Plants build tough cellulose strands one sugar at a time.
PDB entry 6wlb includes a trimer of cellulose synthase, which are arranged here based on electron micrographs of the entire rosette. Each subunit synthesizes a cellulose strand, shown here in tan.Download high quality TIFF image
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.
Reinforcing the Wall
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.
Artistic conception of a plant cell wall. The plasma membrane is shown in green, with two cellulose synthase complexes building cellulose fibers (tan). Xyloglucans and pectins are in darker brown filling in the spaces between the cellulose fibers at the top, and a microtubule is shown running horizontally just under the membrane.Download high quality TIFF image
Cellulose Synthase in Action
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.
Exploring the Structure
Bacterial Cellulose Synthase
Some bacteria also build cellulose to help them form protective biofilms. The cellulose synthase shown here (PDB entry 4hg6) is from a photosynthetic purple bacterium. Similarly to the plant enzyme, it is embedded in the cell membrane, using UDP-glucose from the cytoplasm and extruding cellulose outside the cell. This amazing structure includes a nascent strand of cellulose and a UDP molecule, capturing the complex just after a glucose has been removed from UDP-glucose and added to the growing strand. To explore this structure in more detail, click on the image for an interactive JSmol.
- 6wlb: Purushotham, P., Ho, R., Zimmer, J. (2020) Architecture of a catalytically active homotrimeric plant cellulose synthase complex. Science 369: 1089-1094
- Hofte, H., Voxeu, A. (2017) Plant cell walls. Curr. Biology 27, R865-R870
- 4hg6: Morgan, J.L., Strumillo, J., Zimmer, J. (2012) Crystallographic snapshot of cellulose synthesis and membrane translocation. Nature 493: 181-186
- 3s27: Zheng, Y., Anderson, S., Zhang, Y., Garavito, R.M. (2011) The Structure of Sucrose Synthase-1 from Arabidopsis thaliana and Its Functional Implications. J Biol Chem 286: 36108-36118
- Brett, C. T. (2010) Cellulose microfibrils in plants: synthesis, deposition and integration into the cell wall. Int. Rev. Cytology 199: 161-199.
- 2icy: McCoy, J.G., Bitto, E., Bingman, C.A., Wesenberg, G.E., Bannen, R.M., Kondrashov, D.A., Phillips Jr., G.N. (2007) Structure and Dynamics of UDP-Glucose Pyrophosphorylase from Arabidopsis thaliana with Bound UDP-Glucose and UTP. J Mol Biol 366: 830-841
- Somerville, C. (2006) Cellulose synthesis in higher plants. Annu. Rev. Cell Dev. Biol. 22:53-78
February 2021, David Goodsell