Cellulosome assembly revealed by the crystal structure of the cohesin-dockerin complex

The utilization of organized supramolecular assemblies to exploit the synergistic interactions afforded by close proximity, both for enzymatic synthesis and for the degradation of recalcitrant substrates, is an emerging theme in cellular biology. Anaerobic bacteria harness a multiprotein complex, termed the "cellulosome," for efficient degradation of the plant cell wall. This megadalton catalytic machine organizes an enzymatic consortium on a multifaceted molecular scaffold whose "cohesin" domains interact with corresponding "dockerin" domains of the enzymes. Here we report the structure of the cohesin-dockerin complex from Clostridium thermocellum at 2.2-A resolution. The data show that the beta-sheet cohesin domain interacts predominantly with one of the helices of the dockerin. Whereas the structure of the cohesin remains essentially unchanged, the loop-helix-helix-loop-helix motif of the dockerin undergoes conformational change and ordering compared with its solution structure, although the classical 12-residue EF-hand coordination to two calcium ions is maintained. Significantly, internal sequence duplication within the dockerin is manifested in near-perfect internal twofold symmetry, suggesting that both "halves" of the dockerin may interact with cohesins in a similar manner, thus providing a higher level of structure to the cellulosome and possibly explaining the presence of "polycellulosomes." The structure provides an explanation for the lack of cross-species recognition between cohesin-dockerin pairs and thus provides a blueprint for the rational design, construction, and exploitation of these catalytic assemblies.

Genomic DNA analysis of genes encoding (hemi-)cellulolytic enzymes of the anaerobic fungus Piromyces sp. E2

Anaerobic fungi contain more than one copy of genes encoding (hemi-)cellulases in their genome. The arrangement of these genes on the chromosomes was not known. A genomic DNA (gDNA) library of Piromyces sp. E2 was screened with different probes specific for (hemi-)cellulolytic enzymes. This screening resulted in three gDNA clones with genes encoding glycoside hydrolase enzymes of families 1 (beta-glucosidase), 6 (exoglucanase) and 26 (mannanase). Each clone contained two or more genes of the same family. Comparison of the gene copies on a clone revealed that they were highly homologous, and in addition, 54-75% of the substitutions was synonymous. One of the mannanase genes contained an intron. PCR with selected primers resulted in a gDNA clone with a new representative (cel9B) of glycoside hydrolase family 9 (endoglucanase). Comparison with cel9A revealed that cel9B had 67% homology on the nucleotide level. Furthermore, three introns were present. All results of this paper taken together provided evidence for duplications of (hemi-)cellulolytic genes, which resulted in clusters of almost identical genes arranged head-to-tail on the genome. In contrast to other eukaryotes, this phenomenon appears frequently in anaerobic fungi.

Identification and functional characterization of a type I signal peptidase gene of Bacillus megaterium DSM319

The sipM gene of Bacillus megaterium encoding a type I signal peptidase (SPase) was isolated and structurally characterized. RNA analysis revealed a transcript size in accordance with a bicistronic operon comprising sipM and an adjacent open reading frame. Inactivation of sipM by targeted gene disruption could not be achieved, indicating its essential role for cell viability since there might be no other type I SPase of major importance present in B. megaterium. Plasmid-assisted amplification of the gene resulted in an increase in activity of the heterologous glucanase used as an extracellular reporter, suggesting a potential bottleneck for protein secretion within this species.

Regulation of expression of cellulosomal cellulase and hemicellulase genes in Clostridium cellulovorans

