Akihiko Kosugi

Cellulosomes: plant-cell-wall-degrading enzyme complexes

Cellulose, the main structural component of plant cell walls, is the most abundant carbohydrate polymer in nature. Although abundant, it is extremely difficult to degrade, as it is insoluble and is present as hydrogen-bonded crystalline fibres. Anaerobic microorganisms have evolved a system to break down plant cell walls that involves the formation of a large extracellular enzyme complex called the cellulosome, which consists of a scaffolding protein and many bound cellulases. Cellulosomes have many potential biotechnological applications as the conversion of cellulosic biomass into sugars by cellulosomes could result in the production of high-value products such as ethanol or organic acids from inexpensive renewable resources. Rapid advances in cellulosome research are providing basic information for the development of both in vitro and in vivo systems to achieve such goals.

Cellulosomes from Mesophilic Bacteria

Plant cell wall-degrading enzymes have become increasingly important, since the development of efficient biomass degradation methods and the conversion of sugars to valuable products such as butanol and amino acids and utilizable forms of energy such as ethanol and methane could lead to less dependence on imported petroleum as a fuel and chemical source. Plant biomass is an abundant renewable resource. Since cellulose and hemicellulose comprise about 40 to 50% of plant cell walls and are considered to be the largest components of the earths biomass, efficient conversion of this material by engineered enzymes and/or microorganisms would be highly desirable. The rate-limiting step in biomass degradation is the conversion of the cellulose and hemicellulose polymers to sugars.

Thermostabilization of cellulosomal endoglucanase EngB from Clostridium cellulovorans by in vitro DNA recombination with non-cellulosomal endoglucanase EngD

Enhancement of enzyme thermostability by protein engineering gives us information about the thermostabilization mechanism as well as advantages for industrial use of enzymes. In this study, we enhanced the thermostability of endoglucanase EngB, one component of the cellulase complex (cellulosome) from Clostridium cellulovorans, by the directed evolution technique. The library was constructed by in vitro recombination of the genes for EngB and non‐cellulosomal cellulase EngD, based on the fact that the catalytic domains of both cellulases were highly homologous. To obtain thermostable clones without loss of activity, the library was screened by a com‐bination of activity and thermostability screening. We obtained three mutants out of 8000 selected clones that showed significantly higher thermostability than those of EngB and EngD without compromising their endoglucanase activities. One of the mutants possessed a sevenfold higher thermostability than EngB. The possible mechanisms of thermostabilization are discussed.

Hydrophilic Domains of Scaffolding Protein CbpA Promote Glycosyl Hydrolase Activity and Localization of Cellulosomes to the Cell Surface of Clostridium cellulovorans

CbpA, the scaffolding protein of Clostridium cellulovorans cellulosomes, possesses one family 3 cellulose binding domain, nine cohesin domains, and four hydrophilic domains (HLDs). Among the three types of domains, the function of the HLDs is still unknown. We proposed previously that the HLDs of CbpA play a role in attaching the cellulosome to the cell surface, since they showed some homology to the surface layer homology domains of EngE. Several recombinant proteins with HLDs (rHLDs) and recombinant EngE (rEngE) were examined to determine their binding to the C. cellulovorans cell wall fraction. Tandemly linked rHLDs showed higher affinity for the cell wall than individual rHLDs showed. EngE was shown to have a higher affinity for cell walls than rHLDs have. C. cellulovorans native cellulosomes were found to have higher affinity for cell walls than rHLDs have. When immunoblot analysis was carried out with the native cellulosome fraction bound to cell wall fragments, the presence of EngE was also confirmed, suggesting that the mechanism anchoring CbpA to the C. cellulovorans cell surface was mediated through EngE and that the HLDs play a secondary role in the attachment of the cellulosome to the cell surface. During a study of the role of HLDs on cellulose degradation, the mini-cellulosome complexes with HLDs degraded cellulose more efficiently than complexes without HLDs degraded cellulose. The rHLDs also showed binding affinity for crystalline cellulose and carboxymethyl cellulose. These results suggest that the CbpA HLDs play a major role and a minor role in C. cellulovorans cellulosomes. The primary role increases cellulose degradation activity by binding the cellulosome complex to the cellulose substrate; secondarily, HLDs aid the binding of the CbpA/cellulosome to the C. cellulovorans cell surface.

Solubilization of cellulosomal cellulases by fusion with cellulose-binding domain of noncellulosomal cellulase engd from Clostridium cellulovorans.

Clostridium cellulovorans produces a cellulase complex (cellulosome) as well as noncellulosomal cellulases. In this study, we determined a factor that affected the solubility of the cellulosomal cellulase EngB and the noncellulosomal EngD when they were expressed in Escherichia coli. The catalytic domains of EngB and EngD formed inclusion bodies when expressed in E. coli. On the other hand, both catalytic domains containing the C‐terminal cellulose‐binding domain (CBD) of EngD were expressed in soluble form. Fusion with the CBD of EngD also helped increased the solubility of cellulosomal cellulase EngL upon expression in E. coli. These results indicate that the CBD of EngD plays an important role in the soluble expression of the catalytic domains of EngB, EngL, and EngD. The possible mechanisms of solubilization by fusion of the catalytic domain with the CBD from EngD are discussed. Proteins 2003;50:620–628.

Formation of Protoplasts from Cultured Tobacco Cells and Arabidopsis thaliana by the Action of Cellulosomes and Pectate Lyase from Clostridium cellulovorans

ABSTRACT The crude culture supernatants from Clostridium cellulovorans were tested for their ability to convert plant cells to protoplasts. The supernatants readily released protoplasts from cultured tobacco cells and Arabidopsis thaliana. The crude culture supernatant from pectin-grown cells was more active than supernatants from glucose-, cellobiose-, xylan-, and locust bean gum-grown cells. After removal of cellulosomes, the crude culture supernatant lost its protoplast formation activity. The protoplast formation activity of the crude culture supernatant from C. cellulovorans was more effective than those of commercial enzymes based on protein content.

Site-Directed Mutagenesis and Expression of the Soluble Form of the Family IIIa Cellulose Binding Domain from the Cellulosomal Scaffolding Protein of Clostridium cellulovorans

The planar and anchoring residues of the family IIIa cellulose binding domain (CBD) from the cellulosomal scaffolding protein of Clostridium cellulovorans were investigated by site-directed mutagenesis and cellulose binding studies. By fusion with maltose binding protein, the family IIIa recombinant wild-type and mutant CBDs from C. cellulovorans were expressed as soluble forms. Cellulose binding tests of the mutant CBDs indicated that the planar strip residues played a major role in cellulose binding and that the anchoring residues played only a minor role.