Limited Addition of the 6-Arm beta 1,2-linked N-Acetylglucosamine (GlcNAc) Residue Facilitates the Formation of the Largest N-Glycan in Plants

Abstract

The most abundant N-glycan in plants is the paucimannosidic N-glycan with core beta 1,2-xylose and alpha 1,3-fucose residues (Man(3)XylFuc(GlcNAc)(2)). Here, we report a mechanism in Arabidopsis thaliana that efficiently produces the largest-N-glycan in plants. Genetic and biochemical evidence indicates that the addition of the 6-arm beta 1,2-GlcNAc residue by N-acetylglucosaminyltransferase II (GnTII) is less effective than additions of the core beta 1,2-xylose and alpha 1,3-fucose residues by XylT, FucTA, and FucTB in Arabidopsis. Furthermore, analysis of gnt2 mutant and 35S:GnTII transgenic plants shows that the addition of the 6-arm non-reducing GlcNAc residue to the common N-glycan acceptor GlcNAcMan(3)(GlcNAc)(2) inhibits additions of the core beta 1,2-xylose and alpha 1,3-fucose residues. Our findings indicate that plants limit the rate of the addition of the 6-arm GlcNAc residue to the common N-glycan acceptor as a mechanism to facilitate formation of the prevalent N-glycans with Man(3)XylFuc(GlcNAc)(2) and (GlcNAc)(2)Man(3)XylFuc(GlcNAc)(2) structures.