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

The binding specificity and affinity determinants of family 1 and family 3 cellulose binding modules

Cellulose binding modules (CBMs) potentiate the action of cellulolytic enzymes on insoluble substrates. Numerous studies have established that three aromatic residues on a CBM surface are needed for binding onto cellulose crystals and that tryptophans contribute to higher binding affinity than tyrosines. However, studies addressing the nature of CBM-cellulose interactions have so far failed to establish the binding site on cellulose crystals targeted by CBMs. In this study, the binding sites of CBMs on Valonia cellulose crystals have been visualized by transmission electron microscopy. Fusion of the CBMs with a modified staphylococcal protein A (ZZ-domain) allowed direct immuno-gold labeling at close proximity of the actual CBM binding site. The transmission electron microscopy images provide unequivocal evidence that the fungal family 1 CBMs as well as the family 3 CBM from Clostridium thermocellum CipA have defined binding sites on two opposite corners of Valonia cellulose crystals. In most samples these corners are worn to display significant area of the hydrophobic (110) plane, which thus constitutes the binding site for these CBMs.

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.

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.

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.

Cellulose-binding domains promote hydrolysis of different sites on crystalline cellulose

The cohesin-dockerin interaction in Clostridium thermocellum cellulosome mediates the tight binding of cellulolytic enzymes to the cellulosome-integrating protein CipA. Here, this interaction was used to study the effect of different cellulose-binding domains (CBDs) on the enzymatic activity of C. thermocellum endoglucanase CelD (1,4-beta-d endoglucanase, EC) toward various cellulosic substrates. The seventh cohesin domain of CipA was fused to CBDs originating from the Trichoderma reesei cellobiohydrolases I and II (CBD(CBH1) and CBD(CBH2)) (1,4-beta-d glucan-cellobiohydrolase, EC), from the Cellulomonas fimi xylanase/exoglucanase Cex (CBD(Cex)) (beta-1,4-d glucanase, EC), and from C. thermocellum CipA (CBD(CipA)). The CBD-cohesin hybrids interacted with the dockerin domain of CelD, leading to the formation of CelD-CBD complexes. Each of the CBDs increased the fraction of cellulose accessible to hydrolysis by CelD in the order CBD(CBH1) < CBD(CBH2) approximately CBD(Cex) < CBD(CipA). In all cases, the extent of hydrolysis was limited by the disappearance of sites accessible to CelD. Addition of a batch of fresh cellulose after completion of the reaction resulted in a new burst of activity, proving the reversible binding of the intact complexes despite the apparent binding irreversibility of some CBDs. Furthermore, burst of activity also was observed upon adding new batches of CelD-CBD complexes that contained a CBD differing from the first one. This complementation between different CBDs suggests that the sites made available for hydrolysis by each of the CBDs are at least partially nonoverlapping. The only exception was CBD(CipA), whose sites appeared to overlap all of the other sites.

Biochemical characterization of MI-ENG1, a family 5 endoglucanase secreted by the root-knot nematode Meloidogyne incognita

A beta-1,4-endoglucanase named MI-ENG1, homologous to the family 5 glycoside hydrolases, was previously isolated from the plant parasitic root-knot nematode Meloidogyne incognita. We describe here the detection of the enzyme in the nematode homogenate and secretion and its complete biochemical characterization. This study is the first comparison of the enzymatic properties of an animal glycoside hydrolase with plant and microbial enzymes. MI-ENG1 shares many enzymatic properties with known endoglucanases from plants, free-living or rumen-associated microorganisms and phytopathogens. In spite of the presence of a cellulose-binding domain at the C-terminus, the ability of MI-ENG1 to bind cellulose could not be demonstrated, whatever the experimental conditions used. The biochemical characterization of the enzyme is a first step towards the understanding of the molecular events taking place during the plant-nematode interaction.

Dynamic interaction of Trichoderma reesei cellobiohydrolases Cel6A and Cel7A and cellulose at equilibrium and during hydrolysis

The binding of cellobiohydrolases to cellulose is a crucial initial step in cellulose hydrolysis. In the search for a detailed understanding of the function of cellobiohydrolases, much information concerning how the enzymes and their constituent catalytic and cellulose-binding domains interact with cellulose and with each other and how binding changes during hydrolysis is still needed. In this study we used tritium labeling by reductive methylation to monitor binding of the two Trichoderma reesei cellobiohydrolases, Cel6A and Cel7A (formerly CBHII and CBHI), and their catalytic domains. Measuring hydrolysis by high-performance liquid chromatography and measuring binding by scintillation counting allowed us to correlate activity and binding as a function of the extent of degradation. These experiments showed that the density of bound protein increased with both Cel6A and Cel7A as hydrolysis proceeded, in such a way that the adsorption points moved off the initial binding isotherms. We also compared the affinities of the cellulose-binding domains and the catalytic domains to the affinities of the intact proteins and found that in each case the affinity of the enzyme was determined by the linkage between the catalytic and cellulose-binding domains. Desorption of Cel6A by dilution of the sample showed hysteresis (60 to 70% reversible); in contrast, desorption of Cel7A did not show hysteresis and was more than 90% reversible. These findings showed that the two enzymes differ with respect to the reversibility of binding.