FIG. 1.

Critical proteins involved in S. cerevisiae N-linked glycosylation. In the ER, Mns1 cleaves mannose from core Man₉GlcNAc₂ on nascent polypeptides to form Man₈GlcNAc₂. In the Golgi apparatus, S. cerevisiae adds mannose residues to this oligosaccharide to form core-type and polymannose-type glycans with up to 200 mannoses per structure. Mnn1 adds terminal α1,3-linked mannose caps (open boxes), Och1 adds the initial α1,6-linked mannose of the polymannose side chain (dark-gray boxes), and Mnn9 elongates the α1,6-linked backbone (light-gray box). The deletion of PMR1 results in aberrant glycosylation with severely truncated polymannose side chains so that the N-glycans are similar to those found in the Δmnn9 mutant (35), like yeast core-type N glycosylation (curved arrow). Throughout processing in the Golgi apparatus, the ER-secreted Man₈GlcNAc₂ remains intact (trapezoidal boxes). Phosphomannose residues were omitted for simplicity.