Improved immobilization of fusion proteins via cellulose-binding domains

Catalytic Synthesis of (S)-CHBE by Directional Coupling and Immobilization of Carbonyl Reductase and Glucose Dehydrogenase

Ethyl (S)-4-chloro-3-hydroxybutyrate ((S)-CHBE) is an important chiral intermediate in the synthesis of the cholesterol-lowering drug atorvastatin. Studying the use of SpyTag/SpyCatcher and SnoopTag/SnoopCatcher systems for the asymmetric reduction reaction and directed coupling coenzyme regeneration is practical for efficiently synthesizing (S)-CHBE. In this study, Spy and Snoop systems were used to construct a double-enzyme directed fixation system of carbonyl reductase (BsCR) and glucose dehydrogenase (BsGDH) for converting 4-chloroacetoacetate (COBE) to (S)-CHBE and achieving coenzyme regeneration. We discussed the enzymatic properties of the immobilized enzyme and the optimal catalytic conditions and reusability of the double-enzyme immobilization system. Compared to the free enzyme, the immobilized enzyme showed an improved optimal pH and temperature, maintaining higher relative activity across a wider range. The double-enzyme immobilization system was applied to catalyze the asymmetric reduction reaction of COBE, and the yield of (S)-CHBE reached 60.1% at 30 °C and pH 8.0. In addition, the double-enzyme immobilization system possessed better operational stability than the free enzyme, and maintained about 50% of the initial yield after six cycles. In summary, we show a simple and effective strategy for self-assembling SpyCatcher/SnoopCatcher and SpyTag/SnoopTag fusion proteins, which inspires building more cascade systems at the interface. It provides a new method for facilitating the rapid construction of in vitro immobilized multi-enzyme complexes from crude cell lysate.

One-step purification and immobilization of recombinant proteins using SpyTag/SpyCatcher chemistry

Based on the specific and spontaneous formation of isopeptide bonds by SpyCatcher/SpyTag, we have developed a one-step method for purification and immobilization of recombinant proteins. The procedure is to immobilize SpyCatcher on glyoxyl agarose gels, and then the SpyCatcher immobilisate can be used to immobilize the SpyTag-fused protein in the crude extract selectively. A mutant of SpyCatcher (mSC), in which a peptide (LysGlyLysGlyLysGly) was added to the C-terminus of SpyCatcher and three lysine residues around the SpyTag/SpyCatcher binding domain were replaced with arginine, was designed to improve the attachment of SpyCatcher to the support. Compared with wild-type SpyCatcher, mSC can be immobilized on the glyoxyl-agarose support more efficiently, which enables the obtained mSC derivative a high binding capacity of the SpyTag-fused protein. The results showed that the target proteins in the crude enzyme extract were purified and immobilized in one step, and the thermal stability of the immobilized target proteins was also remarkably improved.

Modular protein architectures for pH-dependent interactions and switchable assembly of nanocellulose

Protein engineering shows a wide range of possibilities for designing properties in novel materials. Following inspiration from natural systems we have studied how combinations or duplications of protein modules can be used to engineer their interactions and achieve functional properties. Here we used cellulose binding modules (CBM) coupled to spider silk N-terminal domains that dimerize in a pH-sensitive manner. We showed how the pH-sensitive switching into dimers affected cellulose binding affinity in relation to covalent coupling between CBMs. Finally, we showed how the pH-sensitive coupling could be used to assemble cellulose nanofibers in a dynamic pH-dependent way. The work shows how novel proteins can be designed by linking functional domains from widely different sources and thereby achieve new functions in the self-assembly of nanoscale materials.

Synergistic effect of the enzyme complexes comprising agarase, carrageenase and neoagarobiose hydrolase on degradation of the red algae

In the practice of converting red algae biomass into biofuel or valuable biomaterials, the critical step is the decomposition process of the agarose to give fermentable monomeric sugars. In this study, we selected three enzymes such as agarase, carrageenase and neoagarobiose hydrolase to inducible the simultaneous hydrolysis of the major substrates such as agar and carrageenan constituting the pretreated red algae, and expressed the chimeric enzymes and formed a complexes through optimization of addition ratio. As a result, hydrolysis by enzyme complexes showed a maximum sugar release of 679 mg L^-1 with 67.9% saccharification yield from G. verrucosa natural substrate. The difference in the reducing sugar by the enzyme complexes was 3.6-fold higher than that of the monomer enzyme (cAgaB yield 188.6 mg L^-1). The synergistic effect of producing sugars from red algae biomass through these enzyme complexes can be a very important biological tools aimed at bioenergy production.

