Studies of the cellulolytic system of Trichoderma reesei QM 9414. Analysis of domain function in two cellobiohydrolases by limited proteolysis

Characterization of Alternaria alternata glucanase genes expressed during infection of resistant and susceptible persimmon fruits

Summary Preharvest treatment with gibberellic acid (GA(3)) or its inhibitor paclobutrazol (PBZ) can reduce or increase, respectively, the susceptibility of persimmon fruits to Alternaria alternata. This was suggested to be the result of the ability of the fungus and produced endoglucanases to induce symptom development. To evaluate the importance of glucanases during A. alternata attack, five glucanase genes, corresponding to the C, F, and K families, were cloned from A. alternata using 'family-specific' oligonucleotide primers. The genes, present in a single copy, encode for exoglucanases AaC1 and AaC2, endoxylanase AaF1, endoglucanase AaK1, and the mixed-linked glucanase AaMLG1. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis of RNA extracted from persimmon fruits, 2 and 4 days post-infection with A. alternata, showed the expression of all five glucanase genes in GA3- and PBZ-treated fruits. However, transcription levels and enzyme production of the endoglucanase (AaK1) and one exoglucanase (AaC1) were enhanced during A. alternata growth on cell walls from susceptible PBZ-treated fruits, whereas the expression of these genes and their enzyme production were significantly reduced in resistant GA(3)-treated fruits. The present results suggest the involvement of endo- and exoglucanase in symptom development caused by A. alternata in resistant and susceptible persimmon fruits.

Differential oligosaccharide recognition by evolutionarily-related beta-1,4 and beta-1,3 glucan-binding modules

Enzymes active on complex carbohydrate polymers frequently have modular structures in which a catalytic domain is appended to one or more carbohydrate-binding modules (CBMs). Although CBMs have been classified into a number of families based upon sequence, many closely related CBMs are specific for different polysaccharides. In order to provide a structural rationale for the recognition of different polysaccharides by CBMs displaying a conserved fold, we have studied the thermodynamics of binding and three-dimensional structures of the related family 4 CBMs from Cellulomonas fimi Cel9B and Thermotoga maritima Lam16A in complex with their ligands, beta-1,4 and beta-1,3 linked gluco-oligosaccharides, respectively. These two CBMs use a structurally conserved constellation of aromatic and polar amino acid side-chains that interact with sugars in two of the five binding subsites. Differences in the length and conformation of loops in non-conserved regions create binding-site topographies that complement the known solution conformations of their respective ligands. Thermodynamics interpreted in the light of structural information highlights the differential role of water in the interaction of these CBMs with their respective oligosaccharide ligands.

Modification of polysaccharides and plant cell wall by endo-1,4-beta-glucanase and cellulose-binding domains

Cellulose is one of the most abundant polymers in nature. Different living systems evolved simultaneously, using structurally similar proteins to synthesize and metabolize polysaccharides. In the growing plant, cell wall loosening, together with cellulose biosynthesis, enables turgor-driven cell expansion. It has been postulated that endo-1,4-beta-glucanases (EGases) play a central role in these complex activities. Similarly, microorganisms use a consortium of lytic enzymes to convert cellulose into soluble sugars. Most, if not all, cellulases have a modular structure with two or more separate independent functional domains. Binding to cellulose is mediated by a cellulose-binding domain (CBD), whereas the catalytic domain mediates hydrolysis. Today, EGases and CBDs are known to exist in a wide range of species and it is evident that both possess immense potential in modifying polysaccharide materials in-vivo and in-vitro. The hydrolytic function is utilized for polysaccharide degradation in microbial systems and cell wall biogenesis in plants. The CBDs exerts activity that can be utilized for effective degradation of crystalline cellulose, plant cell wall relaxation, expansion and cell wall biosynthesis. Applications range from modulating the architecture of individual cells to an entire organism. These genes, when expressed under specific promoters and appropriate trafficking signals can be used to alter the nutritional value and texture of agricultural crop and their final products. EGases and CBDs may also find applications in the modification of physical and chemical properties of composite materials to create new materials possessing improved properties.

Kinetic parameters and mode of action of the cellobiohydrolases produced by Talaromyces emersonii

Three forms of cellobiohydrolase (EC 3.2.1.91), CBH IA, CBH IB and CBH II, were isolated to apparent homogeneity from culture filtrates of the aerobic fungus Talaromyces emersonii. The three enzymes are single sub-unit glycoproteins, and unlike most other fungal cellobiohydrolases are characterised by noteworthy thermostability. The kinetic properties and mode of action of each enzyme against polymeric and small soluble oligomeric substrates were investigated in detail. CBH IA, CBH IB and CBH II catalyse the hydrolysis of microcrystalline cellulose, albeit to varying extents. Hydrolysis of a soluble cellulose derivative (CMC) and barley 1,3;1,4-beta-D-glucan was not observed. Cellobiose (G2) is the main reaction product released by CBH IA, CBH IB, and CBH II from microcrystalline cellulose. All three CBHs are competitively inhibited by G2; inhibition constant values (K(i)) of 2.5 and 0.18 mM were obtained for CBH IA and CBH IB, respectively (4-nitrophenyl-beta-cellobioside as substrate), while a K(i) of 0.16 mM was determined for CBH II (2-chloro-4-nitrophenyl-beta-cellotrioside as substrate). Bond cleavage patterns were determined for each CBH on 4-methylumbelliferyl derivatives of beta-cellobioside and beta-cellotrioside (MeUmbG(n)). While the Tal. emersonii CBHs share certain properties with their counterparts from Trichoderma reesei, Humicola insolens and other fungal sources, distinct differences were noted.

EglC, a new endoglucanase from Aspergillus niger with major activity towards xyloglucan

A novel gene, eglC, encoding an endoglucanase, was cloned from Aspergillus niger. Transcription of eglC is regulated by XlnR, a transcriptional activator that controls the degradation of polysaccharides in plant cell walls. EglC is an 858-amino-acid protein and contains a conserved C-terminal cellulose-binding domain. EglC can be classified in glycoside hydrolase family 74. No homology to any of the endoglucanases from Trichoderma reesei was found. In the plant cell wall xyloglucan is closely linked to cellulose fibrils. We hypothesize that the EglC cellulose-binding domain anchors the enzyme to the cellulose chains while it is cleaving the xyloglucan backbone. By this action it may contribute to the degradation of the plant cell wall structure together with other enzymes, including hemicellulases and cellulases. EglC is most active towards xyloglucan and therefore is functionally different from the other two endoglucanases from A. niger, EglA and EglB, which exhibit the greatest activity towards beta-glucan. Although the mode of action of EglC is not known, this enzyme represents a new enzyme function involved in plant cell wall polysaccharide degradation by A. niger.

