ABSTRACT
Nearly all proteins trafficked through the secretory pathway, whether se creted from the cell or destined to become membrane-or extracellular ma trix-associated resident components, are glycoproteins. The three major classes of protein-associated carbohydrates are found AMinked to specific asparagine residues in-Asn-Xaa-Thr/Ser-“sequons,” O-linked to certain serine or threonine residues, or linked to the C terminus of some membrane proteins in the form of a glycosylphosphatidyl inositol, or “GPI,” anchors. Although there are clearly differences in the structures and complexity in each of these three classes of glycans between the lower and more developed eukaryotes, there has been a remarkable conservation throughout the evo lution of the fundamental cellular components involved in the biosynthesis of each type of glycosylation.
