaves, callose was also induced in longitudinal veins basipetal to the aphid-infested tissue, whereas in sink leaves, more callose deposition was found above the feeding sites. Eight putative callose synthase genes were identified in a database search, of which seven were expressed in the leaves, but with similar transcript accumulation in control and aphid-infested tissue. Five out of 12 examined beta-1,3-glucanases were expressed in the leaves. All five were upregulated in RWA-infested tissue, but only two in BCA-infested tissue, and to a lesser extent than by RWA. The results suggest that callose accumulation may be partly responsible for the symptoms resulting from RWA infestation and that a callose-inducing signal may be transported in the phloem. Furthermore, it is concluded that the absence of callose deposition in BCA-infested leaves is not because of a stronger upregulation of callose-degrading beta-1,3-glucanases in this tissue, as compared to RWA-infested leaves.

Isolation and characterization of the Aspergillus nidulans eglC gene encoding a putative β-1,3-endoglucanase

The Aspergillus nidulans eglC gene, which encodes a putative beta-1,3-endoglucanase, was isolated from a chromosome-specific library by using an expressed sequence tag, esd0113. The EglC open reading frame encodes a 465 amino acid polypeptide, of which the amino acid sequence showed 46% similarity to that of Saccharomyces cerevisiae beta-1,3-endoglucanase. The eglC transcript level at the early stages of asexual and sexual developments was dependent on the presence of the nsdD gene that encodes a GATA-type transcription factor, confirming that the nsdD gene is necessary for full accumulation of the eglC transcript. Deletion of the eglC gene did not affect the radial growth rate, the germination rate of conidia, and both of asexual and sexual development. However, deletion of the gene led to hyphae more resistant to a cell wall-lyzing enzyme, implying that the cell wall structure of the eglC-null mutant is altered from a wild type one. Furthermore, deletion of the fadA and sfaD genes, that encode a Galpha and a Gbeta subunits of a heterotrimeric G protein, respectively, did not affect the eglC transcript level at the early developmental stages. In contrast, deletion of the flbA gene, that codes for a regulatory protein having an RGS (regulator of G protein signaling) motif, led to decrease in the eglC transcript level. The eglC transcript level was not higher in a creA mutant than in a wild type, indicating that the eglC gene is not sensitive to carbon-catabolite repression.

Mutant barley (1-->3,1-->4)-beta-glucan endohydrolases with enhanced thermostability

The similar three-dimensional structures of barley (1-->3)-beta-glucan endohydrolases and (1-->3,1-->4)-beta-glucan endohydrolases indicate that the enzymes are closely related in evolutionary terms. However, the (1-->3)-beta-glucanases hydrolyze polysaccharides of the type found in fungal cell walls and are members of the pathogenesis-related PR2 group of proteins, while the (1-->3,1-->4)-beta-glucanases function in plant cell wall metabolism. The (1-->3)-beta-glucanases have evolved to be significantly more stable than the (1-->3,1-->4)-beta-glucanases, probably as a consequence of the hostile environments imposed upon the plant by invading microorganisms. In attempts to define the molecular basis for the differences in stability, eight amino acid substitutions were introduced into a barley (1-->3,1-->4)-beta-glucanase using site-directed mutagenesis of a cDNA that encodes the enzyme. The amino acid substitutions chosen were based on structural comparisons of the barley (1-->3)- and (1-->3,1-->4)-beta-glucanases and of other higher plant (1-->3)-beta-glucanases. Three of the resulting mutant enzymes showed increased thermostability compared with the wild-type (1-->3,1-->4)-beta-glucanase. The largest increase in stability was observed when the histidine at position 300 was changed to a proline (mutant H300P), a mutation that was likely to decrease the entropy of the unfolded state of the enzyme. Furthermore, the three amino acid substitutions which increased the thermostability of barley (1-->3,1-->4)-beta-glucanase isoenzyme EII were all located in the COOH-terminal loop of the enzyme. Thus, this loop represents a particularly unstable region of the enzyme and could be involved in the initiation of unfolding of the (1-->3,1-->4)-beta-glucanase at elevated temperatures.

