Enzymatic diversity of the Clostridium thermocellum cellulosome is crucial for the degradation of crystalline cellulose and plant biomass.

The cellulosome is a supramolecular multienzyme complex comprised of a wide variety of polysaccharide-degrading enzymes and scaffold proteins. The cellulosomal enzymes that bind to the scaffold proteins synergistically degrade crystalline cellulose. Here, we report in vitro reconstitution of the Clostridium thermocellum cellulosome from 40 cellulosomal components and the full-length scaffoldin protein that binds to nine enzyme molecules. These components were each synthesized using a wheat germ cell-free protein synthesis system and purified. Cellulosome complexes were reconstituted from 3, 12, 30, and 40 components based on their contents in the native cellulosome. The activity of the enzyme-saturated complex indicated that greater enzymatic variety generated more synergy for the degradation of crystalline cellulose and delignified rice straw. Surprisingly, a less complete enzyme complex displaying fewer than nine enzyme molecules was more efficient for the degradation of delignified rice straw than the enzyme-saturated complex, despite the fact that the enzyme-saturated complex exhibited maximum synergy for the degradation of crystalline cellulose. These results suggest that greater enzymatic diversity of the cellulosome is crucial for the degradation of crystalline cellulose and plant biomass, and that efficient degradation of different substrates by the cellulosome requires not only a different enzymatic composition, but also different cellulosome structures.

Stoichiometric Assembly of the Cellulosome Generates Maximum Synergy for the Degradation of Crystalline Cellulose, as Revealed by In Vitro Reconstitution of the Clostridium thermocellum Cellulosome.

The cellulosome is a supramolecular multienzyme complex formed by species-specific interactions between the cohesin modules of scaffoldin proteins and the dockerin modules of a wide variety of polysaccharide-degrading enzymes. Cellulosomal enzymes bound to the scaffoldin protein act synergistically to degrade crystalline cellulose. However, there have been few attempts to reconstitute intact cellulosomes due to the difficulty of heterologously expressing full-length scaffoldin proteins. We describe the synthesis of a full-length scaffoldin protein containing nine cohesin modules, CipA; its deletion derivative containing two cohesin modules, ΔCipA; and three major cellulosomal cellulases, Cel48S, Cel8A, and Cel9K, of the Clostridium thermocellum cellulosome. The proteins were synthesized using a wheat germ cell-free protein synthesis system, and the purified proteins were used to reconstitute cellulosomes. Analysis of the cellulosome assembly using size exclusion chromatography suggested that the dockerin module of the enzymes stoichiometrically bound to the cohesin modules of the scaffoldin protein. The activity profile of the reconstituted cellulosomes indicated that cellulosomes assembled at a CipA/enzyme molar ratio of 1/9 (cohesin/dockerin = 1/1) and showed maximum synergy (4-fold synergy) for the degradation of crystalline substrate and ∼2.4-fold-higher synergy for its degradation than minicellulosomes assembled at a ΔCipA/enzyme molar ratio of 1/2 (cohesin/dockerin = 1/1). These results suggest that the binding of more enzyme molecules on a single scaffoldin protein results in higher synergy for the degradation of crystalline cellulose and that the stoichiometric assembly of the cellulosome, without excess or insufficient enzyme, is crucial for generating maximum synergy for the degradation of crystalline cellulose.

Cell-free protein synthesis and substrate specificity of full-length endoglucanase CelJ (Cel9D-Cel44A), the largest multi-enzyme subunit of the Clostridium thermocellum cellulosome.

Endoglucanase CelJ (Cel9D-Cel44A) is the largest multi-enzyme subunit of the Clostridium thermocellum cellulosome and is composed of glycoside hydrolase (GH) families 9 and 44 (GH9 and GH44) and carbohydrate-binding module (CBM) families 30 and 44 (CBM30 and CBM44). The study of CelJ has been hampered by the inability to isolate full-length CelJ from recombinant Escherichia coli cells. Here, full-length CelJ and its N- and C-terminal segments, CBM30-GH9 (Cel9D) and GH44-CBM44 (Cel44A), were synthesized using a wheat germ cell-free protein synthesis system and then were purified to homogeneity. Analysis of the substrate specificities of CelJ and its derivatives demonstrated that the fusion of Cel9D and Cel44A results in threefold synergy for the degradation of xyloglucan, one of the major structural polysaccharides of plant cell walls. Because CelJ displayed broad substrate specificity including significant carboxymethylcellulase (CMCase) and xylanase activities in addition to high xyloglucanase activity, CelJ may play an important role in the degradation of plant cell walls, which are composed of highly heterogeneous polysaccharides. Furthermore, because Cel9D, but not Cel44A, acts as a semi-processive endoglucanase, the different modes of action between Cel9D and Cel44A may be responsible for the observed synergistic effect on the activity of CelJ (Cel9D-Cel44A).