Designer cellulosomes consist of chimeric cohesin-bearing scaffoldins for the controlled incorporation of recombinant dockerin-containing enzymes. The largest designer cellulosome reported to date is a chimeric scaffoldin that contains 6 cohesins. This scaffoldin represented a technical limit of sorts, since adding another cohesin proved problematic, owing to resultant low expression levels, instability (cleavage) of the scaffoldin polypeptide, and limited numbers of available cohesin-dockerin specificities-the hallmark of designer cellulosomes. Nevertheless, increasing the number of enzymes integrated into designer cellulosomes is critical, in order to further enhance degradation of plant cell wall material. Adaptor scaffoldins comprise an intermediate type of scaffoldin that can both incorporate various enzymes and attach to an additional scaffoldin. Using this strategy, we constructed an efficient form of adaptor scaffoldin that possesses three type I cohesins for enzyme integration, a single type II dockerin for interaction with an additional scaffoldin, and a carbohydrate-binding module for targeting to the cellulosic substrate. In parallel, we designed a hexavalent scaffoldin capable of connecting to the adaptor scaffoldin by the incorporation of an appropriate type II cohesin. The resultant extended designer cellulosome comprised 8 recombinant enzymes-4 xylanases and 4 cellulases-thereby representing a potent enzymatic cocktail for solubilization of natural lignocellulosic substrates. The contribution of the adaptor scaffoldin clearly demonstrated that proximity between the two scaffoldins and their composite set of enzymes is crucial for optimized degradation. After 72 h of incubation, the performance of the extended designer cellulosome was determined to be approximately 70% compared to that of native cellulosomes.
Importance: Plant cell wall residues represent a major source of renewable biomass for the production of biofuels such as ethanol via breakdown to soluble sugars. The natural microbial degradation process, however, is inefficient for achieving cost-effective processes in the conversion of plant-derived biomass to biofuels, either from dedicated crops or human-generated cellulosic wastes. The accumulation of the latter is considered a major environmental pollutant. The development of designer cellulosome nanodevices for enhanced plant cell wall degradation thus has major impacts in the fields of environmental pollution, bioenergy production, and biotechnology in general. The findings reported in this article comprise a true breakthrough in our capacity to produce extended designer cellulosomes via synthetic biology means, thus enabling the assembly of higher-order complexes that can supersede the number of enzymes included in a single multienzyme complex.
Copyright © 2016 Stern et al.