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Molecular architecture and structural transitions of a Clostridium thermocellum mini-cellulosome
Authors:García-Alvarez Begoña  Melero Roberto  Dias Fernando M V  Prates José A M  Fontes Carlos M G A  Smith Steven P  Romão Maria João  Carvalho Ana Luísa  Llorca Oscar
Institution:
  • 1 Centro de Investigaciones Biológicas, Spanish National Research Council (Consejo Superior de Investigaciones Científicas), Ramiro de Maetzu 9, 28040 Madrid, Spain
  • 2 Centro Interdisciplinar de Investigação em Sanidade Animal, Faculdade de Medicina Veterinária, Universidade Técnica de Lisboa, Avenida da Universidade Técnica, 1300-477 Lisboa, Portugal
  • 3Department of Biochemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6
  • 4 REQUIMTE/CQFB, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
  • Abstract:The cellulosome is a highly elaborate cell-bound multienzyme complex that efficiently orchestrates the deconstruction of cellulose and hemicellulose, two of the nature's most abundant polymers. Understanding the intricacy of these nanomachines evolved by anaerobic microbes could sustain the development of an effective process for the conversion of lignocellulosic biomass to bio-ethanol. In Clostridium thermocellum, cellulosome assembly is mediated by high-affinity protein:protein interactions (> 109 M− 1) between dockerin modules found in the catalytic subunits and cohesin modules located in a non-catalytic protein scaffold termed CipA. Whereas the atomic structures of several cellulosomal components have been elucidated, the structural organization of the complete cellulosome remains elusive. Here, we reveal that a large fragment of the cellulosome presents a mostly compact conformation in solution, by solving the three-dimensional structure of a C. thermocellum mini-cellulosome comprising three consecutive cohesin modules, each bound to one Cel8A cellulase, at 35 Å resolution by cryo-electron microscopy. Interestingly, the three cellulosomal catalytic domains are found alternately projected outward from the CipA scaffold in opposite directions, in an arrangement that could expand the area of the substrate accessible to the catalytic domains. In addition, the cellulosome can transit from this compact conformation to a multitude of diverse and flexible structures, where the linkers between cohesin modules are extended and flexible. Thus, structural transitions controlled by changes in the degree of flexibility of linkers connecting consecutive cohesin modules could regulate the efficiency of substrate recognition and hydrolysis.
    Keywords:SAXS  small-angle X-ray scattering  3D  three-dimensional  EM  electron microscopy  2D  two-dimensional  RCT  random conical tilt  PDB  Protein Data Bank
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