Figure 2.

Branch elongation and structural diversity of N-glycans. The Glc₃Man₉GlcNAc₂-P-P-dolichol LLO structure is synthesized in the ER where it is further processed and transferred to the Golgi resulting in high mannose (e.g., Man₅GlcNAc₂), hybrid, and complex type N-glycans that undergo branching via Mgat1, 2, 4, and 5 GlcNAc transferase activity that respectively creates di-, tri-, or tetra-antennary structures. Following the initial branching step, the glycan structure may be fucosylated or undergo additional elongation and capping modifications (Top panel). Alternatively, Mgat3 may add a bisecting GlcNAc residue which blocks Mgat4 and 5 activity thereby preventing tri- and tetra-antennary and further terminal diversification (bottom). The presence of a bisecting GlcNAc also hinders core fucosylation (red triangle) and reduces the capacity for downstream elongation and capping. [All glycan symbol structures in this figure and throughout this document were made using software from Cheng and coauthors (30)].