A Small Cellulose-Binding-Domain Protein (CBD1) in Phytophthora is Highly Variable in the Non-binding Amino Terminus

The small cellulose-binding-domain protein CBD1 is tightly bound to the cellulosic cell wall of the plant pathogenic stramenopile Phytophthora infestans. Transgene expression of the protein in potato plants also demonstrated binding to plant cell walls. A study was undertaken using 47 isolates of P. infestans from a worldwide collection, along with 17 other Phytophthora species and a related pathogen Plasmopara halstedii, to determine if the critical cell wall protein is subject to amino acid variability. Within the amino acid sequence of the secreted portion of CBD 1, encoded by the P. infestans isolates, 30 were identical with each other, and with P. mirabilis. Four isolates had one amino acid difference, each in a different location, while one isolate had two amino acid substitutions. The remaining 13 isolates had five amino acid changes that were each in identical locations (D17/G, D31/G, I32/S, T43/A, and G50/A), suggesting a single origin. Comparison of P. infestans CBD1 with other Phytophthora species identified extensive amino acid variation among the 60 amino acids at the amino terminus of the protein, and a high level of conservation from G61, where the critical cellulose-binding domain sequences begin, to the end of the protein (L110). While the region needed to bind to cellulose is conserved, the region that is available to interact with other cell wall components is subject to considerable variation, a feature that is evident even in the related genus Plasmopara. Specific changes can be used in determining intra- and inter-species relatedness. Application of this information allowed for the design of species-specific primers for PCR detection of P. infestans and P. sojae, by combining primers from the highly conserved and variable regions of the CBD1 gene.

Elastic and pH-Responsive Hybrid Interfaces Created with Engineered Resilin and Nanocellulose

We investigated how a genetically engineered resilin fusion protein modifies cellulose surfaces. We characterized the pH-responsive behavior of a resilin-like polypeptide (RLP) having terminal cellulose binding modules (CBM) and showed its binding to cellulose nanofibrils (CNF). Characterization of the resilin fusion protein at different pHs revealed substantial conformational changes of the protein, which were observed as swelling and contraction of the protein layer bound to the nanocellulose surface. In addition, we showed that employment of the modified resilin in cellulose hydrogel and nanopaper increased their modulus of stiffness through a cross-linking effect.

Cellulose membrane as a biomaterial: from hydrolysis to depolymerization with electron beam

The cellulose membrane (CM) is a major component of plant cell walls and is both a chemically and mechanically stable synthetic polymer with many applications for use in tissue engineering. However, due to its dissolution difficulty, there are no known physiologically relevant or pharmaceutically clinical applications for this polymer. Thus, research is underway on controlled and adjusted forms of cellulose depolymerization.

To advance the study of applying CM for tissue engineering, we have suggested new possibilities for electron beam (E-beam) treatment of CM. Treatment of CM with an E-beam can modify physical, chemical, molecular and biological properties, so it can be studied continuously to improve its usefulness and to enhance value.

We review clinical applications of CM, cellulose binding domains, cellulose crosslinking proteins, conventional hydrolysis of cellulose, and depolymerization with radiation and focus our experiences with depolymerization of E-beam irradiated CM in this article.

Designed biomolecule–cellulose complexes for palladium recovery and detoxification

Genetically engineered biomolecules assembled on cellulose are used for palladium recovery and detoxification.

Efficient enzymatic degradation process for hydrolysis activity of the Carrageenan from red algae in marine biomass

Carrageenan is a generic name for a family of polysaccharides obtained from certain species of red algae. New methods to produce useful cost-efficiently materials from red algae are needed to convert enzymatic processes into fermentable sugars. In this study, we constructed chimeric genes cCgkA and cCglA containing the catalytic domain of κ-carrageenase CgkA and λ-carrageenase CglA from Pseudoalteromonas carrageenovora fused with a dockerin domain. Recombinant strains expressing the chimeric carrageenase resulted in a halo formation on the carrageenan plate by alcian blue staining. The recombinant cCgkA and cCglA were assembled with scaffoldin miniCbpA via cohesin and dockerin interaction. Carbohydrate binding module (CBM) in scaffoldin was used as a tag for cellulose affinity purification using cellulose as a support. The hydrolysis process was monitored by the amount of reducing sugar released from carrageenan. Interestingly, these results indicated that miniCbpA, cCgkA and cCglA assembled into a complex and that the dockerin-fused enzymes on the scaffoldin had synergistic activity in the degradation of carrageenan. The observed enhancement of activity by carrageenolytic complex was 3.1-fold-higher compared with the corresponding enzymes alone. Thus, the assemblies of advancement of active enzyme complexes will facilitate the commercial production of useful products from red algae biomass which represents inexpensive and sustainable feed-stocks.