Analysis and mapping of gene families encoding beta-1,3-glucanases of soybean

Oligonucleotide primers designed for conserved sequences from coding regions of beta-1,3-glucanase genes from different species were used to amplify related sequences from soybean [Glycine max (L.) Merr.]. Sequencing and cross-hybridization of amplification products indicated that at least 12 classes of beta-1,3-glucanase genes exist in the soybean. Members of classes mapped to 34 loci on five different linkage groups using an F(2) population of 56 individuals. beta-1,3-Glucanase genes are clustered onto regions of five linkage groups. Data suggest that more closely related genes are clustered together on one linkage group or on duplicated regions of linkage groups. Northern blot analyses performed on total RNA from root, stem, leaf, pod, flower bud, and hypocotyl using DNA probes for the different classes of beta-1,3-glucanase genes revealed that the mRNA levels of all classes were low in young leaves. SGlu2, SGlu4, SGlu7, and SGlu12 mRNA were highly accumulated in young roots and hypocotyls. SGlu7 mRNA also accumulated in pods and flower buds.

Characterization of rice endo-beta-glucanase genes (Gns2-Gns14) defines a new subgroup within the gene family

Thirteen new beta-glucanase-encoding genes have been identified in the rice genome. These genes, together with other monocot beta-glucanases, have now been classified into four subfamilies based on the structure and function of the genes. Two tandem gene clusters, Gns2-Gns3-Gns4 and Gns5-Gns6, were classified in the defense-related Subfamily A. Growth-related 1,3;1,4-beta-glucanase Gns1 was classified in Subfamily B. Gns7 and Gns8, together with the barley genes GVI and Hv34, represent Subfamily C. Gns9 and a beta-glucanase gene from wheat were grouped in Subfamily D. Genes in Subfamilies C and D have structures that are distinct from those of the other subfamilies, but there are very little data available on the biochemical or physiological roles of these genes. Gene expression in growing tissues and lack of gene induction in response to disease-related treatments suggest that Subfamilies C and D may function in control of plant growth.

Molecular size and net charge of pathogenesis-related enzymes from barley (Hordeum vulgare L., v. Karat) infected with Drechslera teres f. teres (Sacch.) Shoem

Molecular size and net charge of isoforms of pathogenesis-related (PR) chitinase, beta-1,3-glucanase and peroxidase were studied in uninfected barley (Hordeum vulgare L., v. Karat) leaves and in barley leaves infected with the pathogenic fungus Drechslera teres f. teres (Sacch.) Shoem. Molecular characteristics were determined by time-dependent polyacrylamide gradient gel electrophoresis under native conditions and by applying an extended version of the computer program MOL-MASS (Rothe, G. M., Weidmann, H., Electrophoresis 1991, 12, 703-709). Uninfected barley leaves contained predominantly one peroxidase isozyme but also three very weak peroxidases. Activities of all of these three peroxidases increased considerably after infection with Drechslera teres. The molecular masses of peroxidases 1 and 3 were estimated to be 38 +/- 5 and 42 +/- 7 kDa and their apparent valences at pH 8.4 were Z = 3.13 and 3.20, respectively. Amongst the chitinase isoforms, chitinase 1 and chitinase 2 appeared after infection, while chitinase 3 was also observed in uninfected leaves of barley. The molecular mass of chitinase 3 (31 +/- 6 kDa; f/fo = 1.20) was larger than that of chitinase 1 (20 +/- 2 kDa; f/fo = 1.04) and chitinase 2 (23 +/- 3 kDa; f/fo = 1.06). The valence of constitutive chitinase 3 (Z = 1.44 +/- 0.81) at pH 8.4 was lower than that of adaptive chitinase 1 (Z = 3.27 +/- 1.02) and chitinase 2 (Z = 2.96 +/- 1.38). Infection of barley leaves with Drechslera teres also induced the hydrolytic enzyme beta-1,3-glucanase 1; beta-1,3-glucanase 2 appeared in uninfected and in infected leaves. Constitutive beta-1,3-glucanase 2 was smaller (molecular mass 19 +/- kDa; f/fo = 1.05) than adaptive beta-1,3-glucanase 1 (molecular mass 26 +/- 4 kDa; f/fo = 1.07). The valence of adaptive beta-1,3-glucanase 1 (Z = 9.58 +/- 4.17) was approximately threefold that of beta-1,3-glucanase 2 (Z = 2.80 +/- 0.93).

