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Small Angle X-ray Scattering Analysis of Clostridium thermocellum Cellulosome N-terminal Complexes Reveals a Highly Dynamic Structure*

M. Currie,K. Cameron,F. Dias,Holly L. Spencer,E. Bayer,C. Fontes,Steven P. Smith,Z. Jia

Published 2013 in Journal of Biological Chemistry

ABSTRACT

Background: The cellulosome N terminus contains the sole substrate-binding module of the cellulosome scaffoldin. Results: Two N-terminal cellulosomal fragments are devoid of intermodular interactions, are highly dynamic, and inhabit compact and elongated conformations equally. Conclusion: The characteristics of the cellulosome N terminus may facilitate its role in substrate binding. Significance: Information on cellulosome structure and dynamics aids in engineering designer cellulosomes. Clostridium thermocellum produces the prototypical cellulosome, a large multienzyme complex that efficiently hydrolyzes plant cell wall polysaccharides into fermentable sugars. This ability has garnered great interest in its potential application in biofuel production. The core non-catalytic scaffoldin subunit, CipA, bears nine type I cohesin modules that interact with the type I dockerin modules of secreted hydrolytic enzymes and promotes catalytic synergy. Because the large size and flexibility of the cellulosome preclude structural determination by traditional means, the structural basis of this synergy remains unclear. Small angle x-ray scattering has been successfully applied to the study of flexible proteins. Here, we used small angle x-ray scattering to determine the solution structure and to analyze the conformational flexibility of two overlapping N-terminal cellulosomal scaffoldin fragments comprising two type I cohesin modules and the cellulose-specific carbohydrate-binding module from CipA in complex with Cel8A cellulases. The pair distribution functions, ab initio envelopes, and rigid body models generated for these two complexes reveal extended structures. These two N-terminal cellulosomal fragments are highly dynamic and display no preference for extended or compact conformations. Overall, our work reveals structural and dynamic features of the N terminus of the CipA scaffoldin that may aid in cellulosome substrate recognition and binding.

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