Ruminococcus flavefaciens, a cellulolytic bacterium found in the rumen of herbivores plays an important role in cellulose degradation. It produces one of the most elaborate cellulosome systems thus far described in nature, with an unprecedented number of genes encoding cellulosomal proteins. Cellullosome is composed of numerous kinds of cellulases and related enzymes bearing a dockerin module which are assembled by a high-affinity interaction to a scaffoldin subunit consisting of multiple repeats of cohesin modules. The scaffoldins associate with the cell surface and with carbohydrate substrates.
The R. flavefaciens cellulosome is attached to the bacterial cell wall via a covalently bound cohesin of ScaE and several alternative dockerins of various proteins. Additionally to ScaE, these dockerins bind a single cohesin protein, COHG. The binding affinity of COHG to the dockerin partners is relatively weak, compared to that of ScaE. The X-ray structures of ScaE and COHG cohesins were determined, exhibiting to be the most complex cohesin structures known up to date, decorated by several alpha-helixes. The structures of the cohesins with Xdoc(Ctta) were also determined. The different hill shaped topographical features on the surface of these cohesins, appear to direct the dockerin binding orientation, in dual binding mode. The dockerin orientation in the COHG complex is rotated 180o with respect to that in the ScaE complex with the same dockerin.
Taking into consideration the relatively lower binding affinity of COHG to various dockerin domains, we suggest that cellulosomal components may transiently bind to COHG, before they are assembled properly into the cellulosome. While COHG is attached to the dockerin from one side, the second binding site of this dockerin is still available to bind ScaE. The COHG is then released and may recycle in the process of cellulosome assembly, thus, protecting the dockerins from their typical tendency to unfold.
