Tr. Gemmill et Rb. Trimble, Overview of N- and O-linked oligosaccharide structures found in various yeast species, BBA-GEN SUB, 1426(2), 1999, pp. 227-237
Yeast and most higher eukaryotes utilize an evolutionarily conserved N-link
ed oligosaccharide biosynthetic pathway that involves the formation of a Gl
c(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 process
ing glycohydrolases remove the three glucoses and, with the exception of Sc
hizosaccharomyces pombe, a single, specific mannose residue. Processing sug
ar 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 'b
ackbone' that is derivatized with various carbohydrate side chains in a spe
cies-specific manner (Hex(50-200)GlnNAc(2)). In some cases these are short
alpha 1,2-linked Man chains with (Saccharomyces cerevisiae) or without (Pic
hia 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-li
nked pyruvylated Gal residues. Charged groups are also found in S. cerevisi
ae and P. pastoris N-glycans in the form of mannose phosphate diesters. Som
e pathogenic yeast (Candida albicans) add poly-beta 1,2-Man extension throu
gh a phosphate diester to their N-glycans, which appears involved in virule
nce. 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 additi
on 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 b
e capped with one or two alpha 1,3-linked mannoses (S. cerevisiae). S. pomb
e is somewhat unique in that it synthesizes a family of mixed O-glycans wit
h additional alpha 1,2-linked Man and alpha 1,2- and 1,3-linked Gal residue
s. (C) 1999 Elsevier Science B.V. All rights reserved.