Abstract

A metabolic isotope-labelling strategy was used in conjunction with nanoLC-ESIMS peptide sequencing to assess quantitative alterations in the expression patterns of subunits within cellulosomes of the cellulolytic bacterium Clostridium thermocellum, grown on either cellulose or cellobiose. The effects of adding xylan, pectin and galactomannan to these cultures were also explored. In total, 55 cellulosomal proteins were detected, including 50 type I dockerin-containing proteins, which count among them all but one of the known docking components and 28 new subunits. All differential expression data was normalized to scaffoldin CipA such that protein-per-cellulosome was compared for growth between the different substrates. Proteins that exhibited higher expression in cellulosomes from cellulose-grown cells as compared to cellobiose-grown cells were: cell-surface anchor protein OlpB; exoglucanases CeIS and CeIK; and GH9 endoglucanase CeIJ. Conversely, lower expression in cellulosomes from cells grown on cellulose as compared to cellobiose was observed for GH8 endoglucanase CeIA; GH5 endoglucanases Ce1B, CelE, CelG; and hemicellulases XynA, XynC, XynZ, XghA. GH9 cellulases were the most abundant group of enzymes per CipA when cells were grown on cellulose, while hemicellulases were the most abundant group on cellobiose. The results support the existing theory that expression of scaffoldin-related proteins is coordinately regulated by a catabolite repression type of mechanism, as well as the prior observation that xylanase expression is subject to a growth rate-independent type of regulation. However, concerning transcriptional control of cellulases, which had also been previously shown to be subject to catabolite repression, a novel distinction was observed with respect to endoglucanases.