Cell walls made mainly of polysaccharides are a distinguishing feature of plants. They play vital roles in adaptation today and during pivotal evolutionary events, such as colonization of dry land and development of a water-conducting vascular system.
Walls of growing plants are extremely complex and sophisticated composite materials incorporating a dynamic assembly of polysaccharides, proteins, and phenolics. Among the polysaccharides, the pectins encompass a group of acidic heteropolysaccharides, with distinct structural and functional domains. Pectins offer a repertoire of structural complexity ; they can nevertheless be described in terms of a canonical structure which form the distinct specific domains. These domains are named after their major monosaccharide constituents. Rhamnogalacturonan I is one of these domains. Its backbone consists of the repeating disaccharide unit which consists of the repeating disaccharide unit [→ 2)-α-L-Rhap-(1 → 4)-α-D-GalpA-(1→ ].
Four RG-I : rhamnosyltransferases (RRTs), which transfer the rhamnose residue from UDP-β -l-rhamnose to RG-I oligosaccharides have been identified and characterized.
Among these enzymes, RRT1 is a single-spanning transmembrane protein, localized in the Golgi apparatus. RRT1 is an inverting glycosyltransferase that transfers the rhamnose residue from UDP-β-L-rhamnose to RG-I oligosaccharides. It is not used for the initiation of RG-I synthesis but is instead used for elongation of the RG-I backbone. Sequence analysis of RRT1 unveils the existence of a novel glycosyltransferase family, now designated GT106. This is a sizeable plant-specific family, and glycosyltransferases in this family seem to have plant-specific roles, such as biosynthesis of plant cell wall polysaccharides.