Figure 1

N-glycosylation in wild type strains and the approach used to engineer the yeast pathway. (A) Standard N-glycosylation in yeast and mammalian cells. The early N-glycan steps in the ER are strongly conserved between higher and lower eukaryotes until the stage at which folded proteins bearing the eukaryotic common high mannose Man₈GlcNAc₂ glycan structure (isomer B) leave the ER and enter the Golgi apparatus. At this point, the glycans undergo further species and cell-type specific processing. The pathways in the Golgi complex diverge notably. In higher eukaryotes, the Man₈GlcNAc₂ structure is further trimmed to Man₅GlcNAc₂ by mannosidase I and can then be further modified to complex glycans (bottom). In yeast, the Man₈GlcNAc₂ glycan structure is further elongated with mannoses by mannosyl- and phosphomannosyltransferases, and in some cases this results in hypermannosylation (top). (B) Glyco-engineering in yeast. A knock-out of the OCH1 gene prevents the elongation of Man₈GlcNAc₂ glycans (isomer B), and upon expression of the HDEL-tagged α-1,2-mannosidase, Man₅GlcNAc₂ glycans are formed. Conforming to the representation proposed by the Consortium for Functional Glycomics Nomenclature Committee, the green and yellow spheres represent mannose (Man) and galactose (Gal), respectively, blue squares represent N-acetylglucosamine (GlcNAc) residues and the red diamonds represent sialic acid (Sia).