A novel two-step extraction method with detergent/polymer systems for primary recovery of the fusion protein endoglucanase I-hydrophobin I

Extraction systems for hydrophobically tagged proteins have been developed based on phase separation in aqueous solutions of non-ionic detergents and polymers. The systems have earlier only been applied for separation of membrane proteins. Here, we examine the partitioning and purification of the amphiphilic fusion protein endoglucanase I(core)-hydrophobin I (EGI(core)-HFBI) from culture filtrate originating from a Trichoderma reesei fermentation. The micelle extraction system was formed by mixing the non-ionic detergent Triton X-114 or Triton X-100 with the hydroxypropyl starch polymer, Reppal PES100. The detergent/polymer aqueous two-phase systems resulted in both better separation characteristics and increased robustness compared to cloud point extraction in a Triton X-114/water system. Separation and robustness were characterized for the parameters: temperature, protein and salt additions. In the Triton X-114/Reppal PES100 detergent/polymer system EGI(core)-HFBI strongly partitioned into the micelle-rich phase with a partition coefficient (K) of 15 and was separated from hydrophilic proteins, which preferably partitioned to the polymer phase. After the primary recovery step, EGI(core)-HFBI was quantitatively back-extracted (K(EGIcore-HFBI)=150, yield=99%) into a water phase. In this second step, ethylene oxide-propylene oxide (EOPO) copolymers were added to the micelle-rich phase and temperature-induced phase separation at 55 degrees C was performed. Total recovery of EGI(core)-HFBI after the two separation steps was 90% with a volume reduction of six times. For thermolabile proteins, the back-extraction temperature could be decreased to room temperature by using a hydrophobically modified EOPO copolymer, with slightly lower yield. The addition of thermoseparating co-polymer is a novel approach to remove detergent and effectively releases the fusion protein EGI(core)-HFBI into a water phase.

Family 7 cellobiohydrolases from Phanerochaete chrysosporium: crystal structure of the catalytic module of Cel7D (CBH58) at 1.32 A resolution and homology models of the isozymes

Cellobiohydrolase 58 (Cel7D) is the major cellulase produced by the white-rot fungus Phanerochaete chrysosporium, constituting approximately 10 % of the total secreted protein in liquid culture on cellulose. The enzyme is classified into family 7 of the glycosyl hydrolases, together with cellobiohydrolase I (Cel7A) and endoglucanase I (Cel7B) from Trichoderma reesei. Like those enzymes, it catalyses cellulose hydrolysis with net retention of the anomeric carbon configuration. The structure of the catalytic module (431 residues) of Cel7D was determined at 3.0 A resolution using the structure of Cel7A from T. reesei as a search model in molecular replacement, and ultimately refined at 1.32 A resolution. The core structure is a beta-sandwich composed of two large and mainly antiparallel beta-sheets packed onto each other. A long cellulose-binding groove is formed by loops on one face of the sandwich. The catalytic residues are conserved and the mechanism is expected to be the same as for other family members. The Phanerochaete Cel7D binding site is more open than that of the T. reesei cellobiohydrolase, as a result of deletions and other changes in the loop regions, which may explain observed differences in catalytic properties. The binding site is not, however, as open as the groove of the corresponding endoglucanase. A tyrosine residue at the entrance of the tunnel may be part of an additional subsite not present in the T. reesei cellobiohydrolase. The Cel7D structure was used to model the products of the five other family 7 genes found in P. chrysosporium. The results suggest that at least two of these will have differences in specificity and possibly catalytic mechanism, thus offering some explanation for the presence of Cel7 isozymes in this species, which are differentially expressed in response to various growth conditions.

Enzymatic synthesis of aliphatic beta-lactosides as mimic units of glycosphingolipids by use of Trichoderma reesei cellulase

Aliphatic beta-lactosides were directly synthesized by beta-lactosyl transfer reaction from p-nitrophenyl beta-lactoside (Lac beta-pNP) to various 1-alkanols (n = 2-12), utilizing commercially available cellulase preparation of Trichoderma reesei C1. With ethanol acceptor, the enzyme induced ethyl beta-lactoside (1) in 18% yield based on the donor added in aqueous buffer system. When 1-octanol and dodecanol were acceptors, octyl beta-lactoside (2) and dodecyl beta-lactoside (3) were also obtained as transfer products, respectively. In both cases, the addition of sodium cholate as detergent to the reaction system ensured a sufficient solubility of these acceptors and resulted in a remarkable increase of the desired compounds (5-13% yields based on the donor added). Furthermore, the enzyme catalyzed the N-acetyllactosaminyl transfer reaction from p-nitrophenyl beta-N-acetyllactosaminide (LacNAc beta-pNP) not only to 1-alkanol, but also to the OH-4 position of Man and Glc to produce the trisaccharides, Gal beta1-4GlcNAc beta1-4Man (4) and Gal beta1-4GlcNAc beta1-4Glc (5), respectively. The enzyme activities transferring lactosyl and N-acetyllactosaminyl groups were not separated by chromatographies using DEAE-Sepharose Fast Flow and Sephadex 75 pg columns, indicating that the two reactions were catalyzed by a single enzyme. It was specified that a single enzyme works both transglycosylations, based on the substrate competition assay on hydrolysis.

Engineering of a glycosidase Family 7 cellobiohydrolase to more alkaline pH optimum: the pH behaviour of Trichoderma reesei Cel7A and its E223S/ A224H/L225V/T226A/D262G mutant

The crystal structures of Family 7 glycohydrolases suggest that a histidine residue near the acid/base catalyst could account for the higher pH optimum of the Humicola insolens endoglucanase Cel7B, than the corresponding Trichoderma reesei enzymes. Modelling studies indicated that introduction of histidine at the homologous position in T. reesei Cel7A (Ala(224)) required additional changes to accommodate the bulkier histidine side chain. X-ray crystallography of the catalytic domain of the E223S/A224H/L225V/T226A/D262G mutant reveals that major differences from the wild-type are confined to the mutations themselves. The introduced histidine residue is in plane with its counterpart in H. insolens Cel7B, but is 1.0 A (=0.1 nm) closer to the acid/base Glu(217) residue, with a 3.1 A contact between N(epsilon2) and O(epsilon1). The pH variation of k(cat)/K(m) for 3,4-dinitrophenyl lactoside hydrolysis was accurately bell-shaped for both wild-type and mutant, with pK(1) shifting from 2.22+/-0.03 in the wild-type to 3.19+/-0.03 in the mutant, and pK(2) shifting from 5.99+/-0.02 to 6.78+/-0.02. With this poor substrate, the ionizations probably represent those of the free enzyme. The relative k(cat) for 2-chloro-4-nitrophenyl lactoside showed similar behaviour. The shift in the mutant pH optimum was associated with lower k(cat)/K(m) values for both lactosides and cellobiosides, and a marginally lower stability. However, k(cat) values for cellobiosides are higher for the mutant. This we attribute to reduced non-productive binding in the +1 and +2 subsites; inhibition by cellobiose is certainly relieved in the mutant. The weaker binding of cellobiose is due to the loss of two water-mediated hydrogen bonds.

Addition of substrate-binding domains increases substrate-binding capacity and specific activity of a chitinase from Trichoderma harzianum

Chitinase Chit42 from Trichoderma harzianum CECT 2413 is considered to play an important role in the biocontrol activity of this fungus against plant pathogens. Chit42 lacks a chitin-binding domain (ChBD). We have produced hybrid chitinases with stronger chitin-binding capacity by fusing to Chit42 a ChBD from Nicotiana tabacum ChiA chitinase and the cellulose-binding domain from cellobiohydrolase II of Trichoderma reesei. The chimeric chitinases had similar activities towards soluble substrate but higher hydrolytic activity than the native chitinase on high molecular mass insoluble substrates such as ground chitin or chitin-rich fungal cell walls.