Efficiency and stability enhancement of cis-epoxysuccinic acid hydrolase by fusion with a carbohydrate binding module and immobilization onto cellulose

Cis-epoxysuccinic acid hydrolase (CESH) is an enzyme that catalyzes cis-epoxysuccinic acid to produce enantiomeric L(+)-tartaric acid. The production of tartaric acid by using CESH would be valuable in the chemical industry because of its high yield and selectivity, but the low stability of CESH hampers its application. To improve the stability of CESH, we fused five different carbohydrate-binding modules (CBMs) to CESH and immobilized the chimeric enzymes on cellulose. The effects of the fusion and immobilization on the activity, kinetics, and stability of CESH were compared. Activity measurements demonstrated that the fusion with CBMs and the immobilization on cellulose increased the pH and temperature adaptability of CESH. The chimeric enzymes showed significantly different enzyme kinetics parameters, among which the immobilized CBM30-CESH exhibited twofold catalytic efficiency compared with the native CESH. The half-life measurements indicated that the stability of the enzyme in its free form was slightly increased by the fusion with CBMs, whereas the immobilization on cellulose significantly increased the stability of the enzyme. The immobilized CBM30-CESH showed the longest half-life, which is more than five times the free native CESH half-life at 30 °C. Therefore, most CBMs can improve enzymatic properties, and CBM30 is the best fusion partner for CESH to improve both its enzymatic efficiency and its stability.

Cellulose affinity purification of fusion proteins tagged with fungal family 1 cellulose-binding domain

N- or C-terminal fusions of red-fluorescent protein (RFP) with various fungal cellulose-binding domains (CBDs) belonging to carbohydrate binding module (CBM) family 1 were expressed in a Pichia pastoris expression system, and the resulting fusion proteins were used to examine the feasibility of large-scale affinity purification of CBD-tagged proteins on cellulose columns. We found that RFP fused with CBD from Trichoderma reesei CBHI (CBD(Tr)(CBHI)) was expressed at up to 1.2g/l in the culture filtrate, which could be directly injected into the cellulose column. The fusion protein was tightly adsorbed on the cellulose column in the presence of a sufficient amount of ammonium sulfate and was efficiently eluted with pure water. Bovine serum albumin (BSA) was not captured under these conditions, whereas both BSA and the fusion protein were adsorbed on a phenyl column, indicating that the cellulose column can be used for the purification of not only hydrophilic proteins but also for hydrophobic proteins. Recovery of various fusion proteins exceeded 80%. Our results indicate that protein purification by expression of a target protein as a fusion with a fungal family 1 CBD tag in a yeast expression system, followed by affinity purification on a cellulose column, is simple, effective and easily scalable.

N-terminal fusion of a hyperthermophilic chitin-binding domain to xylose isomerase from Thermotoga neapolitana enhances kinetics and thermostability of both free and immobilized enzymes

Immobilization of a thermostable D-xylose isomerase (EC 5.3.1.5) from Thermotoga neapolitana 5068 (TNXI) on chitin beads was accomplished via a N-terminal fusion with a chitin-binding domain (CBD) from a hyperthermophilic chitinase produced by Pyrococcus furiosus (PF1233) to create a fusion protein (CBD-TNXI). The turnover numbers for glucose to fructose conversion for both unbound and immobilized CBD-TNXI were greater than the wild-type enzyme: k(cat) (min(-1)) was approximately 1,000, 3,800, and 5,800 at 80 degrees C compared to 1,140, 10,350, and 7,000 at 90 degrees C, for the wild-type, unbound, and immobilized enzymes, respectively. These k(cat) values for the glucose to fructose isomerization measured are the highest reported to date for any XI at any temperature. Enzyme kinetic inactivation at 100 degrees C, as determined from a bi-phasic inactivation model, showed that the CBD-TNXI bound to chitin had a half-life approximately three times longer than the soluble wild-type TNXI (19.9 hours vs. 6.8 hours, respectively). Surprisingly, the unbound soluble CBD-TNXI had a significantly longer half-life (56.5 hours) than the immobilized enzyme. Molecular modeling results suggest that the N-terminal fusion impacted subunit interactions, thereby contributing to the enhanced thermostability of both the unbound and immobilized CBD-TNXI. These interactions likely also played a role in modifying active site structure, thereby diminishing substrate-binding affinities and generating higher turnover rates in the unbound fusion protein.

