Revisiting the NMR solution structure of the Cel48S type-I dockerin module from Clostridium thermocellum reveals a cohesin-primed conformation

Dockerin modules of the cellulosomal enzyme subunits play an important role in the assembly of the cellulosome by binding tenaciously to cohesin modules of the scaffoldin subunit. A previously reported NMR-derived solution structure of the type-I dockerin module from Cel48S of Clostridium thermocellum, which utilized two-dimensional homonuclear 1H-1H NOESY and three-dimensional 15N-edited NOESY distance restraints, displayed substantial conformational differences from subsequent structures of dockerin modules in complex with their cognate cohesin modules, raising the question whether the source of the observed differences resulted from cohesin-induced structural rearrangements. Here, we determined the solution structure of the Cel48S type-I dockerin based on 15N- and 13C-edited NOESY-derived distance restraints. The structure adopted a fold similar to X-ray crystal structures of dockerin modules in complex with their cohesin partners. A unique cis-peptide bond between Leu-65 and Pro-66 in the Cel48S type-I dockerin module was also identified in the present structure. Our structural analysis of the Cel48S type-I dockerin module indicates that it does not undergo appreciable cohesin-induced structural alterations but rather assumes an inherent calcium-dependent cohesin-primed conformation.