Cell Wall Biosynthesis
The cell wall is a defining feature of plants, affecting all aspects of their morphology, growth, and development. Cell walls also impact human society in many ways, as food, fuel, and fiber. However, despite impressive advances in our understanding of the structure of plant cell walls, much remains to be learned about the biosynthesis and metabolism of their component parts.
Cell walls are a complex composite of polysaccharides, proteins, and lignin. The polysaccharide components consist of three broad categories-pectins, hemicelluloses, and cellulose. Whereas cellulose is the major polymer found in all plant cell walls, the hemicelluloses are chemically more complex, undergo dramatic changes during growth and development, and vary greatly between cell types and between plant species.
A full understanding of the biosynthesis of the cell wall remains a major research objective in plant biology. Biochemical approaches to identification of the enzymes and genes involved have been hindered by the lability of the enzymes and our ignorance of the underlying biosynthetic mechanisms. Glycan synthases are enzymes that link together sugars to make up polysaccharide backbones. Little is known about the glycan synthases involved in hemicellulose biosynthesis other than that they are located in the Golgi. None of the glycan synthases involved in plant cell wall biosynthesis have been purified or characterized in detail and, until recently, none of the genes encoding these enzymes had been identified. Glycosyltransferases are enzymes that add sugars to the polysaccharide backbones that are products of the glycan synthases. Glycosyltransferases are usually type II integral membrane proteins containing a short amino-terminal domain facing the cytoplasm, a single transmembrane domain, and a hydrophilic carboxy terminal domain containing the active site within the lumen of the Golgi apparatus. The genes encoding several glycosyltransferases have been identified in the past few years. Our group is using new tools of genomics and proteomics to identify genes and enzymes involved in the biosynthesis and processing of hemicelluloses of plant cell walls. Because of their different wall composition and complementary advantages as experimental systems, we are studying hemicellulose biosynthesis in representative dicotyledonous plants (Arabidopsis, nasturtium, and cotton) and cereals (maize and rice). Our approaches are biochemical, proteomic, and genomic, and exploit the resources made available by the recently complete or near-complete sequencing of the genomes of Arabidopsis, rice, and maize.
SELECTED PUBLICATIONS
Faik A, Bar-Peled M, DeRocher AE, Zeng W, Perrin RM, Wilkerson C, Raikhel NV, Keegstra K (2000) Biochemical characterization and molecular cloning of an a-1,2-fucosyltransferase that catalyzes the last step of cell wall xyloglucan biosynthesis in pea. J Biol Chem 275: 15082-15089
Faik A, Price NJ, Raikhel NV, Keegstra K (2002) An Arabidopsis gene encoding an a-xylosyltransferase involved in xyloglucan biosynthesis. Proc Natl Acad Sci USA 99: 7797-7802
Keegstra K, Raikhel N (2001) Plant glycosyltransferases. Curr Opin Plant Biol 4: 219-224 Perrin RM, DeRocher AE, Bar-Peled M, Zeng W, Norambuena L, Orellana A, Raikhel NV, Keegstra K (1999) Xyloglucan fucosyltransferase, an enzyme involved in plant cell wall biosynthesis. Science 284: 1976-1979
Perrin R, Wilkerson C, Keegstra K (2001) Golgi enzymes that synthesize plant cell wall polysaccharides: Finding and evaluating candidates in the genomic era. Plant Mol Biol 47: 115-130
Perrin RM, Jia Z, Wagner TA, O'Neill MA, Sarria R, York WS, Raikhel NV, Keegstra K (2003) Analysis of xyloglucan fucosylation in Arabidopsis. Plant Physiol 132: 768-778 Sarria R, Wagner TA, O'Neill MA, Faik A, Wilkerson CG, Keegstra K, Raikhel NV (2001) Characterization of a family of Arabidopsis genes related to xyloglucan fucosyltransferase1. Plant Physiol 127: 1595-1606
Vanzin GF, Madson M, Carpita NC, Raikhel NV, Keegstra K, Reiter W-D (2002) The mur2 mutant of Arabidopsis thaliana lacks fucosylated xyloglucan due to a lesion in fucosyltransferase AtFUT1. Proc Natl Acad Sci USA 99: 3340-3345