Immobilization of Lactococcus lactis to cellulosic material by cellulose-binding domain of Cellvibrio japonicus

AIMS

Immobilization of whole cells can be used to accumulate cells in a bioreactor and thus increase the cell density and potentially productivity, also. Cellulose is an excellent matrix for immobilization purposes because it does not require chemical modifications and is commercially available in many different forms at low price. The aim of this study was to construct a Lactococcus lactis strain capable of immobilizing to a cellulosic matrix.

METHODS AND RESULTS

In this study, the Usp45 signal sequence fused with the cellulose-binding domain (CBD) (112 amino acids) of XylA enzyme from Cellvibrio japonicus was fused with PrtP or AcmA anchors derived from L. lactis. A successful surface display of L. lactis cells expressing these fusion proteins under the P45 promoter was achieved and detected by whole-cell ELISA. A rapid filter paper assay was developed to study the cellulose-binding capability of these recombinant strains. As a result, an efficient immobilization to filter paper was demonstrated for the L. lactis cells expressing the CBD-fusion protein. The highest immobilization (92%) was measured for the strain expressing the CBD in fusion with the 344 amino acid PrtP anchor.

CONCLUSIONS

The result from the binding tests indicated that a new phenotype for L. lactis with cellulose-binding capability was achieved with both PrtP (LPXTG type anchor) and AcmA (LysM type anchor) fusions with CBD.

SIGNIFICANCE AND IMPACT OF THE STUDY

We demonstrated that an efficient immobilization of recombinant L. lactis cells to cellulosic matrix is possible. This is a step forward in developing efficient immobilization systems for lactococcal strains for industrial-scale fermentations.

Model cellulose films exposed to H. insolens glucoside hydrolase family 45 endo-cellulase--the effect of the carbohydrate-binding module

The effects of enzyme structure and activity on the degradation of model cellulose substrates were investigated by ellipsometry for the cellulase Humicola insolens GH45. The inactive variant D10N was found to adsorb at the cellulose surface but also to be incorporated into the cellulose films to an extent that depended on pH. For the native protein, the initial adsorption monitored for the inactive variant D10N was followed by enzyme-mediated degradation of the cellulose films. Again, a dependence on pH was found, such that higher pH resulted in slower enzymatic degradation. Removing the carbohydrate-binding module eliminated this pH dependence but also resulted in a decreased adsorption to the cellulose surface, and in a decreased net catalytic effect.

Construction and evaluation of novel fusion proteins for targeted delivery of micro particles to cellulose surfaces

The use of IgG antibodies and fragments has been limited to specific sectors of the biotechnology industry due to the high cost of producing large batches of product necessary for alternative applications. A novel class of Camelid antibodies, known as V(HH) offer a more economical opportunity to meet a wider application in industry. In this study, we report the evaluation of four llama V(HH)-cellulose binding domain fusion proteins displaying varying formats of V(HH) and CBD domains. Proteins were characterized in a targeted particle delivery system as a method of delivering agents such as perfume to laundry in the wash cycle. Fusion proteins were shown to be stable at high pH and in the presence of a detergent base. They were also shown to bind effectively to both the designated antigen, the azo-dye reactive-red 6 (either conjugated to BSA or attached to coacervate microparticles), and cellulose. Binding strength differences were observed between the different fusion protein formats using surface plasmon resonance. The effect of key laundry ingredients was also studied. Combining the fusion proteins and particles into a delivery and deposition study generated clear microscopy evidence for bifunctionality. Confirmation of this was validated by GC-MS analysis of retained fragrance. This research, reporting the construction and characterization of a variety of fusion proteins, illustrates that the single multidomain fusion protein route offers a new technology for successful targeted delivery of encapsulated benefit agents. Furthermore, the potential to modify or select for proteins to recognize a wide range of surfaces is also possible.

