9.B.231.  The Die2/ALG10 Glycosyl Transferase (Die2/ALG10) Family 

The biosynthesis of the lipid-linked oligosaccharide substrate for N-linked protein glycosylation follows a highly conserved pathway at the membrane of the endoplasmic reticulum in eukaryotes. Based on the synthetic growth defect in combination with a reduced oligosaccharyltransferase activity (wbp1), Burda and Aebi 1998 identified alg10 mutant strains which accumulate lipid-linked Glc2Man9GlcNAc2. They cloned the corresponding wild-type gene and showed that Alg10p is a dolichyl-phosphoglucose-dependent glucosyltransferase which adds the terminal alpha-1,2 glucose to the lipid-linked Glc2Man9GlcNAc2 oligosaccharide. Hypoglycosylation of secreted proteins in alg10 deletion strains demonstrated that the terminal alpha-1,2-linked glucose residue is a key element in substrate recognition by the oligosaccharyltransferase. This ensures that completely assembled oligosaccharide is transferred to protein (Burda and Aebi 1998).