Genetically engineered peptide fusions for improved protein partitioning in aqueous two-phase systems. Effect of fusion localization on endoglucanase I of Trichoderma reesei

Genetic engineering has been used for fusion of the peptide tag, Trp-Pro-Trp-Pro, on a protein to study the effect on partitioning in aqueous two-phase systems. As target protein for the fusions the cellulase, endoglucanase I (endo-1,4-beta-Dglucan-4-glucanohydrolase, EC 3.2.1.4, EGI, Cel7B) of Trichoderma reesei was used. For the first time a glycosylated two-domain enzyme has been utilized for addition of peptide tags to change partitioning in aqueous two-phase systems. The aim was to find an optimal fusion localization for EGI. The peptide was (1) attached to the C-terminus end of the cellulose binding domain (CBD), (2) inserted in the glycosylated linker region, (3) added after a truncated form of EGI lacking the CBD and a small part of the linker. The different constructs were expressed in the filamentous fungus T. reesei under the gpdA promoter from Aspergillus nidulans. The expression levels were between 60 and 100 mg/l. The partitioning behavior of the fusion proteins was studied in an aqueous two-phase model system composed of the thermoseparating ethylene oxide (EO)-propylene oxide (PO) random copolymer EO-PO (50:50) (EO50PO50) and dextran. The Trp-Pro-Trp-Pro tag was found to direct the fusion protein to the top EO50PO50 phase. The partition coefficient of a fusion protein can be predicted with an empirical correlation based on independent contributions from partitioning of unmodified protein and peptide tag in this model system. The fusion position at the end of the CBD, with the spacer Pro-Gly, was shown to be optimal with respect to partitioning and tag efficiency factor (TEF) was 0.87, where a fully exposed tag would have a TEF of 1.0. Hence, this position can further be utilized for fusion with longer tags. For the other constructs the TEF was only 0.43 and 0.10, for the tag fused to the truncated EGI and in the linker region of the full length EGI, respectively.

Directed immobilization of recombinant staphylococci on cotton fibers by functional display of a fungal cellulose-binding domain

The immobilization of recombinant staphylococci onto cellulose fibers through surface display of a fungal cellulose-binding domain (CBD) was investigated. Chimeric proteins containing the CBD from Trichoderma reesei cellulase Cel6A were found to be correctly targeted to the cell wall of Staphylococcus carnosus cells, since full-length proteins could be extracted and affinity-purified. Furthermore, surface accessibility of the CBD was verified using a monoclonal antibody and functionality in terms of cellulose-binding was demonstrated in two different assays in which recombinant staphylococci were found to efficiently bind to cotton fibers. The implications of this strategy of directed immobilization for the generation of whole-cell microbial tools for different applications will be discussed.

Protein-based chiral stationary phases for high-performance liquid chromatography enantioseparations

The enantioseparations of various compounds using proteins as the chiral selectors in high-performance liquid chromatography (HPLC) are considered in this review. The proteins used include albumins such as bovine serum albumin and human serum albumin, glycoproteins such as alpha1-acid glycoprotein, ovomucoid, ovoglycoprotein, avidin and riboflavin binding protein, enzymes such as trypsin, alpha-chymotrypsin, cellobiohydrolase I, lysozyme, pepsin and amyloglucosidase, and other proteins such as ovotransferrin and beta-lactoglobulin. This review deals with the properties of HPLC chiral stationary phases based on proteins, and the enantioselective properties and chiral recognition mechanisms of these stationary phases.

Structural basis for enantiomer binding and separation of a common beta-blocker: crystal structure of cellobiohydrolase Cel7A with bound (S)-propranolol at 1.9 A resolution

Cellobiohydrolase Cel7A (previously called CBH 1), the major cellulase produced by the mould fungus Trichoderma reesei, has been successfully exploited as a chiral selector for separation of stereo-isomers of some important pharmaceutical compounds, e.g. adrenergic beta-blockers. Previous investigations, including experiments with catalytically deficient mutants of Cel7A, point unanimously to the active site as being responsible for discrimination of enantiomers. In this work the structural basis for enantioselectivity of basic drugs by Cel7A has been studied by X-ray crystallography. The catalytic domain of Cel7A was co-crystallised with the (S)-enantiomer of a common beta-blocker, propranolol, at pH 7, and the structure of the complex was determined and refined at 1. 9 A resolution. Indeed, (S)-propranolol binds at the active site, in glucosyl-binding subsites -1/+1. The catalytic residues Glu212 and Glu217 make tight salt links with the secondary amino group of (S)-propranolol. The oxygen atom attached to the chiral centre of (S)-propranolol forms hydrogen bonds to the nucleophile Glu212 O(epsilon1) and to Gln175 N(epsilon2), whereas the aromatic naphthyl moiety stacks with the indole ring of Trp376 in site +1. The bidentate charge interaction with the catalytic glutamate residues is apparently crucial, since no enantioselectivity has been obtained with the catalytically deficient mutants E212Q and E217Q. Activity inhibition experiments with wild-type Cel7A were performed in conditions close to those used for crystallisation. Competitive inhibition constants for (R)- and (S)-propranolol were determined at 220 microM and 44 microM, respectively, corresponding to binding free energies of 20 kJ/mol and 24 kJ/mol, respectively. The K(i) value for (R)-propranolol was 57-fold lower than the highest concentration, 12.5 mM, used in co-crystallisation experiments. Still several attempts to obtain a complex with the (R)-enantiomer have failed. By using cellobiose as a selective competing ligand, the retention of the enantiomers of propranolol on the chiral stationary phase (CSP) based on Cel7A mutant D214N were resolved into enantioselective and non- selective binding. The enantioselective binding was weaker for both enantiomers on D214N-CSP than on wild-type-CSP.

Three-dimensional structure of the catalytic core of acetylxylan esterase from Trichoderma reesei: insights into the deacetylation mechanism

Acetylxylan esterase from Trichoderma reesei removes acetyl side groups from xylan. The crystal structure of the catalytic core of the enzyme was solved at 1.9 A resolution. The core has an alpha/beta/alpha sandwich fold, similar to that of homologous acetylxylan esterase from Penicillium purpurogenum and cutinase from Fusarium solani. All three enzymes belong to family 5 of the carbohydrate esterases and the superfamily of the alpha/beta hydrolase fold. Evidently, the enzymes have diverged from a common ancestor and they share the same catalytic mechanism. The catalytic machinery of acetylxylan esterase from T. reesei was studied by comparison with cutinase, the catalytic site of which is well known. Acetylxylan esterase is a pure serine esterase having a catalytic triad (Ser90, His187, and Asp175) and an oxyanion hole (Thr13 N, and Thr13 O gamma). Although the catalytic triad of acetylxylan esterase has been reported previously, there has been no mention of the oxyanion hole. A model for the binding of substrates is presented on the basis of the docking of xylose. Acetylxylan esterase from T. reesei is able to deacetylate both mono- and double-acetylated residues, but it is not able to remove acetyl groups located close to large side groups such as 4-O-methylglucuronic acid. If the xylopyranoside residue is double-acetylated, both acetyl groups are removed by the catalytic triad: first one acetyl group is removed and then the residue is reorientated so that the nucleophilic oxygen of serine can attack the second acetyl group.

