Biochemistry

A Microbial Spinning Jenny

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Science  22 Oct 2010:
Vol. 330, Issue 6003, pp. 428
DOI: 10.1126/science.330.6003.428-b
CREDIT: MIDORI

Forests contain a lot of carbon, and much of that carbon is used to make the structural polymer cellulose. Like other biopolymers, cellulose is synthesized inside cells, yet it must be extruded across cell membranes as individual chains that are wound together, first into smaller fibrils and then into larger ones. The biological and physical properties of cellulose are remarkable, and Acetobacter xylinium is a prime bacterial source for medical and industrial applications (as well as the tasty dessert shown below). Hu et al. describe the crystal structure of subunit D of A. xylinium cellulose synthase, which is involved in the assembly and extrusion of the intact polymer. Subunit D is a toroidal tetramer of dimers, where the dimer-dimer interfaces spiral around the central axis. Soaking these crystals in cellopentaose solution revealed binding of four molecules to each subunit D octamer at the sites where the dimer interfaces open into the central pore, suggesting that the passage of four glucan chains across the membrane may be coupled to a twist that promotes fibril assembly.

Proc. Natl. Acad. Sci. U.S.A. 107, 10.1073/pnas.1000601107 (2010).

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