The regulation of expression of the genes encoding the cellulases and hemicellulases of Clostridium cellulovorans was studied at the mRNA level with cells grown under various culture conditions. A basic pattern of gene expression and of relative expression levels was obtained from cells grown in media containing poly-, di- or monomeric sugars. The cellulase (cbpA and engE) and hemicellulase (xynA) genes were coordinately expressed in medium containing cellobiose or cellulose. Growth in the presence of cellulose, xylan, and pectin gave rise to abundant expression of most genes (cbpA-exgS, engH, hbpA, manA, engM, engE, xynA, and/or pelA) studied. Moderate expression of cbpA, engH, manA, engE, and xynA was observed when cellobiose or fructose was used as the carbon source. Low levels of mRNA from cbpA, manA, engE, and xynA were observed with cells grown in lactose, mannose, and locust bean gum, and very little or no expression of cbpA, engH, manA, engE, and xynA was detected in glucose-, galactose-, maltose-, and sucrose-grown cells. The cbpA-exgS and engE genes were most frequently expressed under all conditions studied, whereas expression of xynA and pelA was more specifically induced at higher levels in xylan- or pectin-containing medium, respectively. Expression of the genes (cbpA, hbpA, manA, engM, and engE) was not observed in the presence of most soluble di- or monosaccharides such as glucose. These results support the hypotheses that there is coordinate expression of some cellulases and hemicellulases, that a catabolite repression type of mechanism regulates cellulase expression in rapidly growing cells, and that the presence of hemicelluloses has an effect on cellulose utilization by the cell.

Characterization of the hrp pathogenicity cluster of Erwinia carotovora subsp. carotovora: high basal level expression in a mutant is associated with reduced virulence

Extracellularly targeted proteins are crucial for virulence of gram-negative phytopathogenic bacteria. Erwinia carotovora subsp. carotovora employs the so-called type II (GSP) pathway to secrete a number of pectinases and cellulases, which cause the typical tissue maceration symptoms of soft-rot disease. The type III (hrp) pathway is the major virulence determinant in the genera Pseudomonas, Ralstonia and Xanthomonas, and in non-macerating species of Erwinia. The hrp cluster was recently partially characterized from E. carotovora sp. carotovora, and shown to affect virulence during early stages of infection. Here we have isolated and characterized 15 hrp genes comprising the remaining part of the cluster. The genes hrpL, hrpXY and hrpS were deduced to be transcribed as separate units, whereas the 11 remaining genes from hrpJ to hrcU form a single large operon. The hrpX gene, which codes for the sensory kinase of the two-component regulatory locus hrpXY was insertionally inactivated by placing a transposon (entranceposon) in the gene. The resulting mutant bacterium expresses the hrp genes at high basal level even in a non-inducing medium. This relative overexpression was shown to be due to the hrpX::entranceposon insertion causing enhanced transcription of the downstream hrpY gene. The hrpX(-)-hrpYC mutant bacterium exhibited a slower growth rate and the appearance of disease symptoms in infected Arabidopsis plants was delayed, as compared to the wild-type strain. The need for hrp gene expression for virulence has been documented in both non-macerating plant pathogens and in soft-rotting Erwinia sp. but this is the first demonstration that high basal-level expression of hrp -regulated genes may actually have a negative impact on disease progress in a susceptible host plant.

Nucleosome transactions on the Hypocrea jecorina (Trichoderma reesei) cellulase promoter cbh2 associated with cellulase induction

The 5' regulatory region of the cbh2 gene of Hypocrea jecorina contains the cbh2 activating element (CAE) which is essential for induction of cbh2 gene expression by sophorose and cellulose. The CAE consists of two motifs, a CCAAT box on the template strand and a GTAATA box on the coding strand, which cooperate during induction. Northern analyses of cbh2 gene expression has revealed an absolute dependence on induction, but no direct effect of Cre1-mediated carbon catabolite repression. Investigation of the chromatin structure in the wild-type strain showed that, under repressing conditions, there is a nucleosome free region (nfr) around the CAE, which is flanked by strictly positioned nucleosomes. Induction results in a loss of positioning of nucleosomes -1 and -2 downstream of the CAE, thus making the TATA box accessible. Simultaneous mutation of both motifs of the CAE, or of the CCAAT-box alone, also leads to shifting of nucleosome -1, which normally covers the TATA-box under repressing conditions, whereas mutation of the GTAATA element results in a narrowing of the nfr, indicating that the proteins that bind to both motifs in the CAE interact with chromatin, although in different ways. A cellulase-negative mutant strain, which has previously been shown to be altered in protein binding to the CAE, still displayed the induction-specific changes in nucleosome structure, indicating that none of the proteins that directly interact with CAE are affected, and that nucleosome rearrangement and induction of cbh2 expression are uncoupled. Interestingly, the carbon catabolite repressor Cre1 is essential for strict nucleosome positioning in the 5' regulatory sequences of cbh2 under all of the conditions tested, and induction can occur in a promoter that lacks positioned nucleosomes. These data suggest that Cre1, the Hap2/3/5 complex and the GTAATA-binding protein are all involved in nucleosome assembly on the cbh2 promoter, and that the latter two respond to inducing conditions by repositioning nucleosome -1.

