(Photo courtesy of the University of Virginia)
Department of Molecular Physiology & Biological Physics - University of Virginia
Host: Olga Zabotina
"Mechanisms of Cellulose Biosynthesis in Bacteria and Plants'"
Under certain conditions, essentially all biopolymers, such as nucleic acids, polypeptides 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.
Expanding these analyses towards plant cellulose synthases, we demonstrated that poplar cellulose synthase-8 is sufficient for cellulose microfibril formation in vitro. Our analyses suggest that, while the mechanism of glucan biosynthesis is likely evolutionarily conserved, the plant enzymes contain features facilitating bundling of the nascent polymers into cable-like microfibrils.
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