Development of a High-Throughput Format for Solid-Phase Extraction of Enantiomers Using an Immunosorbent in 384-Well Plates

An antibody-based solid-phase extraction method for filtered 384-well plates was developed for a medical drug candidate having two enantiomeric forms in order to demonstrate the potential of the use of recombinant antibody fragments as specific and efficient immunosorbents. An immobilization method using a six-histidine tag of the antibody fragment and mild oxidation was applied in order to immobilize antibody fragments in an oriented and kinetically stable way that ensured high capacity of the antibody support. Phosphate buffer or plasma spiked with enantiomers were used as samples. Selective solid-phase extraction was followed by liquid chromatography-mass spectrometry analysis. Average recoveries for buffer and plasma samples ranged from 79 to 122% and 80 to 108%, respectively. Good linearity was observed in the concentration range of 30-3000 ng/mL of the enantiomer.

Effects of the PT region of EngD and HLD of CbpA on solubility, catalytic activity and purification characteristics of EngD-CBDCbpA fusions from Clostridium cellulovorans

Chimeric proteins combining the catalytic N-terminal region of native EngD with its proline-threonine-threonine (PT) linker region, hydrophilic domain (HLD) and cellulose binding domain (CBD) of cellulose binding protein A (CbpA) from Clostridium cellulovorans were constructed, expressed, and analyzed. The chimeric proteins with CBD(CbpA) all demonstrated strong affinity to Avicel. The chimeric protein with the PT region of EngD and the HLD had the best catalytic activity and the highest estimated percentage of soluble protein amongst the chimeric proteins. Native EngD and two of the chimeric proteins (EngD-PT-HLD-CBD and EngD-CBD) were purified and their characteristics analyzed. Their binding affinities to Avicel as well as their enzymatic activities against various substrates were found to be consistent with the results we saw from protein lysate samples, which was good binding to Avicel but a decrease in solubility and catalytic activities in chimeric proteins without PT and/or HLD. The reasons for these are discussed. These fusion proteins may be important in applications, such as immobilization to solid cellulose substrate for purification of proteins and enrichment/aggregation of protein complexes.

Cellulose-binding domains

Many researchers have acknowledged the fact that there exists an immense potential for the application of the cellulose-binding domains (CBDs) in the field of biotechnology. This becomes apparent when the phrase "cellulose-binding domain" is used as the key word for a computerized patent search; more then 150 hits are retrieved. Cellulose is an ideal matrix for large-scale affinity purification procedures. This chemically inert matrix has excellent physical properties as well as low affinity for nonspecific protein binding. It is available in a diverse range of forms and sizes, is pharmaceutically safe, and relatively inexpensive. Present studies into the application of CBDs in industry have established that they can be applied in the modification of physical and chemical properties of composite materials and the development of modified materials with improved properties. In agro-biotechnology, CBDs can be used to modify polysaccharide materials both in vivo and in vitro. The CBDs exert nonhydrolytic fiber disruption on cellulose-containing materials. The potential applications of "CBD technology" range from modulating the architecture of individual cells to the modification of an entire organism. Expressing these genes under specific promoters and using appropriate trafficking signals, can be used to alter the nutritional value and texture of agricultural crops and their final products.

Use of recombinant cellulose-binding domains of Trichoderma reesei cellulase as a selective immunocytochemical marker for cellulose in protozoa

Some unicellular organisms are able to encyst as a protective response to a harmful environment. The cyst wall usually contains chitin as its main structural constituent, but in some cases, as in Acanthamoeba, it consists of cellulose instead. Specific cytochemical differentiation between cellulose and chitin by microscopy has not been possible, due to the similarity of their constituent beta-1,4-linked hexose backbones. Thus, various fluorescent brightening agents and lectins bind to both cellulose and chitin. We have used a recombinant cellulose-binding protein consisting of two cellulose-binding domains (CBDs) from Trichoderma reesei cellulases linked together in combination with monoclonal anticellulase antibodies and anti-mouse immunoglobulin fluorescein conjugate to specifically stain cellulose in the cysts of Acanthamoeba strains for fluorescence microscopy imaging. Staining was observed in ruptured cysts and frozen sections of cysts but not in intact mature cysts. No staining reaction was observed with the chitin-containing cyst walls of Giardia intestinalis, Entamoeba dispar, or Pneumocystis carinii. Thus, the recombinant CBD can be used as a marker to distinguish between cellulose and chitin. Thirteen of 25 environmental or clinical isolates of amoebae reacted in the CBD binding assay. All 13 isolates were identified as Acanthamoeba spp. Five isolates of Hartmannella and seven isolates of Naegleria tested negative in the CBD binding assay. Whether cyst wall cellulose really is a unique property of Acanthamoeba spp. among free-living amoebae, as suggested by our findings, remains to be shown in more extensive studies.

