Crystal structure of a cohesin module from Clostridium cellulolyticum: Implications for dockerin recognition

In the assembly of the Clostridium cellulolyticum cellulosome, the multiple cohesin modules of the scaffolding protein CipC serve as receptors for cel lulolytic enzymes which bear a dockerin module. The X-ray structure of a ty pe I C. cellulolyticum cohesin module (Cc-cohesin) has been solved using mo lecular replacement, and refined at 2.0 Angstrom resolution. Despite a rath er low sequence identity of 32%, this module has a fold close to those of t he two Clostridium thermocellum cohesin (Ct-cohesin) modules whose 3D struc tures have been determined previously. Cc-cohesin forms a dimer in the crys tal, as do the two Ct-cohesins. We show here that the dimer exists in solut ion and that addition of dockerin-containing proteins dissociates the dimer . This suggests that the dimerization interface and the cohesin/dockerin in terface may overlap. The nature of the overall surface and of the dimer int erface of Cc-cohesin differ notably from those of the Ct-cohesin modules, b eing much less polar, and this may explain the species specificity observed in the cohesin/dockerin interaction of C. cellulolyticum and C. thermocell um. We have produced a topology model of a C. cellulolyticum dockerin and o f a Cc-cohesin/dockerin complex using homology modeling and available bioch emical data. Our model suggests that a special residue pair, already identi fied in dockerin sequences, is located at the center of the cohesin surface putatively interacting with the dockerin. (C) 2000 Academic Press.