Overview of N- and O-linked oligosaccharide structures found in various yeast species

Yeast and most higher eukaryotes utilize an evolutionarily conserved N-linked oligosaccharide biosynthetic pathway that involves the formation of a Glc(3)Man(9)GlcNAc(2)-PP-dolichol lipid-linked precursor, the glycan portion of which is cotranslationally transferred in the endoplasmic reticulum (ER) to suitable Asn residues on nascent polypeptides. Subsequently, ER processing glycohydrolases remove the three glucoses and, with the exception of Schizosaccharomyces pombe, a single, specific mannose residue. Processing sugar transferases in the Golgi lead to the formation of core-sized structures (Hex(<15)GlcNa(2)) as well as cores with an extended poly-alpha 1,6-Man 'backbone' that is derivatized with various carbohydrate side chains in a species-specific manner (Hex(50-200)GlnNAc(2)). In some cases these are short alpha 1,2-linked Man chains with (Saccharomyces cerevisiae) or without (Pichia pastoris) alpha 1,3-Man caps, while in other yeast (S. pombe), the side chains are alpha 1,2-linked Gal, some of which are capped with beta-1,3-linked pyruvylated Gal residues. Charged groups are also found in S. cerevisiae and P. pastoris N-glycans in the form of mannose phosphate diesters. Some pathogenic yeast (Candida albicans) add poly-beta 1,2-Man extension through a phosphate diester to their N-glycans, which appears involved in virulence. O-Linked glycan synthesis in yeast, unlike in animal cells where it is initiated in the Golgi using nucleotide sugars, begins in the ER by addition of a single mannose from Man-P-dolichol to selected Ser/Thr residues in newly made proteins. Once transported to the Golgi, sugar transferases add one (C. albicans) or more (P. pastoris) alpha 1,2-linked mannose that may be capped with one or two alpha 1,3-linked mannoses (S. cerevisiae). S. pombe is somewhat unique in that it synthesizes a family of mixed O-glycans with additional alpha 1,2-linked Man and alpha 1,2- and 1,3-linked Gal residues. (C) 1999 Elsevier Science B.V. All rights reserved.