Diversification of exogenous genes in vivo in Neurospora

We have adapted the meiotic recombination hotspot cog of Neurospora crassa for shuffling exogenous DNA, providing a means of generating novel genes in situ from sequences introduced into chromosomes. Genes to be diversified are inserted between the his-3 locus and cog. Diversification crosses are heterozygous both for alleles of the exogenous DNA and for auxotrophic alleles of his-3. Progeny selected for ability to grow without histidine supplementation are enriched for exchange events within the exogenous DNA. Exchange events initiated by cog can propagate past DNA sequences mismatched for more than 370 bp and complete exchanges in patches of matched sequence as short as 24 bp, parameters that make the system suited for use in the directed evolution of genes for protein engineering. Here we demonstrate the system by shuffling human immunoglobulin kappa chain genes and also endoglucanase genes derived from different species of fungi.

Isolation of a multi-functional endogenous cellulase gene from mollusc, Ampullaria crossean

The cellulase genes of some animals, most coding for endo-beta-1,4-glucanases, were found and cloned. There has been no reports about genes encoding exo-beta-1,4-glucanase or endo- -1,4-xylanase from animal. Here we cloned the cDNA of a cellulase designated as EGX from mollusc, Ampullaria crossean, and expressed it in Pichia pastoris for the first time. The cellulase EGX is a multi-functional beta cellulase with the activities of exo-beta-1,4-glucanase, endo-beta-1,4-glucanase and endo-beta-1,4-xylanase. The opening reading frame of EGX cDNA is 1185 bp and encodes 395 amino acids. The EGX gene can also be amplificated from the genomic DNA by PCR, which verified the endogenous origin of this gene. This EGX gene was the first multi-functional cellulase gene that was directly isolated from animals.

A comparative study between an endoglucanase IV and its fused protein complex Cel5-CBM6

The recombinant endoglucanase IV (Cel5; encoded by egIV) of Ruminococcus albus was compared with protein Cel5-CBM6 comprised of Cel5 fused at the C-terminus with the single-cellulose binding domain II (CBM6) of Clostridium stercorarium xylanase A, in order to improve its binding ability. Previous analyses using ball-milled cellulose had suggested that a cellulose binding domain of xylanase A could enhance cellulase activity, especially with insoluble substrates. Comparison of the catalytic activities of Cel5 and Cel5-CBM6 were determined using carboxymethylcellulose, Avicel, and filter paper as substrates. This study confirmed previous findings, and provided further evidence suggesting that Cel5-CBM6 exhibits enhanced activity with insoluble cellulose compared to native Cel5. However, its hydrolytic activity with soluble substrates such as carboxymethylcellulose was comparable to Cel5. For both cellulases, central linkages of cellulooligosaccharides (up to six glucose residues) were found to be the preferred points of cleavage. The rates of hydrolysis with both cellulases increased with cellulooligosaccharide chain length, and at least three consecutive glycosyl residues seemed to be necessary for hydrolysis to occur. Cel5-CBM6 showed a higher affinity for cellulose substrates than did Cel5, as demonstrated by transmission electron microscopy. Taken together, these results suggest that CBM6 increases the affinity of Cel5 for insoluble substrates, and this increased binding capacity seems to result in increased catalytic activity.

Expression and Characterization of the celcflB Gene from Cellulomonas flavigena Encoding an Endo-?-1,4-Glucanase

An endoglucanase gene called celcflB was isolated from a genomic library of C. flavigena. Its nucleotide sequence showed an ORF of 1725 bp with a GTG start codon. A promoter sequence was found inside the adjacent gene upstream from the start point of translation of celcflB gene. The gene celcflB was induced by sugarcane bagasse and repressed by glucose when C. flavigena was grown on these carbon sources, suggesting that celcflB gene is regulated at transcriptional level. The predicted amino acid sequence of the CelcflB protein presented a catalytic domain with a high homology to family 5 of glycosil hydrolases, and a cellulose binding domain identical to cellulose binding domains type II. The coding region of the celcflB gene was cloned into the expression vector pQE30 and the recombinant protein of 58 kDa presented endoglucanase activity towards carboxymethyl cellulose (CMC).

