Clostridium thermocellum is an anaerobic bacterium which is known to bind plant cell walls mainly by its surface cellulosome. Alternative binding mechanisms may be present.
In this work, we analyzed a novel pivotal 300 aa surface protein (product of Cthe_0271), containing a carboxy-terminal family 3 carbohydrate binding module (CBM3). We have heterologously expressed in Escherichia coli and purified the CBM3 of Cthe_0271 (CBM3-0271). It was found that the protein does not bind to cellulose despite its high expression in C. thermocellum grown on cellulose or cellobiose (Riederer et al., 2011). The protein CBM3-0271 binds to at least two different kinds of xylan (from oat spelt or birch wood) and to fiber of alfalfa cell walls, banana fruit stem and wheat straw.
The CBM is an extremely soluble protein (>200 mg/ml) which we were able to successfully crystalize and solve its 3D structure by X-ray diffraction. Unlike most of the solved CBM3 modules, CBM3-0271 lacks the typical planar surface and the cellulose binding aromatic amino acids strip. In order to understand the mode of binding to hemicellulose, we are currently analyzing crystals that were obtained in the presence of xylose or xylobiose. We are also performing comparative work with the CBM3-0271 homolog from C. clariflavum with encouraging results.
We postulated that in spite of the fact that the above clostridia do not grow on hemicellulose alone as a substrate, the subject surface CBM protein may be involved in the initial attachment to the plant cell wall.
