EndB, a multidomain family 44 cellulase from Ruminococcus flavefaciens 17,binds to cellulose via a novel cellulose-binding module and to another R-flavefaciens protein via a dockerin domain

The mechanisms by which cellulolytic enzymes and enzyme complexes in Rumino coccus spp. bind to cellulose are not fully understood. The product of the newly isolated cellulase gene endB from Runtinococcus flavefaciens 17 was p urified as a His-tagged product after expression in Escherichia coli and fo und to be able to bind directly to crystalline cellulose. The ability to bi nd cellulose is shown to be associated with a novel cellulose-binding modul e (CBM) located within a region of 200 amino acids that is unrelated to kno wn protein sequences. EndB (808 amino acids) also contains a catalytic doma in belonging to glycoside hydrolase family 44 and a C-terminal dockerin-lik e domain. Purified EndB is also shown to bind specifically via its dockerin domain to a polypeptide of ca. 130 kDa present among supernatant proteins from Avicel-grown R. flavefaciens that attach to cellulose. The protein to which EndB attaches is a strong candidate for the scaffolding component of a cellulosome-like multienzyme complex recently identified in this species (S.-Y. Ding et al., J. Bacteriol. 183:1945-1953, 2001). It is concluded tha t binding of EndB to cellulose may occur both through its own CBM and poten tially also through its involvement in a cellulosome complex.