Detecting cellulase and esterase enzyme activities encoded by novel genes present in environmental DNA libraries

A genomic DNA library was made from the alkaliphilic cellulase-producing Bacillus agaradhaerans in order to prove our technologies for gene isolation prior to using them with samples of DNA isolated directly from environmental samples. Clones expressing a cellulase activity were identified and sequenced. A new cellulase gene was identified. Genomic DNA libraries were then made from DNA isolated directly from the Kenyan soda lakes, Lake Elmenteita and Crater Lake. Crater Lake clones expressing a cellulase activity and Lake Elmenteita clones expressing a lipase/esterase activity were identified and sequenced. These were encoded by novel genes as judged by DNA sequence comparisons. Genomic DNA libraries were also made from laboratory enrichment cultures of Lake Nakuru and Lake Elmenteita samples. Selective enrichment cultures were grown in the presence of carboxymethylcellulose (CMC) and olive oil. A number of new cellulase and lipase/esterase genes were discovered in these libraries. Cellulase-positive clones from Lake Nakuru were isolated at a frequency of 1 in 15,000 from a library made from a CMC enrichment as compared to 1 in 60,000 from a minimal medium enrichment. Esterase/lipase-positive clones from Lake Elmenteita were isolated with a frequency of 1 in 30,000 from a library made from an olive-oil enrichment as compared to 1 in 100,000 from an environmental library.

Caste- and development-associated gene expression in a lower termite

Social insects such as termites express dramatic polyphenism (the occurrence of multiple forms in a species on the basis of differential gene expression) both in association with caste differentiation and between castes after differentiation. We have used cDNA macroarrays to compare gene expression between polyphenic castes and intermediary developmental stages of the termite Reticulitermes flavipes.

Background

Social insects such as termites express dramatic polyphenism (the occurrence of multiple forms in a species on the basis of differential gene expression) both in association with caste differentiation and between castes after differentiation. We have used cDNA macroarrays to compare gene expression between polyphenic castes and intermediary developmental stages of the termite Reticulitermes flavipes.

Results

We identified differentially expressed genes from nine ontogenic categories. Quantitative PCR was used to quantify precise differences in gene expression between castes and between intermediary developmental stages. We found worker and nymph-biased expression of transcripts encoding termite and endosymbiont cellulases; presoldier-biased expression of transcripts encoding the storage/hormone-binding protein vitellogenin; and soldier-biased expression of gene transcripts encoding two transcription/translation factors, two signal transduction factors and four cytoskeletal/muscle proteins. The two transcription/translation factors showed significant homology to the bicaudal and bric-a-brac developmental genes of Drosophila.

Conclusions

Our results show differential expression of regulatory, structural and enzyme-coding genes in association with termite castes and their developmental precursor stages. They also provide the first glimpse into how insect endosymbiont cellulase gene expression can vary in association with the caste of a host. These findings shed light on molecular processes associated with termite biology, polyphenism, caste differentiation and development and highlight potentially interesting variations in developmental themes between termites, other insects, and higher animals.

Regulation of Trichoderma cellulase formation: lessons in molecular biology from an industrial fungus. A review

The present article reviews the current understanding of regulation of cellulase gene transcription in Hypocrea jecorina (= Trichoderma reesei). Special emphasis is put on the mechanism of action of low molecular weight inducers of cellulase formation, the presence and role of recently identified transactivating proteins (Ace1, Ace2, Hap2/3/5), and the role of the carbon catabolite repressor Cre1. We also report on some recent genomic approaches towards understanding how cellulase inducers signal their presence to the transcriptional apparatus.

