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 Glc3Man9GlcNAc2-PP-dolichol lipid-linked precursor, the glycan portion of which is co-translationally 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<15GlcNac2) as well as cores with an extended poly-alpha1,6-Man 'backbone' that is derivatized with various carbohydrate side chains in a species-specific manner (Hex50-200GlnNAc2). In some cases these are short alpha1,2-linked Man chains with (Saccharomyces cerevisiae) or without (Pichia pastoris) alpha1,3-Man caps, while in other yeast (S. pombe), the side chains are alpha1,2-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-beta1,2-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) alpha1,2-linked mannose that may be capped with one or two alpha1,3-linked mannoses (S. cerevisiae). S. pombe is somewhat unique in that it synthesizes a family of mixed O-glycans with additional alpha1,2-linked Man and alpha1,2- and 1, 3-linked Gal residues.
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Wadsworth Center C-547, New York State Department of Health, P.O. Box 509, Albany, NY 12201-0509, USA.
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D002176:Candida albicans / Q000737:chemistry; D002240:Carbohydrate Sequence; D006023:Glycoproteins / Q000032:analysis*; D006031:Glycosylation; D008969:Molecular Sequence Data; D009844:Oligosaccharides / Q000032:analysis*; D010843:Pichia / Q000737:chemistry; D012441:Saccharomyces cerevisiae / Q000737:chemistry; D012568:Schizosaccharomyces / Q000737:chemistry; D015003:Yeasts / Q000737:chemistry* / Q000235:genetics / Q000648:ultrastructure
D006023:Glycoproteins; D009844:Oligosaccharides