Purification and characterization of a (1-->3)-beta-D-glucan endohydrolase from rice (Oryza sativa) bran

A (1-->3)-beta-glucanase with an apparent M(r) of 29,000 and an isoelectric point of 4.0 has been purified 2000-fold from extracts of rice bran, using fractional precipitation with ammonium sulfate, anion exchange chromatography, size-exclusion chromatography, chromatofocussing, and hydrophobic interaction chromatography. The enzyme can be classified with the EC 3.2.1.39 group, because it releases laminarabiose and higher laminara-oligosaccharides from linear (1-->3)-beta-D-glucans with an action pattern that is typical of (1-->3)-beta-D-glucan endohydrolases. However, the introduction of substituents or branching in the (1-->3)-beta-D-glucan substrates causes a marked decrease in the rate of hydrolysis. Thus, substituted or branched (1-->3)-beta-D-glucans of the kind commonly found in fungal cell walls are less susceptible to hydrolysis than essentially linear (1-->3)-beta-D-glucans. Kinetic analyses indicate an apparent Km of 42 microM, a kcat constant of 67 s-1, and a pH optimum of 5.0 during hydrolysis of the (1-->3)-beta-D-glucan, laminaran, from Laminaria digitata. The first 60 NH2-terminal amino acid residues of the purified rice (1-->3)-beta-glucanase contain blocks of amino acids that are conserved in other cereal (1-->3)-beta-glucanases. Although the precise tissue location and function of the enzyme in rice bran are not known, it is likely that it is concentrated in the aleurone layer and that it plays a preemptive role in the protection of ungerminated grain against pathogen attack.

Molecular cloning of cDNAs encoding (1-->4)-beta-xylan endohydrolases from the aleurone layer of germinated barley (Hordeum vulgare)

Heteroxylans are major constituents of cell walls in the graminaceous monocotyledons. Degradation of walls in the starchy endosperm of germinated cereal grains is mediated, in part at least, by the action of (1-->4)-beta-xylan endohydrolases (EC 3.2.1.8). Complementary DNAs encoding (1-->4)-beta-xylan endohydrolases from the aleurone layer of germinated barley have been isolated and characterized. Southern blot analyses suggest that the enzymes are derived from a family of 3 or 4 genes, and cDNAs corresponding to two of these genes have been sequenced. The amino acid sequence deduced from one cDNA almost exactly matches the amino acid sequence determined previously from the purified enzyme. This enzyme is designated (1-->4)-beta-xylan endohydrolase isoenzyme X-I. The mature enzyme consists of 395 amino acid residues, has a calculated M(r) of ca. 44600 and an isoelectric point of 6.1, and is likely to adopt an (alpha/beta)8 barrel conformation. The amino acid sequence of the barley (1-->4)-beta-xylan endohydrolase encoded by the other cDNA, which is designated isoenzyme X-II, shows ca. 13% sequence divergence compared with isoenzyme X-I. Both enzymes exhibit sequence and structural similarities with microbial xylanases. Expression of the genes in germinated grain appears to be confined largely to the aleurone layer, and no mRNA transcripts could be detected in young vegetative tissues.

STRUCTURE AND BIOGENESIS OF THE CELL WALLS OF GRASSES

The chemical structures of the primary cell walls of the grasses and their progenitors differ from those of all other flowering plant species. They vary in the complex glycans that interlace and cross-link the cellulose microfibrils to form a strong framework, in the nature of the gel matrix surrounding this framework, and in the types of aromatic substances and structural proteins that covalently cross-link the primary and secondary walls and lock cells into shape. This review focuses on the chemistry of the unique polysaccharides, aromatic substances, and proteins of the grasses and how these structural elements are synthesized and assembled into dynamic and functional cell walls. Despite wide differences in wall composition, the developmental physiology of grasses is similar to that of all flowering plants. Grass cells respond similarly to environmental cues and growth regulators, exhibit the same alterations in physical properties of the wall to allow cell growth, and possess similar patterns of wall biogenesis during the development of specific cell and tissue types. Possible unifying mechanisms of growth are suggested to explain how grasses perform the same wall functions as other plants but with different constituents and architecture.

Cloning and characterization of the genomic region encoding toxin IV-5 from the scorpion Tityus serrulatus Lutz and Mello

By means of PCR and using synthetic oligonucleotides designed from the reported cDNA, we amplified the gene that codes for toxin IV-5 from the Brazilian scorpion Tityus serrulatus. The analysis of the nucleotide sequence shows that the amplified genomic region is composed of 659 base pairs (bp) comprising two exons (28 and 284 bp) and an intron of 347 bp interrupting the region that encodes the signal peptide of the precursor toxin. Based on these findings a model for the structural organization of scorpion toxin genes is proposed.

