Characterization of large oligosaccharide-lipids synthesized in vitro by microsomes from Saccharomyces cerevisiae.

Conditions are described for optimizing the synthesis of large oligosaccharide-lipids in microsomal preparations from Saccharomyces cerevisiae. On incubating microsomes, with GDP-[14C]Man, the major product obtained was Man9GlcNAc2-P-P-dolichol, but when both GDP-[14C]Man and UDP-[3H]Glc were present in the incubation mixture about half of the Man9GlcNAc2 was elongated to Glc3Man9GlcNAc2-P-P-dolichol. Unlike particulate fractions from mammalian systems, little glucosylation of the yeast microsomal oligosaccharide-lipid was obtained when the concentration of UDP-Glc was less than 10 microM, but the synthesis of this product could be maximized by raising the concentration of UDP-Glc to 50 microM. Analysis of the yeast Man9GlcNAc2 species confirmed that 8 of the 9 mannose residues could be released with alpha-mannosidase, while the remaining mannosyl residue was in the core trisaccharide, Manbeta 1 leads to 4GlcNAc beta 1 leads to 4GlcNAc. Treatment of Glc3Man9GlcNAc2 with alpha-mannosidase released 5 of 9 mannose residues and yielded Glc3Man4GlcNAc2. This product appeared to be identical with that obtained in parallel experiments with double labeled oligosaccharide-lipid synthesized in oviduct microsomes. Streptomyces plicatus endo-beta-N-acetylglucosaminidase H (Endo-H) treatment of yeast microsomal glycoproteins that were labeled with sugar nucleotides established that 15% of the label was associated with N-linked oligosaccharides. The remaining labeled sugars were released with alkali, indicating that they were linked to serine or threonine. Based on the size and distribution of [3H]glucose and [14C]mannose in the Endo-H-released oligosaccharides, it was concluded that Glc3Man9GlcNAc2 was the primary species transferred to proteins in the yeast system.