Probing pH-dependent functional elements in proteins: modification of carboxylic acid pairs in Trichoderma reesei cellobiohydrolase Cel6A

Two carboxylic acid side chains can, depending on their geometry and environment, share a proton in a hydrogen bond and form a carboxyl-carboxylate pair. In the Trichoderma reesei cellobiohydrolase Cel6A structure, five carboxyl-carboxylate pairs are observed. One of these pairs (D175-D221) is involved in catalysis, and three other pairs are found in, or close to the two surface loops covering the active site tunnel of the catalytic domain. To stabilize Cel6A at alkaline pH values, where deprotonation of the carboxylic acids leads to repulsion of their side chains, we designed two mutant enzymes. In the first mutant, one carboxyl-carboxylate pair (E107-E399) was replaced by a corresponding amide-carboxylate pair (Q107-E399), and in the second mutant, all three carboxyl-carboxylate pairs (E107-E399, D170-E184, and D366-D419) were mutated in a similar manner. The unfolding studies using both intrinsic tryptophan fluorescence and far-ultraviolet circular dichroism spectroscopy at different pH values demonstrate that the unfolding temperature (T(m)) of both mutants has changed, resulting in destabilization of the mutant enzymes at acidic pH and stabilization at alkaline pH. The effect of stabilization seems additive, as a Cel6A triple mutant is the most stable enzyme variant. This increased stability is also reflected in the 2- or 4-fold increased half-life of the two mutants at alkaline pH, while the catalytic rate on cellotetraose (at t = 0) has not changed. Increased operational stability at alkaline pH was also observed on insoluble cellulosic substrates. Local conformational changes are suggested to take place in the active site loops of Cel6A wild-type enzyme at elevated pHs (pH 7), affecting to the end-product spectrum on insoluble cellulose. The triple mutant does not show such pH-dependent behavior. Overall, our results demonstrate that carboxyl-carboxylate pair engineering is a useful tool to alter pH-dependent protein behavior.

A thermoacidophilic endoglucanase (CelB) from Alicyclobacillus acidocaldarius displays high sequence similarity to arabinofuranosidases belonging to family 51 of glycoside hydrolases

A 100-kDa protein with endoglucanase activity was purified from Triton X-100 extract of cells of the thermoacidophilic Gram-positive bacterium Alicyclobacillus acidocaldarius. The enzyme exhibited activity towards carboxy methyl cellulose and oat spelt xylan with pH and temperature optima of 4 and 80 degrees C, respectively. Cloning and nucleotide sequence analysis of the corresponding gene (celB) revealed an ORF encoding a preprotein of 959 amino acids which is consistent with an extracellular localization. Purified recombinant CelB and a variant lacking the C-terminal 203 amino acid residues (CelBtrunc) displayed similar enzymatic properties as the wild-type protein. Analysis of product formation suggested an endo mode of action. Remarkable stability was observed at pH values between 1 and 7 and 60% of activity were retained after incubation for 1 h at 80 degrees C. CelB displayed homology to members of glycoside hydrolase family 51, being only the second entry with activity typical of an endoglucanase but lacking activity on p-nitrophenyl-alpha-l-arabinofuranoside (pNPAraf). Highest sequence similarity was found towards the other endoglucanase F from Fibrobacter succinogenes (EGF), forming a distinct group in the phylogenetic tree of this family. Analysis of the amino acid composition of the catalytic domains demonstrated that CelB contains fewer charged amino acids than its neutrophilic counterparts, which is in line with adaptation to low pH. Wild-type and full-length recombinant CelB were soluble only in Triton X-100. In contrast, CelBtrunc was completely water soluble, suggesting a role of the C-terminal region in cell association. This C-terminal hydrophobic region displayed local sequence similarities to an alpha-amylase from the same organism.

Evidence for the presence of a cellulase gene in the last common ancestor of bilaterian animals

Until recently, the textbook view of cellulose hydrolysis in animals was that gut-resident symbiotic organisms such as bacteria or unicellular eukaryotes are responsible for the cellulases produced. This view has been challenged by the characterization and sequencing of endogenous cellulase genes from some invertebrate animals, including plant-parasitic nematodes, arthropods and a mollusc. Most of these genes are completely unrelated in terms of sequence, and their evolutionary origins remain unclear. In the case of plant-parasitic nematodes, it has been suggested that their ancestor obtained a cellulase gene via horizontal gene transfer from a prokaryote, and similar suggestions have been made about a cellulase gene recently discovered in a sea squirt. To improve understanding about the evolution of animal cellulases, we searched for all known types of these enzymes in GenBank, and performed phylogenetic comparisons. Low phylogenetic resolution was found among most of the sequences examined, however, positional identity in the introns of cellulase genes from a termite, a sea squirt and an abalone provided compelling evidence that a similar gene was present in the last common ancestor of protostomes and deuterostomes. In a different enzyme family, cellulases from beetles and plant-parasitic nematodes were found to cluster together. This result questions the idea of lateral gene transfer into the ancestors of the latter, although statistical tests did not allow this possibility to be ruled out. Overall, our results suggest that at least one family of endogenous cellulases may be more widespread in animals than previously thought.