Subsite affinities and disposition of catalytic amino acids in the substrate-binding region of barley 1,3-beta-glucanases. Implications in plant-pathogen interactions

Oligo-1,3-beta-glucosides with degrees of polymerization of 2-9 were labeled at their reducing terminal residues by catalytic tritiation. These substrates were used in detailed kinetic and thermodynamic analyses to examine substrate binding in 1,3-beta-D-glucan glucanohydrolase (EC 3.2.1.39) isoenzymes GI, GII, and GIII from young seedlings of barley (Hordeum vulgare). Bond-cleavage frequencies, together with the kinetic parameter kcat/Km, have been calculated as a function of substrate chain length to define the number of subsites that accommodate individual beta-glucosyl residues and to estimate binding energies at each subsite. Each isoenzyme has eight beta-glucosyl-binding subsites. The catalytic amino acids are located between the third and fourth subsite from the nonreducing terminus of the substrate. Negative binding energies in subsites adjacent to the hydrolyzed glycosidic linkage suggest that some substrate distortion may occur in this region during binding and that the resultant strain induced in the substrate might facilitate hydrolytic cleavage. If the 1,3-beta-glucanases exert their function as pathogenesis-related proteins by hydrolyzing the branched or substituted 1,3;1,6-beta-glucans of fungal walls, it is clear that relatively extended regions of the cell wall polysaccharide must fit into the substrate-binding cleft of the enzyme.

A tetrad of ionizable amino acids is important for catalysis in barley beta-glucanases

Determination of the crystal structures of a 1,3-beta-D-glucanase (E.C. 3.2.1.39) and a 1,3-1,4-beta-D-glucanase (E.C. 3.2.1.73) from barley (Hordeum vulgare) (Varghese, J.N, Garrett, T. P. J., Colman, P. M., Chen, L., Høj, P. B., and Fincher, G. B. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 2785-2789) showed the spatial positions of the catalytic residues in the substrate-binding clefts of the enzymes and also identified highly conserved neighboring amino acid residues. Site-directed mutagenesis of the 1,3-beta-glucanase has now been used to investigate the importance of these residues. Substitution of glutamine for the catalytic nucleophile Glu231 (mutant E231Q) reduced the specific activity about 20,000-fold. In contrast, substitution of glutamine for the catalytic acid Glu288 (mutant E288Q) had less severe consequences, reducing kcat approximately 350-fold with little effect on Km. Substitution of two neighboring and strictly conserved active site-located residues Glu279 (mutant E279Q) and Lys282 (mutant K282M) led to 240- and 2500-fold reductions of Kcat, respectively, with small increases in Km. Thus, a tetrad of ionizable amino acids is required for efficient catalysis in barley beta-glucanases. The active site-directed inhibitor 2,3-epoxypropyl beta-laminaribioside was soaked into native crystals. Crystallographic refinement revealed all four residues (Glu231, Glu279, Lys282, and Glu288) to be in contact with the bound inhibitor, and the orientation of bound substrate in the active site of the glucanase was deduced.

Heterologous expression of cDNAs encoding barley (Hordeum vulgare) (1-->3)-beta-glucanase isoenzyme GV

Two cDNAs have been isolated from libraries generated from poly(A)+RNA of young barley roots and leaves, using a cDNA encoding barley (1-->3)-beta-glucanase isoenzyme GII as a probe. Nucleotide sequence analyses and ribonuclease protection assays show that the two cDNAs differ only in the length of their 3'-untranslated regions; the corresponding mRNAs are likely to originate from a single gene by tissue-specific processing at separate polyadenylation sites. When the coding region of the cDNA is expressed in E. coli, the resultant protein catalyses the hydrolysis of (1-->3)-beta-glucan with an action pattern characteristic of a (1-->3)-beta-glucan endohydrolase (EC 3.2.1.39). The enzyme has been designated isoenzyme GV of the barley (1-->3)-beta-glucanase family).

