Pectin RG-I rhamnosyltransferases represent a novel plant-specific glycosyltransferase family.

Pectin is one of the three key cell wall polysaccharides in land plants and consists of three major structural domains: homogalacturonan, rhamnogalacturonan I (RG-I) and RG-II. Although the glycosyltransferase required for the synthesis of the homogalacturonan and RG-II backbone was identified a decade ago, those for the synthesis of the RG-I backbone, which consists of the repeating disaccharide unit [→2)-α-L-Rha-(1 → 4)-α-D-GalUA-(1→], have remained unknown. Here, we report the identification and characterization of Arabidopsis RG-I:rhamnosyltransferases (RRTs), which transfer the rhamnose residue from UDP-ß-L-rhamnose to RG-I oligosaccharides. RRT1, which is one of the four Arabidopsis RRTs, is a single-spanning transmembrane protein, localized to the Golgi apparatus. RRT1 was highly expressed during formation of the seed coat mucilage, which is a specialized cell wall with abundant RG-I. Loss-of-function mutation in RRT1 caused a reduction in the level of RG-I in the seed coat mucilage. The RRTs belong to a novel glycosyltransferase family, now designated GT106. This is a large plant-specific family, and glycosyltransferases in this family seem to have plant-specific roles, such as biosynthesis of plant cell wall polysaccharides.

Biochemical characterization of rhamnosyltransferase involved in biosynthesis of pectic rhamnogalacturonan I in plant cell wall.

The pectin in plant cell walls consists of three domains: homogalacturonan, rhamnogalacturonan (RG)-I, and RG-II. It is predicted that around 50 different glycosyltransferases are required for their biosynthesis. Among these, the activities of only a few glycosyltransferases have been detected because pectic oligosaccharides are not readily available for use as substrates. In this study, fluorogenic pyridylaminated RG-I-backbone oligosaccharides (PA-RGs) with 3-14 degrees of polymerization (DP) were prepared. Using these oligosaccharides, the activity of RG-I:rhamnosyltransferase (RRT), involved in the biosynthesis of the RG-I backbone diglycosyl repeating units (-4GalUAα1-2Rhaα1-), was detected from the microsomes of azuki bean epicotyls. RRT was found to prefer longer acceptor substrates, PA-RGs with a DP > 7, and it does not require any metal ions for its activity. RRT is located in the Golgi and endoplasmic reticulum. The activity of RRT coincided with epicotyl growth, suggesting that RG-I biosynthesis is involved in plant growth.