CelF of Orpinomyces PC-2 has an intron and encodes a cellulase (CelF) containing a carbohydrate-binding module

A cDNA, designated celF, encoding a cellulase (CelF) was isolated from the anaerobic fungus Orpinomyces PC-2. The open reading frame contains regions coding for a signal peptide, a carbohydrate-binding module (CBM), a linker, and a catalytic domain. The catalytic domain was homologous to those of CelA and CelC of the same fungus and to that of the Neocallimastix patriciarum CELA, but CelF lacks a docking domain, characteristic for enzymes of cellulosomes. It was also homologous to the cellobiohydrolase IIs and endoglucanases of aerobic organisms. The gene has a 111-bp intron, located within the CBM-coding region. Some biochemical properties of the purified recombinant enzyme are described.

The cellulosome system of Acetivibrio cellulolyticus includes a novel type of adaptor protein and a cell surface anchoring protein

A scaffoldin gene cluster was identified in the mesophilic cellulolytic anaerobe Acetivibrio cellulolyticus. The previously described scaffoldin gene, cipV, encodes an N-terminal family 9 glycoside hydrolase, a family 3b cellulose-binding domain, seven cohesin domains, and a C-terminal dockerin. The gene immediately downstream of cipV was sequenced and designated scaB. The protein encoded by this gene has 942 amino acid residues and a calculated molecular weight of 100,358 and includes an N-terminal signal peptide, four type II cohesions, and a C-terminal dockerin. ScaB cohesins 1 and 2 are very closely linked. Similar, but not identical, 39-residue Thr-rich linker segments separate cohesin 2 from cohesin 3 and cohesin 3 from cohesin 4, and an 84-residue Thr-rich linker connects the fourth cohesin to a C-terminal dockerin. The scaC gene downstream of scaB codes for a 1,237-residue polypeptide that includes a signal peptide, three cohesins, and a C-terminal S-layer homology (SLH) module. A long, ca. 550-residue linker separates the third cohesin and the SLH module of ScaC and is characterized by an 18-residue Pro-Thr-Ala-Ser-rich segment that is repeated 27 times. The calculated molecular weight of the mature ScaC polypeptide (excluding the signal peptide) is 124,162. The presence of the cohesins and the conserved SLH module implies that ScaC acts as an anchoring protein. The ScaC cohesins are on a separate branch of the phylogenetic tree that is close to, but distinct from, the type I cohesins. Affinity blotting with representative recombinant probes revealed the following specific intermodular interactions: (i) an expressed CipV cohesin binds selectively to an enzyme-borne dockerin, (ii) a representative ScaB cohesin binds to the CipV band of the cell-free supernatant fraction, and (iii) a ScaC cohesin binds to the ScaB dockerin. The experimental evidence thus indicates that CipV acts as a primary (enzyme-recognizing) scaffoldin, and the protein was also designated ScaA. In addition, ScaB is thought to assume the role of an adaptor protein, which connects the primary scaffoldin (ScaA) to the cohesin-containing anchoring scaffoldin (ScaC). The cellulosome system of A. cellulolyticus thus appears to exhibit a special type of organization that reflects the function of the ScaB adaptor protein. The intercalation of three multiple cohesin-containing scaffoldins results in marked amplification of the number of enzyme subunits per cellulosome unit. At least 96 enzymes can apparently be incorporated into an individual A. cellulolyticus cellulosome. The role of such amplified enzyme incorporation and the resultant proximity of the enzymes within the cellulosome complex presumably contribute to the enhanced synergistic action and overall efficient digestion of recalcitrant forms of cellulose. Comparison of the emerging organization of the A. cellulolyticus cellulosome with the organizations in other cellulolytic bacteria revealed the diversity of the supramolecular architecture.

