Abstract
Cellulosomes are highly sophisticated molecular nanomachines that participate in the deconstruction of complex polysaccharides, notably cellulose and hemicellulose. Cellulosomal assembly is orchestrated by the interaction of enzyme-borne dockerin (Doc) modules to tandem cohesin (Coh) modules of a non-catalytic primary scaffoldin. In some cases, as exemplified by the cellulosome of the major cellulolytic ruminal bacterium Ruminococcus flavefaciens, primary scaffoldins bind to adaptor scaffoldins that further interact with the cell surface via anchoring scaffoldins, thereby increasing cellulosome complexity. Here we elucidate the structure of the unique Doc of R. flavefaciens FD-1 primary scaffoldin ScaA, bound to Coh 5 of the adaptor scaffoldin ScaB. The RfCohScaB5-DocScaA complex has an elliptical architecture similar to previously described complexes from a variety of ecological niches. ScaA Doc presents a single-binding mode, analogous to that described for the other two Coh-Doc specificities required for cellulosome assembly in R. flavefaciens. The exclusive reliance on a single-mode of Coh recognition contrasts with the majority of cellulosomes from other bacterial species described to date, where Docs contain two similar Coh-binding interfaces promoting a dual-binding mode. The discrete Coh-Doc interactions observed in ruminal cellulosomes suggest an adaptation to the exquisite properties of the rumen environment.
Highlights
Between dockerin (Doc) modules, located in the cellulosomal enzymes, and cohesin (Coh) modules of the scaffoldin molecule are the primary force for cellulosomal assembly
More recently three additional structures of R. flavefaciens strain FD-1 Coh-Doc complexes, namely ScaC Coh bound to a group 3 Doc[22], a ScaA Coh bound to a group 1b Doc and a ScaB Coh bound to a group 1a Doc[28], revealed the exquisite properties of rumen cellulosomes
Previous studies have shown that the Doc module of R. flavefaciens strain FD-1primary scaffoldin ScaA (RfDocScaA) interacts exclusively with Cohs 5 to 9 of scaffoldin ScaB21,24
Summary
Between dockerin (Doc) modules, located in the cellulosomal enzymes, and cohesin (Coh) modules of the scaffoldin molecule are the primary force for cellulosomal assembly. ScaA Doc, the unique member of group 5, binds exclusively to ScaB Cohs 5–9 This latter interaction has a central role in cellulosomal assembly as it allows the binding of up to five ScaA primary scaffoldins to ScaB and up to 10 more enzymes to a single cellulosome (Fig. 1). While the first of the three is very similar to the previously described type I complexes, Coh-Doc complexes involving R. flavefaciens strain FD-1group 1 Docs do not bear much homology with any other complexes described to date These three complexes are responsible for the integration of enzymes into the primary scaffoldins, either directly or through an adaptor scaffoldin, none of them possesses a dual-binding mode as observed in other cellulosomes[22,28]. In contrast to the other known cellulosomes, this work supports the view that in R. flavefaciens cellulosome, protein assembly is the result of exclusively single-binding mode Coh-Doc interactions
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