Affinity electrophoresis for the identification and characterization of soluble sugar binding by carbohydrate-binding modules

Affinity electrophoresis was used to identify and quantify the interaction of carbohydrate-binding modules (CBMs) with soluble polysaccharides. Association constants determined by AE were in excellent agreement with values obtained by isothermal titration calorimetry and fluorescence titration. The method was adapted to the identification, study and characterization of mutant carbohydrate-binding modules with altered affinities and specificities. Competition affinity electrophoresis was used to monitor binding of small, soluble mono- and disaccharides to one of the modules.

PCR with degenerate primers amplifies a subgenomic DNA fragment from the endoglucanase gene(s) of Torula thermophila, a thermophilic fungus

The aim of this study was to enable the polymerase chain reaction (PCR) amplification of DNA fragments within endoglucanase gene(s) of Torula thermophila, by using degenerate primers so that the amplified fragment(s) could be used as homologous probe(s) for cloning of full-length endoglucanase gene(s). The design of the degenerate PCR primers was mainly based on the endoglucanase sequences of other fungi. The endoglucanase gene sequence of Humicola insolens was the only sequence from a thermophilic fungus publicly available in the literature. Therefore, the endoglucanase sequences of the two Trichoderma species, Trichoderma reesei and Trichoderma longibrachiatum, were used to generalize the primers. PCR amplification of T. thermophila genomic DNA with these primers resulted in a specific amplification. The specificity of the amplified fragment was shown by Southern hybridization analysis using egl3 gene of T. reesei as probe. This result suggested that the degenerate primers used in this study may be of value for studies aimed at cloning of endoglucanase genes from a range of related fungi.

Specificity and affinity of substrate binding by a family 17 carbohydrate-binding module from Clostridium cellulovorans cellulase 5A

The C-terminal carbohydrate-binding module (CBM17) from Clostridium cellulovorans cellulase 5A is a beta-1,4-glucan binding module with a preference for soluble chains. CBM17 binds to phosphoric acid swollen Avicel (PASA) and Avicel with association constants of 2.9 (+/-0.2) x 10(5) and 1.6 (+/-0.2) x 10(5) M(-1), respectively. The capacity values for PASA and Avicel were 11.9 and 0.4 micromol/g of cellulose, respectively. CBM17 did not bind to crystalline cellulose. CBM17 bound tightly to soluble barley beta-glucan and the derivatized celluloses HEC, EHEC, and CMC. The association constants for binding to barley beta-glucan, HEC, and EHEC were approximately 2.0 x 10(5) M(-1). Significant binding affinities were found for cello-oligosaccharides greater than three glucose units in length. The affinities for cellotriose, cellotetraose, cellopentaose, and cellohexaose were 1.2 (+/-0.3) x 10(3), 4.3 (+/-0.4) x 10(3), 3.8 (+/-0.1) x 10(4), and 1.5 (+/-0.0) x 10(5) M(-1), respectively. Fluorescence quenching studies and N-bromosuccinimide modification indicate the participation of tryptophan residues in ligand binding. The possible architecture of the ligand-binding site is discussed in terms of its binding specificity, affinity, and the participation of tryptophan residues.

Baculoviral display of functional scFv and synthetic IgG-binding domains

Viral vectors displaying specific ligand binding moities such as scFv fragments or intact antibodies hold promise for the development of targeted gene therapy vectors. In this report we describe baculoviral vectors displaying either functional scFv fragments or the synthetic Z/ZZ IgG binding domain derived from protein A. Display on the baculovirus surface was achieved via fusion of the scFv fragment or Z/ZZ domain to the N-terminus of gp64, the major envelope protein of the Autographa californica nuclear polyhedrosis virus, AcNPV. As examples of scFv fragments we have used a murine scFv specific for the hapten 2-phenyloxazolone and a human scFv specific for carcinoembryonic antigen. In principle, the Z/ZZ IgG binding domain displaying baculoviruses could be targeted to specific cell types via the binding of an appropriate antibody. We envisage applications for scFv and Z/ZZ domain displaying baculoviral vectors in the gene therapy field.

Binding site analysis of cellulose binding domain CBD(N1) from endoglucanse C of Cellulomonas fimi by site-directed mutagenesis

Endoglucanase C (CenC), a beta1,4 glucanase from the soil bacterium Cellulomonas fimi, binds to amorphous cellulose via two homologous cellulose binding domains, termed CBD(N1) and CBD(N2). In this work, the contributions of 10 amino acids within the binding cleft of CBD(N1) were evaluated by single site-directed mutations to alanine residues. Each isolated domain containing a single mutation was analyzed for binding to an insoluble amorphous preparation of cellulose, phosphoric acid swollen Avicel (PASA), and to a soluble glucopyranoside polymer, barley beta-glucan. The effect of any given mutation on CBD binding was similar for both substrates, suggesting that the mechanism of binding to soluble and insoluble substrates is the same. Tyrosines 19 and 85 were essential for tight binding by CBD(N1) as their replacement by alanine results in affinity decrements of approximately 100-fold on PASA, barley beta-glucan, and soluble cellooligosaccharides. The tertiary structures of unbound Y19A and Y85A were assessed by heteronuclear single quantum coherence (HSQC) spectroscopy. These studies indicated that the structures of both mutants were perturbed but that all perturbations are very near to the site of mutation.

Substrate-dependent differential expression of Humicola grisea var. thermoidea cellobiohydrolase genes

Transcription of fungal cellulase genes may be affected by substrate induction. We studied the expression of Humicola grisea var. thermoidea cellobiohydrolase genes (cbh1.1 and cbh1.2) under induction by several soluble and insoluble carbon sources. Using the RT-PCR technique, the cbh1.2 transcript was detected in all the conditions assayed along the growth curve. Catabolite repression, which frequently occurs in other fungal celluloytic systems, was not observed. On the other hand, cbh1.1 transcription was shown to be driven by insoluble and complex lignocellulosic substrates. In summary, the cbh1.2 gene product is constitutively produced, while cbh1.1 seems to respond to a distinct regulatory mechanism.

Expression and characterization of the chitin-binding domain of chitinase A1 from Bacillus circulans WL-12

Chitinase A1 from Bacillus circulans WL-12 comprises an N-terminal catalytic domain, two fibronectin type III-like domains, and a C-terminal chitin-binding domain (ChBD). In order to study the biochemical properties and structure of the ChBD, ChBD(ChiA1) was produced in Escherichia coli using a pET expression system and purified by chitin affinity column chromatography. Purified ChBD(ChiA1) specifically bound to various forms of insoluble chitin but not to other polysaccharides, including chitosan, cellulose, and starch. Interaction of soluble chitinous substrates with ChBD(ChiA1) was not detected by means of nuclear magnetic resonance and isothermal titration calorimetry. In addition, the presence of soluble substrates did not interfere with the binding of ChBD(ChiA1) to regenerated chitin. These observations suggest that ChBD(ChiA1) recognizes a structure which is present in insoluble or crystalline chitin but not in chito-oligosaccharides or in soluble derivatives of chitin. ChBD(ChiA1) exhibited binding activity over a wide range of pHs, and the binding activity was enhanced at pHs near its pI and by the presence of NaCl, suggesting that the binding of ChBD(ChiA1) is mediated mainly by hydrophobic interactions. Hydrolysis of beta-chitin microcrystals by intact chitinase A1 and by a deletion derivative lacking the ChBD suggested that the ChBD is not absolutely required for hydrolysis of beta-chitin microcrystals but greatly enhances the efficiency of degradation.