Purification, characterization, cDNA cloning and nucleotide sequencing of a cellulase from the yellow-spotted longicorn beetle, Psacothea hilaris

A cellulase (endo-beta-1,4-glucanase, EC 3.2.1.4) was purified from the gut of larvae of the yellow-spotted longicorn beetle Psacothea hilaris by acetone precipitation and elution from gels after native PAGE and SDS/PAGE with activity staining. The purified protein formed a single band, and the molecular mass was estimated to be 47 kDa. The purified cellulase degraded carboxymethylcellulose (CMC), insoluble cello-oligosaccharide (average degree of polymerization 34) and soluble cello-oligosaccharides longer than cellotriose, but not crystalline cellulose or cellobiose. The specific activity of the cellulase against CMC was 150 micro mol.min-1.(mg protein)-1. TLC analysis showed that the cellulase produces cellotriose and cellobiose from insoluble cello-oligosaccharides. However, a glucose assay linked with glucose oxidase detected a small amount of glucose, with a productivity of 0.072 micro mol.min-1.(mg protein)-1. The optimal pH of P. hilaris cellulase was 5.5, close to the pH in the midgut of P. hilaris larvae. The N-terminal amino-acid sequence of the purified P. hilaris cellulase was determined and a degenerate primer designed, which enabled a 975-bp cDNA clone containing a typical polyadenylation signal to be obtained by PCR and sequencing. The deduced amino-acid sequence of P. hilaris cellulase showed high homology to members of glycosyl hydrolase family 5 subfamily 2, and, in addition, a signature sequence for family 5 was found. Thus, this is the first report of a family 5 cellulase from arthropods.

New integration vector using a cellulase gene as a screening marker for Lactobacillus

The new integration vector for Lactobacillus, pJC4, was developed using the extracellular endoglucanase A gene (celA) of Clostridium thermocellum as a screening marker. pJC4 was transformed into four Lactobacillus species, Lb. johnsonii, Lb. gasseri, Lb. bulgaricus, and Lb. plantarum. In each species, the pJC4 integrants were easily and accurately detected by the appearance of a clear halo on a cellulase screening plate without any false transformants. Polymerase chain reaction and Southern hybridization indicated that all transformants with clear halos contained pJC4 in their chromosomal DNAs. The celA gene could be a useful screening marker for other lactic acid bacteria.

Cel6A, a major exoglucanase from the cellulosome of the anaerobic fungi Piromyces sp. E2 and Piromyces equi

Anaerobic fungi possess high cellulolytic activities, which are organised in high molecular mass (HMM) complexes. Besides catalytic modules, the cellulolytic enzyme components of these complexes contain non-catalytic modules, known as dockerins, that play a key role in complex assembly. Screening of a genomic and a cDNA library of two Piromyces species resulted in the isolation of two clones containing inserts of 5.5 kb (Piromyces sp. E2) and 1.5 kb (Piromyces equi). Both clones contained the complete coding region of a glycoside hydrolase (GH) from family 6, consisting of a 20 amino acid signal peptide, a 76 (sp. E2)/81 (P. equi) amino acid stretch comprising two fungal non-catalytic docking domains (NCDDs), a 24 (sp. E2)/16 (P. equi) amino acid linker, and a 369 amino acid catalytic module. Homology modelling of the catalytic module strongly suggests that the Piromyces enzymes will be processive cellobiohydrolases. The catalytic residues and all nearby residues are conserved. The reaction is thus expected to proceed via a classical single-displacement (inverting) mechanism that is characteristic of this family of GHs. The enzyme, defined as Cel6A, encoded by the full-length Piromyces E2 sequence was expressed in Escherichia coli. The recombinant protein expressed had a molecular mass of 55 kDa and showed activity against Avicel, supporting the observed relationship of the sequence to those of known cellobiohydrolases. Affinity-purified cellulosomes of Piromyces sp. E2 were analysed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and sodium dodecyl sulfate-polyacrylamide gel (SDS-PAGE) electrophoresis. A major band was detected with the molecular weight of Cel6A. A tryptic fingerprint of this protein confirmed its identity.

