Structure of a family IIIa scaffoldin CBD from the cellulosome of Clostridium cellulolyticum at 2.2 angstrom resolution

The crystal structure of the family IIIa cellulose-binding domain (CBD) fro m the cellulosomal scaffoldin subunit (CipC) of Clostridium cellulolyticum has been determined. The structure reveals a nine-stranded jelly-roll topol ogy which exhibits distinctive structural elements consistent with family I II CBDs that bind crystalline cellulose. These include a well conserved cal cium-binding site, a putative cellulose-binding surface and a conserved sha llow groove of unknown function. The CipC CBD structure is very similar to the previously elucidated family IIIa CBD from the CipA scaffoldin of C. th ermocellum, with some minor differences. The CipC CBD structure was also co mpared with other previously described CBD structures from families IIIc an d IV derived from the endoglucanases of Thermomonospora fusca and Cellulomo nas fimi, respectively. The possible functional consequences of structural similarities and differences in the shallow groove and cellulose-binding fa ces among various CBD families and subfamilies are discussed.