Molecule of the Month: Cellulose Synthase

Cellulose is the most abundant biopolymer on the Earth. If we look around, this is really no surprise, since we are surrounded by plants, and plant cells are themselves surrounded by a cell wall composed of cellulose. Cellulose is quite simple, belying its essential function in plant cell structure. It is composed of long chains of glucose, 500 to 15,000 glucose units long. These chains then associate side-by-side to form cable-like fibrils that are strong enough, when combined together, to build a delicate flower or a majestic redwood tree.

Cellulose synthase is a large protein complex found in the plant plasma membrane that constructs cellulose strands and fibrils. The complex is a large rosette with approximate 6-fold symmetry, composed of six trimers that most likely contain three slightly different forms of the enzyme. PDB entry 6wlb includes a trimer of the enzyme from poplar trees. This structure helps to resolve a persistent mystery in cellulose biology: how many strands are bundled together to make up a cellulose fibril? The rosette structure gives an answer to this: since each subunit builds one strand in close proximity to the others, the final fibril likely includes 18 aligned strands.

Cell walls are built much like reinforced concrete. Concrete walls are built by embedding tough iron reinforcing rods within a solid, but relatively brittle, matrix of concrete. Similarly, cellulose fibrils act as reinforcing rods that are embedded in a matrix of other proteins, such as xyloglucans and pectins, that together form a tough but adjustable cell wall.

When we look closely at plant cell walls, we find that the cellulose fibrils usually have specific orientations. This orientation is controlled by microtubules inside the cells. The cellulose synthase complex includes several other proteins that interact with microtubules and guide the movement of the complex during synthesis, and the ultimate location of the cellulose fibril.

Cellulose synthase uses an activated form of glucose, with the sugar attached to a UDP nucleotide, to build cellulose. Several enzymes have the task of creating this building block. UDP-glucose pyrophosphorylase, shown here from PDB entry 2icy, connects glucose-1-phosphate and UTP. Sucrose synthase, shown here from PDB entry 3s27, builds the molecule from sucrose and UDP. This is important because sucrose is the major sugar made in leaves, which is then provided to roots, stems, and other parts of the plant for energy and construction.