GDP-$-L-fucose (GDP-Fuc)

Plant cell wall polysaccharides contain L-fucose (6-deoxy-L-Gal) derived from the sugar — donor, GDP-Fuc. GDP-Fuc synthesis occurs in two enzymatic steps (similar to the synthesis of UDP-Rha from UDP-Glc). These enzymes have been characterized and the corresponding functional genes have been identified in humans, plants, and bacteria. First, GDP-Man 4,6-dehydratase (GMD), converts GDP-Man to a GDP-4keto-6-deoxyMan intermediate. The latter is then converted by GDP-4-keto-6-deoxymannose-3,5-epimerase-4-reductase (GER1, FX) to GDP-Fuc. In Arabidopsis, there are two GMD isoforms (GMD1, At5g66280; GMD2 (mur1) At3g51160) which share 92% aa sequence identity to each other; and two GER isoforms (GER1, At1g73250; GER2, At1g17890) that share high (88%) sequence identity to each other. In some tissues it appeared that GMD isoforms are co-expressed, but in other tissues expression is restricted. For example, GMD2 is expressed in most cell types of the root, but not in the root tip where strong expression of GMD1 is observed (490). Within shoot organs, GMD2 appears to be expressed in most tissues while GMD1 expression is restricted to stipules and pollen grains. The lack of GMD2 above ground (murl mutant) corresponds to an almost complete reduction in Fuc in wall polysaccharides including XG whose Fuc can be substituted by L-Gal presumably as a result of increased GDP-L-Gal availability (491, 492). However, below ground the murl mutation leads to a 40% reduction of Fuc. Some isoforms may have redundant function in a specific cell, but in other tissues of the same plant the existence of isoforms may provide pools of the NDP-sugars to synthesis of different types of glycans.

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