Solution structure of the chitin-binding domain of Bacillus circulans WL-12 chitinase A1

The three-dimensional structure of the chitin-binding domain (ChBD) of chitinase A1 (ChiA1) from a Gram-positive bacterium, Bacillus circulans WL-12, was determined by means of multidimensional heteronuclear NMR methods. ChiA1 is a glycosidase that hydrolyzes chitin and is composed of an N-terminal catalytic domain, two fibronectin type III-like domains, and C-terminal ChBD(ChiA1) (45 residues, Ala(655)-Gln(699)), which binds specifically to insoluble chitin. ChBD(ChiA1) has a compact and globular structure with the topology of a twisted beta-sandwich. This domain contains two antiparallel beta-sheets, one composed of three strands and the other of two strands. The core region formed by the hydrophobic and aromatic residues makes the overall structure rigid and compact. The overall topology of ChBD(ChiA1) is similar to that of the cellulose-binding domain (CBD) of Erwinia chrysanthemi endoglucanase Z (CBD(EGZ)). However, ChBD(ChiA1) lacks the three aromatic residues aligned linearly and exposed to the solvent, which probably interact with cellulose in CBDs. Therefore, the binding mechanism of a group of ChBDs including ChBD(ChiA1) may be different from that proposed for CBDs.

Enantiomer separation of drugs by capillary electrophoresis using proteins as chiral selectors

The separation of drug enantiomers using proteins as the chiral selectors in capillary electrophoresis (CE) is considered in this review. The proteins used include albumins such as bovine serum albumin, human serum albumin and serum albumins from other species, glycoproteins such as alpha1-acid glycoprotein, crude ovomucoid, ovoglycoprotein, avidin and riboflavin binding protein, enzymes such as fungal cellulase, cellobiohydrolase I, pepsin and lysozyme and other proteins such as casein, human serum transferrin and ovotransferrin. Protein-based CE is carried out in two modes: in one proteins are immobilized or adsorbed within the capillary, or protein-immobilized silica gels are packed into the capillary (affinity capillary electrochromatography mode), and in the other proteins are dissolved in the running buffer (affinity CE mode). Furthermore, the advantages and limitations of the two modes and the factors affecting the chiral separations of various drugs by protein-based CE are discussed.

The mechanism of cellulase action on cotton fibers: evidence from atomic force microscopy

Two cellulases from Trichoderma reesei--an exoglucanase, CBH I, and an endoglucanase, EG II--alone and in combination were incubated with cotton fibers. The effects of the cellulases on the surfaces of the cotton fibers were examined by atomic force microscopy. At high magnification, the physical effects on the fibers caused by the two types of enzymes were considerably different. Treatment with CBH I resulted in the appearance of distinct pathways or tracks along the length of the macrofibril. Treatment with EG II appeared to cause peeling and smoothing of the fiber surface. In combination, their effect was observed to be greatest when both enzymes were present simultaneously. When fibers smoothed by treatment with EG II were treated subsequently with CBH I, further evidence of path way formation caused by the action of CBH I along the fibers was observed. Incubation with a cellulase from Thermotoga maritima that lacks a cellulose binding domain had no effect on the surface of cotton fibers. These images provide the first physical evidence of differences in the effect of cellulase components action on the surface of cotton fibers and provide evidence for the movement or tracking of CBH I along the fibers. The first AFM image of CBH I molecules are presented.

A family 26 mannanase produced by Clostridium thermocellum as a component of the cellulosome contains a domain which is conserved in mannanases from anaerobic fungi

Cellulosomes prepared by the cellulose affinity digestion method from Clostridium thermocellum culture supernatant hydrolysed carob galactomannan during incubation at 60 degrees C and pH 6.5. A recombinant phage expressing mannanase activity was isolated from a library of C. thermocellum genomic DNA constructed in lambdaZAPII. The cloned fragment of DNA containing a putative mannanase gene (manA) was sequenced, revealing an ORF of 1767 nt, encoding a protein (mannanase A; Man26A) of 589 aa with a molecular mass of 66816 Da. The putative catalytic domain (CD) of Man26A, identified by gene sectioning and sequence comparisons, displayed up to 32% identity with other mannanases belonging to family 26. Immediately downstream of the CD and separated from it by a short proline/threonine linker was a duplicated 24-residue dockerin motif, which is conserved in all C. thermocellum cellulosomal enzymes described thus far and mediates their attachment to the cellulosome-integrating protein (CipA). Man26A consisting of the CD alone (Man26A") was hyperexpressed in Escherichia coli BL21(DE3) and purified. The truncated enzyme hydrolysed soluble and insoluble mannan, displaying a temperature optimum of 65 degrees C and a pH optimum of 6.5, but exhibited no activity against other plant cell wall polysaccharides. Antiserum raised against Man26A" cross-reacted with a polypeptide with a molecular mass of 70000 Da that is part of the C. thermocellum cellulosome. A second variant of Man26A containing the N-terminal segment of 130 residues and the CD (Man26A") bound to ivory-nut mannan and weakly to soluble Carob galactomannan and insoluble cellulose. Man26A" consisting of the CD alone did not bind to these polysaccharides. These results indicate that the N-terminal 130 residues of mature Man26A may constitute a weak mannan-binding domain. Sequence comparisons revealed a lack of identity between this region of Man26A and other polysaccharide-binding domains, but significant identity with a region conserved in the three family 26 mannanases from the anaerobic fungus Piromyces equi.

Discrimination between enantioselective and non-selective binding sites on cellobiohydrolase-based stationary phases by site specific competing ligands

A systematic study was performed to investigate the influence of cellobiose or lactose on the enantioselective retention behaviour of some beta-blockers in liquid chromatography using Cellobiohydrolase (CHB) I from Trichoderma reesei or Cellobiohydrolase 58 from Phanerochaete chrysosporium immobilized on silica as stationary phases. The results revealed that the retention could be described by the function [equation; see text] where the observed capacity factor corresponds to the sum of an enantioselective mode being influenced by a site specific competing ligand (competitor) and a non-selective mode unaffected by the competitor. A non-constrained non-linear least-square regression gave in all cases virtually identical nondisplacable capacity factors (k'ns) for both enantiomers of the same drug. The experimental capacity factors (k'(x,C)) of the enantiomers all show a close fit to the adapted function. The Kd values calculated for the competitor were also virtually identical for each pair of enantiomers and were in accordance with Ki data determined for the competitors in classical enzyme kinetics experiments, demonstrating that one unique site; namely, the catalytic site, was responsible for the enantioselective binding. Similar results were obtained with the resolution of rac-alprenolol and rac-metoprolol on CBH I phase.

