Observing cellulose biosynthesis and membrane translocation in crystallo

Jacob L. W. Morgan, Joshua T. McNamara, Michael Fischer, Jamie Rich, Hong-Ming Chen, Stephen G. Withers and Jochen Zimmer ()
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Jacob L. W. Morgan: University of Virginia School of Medicine, Center for Membrane Biology, Molecular Physiology and Biological Physics
Joshua T. McNamara: University of Virginia School of Medicine, Center for Membrane Biology, Molecular Physiology and Biological Physics
Michael Fischer: University of British Columbia
Jamie Rich: University of British Columbia
Hong-Ming Chen: University of British Columbia
Stephen G. Withers: University of British Columbia
Jochen Zimmer: University of Virginia School of Medicine, Center for Membrane Biology, Molecular Physiology and Biological Physics

Nature, 2016, vol. 531, issue 7594, 329-334

Abstract: Abstract Many biopolymers, including polysaccharides, must be translocated across at least one membrane to reach their site of biological function. Cellulose is a linear glucose polymer synthesized and secreted by a membrane-integrated cellulose synthase. Here, in crystallo enzymology with the catalytically active bacterial cellulose synthase BcsA–BcsB complex reveals structural snapshots of a complete cellulose biosynthesis cycle, from substrate binding to polymer translocation. Substrate- and product-bound structures of BcsA provide the basis for substrate recognition and demonstrate the stepwise elongation of cellulose. Furthermore, the structural snapshots show that BcsA translocates cellulose via a ratcheting mechanism involving a ‘finger helix’ that contacts the polymer’s terminal glucose. Cooperating with BcsA’s gating loop, the finger helix moves ‘up’ and ‘down’ in response to substrate binding and polymer elongation, respectively, thereby pushing the elongated polymer into BcsA’s transmembrane channel. This mechanism is validated experimentally by tethering BcsA’s finger helix, which inhibits polymer translocation but not elongation.

Date: 2016
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DOI: 10.1038/nature16966

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