Fig. 1.

Pathways for N-linked glycan fucosylation in Drosophila. N-acetylglucosaminyltransferase I (GlcNAc-TI) adds N-acetylglucosamine to the high-mannose glycan Man₅GlcNAc₂ on glycoproteins transported to the Golgi, resulting in glycans with the composition Man₅GlcNAc₃. This octasaccharide is substrate for the production of all hybrid, complex, and paucimannose N-glycans in the Drosophila embryo. Two shorthand nomenclatures are in general use to describe glycan structures that derive from Man₅GlcNAc₃, also known as NM5N2 (Man5Gn; alternative names are in parentheses). Following trimming by Golgi mannosidases (Golgi Man’ase), the NM3N2 (MGn) glycan and its monofucosylated derivative NM3N2F⁶ (MGnF⁶) are substrates for production of the HRP epitopes, NM3N2F³ (MGnF³) and NM3N2F2 (MGnF³F⁶), which are found in very low prevalence in the embryo. The major pathway for HRP epitope synthesis is initiated by addition of fucose (Fuc) in α6-linkage to NM3N2 (MGn) by Fucosyltransferase 6 (FucT⁶), followed by addition of α3-linked Fuc, which is catalyzed by Fucosyltransferase A (FucTA) in vitro. FucTA prefers substrates already containing α6-linked Fuc; thus, most α3-fucosylated glycans are difucosylated. The hexosaminidase known as “Fused lobes” (Fdl) removes non-reducing terminal GlcNAc to produce paucimannose glycans such as M3N2F⁶ (MMF⁶) or M3N2F2 (MMF³F⁶). An unidentified mannosidase activity (Man’ase) removes an additional Man residue from the M3N2F⁶ (MMF⁶) structure to generate M2N2F⁶ (MUF⁶). The boxed glycans are a subset of the HRP epitopes that have been detected in Drosophila embryos, of which M3N2F2 (MMF³F⁶) is the most prevalent. Graphical representations of glycans are consistent with the nomenclature of the Consortium for Functional Glycomics.