Cloning and sequence analysis of a new cellulase gene encoding CelK, a major cellulosome component of Clostridium thermocellum: evidence for gene duplication and recombination

The cellulolytic and hemicellulolytic complex of Clostridium thermocellum, termed cellulosome, consists of up to 26 polypeptides, of which at least 17 have been sequenced. They include 12 cellulases, 3 xylanases, 1 lichenase, and CipA, a scaffolding polypeptide. We report here a new cellulase gene, celK, coding for CelK, a 98-kDa major component of the cellulosome. The gene has an open reading frame (ORF) of 2,685 nucleotides coding for a polypeptide of 895 amino acid residues with a calculated mass of 100,552 Da. A signal peptide of 27 amino acid residues is cut off during secretion, resulting in a mature enzyme of 97,572 Da. The nucleotide sequence is highly similar to that of cbhA (V. V. Zverlov et al., J. Bacteriol. 180:3091-3099, 1998), having an ORF of 3,690 bp coding for the 1,230-amino-acid-residue CbhA of the same bacterium. Homologous regions of the two genes are 86.5 and 84.3% identical without deletion or insertion on the nucleotide and amino acid levels, respectively. Both have domain structures consisting of a signal peptide, a family IV cellulose binding domain (CBD), a family 9 glycosyl hydrolase domain, and a dockerin domain. A striking distinction between the two polypeptides is that there is a 330-amino-acid insertion in CbhA between the catalytic domain and the dockerin domain containing a fibronectin type 3-like domain and family III CBD. This insertion, missing in CelK, is responsible for the size difference between CelK and CbhA. Upstream and downstream flanking sequences of the two genes show no homology. The data indicate that celK and cbhA in the genome of C. thermocellum have evolved through gene duplication and recombination of domain coding sequences. celK without a dockerin domain was expressed in Escherichia coli and purified. The enzyme had pH and temperature optima at 6.0 and 65 degrees C, respectively. It hydrolyzed p-nitrophenyl-beta-D-cellobioside with a Km and a Vmax of 1.67 microM and 15.1 U/mg, respectively. Cellobiose was a strong inhibitor of CelK activity, with a Ki of 0.29 mM. The enzyme was thermostable, after 200 h of incubation at 60 degrees C, 97% of the original activity remained. Properties of the enzyme indicated that it is a cellobiohydrolase.

An alpha-L-arabinofuranosidase from Trichoderma reesei containing a noncatalytic xylan-binding domain

L-Sorbose, an excellent cellulase and xylanase inducer from Trichoderma reesei PC-3-7, also induced alpha-L-arabinofuranosidase (alpha-AF) activity. An alpha-AF induced by L-sorbose was purified to homogeneity, and its molecular mass was revealed to be 35 kDa (AF35), which was not consistent with that of the previously reported alpha-AF. Another species, with a molecular mass of 53 kDa (AF53), which is identical to that of the reported alpha-AF, was obtained by a different purification procedure. Acid treatment of the ammonium sulfate-precipitated fraction at pH 3.0 in the purification steps or pepsin treatment of the purified AF53 reduced the molecular mass to 35 kDa. Both purified enzymes have the same enzymological properties, such as pH and temperature effects on activity and kinetic parameters for p-nitrophenyl-alpha-L-arabinofuranoside (pNPA). Moreover, the N-terminal amino acid sequences of these enzymes were identical with that of the reported alpha-AF. Therefore, it is obvious that AF35 results from the proteolytic cleavage of the C-terminal region of AF53. Although AF35 and AF53 showed the same catalytic constant with pNPA, the former showed drastically reduced specific activity against oat spelt xylan compared to the latter. Furthermore, AF53 was bound to xylan rather than to crystalline cellulose (Avicel), but AF35 could not be bound to any of the glycans. These results suggest that AF53 is a modular glycanase, which consists of an N-terminal catalytic domain and a C-terminal noncatalytic xylan-binding domain.

A family IIb xylan-binding domain has a similar secondary structure to a homologous family IIa cellulose-binding domain but different ligand specificity

BACKGROUND

Many enzymes that digest polysaccharides contain separate polysaccharide-binding domains. Structures have been previously determined for a number of cellulose-binding domains (CBDs) from cellulases.

RESULTS

The family IIb xylan-binding domain 1 (XBD1) from Cellulomonas fimi xylanase D is shown to bind xylan but not cellulose. Its structure is similar to that of the homologous family IIa CBD from C. fimi Cex, consisting of two four-stranded beta sheets that form a twisted 'beta sandwich'. The xylan-binding site is a groove made from two tryptophan residues that stack against the faces of the sugar rings, plus several hydrogen-bonding polar residues.

CONCLUSIONS

The biggest difference between the family IIa and IIb domains is that in the former the solvent-exposed tryptophan sidechains are coplanar, whereas in the latter they are perpendicular, forming a twisted binding site. The binding sites are therefore complementary to the secondary structures of the ligands cellulose and xylan. XBD1 and CexCBD represent a striking example of two proteins that have high sequence similarity but a different function.

Sequence pattern for the occurrence of N-glycosylation in proteins

To further understand the occurrence of N-glycosylation, 21 nonhomologous proteins with Asn-x-Ser/Thr sequence were investigated. The results showed that some oligopeptides with Gly residues (G-x-y or y-x-G) are adjacent to the N-glycosylated sequences. These oligopeptides are not only essential for the structure and function of the proteins, but they are also found to be often proteolytic processing sites. These properties suggest that these oligopeptides may be a "sequence pattern" for the occurrence of N-glycosylation. The implications of the findings for protein structure and function are discussed.

Extracellular proteins in fungi: a cytological and molecular perspective

Protein secretion is a vital process in fungi. For many, the secretion of hydrolytic enzymes provides a crucial step in their nutrition in nature. However, in recent years the list of different types of secreted proteins that have been discovered has extended significantly. These have been shown to have a diversity of functions including toxic molecule transport and control of desiccation. The majority of secreted proteins are glycosylated and our understanding of this aspect of fungal biochemistry has also extended in recent years. This review addresses the process of protein secretion from the cytological, biochemical and genetical standpoints. Advances in technology in many areas of scientific approach have enabled a better and understanding of this important process in fungi.

Widely different off rates of two closely related cellulose-binding domains from Trichoderma reesei

The filamentous fungus Trichoderma reesei produces two cellobiohydrolases (CBHI and CBHII). These, like most other cellulose-degrading enzymes, have a modular structure consisting of a catalytic domain linked to a cellulose-binding domain (CBD). The isolated catalytic domains bind poorly to cellulose and have a much lower activity towards cellulose than the intact enzymes. For the CBDs, no function other than binding to cellulose has been found. We have previously described the reversibility and exchange rate for the binding of the CBD of CBHI to cellulose. In this work, we studied the binding of the CBD of CBHII and showed that it differs markedly from the behaviour of that of CBHI. The apparent binding affinities were similar, but the CBD of CBHII could not be dissociated from cellulose by buffer dilution and did not show a measurable exchange rate. However, desorption could be triggered by shifting the temperature. The CBD of CBHII bound reversibly to chitin. Two variants of the CBHII CBD were made, in which point mutations increased its similarity to the CBD of CBHI. Both variants were found to bind reversibly to cellulose.

