Cellodextrin and laminaribiose ABC transporters in Clostridium thermocellum

Clostridium thermocellum is an anaerobic thermophilic bacterium that grows efficiently on cellulosic biomass. This bacterium produces and secretes a highly active multienzyme complex, the cellulosome, that mediates the cell attachment to and hydrolysis of the crystalline cellulosic substrate. C. thermocellum can efficiently utilize only β-1,3 and β-1,4 glucans and prefers long cellodextrins. Since the bacterium can also produce ethanol, it is considered an attractive candidate for a consolidated fermentation process in which cellulose hydrolysis and ethanol fermentation occur in a single process. In this study, we have identified and characterized five sugar ABC transporter systems in C. thermocellum. The putative transporters were identified by sequence homology of the putative solute-binding lipoprotein to known sugar-binding proteins. Each of these systems is transcribed from a gene cluster, which includes an extracellular solute-binding protein, one or two integral membrane proteins, and, in most cases, an ATP-binding protein. The genes of the five solute-binding proteins were cloned, fused to His tags, overexpressed, and purified, and their abilities to interact with different sugars was examined by isothermal titration calorimetry. Three of the sugar-binding lipoproteins (CbpB to -D) interacted with different lengths of cellodextrins (G₂ to G₅), with disassociation constants in the micromolar range. One protein, CbpA, binds only cellotriose (G₃), while another protein, Lbp (laminaribiose-binding protein) interacts with laminaribiose. The sugar specificity of the different binding lipoproteins is consistent with the observed substrate preference of C. thermocellum, in which cellodextrins (G₃ to G₅) are assimilated faster than cellobiose.