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There is now a CONTENT FREEZE for Mercury while we switch to a new platform. It began on Friday, March 10 at 6pm and will end on Wednesday, March 15 at noon. No new content can be created during this time, but all material in the system as of the beginning of the freeze will be migrated to the new platform, including users and groups. Functionally the new site is identical to the old one. webteam@gatech.edu
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Under certain conditions, essentially all biopolymers, including nucleic acids, proteins and polysaccharides, must be translocated across at least one membrane to reach their final destinations. Cellulose is a linear glucose polymer synthesized and secreted by a membrane-integrated cellulose synthase. We used in crystallo enzymology with the catalytically active bacterial cellulose synthase BcsA–B complex to obtain structural snapshots of a complete cellulose synthesis 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. Our structural snapshots reveal 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. Taken together, our structural and functional analyses reveal how processive membrane integrated glycosyltransferases couple polymer synthesis with translocation.