A comparative study of two retaining enzymes of Trichoderma reesei: transglycosylation of oligosaccharides catalysed by the cellobiohydrolase I, Cel7A, and the beta-mannanase, Man5A

HPLC, MALDI-TOF MS and NMR spectroscopy were used to investigate the hydrolysis of cello- and mannooligosaccharides by Cel7A and Man5A from Trichoderma reesei. The experimental progress curves were analysed by fitting the numerically integrated kinetic equations, which provided cleavage patterns for oligosaccharides. This data evaluation procedure accounts for product inhibition and avoids the initial slope approximation. In addition, a transglycosylation step had to be included in the model to reproduce the experimental progress curves. For the hydrolysis of manno-oligosaccharides, Man4-6, by Man5A no mannose was detected at the beginning of the reaction showing that only the internal linkages are hydrolysed. For cellotriose and cellotetraose hydrolysis by Cel7A, the main product is cellobiose and glucose is released from the non-reducing end of the substrate. Intermediary products longer than the substrates were detected by MALDI-TOF MS when oligosaccharides (Glc4-6 or Man4-6) were hydrolysed by either Cel7A or Man5A. Interestingly, two distinct transglycosylation pathways could be observed. Cel7A produced intermediates that are one unit longer than the substrate, whereas Man5A produced intermediates that are two units longer than the substrate.

Comparison of gene structures and enzymatic properties between two endoglucanases from Humicola grisea

We have cloned two endoglucanase genes (egl3 and egl4) from a thermophilic fungus, Humicola grisea. The coding region of the egl3 gene was interrupted by an intron of 56-bp, and the deduced amino acid sequence of the egl3 gene was 305 amino acids in length and showed 98.4% identity with Humicola insolens EGV. The coding region of the egl4 gene was also interrupted by an intron of 173-bp, which contains 34 TTC repeated sequence units, and the deduced amino acid sequence of the egl4 gene was 227 amino acids in length and showed 61.5% identity with H. grisea EGL3. The typical hinge and the cellulose-binding domain were observed in the C-terminal region of EGL3, but they were not observed in EGL4. In the 5' upstream region of both genes, there were a TATA box or its similar sequence, CAAT motifs, and 6-bp sites which are identical or similar to the consensus sequence for binding a catabolite repressor CREA in Aspergillus nidulans. The egl3 and the egl4 genes were expressed in Aspergillus oryzae, and the translation products were purified. The fusion protein, EGL4CBD, which consists of a catalytic domain of EGL4 and the C-terminal region of EGL3, was also constructed and produced by A. oryzae, and purified. These enzymes showed relatively high activity toward carboxymethyl cellulose (CMC) and could not hydrolyze p-nitrophenyl-beta-D-glucoside and p-nitrophenyl-beta-D-cellobioside. The positive effect of substituting the C-terminal region of EGL4 with that of EGL3 was observed in the hydrolysis of CMC.

Purification, characterization and gene analysis of exo-cellulase II (Ex-2) from the white rot basidiomycete Irpex lacteus

A new exo-type cellulase, named exo-cellulase II (Ex-2), was purified from the crude enzyme preparation of Irpex lacteus. Ex-2 was very similar to the previously characterized exo-cellulase I (Ex-1) with respect to enzymatic features such as optimal pH, temperature, heat stability, and catalytic activity. However, Ex-2 exhibited greater pH stability than Ex-1. The molecular mass and carbohydrate content of Ex-2 (56,000, 4.0%) were different from those of Ex-1 (53,000, 2.0%). A cellulase gene (named cel2) encoding both Ex-2 and Ex-1 was isolated from an I. lacteus genomic library. The cel2 gene was found to consist of 1569 bp with an open reading frame encoding 523 amino acids, interrupted by two introns. The deduced amino acid sequences revealed that cel2 ORF has a modular structure consisting of a catalytic domain and a fungal-type cellulose-binding domain (CBD) separated by a serine-rich linker region. The catalytic domain was homologous to those of fungal cellobiohydrolases belonging to family 7 of the glycosyl hydrolases. Northern blot analysis showed that expression of the cel2 gene was induced by various cellulosic substrates and repressed by glucose, fructose, and lactose.

A xyloglucan-specific endo-beta-1,4-glucanase from Aspergillus aculeatus: expression cloning in yeast, purification and characterization of the recombinant enzyme

A full-length c-DNA encoding a xyloglucan-specific endo -beta-1, 4-glucanase (XEG) has been isolated from the filamentous fungus Aspergillus aculeatus by expression cloning in yeast. The colonies expressing functional XEG were identified on agar plates containing azurine-dyed cross-linked xyloglucan. The cDNA encoding XEG was isolated, sequenced, cloned into an Aspergillus expression vector, and transformed into Aspergillus oryzae for heterologous expression. The recombinant enzyme was purified to apparent homogeneity by anion-exchange and gel permeation chromatography. The recombinant XEG has a molecular mass of 23,600, an isoelectric point of 3.4, and is optimally stable at a pH of 3.4 and temperature below 30 degreesC. The enzyme hydrolyzes structurally diverse xyloglucans from various sources, but hydrolyzes no other cell wall component and can therefore be considered a xyloglucan-specific endo -beta-1, 4-glucanohydrolase. XEG hydrolyzes its substrates with retention of the anomeric configuration. The Kmof the recombinant enzyme is 3.6 mg/ml, and its specific activity is 260 micromol/min per mg protein. The enzyme was tested for its ability to solubilize xyloglucan oligosaccharides from plant cell walls. It was shown that treatment of plant cell walls with XEG yields only xyloglucan oligosaccharides, indicating that this enzyme can be a powerful tool in the structural elucidation of xyloglucans.

Overproduction of recombinant Trichoderma reesei cellulases by Aspergillus oryzae and their enzymatic properties

We have established an expression system of Trichoderma reesei cellulase genes using Aspergillus oryzae as a host. In this system, the expression of T. reesei cellulase genes were regulated under the control of A. oryzae Taka-amylase promoter and the cellulase genes were highly expressed when maltose was used as a main carbon source for inducer. The production of recombinant cellulases by A. oryzae transformants reached a maximum after 3-4 days of cultivation. In some cases, proteolysis of recombinant cellulases was observed in the late stage of cultivation. The recombinant cellulases were purified and characterized. The apparent molecular weights of recombinant cellulases were more or less larger than those of native enzymes. The optimal temperatures and pHs of recombinant cellulases were 50-70 degrees C and 4-5, respectively. Among the recombinant cellulases, endoglucanase I showed broad substrate specificities and high activity when compared with the other cellulases investigated here.

