Structure of novel type I cohesin-dockerin complexes from Clostridium thermocellum displaying unique ligand interacting interfaces

The elucidation of the molecular mechanisms involved in the recognition of type I cohesindockerin (Coh-Doc) interactions constitute a major aspect of the detailed comprehension of cellulosome assembly. Cellulosomes are massive cell-bound multienzyme complexes tethered by a molecular scaffold which centralize anaerobic bacteria‘s effort to hydrolyze plant cell wall polysaccharides. The cellulosome of Clostridium thermocellum constitutes a paradigm for the organization of this megadalton nanomachine. Central to cellulosome assembly processes are the type I interactions between cohesin modules, present in a noncatalytic scaffold protein, and the dockerin domains appended to the catalytic enzymes.
Dockerins display two duplicated segments, sharing a striking sequence similarity on key residues and also structural conservation as they fold into three α-helices, with helices 1 and 3 comprising the first and second duplicated segments, respectively. C. thermocellum’s xylanase 10B dockerin was shown to interact with type I cohesins both through helix 1 (N-terminal) or 3 (C-terminal) and thus displays two cohesin binding interfaces.
Here we report the structure of three recently characterized novel type I Coh-Doc complexes from C. thermocellum: OlpC-Doc435 (NovelI-M9), OlpA-Doc258 (OlpA-M11) and OlpA-Doc918 (OlpA-M10).

A comparative analysis of the structure, molecular interactions and biochemical properties of these Coh-Doc complexes was performed, highlighting differences to the canonical type I Coh-Doc. The three complexes revealed that cohesion recognition is performed uniquely by one dockerin interface. This study reveals that the dual binding mode is not universal in type I cohesin-dockerin complexes.

No relevant conflicts of interest declared.