Figure 4.

Golgi glycans as tags. Glycoproteins in the ER, ERGIC, and cis-Golgi have high mannose N-glycans that are all susceptible to release by Endo H. Processing α-mannosidases generate the Man₅GlcNAc₂Asn N-glycan shown above in the medial-Golgi to which GlcNAcT-I transfers a GlcNAc on the left terminal Man residue. Hybrid N-glycans result from the extension of this GlcNAc but the Man₅GlcNAc₂Asn core is not further processed and remains sensitive to Endo H. Complex N-glycans are formed by the removal of two Man residues following the action of GlcNAcT-I and the addition of a second GlcNAc by GlcNAcT-II. Further extension gives rise to the biantennary complex N-glycan shown in the diagram. Complex N-glycans are resistant to Endo H. Both high mannose and complex N-glycans are sensitive to removal by N-glycanase, whose action generates an Asp in place of Asn, a change that may be used to identify N-glycosylation sites by mass spectrometry.