Structure and function analysis of Pseudomonas plant cell wall hydrolases

Hydrolysis of the major structural polysaccharides of plant cell walls by the aerobic soil bacterium Pseudomonas fluorescens subsp. cellulosa is attributable to the production of multiple extracellular cellulase and hemicellulase enzymes, which are the products of distinct genes belonging to multigene families. Cloning and sequencing of individual genes, coupled with gene sectioning and functional analysis of the encoded proteins have provided a detailed picture of structure/function relationships and have established the cellulase-hemicellulase system of P. fluorescens subsp. cellulosa as a model for the plant cell wall degrading enzyme systems of aerobic cellulolytic bacteria. Cellulose- and xylan-degrading enzymes produced by the pseudomonad are typically modular in structure and contain catalytic and noncatalytic domains joined together by serine-rich linker sequences. The cellulases include a cellodextrinase; a beta-glucan glucohydrolase and multiple endoglucanases, containing catalytic domains belonging to glycosyl hydrolase families 5, 9, and 45; and cellulose-binding domains of families II and X, both of which are present in each enzyme. Endo-acting xylanases, with catalytic domains belonging to families 10 and 11, and accessory xylan-degrading enzymes produced by P. fluorescens subsp. cellulosa contain cellulose-binding domains of families II, X, and XI, which act by promoting close contact between the catalytic domain of the enzyme and its target substrate. A domain homologous with NodB from rhizobia, present in one xylanase, functions as a deacetylase. Mananase, arabinanase, and galactanase produced by the pseudomonad are single domain enzymes. Crystallographic studies, coupled with detailed kinetic analysis of mutant forms of the enzyme in which key residues have been altered by site-directed mutagenesis, have shown that xylanase A (family 10) has 8-fold alpha/beta barrel architecture, an extended substrate-binding cleft containing at least six xylose-binding pockets and a calcium-binding site that protects the enzyme from thermal inactivation, thermal unfolding, and attack by proteinases. Kinetic studies of mutant and wild-type forms of a mannanase and a galactanase from P. fluorescens subsp. cellulosa have enabled the catalytic mechanisms and key catalytic residues of these enzymes to be identified.

Determinants of recombinant production of antimicrobial cationic peptides and creation of peptide variants in bacteria

Cationic peptides possessing antibacterial activity are virtually ubiquitous in nature, and offer exciting prospects as new therapeutic agents. We had previously demonstrated that such peptides could be produced by fusion protein technology in bacteria and several carrier proteins had been tested as fusion partners including glutathione-S-transferase, S. aureus protein A, IgG binding protein and P. aeruginosa outer membrane protein OprF. However these fusion partners, while successfully employed in peptide expression, were not optimized for high level production of cationic peptides (Piers, K., Brow, M. L., and Hancock, R. E. W. 1993, Gene 137, 7-13). In this paper we took advantage of a small replication protein RepA from E. coli and used its truncated version to construct fusion partners. The minimal elements required for high level expression of cationic peptide were defined as a DNA sequence encoding a fusion protein comprising, from the N-terminus, a 68 amino acid carrier region, an anionic prepro domain, a single methionine and the peptide of interest. The 68 amino acid carrier region was a block of three polypeptides consisting of a truncated RepA, a synthetic cellulose binding domain and a hexa histidine domain. The improved system showed high level expression and simplified downstream purification. The active peptide could be yielded by CNBr cleavage of the fusion protein. This novel vector was used to express three classes of cationic peptides including the alpha-helical peptide CEMA, the looped peptide bactenecin and the extended peptide indolicidin. In addition, mutagenesis of the peptide gene to produce peptide variants of CEMA and indolicidin using the improved vector system was shown to be successful.

Tryptophan 272: an essential determinant of crystalline cellulose degradation by Trichoderma reesei cellobiohydrolase Cel6A

Trichoderma reesei cellobiohydrolase Cel6A (formerly CBHII) has a tunnel shaped active site with four internal subsites for the glucose units. We have predicted an additional ring stacking interaction for a sixth glucose moiety with a tryptophan residue (W272) found on the domain surface. Mutagenesis of this residue selectively impairs the enzyme function on crystalline cellulose but not on soluble or amorphous substrates. Our data shows that W272 forms an additional subsite at the entrance of the active site tunnel and suggests it has a specialised role in crystalline cellulose degradation, possibly in guiding a glucan chain into the tunnel.

Properties and gene structure of a bifunctional cellulolytic enzyme (CelA) from the extreme thermophile 'Anaerocellum thermophilum' with separate glycosyl hydrolase family 9 and 48 catalytic domains

A large cellulolytic enzyme (CelA) with the ability to hydrolyse microcrystalline cellulose was isolated from the extremely thermophilic, cellulolytic bacterium 'Anaerocellum thermophilum'. Full-length CelA and a truncated enzyme species designated CelA' were purified to homogeneity from culture supernatants. CelA has an apparent molecular mass of 230 kDa. The enzyme exhibited significant activity towards Avicel and was most active towards soluble substrates such as CM-cellulose (CMC) and beta-glucan. Maximal activity was observed between pH values of 5 and 6 and temperatures of 95 degrees C (CM-cellulase) and 85 degrees C (Avicelase). Cellobiose, glucose and minor amounts of cellotriose were observed as end-products of Avicel degradation. The CelA-encoding gene was isolated from genomic DNA of 'A. thermophilum' by PCR and the nucleotide sequence was determined. The celA gene encodes a protein of 1711 amino acids (190 kDa) starting with the sequence found at the N-terminus of CelA purified from 'A. thermophilum'. Sequence analysis revealed a multidomain structure consisting of two distinct catalytic domains homologous to glycosyl hydrolase families 9 and 48 and three domains homologous to family III cellulose-binding domain linked by Pro-Thr-Ser-rich regions. The enzyme is most closely related to CelA of Caldicellulosiruptor saccharolyticus (sequence identities of 96 and 97% were found for the N- and C-terminal catalytic domains, respectively). Endoglucanase CelZ of Clostridium stercorarium shows 70.4% sequence identity to the N-terminal family 9 domain and exoglucanase CelY from the same organism has 69.2% amino acid identity with the C-terminal family 48 domain. Consistent with this similarity on the primary structure level, the 90 kDa truncated derivative CelA' containing the N-terminal half of CelA exhibited endoglucanase activity and bound to microcrystalline cellulose. Due to the significantly enhanced Avicelase activity of full-length CelA, exoglucanase activity may be ascribed to the C-terminal family 48 catalytic domain.

High-resolution crystal structures reveal how a cellulose chain is bound in the 50 A long tunnel of cellobiohydrolase I from Trichoderma reesei

Detailed information has been obtained, by means of protein X-ray crystallography, on how a cellulose chain is bound in the cellulose-binding tunnel of cellobiohydrolase I (CBHI), the major cellulase in the hydrolysis of native, crystalline cellulose by the fungus Trichoderma reesei. Three high-resolution crystal structures of different catalytically deficient mutants of CBHI in complex with cellotetraose, cellopentaose and cellohexaose have been refined at 1.9, 1.7 and 1.9 A resolution, respectively. The observed binding of cellooligomers in the tunnel allowed unambiguous identification of ten well-defined subsites for glucosyl units that span a length of approximately 50 A. All bound oligomers have the same directionality and orientation, and the positions of the glucosyl units in each binding site agree remarkably well between the different complexes. The binding mode observed here corresponds to that expected during productive binding of a cellulose chain. The structures support the hypothesis that hydrolysis by CBHI proceeds from the reducing towards the non-reducing end of a cellulose chain, and they provide a structural explanation for the observed distribution of initial hydrolysis products.