Selectable marker-free transgenic barley producing a high level of cellulase (1,4-beta-glucanase) in developing grains

The use of barley grains as bioreactors for high-level production of cellulase (1,4-beta-glucanase) was investigated. A hybrid cellulase gene, cel-hyb1, driven by the rice GluB-1 promoter was expressed specifically in developing endosperm. Codon usage optimisation of cel-hyb1 increased its expression in barley grains 527-fold and led to cellulase production of up to 1.5% of total grain protein. CEL-HYB1 enzyme in barley grains was highly stable during post-harvest storage. Selectable marker gene ( hph) was subsequently eliminated from transgenic lines through segregation of hph from synthetic cel-hyb1 ( syn.cel-hyb1) in T1 progeny, using a binary plasmid containing hph and syn.cel-hyb1 in separate T-DNAs. These data suggest that barley grains can potentially be used for the commercial production of cellulase.

Quantitative determination of noncovalent binding interactions using automated nanoelectrospray mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) has proven to be an extremely powerful tool for studying biomolecular structures and noncovalent interactions. Here we report a method using a fully automated, chip-based nanoESI-MS system to determine the dissociation constants (Kd) for the complexes of two different proteins with their ligands. The automated nanoelectrospray system, consisting of the NanoMate and ESI chip, serves functionally as a combination of autosampler and nanoelectrospray ionization source. This system provides all the advantages of conventional nanoelectrospray plus automated, high-throughput analyses without carryover. The automated nanoESI system was used to investigate quantitative noncovalent interactions between ribonuclease A (RNase A) and cytidylic acid ligands (2'-CMP, CTP), a well-characterized model protein-ligand complex, and between an inactive endocellulase mutant (Thermobifida fusca Cel6A D117Acd) and four oligosaccharide ligands (cellotriose, cellotetraose, cellopentaose, cellohexaose). Both titration and competitive binding approaches were performed prior to automated nanoESI-MS analysis with a Q-TOF mass spectrometer. Dissociation constants for each complex were calculated from the sum of ion peak areas of free and complexed proteins during the titration and competition experiments. The measured Kd values for the RNase A-CMP and Cel6A D117Acd-G3 complexes were found to be in excellent agreement with the available published values obtained by standard spectroscopic titration techniques. To our knowledge, this is the first report of using an ESI-MS approach to study the interactions between a cellulase and oligosaccharides. The results provide new insights for understanding the nature of cellulase-cellulose interactions.

Exo-mode of action of cellobiohydrolase Cel48C from Paenibacillus sp. BP-23. A unique type of cellulase among Bacillales

Sequence analysis of a Paenibacillus sp. BP-23 recombinant clone coding for a previously described endoglucanase revealed the presence of an additional truncated ORF with homology to family 48 glycosyl hydrolases. The corresponding 3509-bp DNA fragment was isolated after gene walking and cloned in Escherichia coli Xl1-Blue for expression and purification. The encoded enzyme, a cellulase of 1091 amino acids with a deduced molecular mass of 118 kDa and a pI of 4.85, displayed a multidomain organization bearing a canonical family 48 catalytic domain, a bacterial type 3a cellulose-binding module, and a putative fibronectin-III domain. The cloned cellulase, unique among Bacillales and designated Cel48C, was purified through affinity chromatography using its ability to bind Avicel. Maximum activity was achieved at 45 degrees C and pH 6.0 on acid-swollen cellulose, bacterial microcrystalline cellulose, Avicel and cellodextrins, whereas no activity was found on carboxy methyl cellulose, cellobiose, cellotriose, pNP-glycosides or 4-methylumbeliferyl alpha-d-glucoside. Cellobiose was the major product of cellulose hydrolysis, identifying Cel48C as a processive cellobiohydrolase. Although no chromogenic activity was detected from pNP-glycosides, TLC analysis revealed the release of p-nitrophenyl-glycosides and cellodextrins from these substrates, suggesting that Cel48C acts from the reducing ends of the sugar chain. Presence of such a cellobiohydrolase in Paenibacillus sp. BP-23 would contribute to widen up its range of action on natural cellulosic substrates.