Proteomic identification of CBM37-containing cellulases produced by the rumen cellulolytic bacterium Ruminococcus albus 20 and their putative involvement in bacterial adhesion to cellulose

The objective of this study was to identify and characterize other proteins than fimbrial proteins potentially involved in R. albus 20 adhesion to cellulose using an adhesion-related antiserum preparation (i.e. anti-Adh serum). From protein fractions of R. albus 20 grown on cellulose, the serum recognized at least 10 cellulose-binding proteins (CBPs), among which homologs of glycoside hydrolases (family 5, 9 and 48) of R. albus 8 (i.e. Cel5G, Cel9B and Cel48A) were identified by a proteomic approach. In strain 20, Cel9B and Cel48A were identified as two major CBPs and as bacterial cell-associated proteins. The anti-Adh serum was also shown to target the C-terminal family 37 carbohydrate-binding module (CBM37) of Cel9B and Cel48A, indicating that this module, unique to R. albus, may play a significant role in bacterial adhesion to cellulose as suggested previously for R. albus 8. Overall, our results support the hypothesis of an adhesion mechanism involving the CBM37 of Cel9B and Cel48A. This adhesion mechanism may not be restricted to these two enzymes but may also involve other CBM37-containing proteins such as Cel5G and the other uncharacterised proteins recognized by the anti-Adh serum.