Nucleotide Sugars and the Transporters That Put Them at the Right Place

Nucleotide sugars are the building blocks for plant cell wall polysaccharide biosynthesis. They are the high-energy activated form of monosaccharides, and the substrates for glycosyltransferases and enzymes responsible to link the monosaccharides together to create the polysaccharides. Most glycosyltransferases involved in plant cell wall biosynthesis are located in the Golgi apparatus, with the exception of those involved in the biosynthesis of cellulose which are located in plasma membrane. These Golgi-localised glycosyltransferases are mostly in the lumen of the organelle; however, nucleotide sugars that are synthesised in the cytoplasm must cross the Golgi membrane to become available to the enzymes. Since nucleotide sugars are not membrane permeable, specific membrane proteins known as nucleotide sugar transporters are involved in their transport to the Golgi lumen. These proteins are encoded by a gene family composed of 51 members. To date, several genes have been characterised, and transporters for many nucleotide sugars involved in cell wall biosynthesis have been identified. This chapter provides a historical perspective, as well as a description of the journey leading to the characterisation of nucleotide sugar transporters playing a decisive role in the biosynthesis of the plant cell wall.