Observing cellulose biosynthesis and membrane translocation in crystallo

• Published in: Nature (London) Vol. 531; no. 7594; pp. 329 - 334
• Main Authors: Morgan, Jacob L. W., McNamara, Joshua T., Fischer, Michael, Rich, Jamie, Chen, Hong-Ming, Withers, Stephen G., Zimmer, Jochen
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 17.03.2016; Nature Publishing Group
• Subjects: 631/45/221; 631/535/1266; Analysis; Bacteria; Bacteriology; BASIC BIOLOGICAL SCIENCES; Binding sites; Biopolymers; Biosynthesis; Catalysis; Cellulose; Cellulose - biosynthesis; Cellulose - chemistry; Cellulose - metabolism; Crystallography, X-Ray; Crystals; Glucose - metabolism; Glucosyltransferases - chemistry; Glucosyltransferases - metabolism; Glycobiology; Humanities and Social Sciences; Intracellular Membranes - chemistry; Intracellular Membranes - metabolism; Membranes; Models, Molecular; Movement; multidisciplinary; Physiological aspects; Polymers; Protein Structure, Secondary; Proteolipids - chemistry; Proteolipids - metabolism; Rhodobacter sphaeroides - enzymology; Saccharides; Science; Substrate Specificity; Substrates; Translocation; Translocation (Genetics); X-ray crystallography; United States
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature16966

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. Here the authors use in crystallo enzymology to obtain structural snapshots of a complete cellulose biosynthesis cycle and reveal the mechanism by which the bacterial cellulose synthase BcsA–BcsB translocates the nascent cellulose polymer. Scenes from cellulose biosynthesis Cellulose is a long, linear polysaccharide made from D-glucose molecules. It is an important component of plant cell walls and a starting material for the production of many potential biofuels. These authors have used in crystallo enzymology to obtain structural snapshots of several steps in the cellulose biosynthesis cycle, including structures of the substrate-bound and product-bound states of the bacterial cellulose synthase BcsA–BcsB complex. And by incubating cellulose-bound BcsA–BcsB with UDP-glucose, they demonstrate polymer elongatation in the crystal. The structures suggest that cellulose is translocated via a ratcheting mechanism involving the up/down movement of a key 'finger helix'.