Crystallographic snapshot of cellulose synthesis and membrane translocation

Cellulose, the most abundant biological macromolecule, is an extracellular, linear polymer of glucose molecules. It represents an essential component of plant cell walls but is also found in algae and bacteria. In bacteria, cellulose production frequently correlates with the formation of biofilms, a sessile, multicellular growth form. Cellulose synthesis and transport across the inner bacterial membrane is mediated by a complex of the membrane-integrated catalytic BcsA subunit and the membrane-anchored, periplasmic BcsB protein. Here we present the crystal structure of a complex of BcsA and BcsB from Rhodobacter sphaeroides containing a translocating polysaccharide. The structure of the BcsA–BcsB translocation intermediate reveals the architecture of the cellulose synthase, demonstrates how BcsA forms a cellulose-conducting channel, and suggests a model for the coupling of cellulose synthesis and translocation in which the nascent polysaccharide is extended by one glucose molecule at a time. An X-ray crystal structure of the bacterial cellulose synthase captures the process of cellulose synthesis and membrane translocation; the structure indicates how the synthesis of cellulose and the translocation of the nascent polysaccharide chain across the cell membrane are coupled. Cellulose, a linear polysaccharide made from D-glucose molecules, is an important component of plant cell walls and a starting material for the production of many potential biofuels. In this manuscript, the authors solve the X-ray crystal structure of proteins that catalyse the synthesis of this biopolymer and facilitate its export from the cell. The structure of a complex between catalytic BcsA protein and the periplasmic membrane-anchored BcsB protein from the photosynthetic bacterium Rhodobacter sphaeroides suggests a mechanism for the coupling of cellulose synthesis and membrane transport.