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. 2015 May 22;290(27):16560–16572. doi: 10.1074/jbc.M115.653162

FIGURE 8.

FIGURE 8.

Limited addition of the 6-arm β1,2-GlcNAc is involved in the formation of the largest N-glycan in plants. N-Glycan processing in the ER and Golgi of Arabidopsis is summarized with corresponding enzymes and illustrated structures of the N-glycans (45). The biosynthesis of complex N-glycan initiates with addition of the 3-arm β1,2-GlcNAc residue to the Man5(GlcNAc)2 N-glycan by GnTI at the cis-face of the Golgi complex (23). Subsequently, the outer chain mannose residues are removed by Golgi α-mannosidase II, resulting in the production of the GlcNAcMan3(GlcNAc)2 N-glycan (28, 33). This structure can be used as a common acceptor of the 6-arm β1,2-GlcNAc, core β1,2-xylose, and α1,3-fucose residues by GnTII, XylT, FucTA, and FucTB (17, 28, 30–34) in the medial Golgi compartments. Because the addition of the 6-arm β1,2-GlcNAc residue by GnTII is less efficient than the additions of the core β1,2-xylose and α1,3-fucose residues by XylT, FucTA, and FucTB, the amount of the N-glycan with GlcNAcMan3XylFuc(GlcNAc)2 structure is greater than that with (GlcNAc)2Man3XylFuc(GlcNAc)2 structure in plants. The outer chain GlcNAc residue of the GlcNAcMan3XylFuc(GlcNAc)2 is removed by HEXOI and HEXOIII, resulting in the prevalent PNGXF in plants. The dominant additions of the core β1,2-xylose and α1,3-fucose residues are indicated with thick solid lines, and the limited addition of the 6-arm β1,2-GlcNAc residue is indicated with a thin dashed line. OST, oligosaccharyltransferase.