2-01: Lignocellulose degradation mediated by novel binding proteins from the extremely thermophilic genus Caldicellulosiruptor

Consolidated bio-processing of second generation biofuels from lignocellulose will require that the microbe(s) be able to deconstruct complex polysaccharides from plant biomass.  The extremely thermophilic genus Caldicellulosiruptor has come under renewed interest due to its plant biomass-degrading abilities at high temperatures (T_opt, 70~80^°C). Degradation of plant biomass by this genus uses an alternative to the cellulosomal paradigm, involving freely-secreted modular, multifunctional glycoside hydrolases. While all Caldicellulosiruptor species are capable of growing on plant polysaccharides such as xylan and acid-pretreated plant biomass, growth on insoluble cellulose (Avicel) is variable. Potential genetic determinants associated with the range of growth on Avicel within the genus were previously inferred using draft genome sequences (Blumer-Schuette et al., 2010). Recently, comparative proteo-genomics of eight Caldicellulosiruptor species highlighted additional mechanisms beyond enzymes that appear to be involved in the process of adhering to and degrading insoluble cellulose (Blumer-Schuette et al., 2012). Of note are novel Caldicellulosiruptor adhesins that are located directly downstream of a type IV pilus operon and upstream of the glucan degradation locus. All cellulolytic Caldicellulosiruptor species possess two adhesins; however one particular orthologous group was only detected as being expressed by the strongly cellulolytic species. Both classes of adhesins have been produced recombinantly to confirm and quantify their affinity for cellulose and plant biomass. Taking into account the observed enrichment of non-catalytic proteins on cellulose, we propose that the mechanism by which Caldicellulosiruptor species deconstruct crystalline cellulose is more complex than previously thought and involves attachment to the substrate through numerous cellulose-binding proteins.