Sweet secret of the multicellular life.

Fig. 1. Prion “furball”. Prion protein is a glycoprotein composed of the amino acid backbone, two N-glycans, and a glycosylphosphatidylinositol anchor. The protein part of the molecule is shown in green, dynamics of the two attached Nglycans is shown in light and dark blue, and the glycosylphosphatidylinositol anchor is colored orange. Figure courtesy of M.R. Wormald, R.A. Dwek, and P. M. Rudd, Glycobiology Institute, Oxford, UK. Carbohydrates have been in the center of human interest since the very beginning of civilization, but we have only recently started to understand the importance of complex oligosaccharides (glycans) attached to protein or lipid backbones. This is perhaps not surprising, since the branched structures of sugars make analysis of glycoconjugates significantly more challenging than the analysis of linear DNA and protein sequences. A typical glycan is a complex molecule containing between 10 and 15 monosaccharides linked in a rather complicated manner that many of us have not yet learned to interpret. Two or more such glycans are attached to the protein backbone of an average glycoprotein, and since there is no genetic blueprint for glycans, individual glycan structures can vary depending on the current level of expression and intracellular localization of biosynthetic enzymes (glycosyltransferases and glycosidases). Consequently, slightly different glycan structures can be attached to the same protein backbone, and after the glycosylation of a protein is completed, proteins with the same amino acid sequence can end up in one of several hundred possible glycoforms. Because naturally glycosylated proteins still cannot be produced in vitro, structural analysis must be performed on small quantities of glycoproteins that can be isolated from nature, and considering the complexity of glycosylation, this can be a formidable task. Glycans frequently represent a significant part of glycoproteins or glycolipids. Therefore, in order to understand the function of glycoconjugates, we have to study the structures of their glycans. For example, it is difficult to imagine that we could understand the way prion protein (Fig. 1) functions if we focus only on its protein part. Depending on the structure of a glycan and the protein to which it is attached, glycans can have many different functions: they can be important in proper folding of proteins; they can regulate function of protein backbones by differential processing of glycosylation [1]; they can be strategically placed so that they can provide protease protection without interfering with the function of the protein; they can serve as recognition motifs for specific carbohydrate binding proteins−lectins [2]; they can enable proteins and lipids to “jump” from one cell to another [3]; they can also have many other known and unknown functions. In the course of evolution, life forms learned how to use the potential for variability that glycosylation offers and the complexity of glycans exploded with the appearance of multicellular organisms. The transition