Specific adhesion to cellulose and hydrolysis of organophosphate nerve agents by a genetically engineered Escherichia coli strain with a surface-expressed cellulose-binding domain and organophosphorus hydrolase

A genetically engineered Escherichia coli cell expressing both organophosphorus hydrolase (OPH) and a cellulose-binding domain (CBD) on the cell surface was constructed, enabling the simultaneous hydrolysis of organophosphate nerve agents and immobilization via specific adsorption to cellulose. OPH was displayed on the cell surface by use of the truncated ice nucleation protein (INPNC) fusion system, while the CBD was surface anchored by the Lpp-OmpA fusion system. Production of both INPNC-OPH and Lpp-OmpA-CBD fusion proteins was verified by immunoblotting, and the surface localization of OPH and the CBD was confirmed by immunofluorescence microscopy. Whole-cell immobilization with the surface-anchored CBD was very specific, forming essentially a monolayer of cells on different supports, as shown by electron micrographs. Optimal levels of OPH activity and binding affinity to cellulose supports were achieved by investigating expression under different induction levels. Immobilized cells degraded paraoxon rapidly at an initial rate of 0.65 mM/min/g of cells (dry weight) and retained almost 100% efficiency over a period of 45 days. Owing to its superior degradation capacity and affinity to cellulose, this immobilized-cell system should be an attractive alternative for large-scale detoxification of organophosphate nerve agents.

Self-immobilizing recombinant antibody fragments for immunoaffinity chromatography: generic, parallel, and scalable protein purification

We present the directed immobilization of recombinant antibody fragments as ligands for general immunoaffinity chromatography methods. It is based on fusion proteins of scFv fragments with several chitin-binding domains which can be immobilized directly from a crude bacterial lysate on inexpensive chitin beads for the purification of proteins without any gradient or detector. It has been used with a positive pressure manifold, allowing the parallel processing of 24 different samples on a milligram scale, as convenient as plasmid isolation. The method is demonstrated with several anti-protein antibodies. In addition, methods are presented of using an anti-His tag antibody either alone or directly coupled to IMAC to obtain very pure protein. As those methods are scalable, they should prove very useful in the parallel purification of natural and recombinant proteins on small scales (for proteomics), medium scales (for crystallography and NMR), and very large scales (for therapeutic proteins).

Efficient enantioselective separation of drug enantiomers by immobilised antibody fragments

There is an increasing need for methods for efficient enantioselective separation and purification of chiral drugs. Genetic engineering provides the means for generating recombinant antibodies exhibiting extremely high specificity for even small molecular mass compounds. Here, recombinant antibody fragments have been generated for the drug diarylalkyltriazole that contains two chiral centres. Immobilised antibody fragments has been used successfully for efficient, step-wise separation of two enantiomers of the drug. Owing to the antibody specificity, one enantiomer came out in the flow-through, while the bound enantiomer could be specifically eluted. One of the antibodies tolerated solvents required both for dissolving the target molecules and for their elution for extended times and was shown to function over multiple cycles of the separation process.

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.

Design of a pH-dependent cellulose-binding domain

Protein-carbohydrate interactions typically rely on aromatic stacking interactions of tyrosine, phenylalanine and tryptophan side chains with the sugar rings whereas histidine residues are rarely involved. The small cellulose-binding domain of the Cel7A cellobiohydrolase (formerly CBHI) from Trichoderma reesei binds to crystalline cellulose primarily using a planar strip of three tyrosine side chains. Binding of the wild-type Cel7A CBD is practically insensitive to pH. Here we have investigated how histidine residues mediate the binding interaction and whether the protonation of a histidine side chain makes the binding sensitive to pH. Protein engineering of the Cel7A CBD was thus used to replace the tyrosine residues in two different positions with histidine residues. All of the mutants exhibited a clear pH-dependency of the binding, in clear contrast to the wild-type. Although the binding of the mutants at optimal pH was less than for the wild-type, in one case, Y31H, this binding almost reached the wild-type level.