Three-dimensional structures of two plant beta-glucan endohydrolases with distinct substrate specificities

The three-dimensional structures of (1-->3)-beta-glucanase (EC 3.2.1.39) isoenzyme GII and (1-->3,1-->4)-beta-glucanase (EC 3.2.1.73) isoenzyme EII from barley have been determined by x-ray crystallography at 2.2- to 2.3-A resolution. The two classes of polysaccharide endohydrolase differ in their substrate specificity and function. Thus, the (1-->3)-beta-glucanases, which are classified amongst the plant "pathogenesis-related proteins," can hydrolyze (1-->3)- and (1-->3,1-->6)-beta-glucans of fungal cell walls and may therefore contribute to plant defense strategies, while the (1-->3,1-->4)-beta-glucanases function in plant cell wall hydrolysis during mobilization of the endosperm in germinating grain or during the growth of vegetative tissues. Both enzymes are alpha/beta-barrel structures. The catalytic amino acid residues are located within deep grooves which extend across the enzymes and which probably bind the substrates. Because the polypeptide backbones of the two enzymes are structurally very similar, the differences in their substrate specificities, and hence their widely divergent functions, have been acquired primarily by amino acid substitutions within the groove.

Purification and characterization of (1-->3, 1-->4)-beta-glucan endohydrolases from germinated wheat (Triticum aestivum)

A (1-->3, 1-->4)-beta-glucan 4-glucanohydrolase [(1-->3, 1-->4)-beta-glucanase, EC 3.2.1.73] was purified to homogeneity from extracts of germinated wheat grain. The enzyme, which was identified as an endohydrolase on the basis of oligosaccharide products released from a (1-->3, 1-->4)-beta-glucan substrate, has an apparent pI of 8.2 and an apparent molecular mass of 30 kDa. Western blot analyses with specific monoclonal antibodies indicated that the enzyme is related to (1-->3, 1-->4)-beta-glucanase isoenzyme EI from barley. The complete primary structure of the wheat (1-->3, 1-->4)-beta-glucanase has been deduced from nucleotide sequence analysis of cDNAs isolated from a library prepared using poly(A)+ RNA from gibberellic acid-treated wheat aleurone layers. One cDNA, designated lambda LW2, is 1426 nucleotide pairs in length and encodes a 306 amino acid enzyme, together with a NH2-terminal signal peptide of 28 amino acid residues. The mature polypeptide encoded by this cDNA has a molecular mass of 32,085 and a predicted pI of 8.1. The other cDNA, designated lambda LW1, carries a 109 nucleotide pair sequence at its 5' end that is characteristic of plant introns and therefore appears to have been synthesized from an incompletely processed mRNA. Comparison of the coding and 3'-untranslated regions of the two cDNAs reveals 31 nucleotide substitutions, but none of these result in amino acid substitutions. Thus, the cDNAs encode enzymes with identical primary structures, but their corresponding mRNAs may have originated from homeologous chromosomes in the hexaploid wheat genome.

Post-translational processing of barley beta-glucan endohydrolases in the baculovirus-insect cell expression system

Two cDNAs encoding barley (1-->3,1-->4)-beta-glucanase (EC 3.2.1.73) isoenzymes EI and EII have been expressed in Spodoptera frugiperda (Sf9) cell cultures using the baculovirus AcNPV vector. Modifications to both the 5' and 3' ends of the cDNAs were required before satisfactory levels of expression were obtained. The modified cDNAs directed high levels of (1-->3,1-->4)-beta-glucanase expression in the Sf9 insect cell cultures, with yields of approximately 10 mg/liter of isoenzyme EI (expEI) and 15 mg/liter of isoenzyme EII (expEII). Amino acid sequence analyses showed that the expressed enzymes were processed correctly at their amino termini. However, affinity chromatography of the isoenzyme expEII on concanavalin-A (conA)-Sepharose indicated that, although the enzyme is glycosylated, the structures of the carbohydrate chains differ from those of the native enzyme. When a cDNA encoding the homologous barley (1-->3)-beta-glucanase (EC 3.2.1.39) isoenzyme GII was expressed in insect cells, aberrant amino-terminal processing of the nascent polypeptide was sometimes observed. The forms with incompletely removed signal peptides retained their substrate specificity, but exhibited slightly reduced catalytic efficiency, altered chromatographic behavior, and reduced stability at elevated temperatures. The results show that high levels of expression of recombinant plant proteins can be obtained in insect cells, but they emphasize the need to characterize thoroughly the products that are expressed in the heterologous insect cell system before comparisons are made with the native enzyme or with engineered enzyme mutants.