Abstract
The anaerobic thermophilic bacterium Clostridium thermocellum is a cellulolytic organism capable of hydrolyzing cellulose and fermenting the hydrolysis products to ethanol and other metabolic products. C. thermocellum achieves efficient cellulose hydrolysis using multiprotein extracellular enzymatic complexes, termed the cellulosomes. In this study, we used quantitative proteomics (multidimensional LC-MS/MS and 15N-metabolic labeling) to measure relative changes in levels of cellulosomal subunit proteins (per CipA scaffoldin basis) when C. thermocellum was grown on a variety of carbon sources [dilute-acid pretreated switchgrass, cellobiose, amorphous cellulose, crystalline cellulose (Avicel) and combinations of crystalline cellulose with pectin or xylan or both]. Cellulosome samples isolated from cultures grown on these carbon sources were compared to 15N labeled cellulosome samples isolated from crystalline cellulose grown cultures. In total from all samples, proteomic analysis identified 59 dockerin- and 8 cohesin-module containing components, including 15 previously undetected cellulosomal subunits. Many cellulosomal components showed differential protein abundance in the presence of non-cellulose substrates in the growth medium. Cellulosome samples from amorphous cellulose, cellobiose and pretreated switchgrass grown cultures displayed the most distinct differences in composition as compared to cellulosome samples from crystalline cellulose grown cultures. While Glycoside Hydrolase Family 9 enzymes showed increased levels in the presence of crystalline cellulose, and pretreated switchgrass in particular, GH5 enzymes showed increased levels in response to the presence of cellulose in general, amorphous or crystalline. Overall, the results suggest a coordinated substrate-specific regulation of cellulosomal composition in C. thermocellum.