Biosynthesis of non-cellulosic polysaccharides in the Golgi apparatus. Topological considerations

The biosynthesis of hemicelluloses and pectins take place in the Golgi apparatus. Most of the glycosyltransferases involved in this process are facing the lumen of the organelle, thus NDP-sugars have to be available in the lumen of the Golgi. Nucleotide sugar transporters are responsible for the transfer of NDP-sugars from the cytosol into the lumen of the organelle, therefore, this class ofp roteins may have an important role in the biosynthetic process. Recently, it has been proposed that glycosyltransferases belonging to the CsI family could be involved in the biosynthesis of the backbone of some hemicelluloses. However, these kinds of glycosyltransferases may use NDP-sugars directly from the cytosol. The topological considerations associated to the biosynthesis of polysaccharides in the Golgi apparatus are discussed in this review.