module: dmel_n_glycan_processing title: N-glycan trimming and processing in Drosophila melanogaster label: N-glycan processing module_type: METABOLIC_PATHWAY description: >- After oligosaccharyltransferase transfers Glc3Man9GlcNAc2 to asparagine residues of nascent proteins, the N-glycan is trimmed in the endoplasmic reticulum and then remodelled in the Golgi apparatus. In the ER, glucosidase I (Gcs1) removes the outer glucose and the glucosidase II heterodimer (Gcs2alpha catalytic and Gcs2beta regulatory subunits) removes the two inner glucoses; these steps also control entry into and exit from the calnexin-calreticulin folding cycle. Class I alpha-1,2-mannosidases trim mannoses in the ER and cis-Golgi. In the Golgi, GlcNAc transferase I (Mgat1) adds GlcNAc to the alpha-1,3 arm, which allows alpha-mannosidase II to remove two further mannoses. In insects most N-glycans are then converted to paucimannose glycans by the Golgi beta-N-acetylglucosaminidase Fused lobes (fdl), while a smaller fraction is extended to hybrid and complex glycans by GlcNAc transferase II, core fucosyltransferases, galactosyl-, GalNAc-, glucuronyl- and sialyltransferases. summary: >- ER and Golgi trimming of protein-linked Glc3Man9GlcNAc2 and its conversion to paucimannose, hybrid and complex N-glycans. concepts: - {id: GO:0006491, label: N-glycan processing} parts: - id: glucosidase_i label: Glucosidase I trimming role: step 1, removal of the terminal alpha-1,2-glucose module_type: REACTION genes: [Gcs1] function: {id: GO:0004573, label: Glc3Man9GlcNAc2 oligosaccharide glucosidase activity} processes: - {id: GO:0140277, label: endoplasmic reticulum N-glycan trimming} - id: glucosidase_ii label: Glucosidase II complex role: step 2, removal of the two inner alpha-1,3-glucoses module_type: PROTEIN_COMPLEX complex: true group: GLU2ABII complex_term: {id: GO:0017177, label: glucosidase II complex} unit_functions: Gcs2alpha: {id: GO:0106407, label: Glc2Man9GlcNAc2 oligosaccharide glucosidase activity} roles: Gcs2alpha: catalytic alpha subunit Gcs2beta: regulatory beta subunit that retains the complex in the ER - id: er_mannosidase label: ER alpha-1,2-mannosidase trimming role: step 3, ER mannose trimming module_type: REACTION genes: [alpha-Man-Ib] function: {id: GO:0004571, label: "mannosyl-oligosaccharide 1,2-alpha-mannosidase activity"} processes: - {id: GO:0140277, label: endoplasmic reticulum N-glycan trimming} - id: golgi_mannosidase_i label: Golgi alpha-1,2-mannosidase I trimming role: step 4, trimming to Man5GlcNAc2 module_type: REACTION genes: [alpha-Man-Ia] function: {id: GO:0004571, label: "mannosyl-oligosaccharide 1,2-alpha-mannosidase activity"} processes: - {id: GO:1904381, label: Golgi apparatus N-glycan mannose trimming} - id: gnt1 label: GlcNAc transferase I role: step 5, GlcNAc addition to the alpha-1,3 mannose arm module_type: REACTION genes: [Mgat1] function: {id: GO:0003827, label: "alpha-1,3-mannosylglycoprotein 2-beta-N-acetylglucosaminyltransferase activity"} - id: golgi_mannosidase_ii label: Golgi alpha-mannosidase II trimming role: step 6, removal of the alpha-1,3 and alpha-1,6 mannoses of the 6-arm module_type: REACTION genes: [alpha-Man-IIa] function: {id: GO:0004572, label: "mannosyl-oligosaccharide 1,3-1,6-alpha-mannosidase activity"} processes: - {id: GO:1904381, label: Golgi apparatus N-glycan mannose trimming} - id: paucimannose_formation label: Paucimannose formation by Fused lobes role: step 7a, removal of the GlcNAc added by GlcNAc transferase I module_type: REACTION genes: [fdl] function: {id: GO:0016231, label: beta-N-acetylglucosaminidase activity} - id: golgi_diversification label: Golgi N-glycan diversification role: step 7b, extension to hybrid and complex N-glycans module_type: METABOLIC_PATHWAY processes: - {id: GO:0016258, label: Golgi apparatus N-glycan diversification} parts: - id: gnt2 label: GlcNAc transferase II role: GlcNAc addition to the alpha-1,6 mannose arm module_type: REACTION genes: [Mgat2] function: {id: GO:0008455, label: "alpha-1,6-mannosylglycoprotein 2-beta-N-acetylglucosaminyltransferase activity"} - id: alternative_mannosidase_ii label: Alternative alpha-mannosidase II role: alternative Golgi mannose trimming module_type: REACTION genes: [alpha-Man-IIb] - id: core_fucosylation label: Core fucosylation role: alpha-1,6 and alpha-1,3 fucosylation of the core GlcNAc module_type: REACTION genes: [Fut6, FutA] unit_functions: Fut6: {id: GO:0008424, label: glycoprotein 6-alpha-L-fucosyltransferase activity} FutA: {id: GO:0018392, label: glycoprotein 3-alpha-L-fucosyltransferase activity} - id: antenna_elongation label: Antenna elongation role: galactose, GalNAc and glucuronic acid addition to antennae module_type: REACTION genes: [GalT1, beta4GalNAcTA, GlcAT-P, GlcAT-S] unit_functions: beta4GalNAcTA: {id: GO:0033842, label: N-acetyl-beta-glucosaminyl-derivative 4-beta-N-acetylgalactosaminyltransferase activity} - id: sialylation label: Sialylation role: terminal alpha-2,6 sialylation module_type: REACTION genes: [Siat] function: {id: GO:0003835, label: "beta-galactoside alpha-2,6-sialyltransferase activity"} connections: - source: gnt2 target: antenna_elongation connection_type: PRECEDES description: GlcNAc residues on both arms are substrates for antenna extension. - source: antenna_elongation target: sialylation connection_type: PRECEDES description: Sialic acid is added to terminal galactose. connections: - source: glucosidase_i target: glucosidase_ii connection_type: PRECEDES description: Glc2Man9GlcNAc2 is the substrate of glucosidase II. - source: glucosidase_ii target: er_mannosidase connection_type: PRECEDES description: Deglucosylated Man9GlcNAc2 is trimmed by ER mannosidase. - source: er_mannosidase target: golgi_mannosidase_i connection_type: PRECEDES description: Man8GlcNAc2 is trimmed further to Man5GlcNAc2 in the Golgi. - source: golgi_mannosidase_i target: gnt1 connection_type: PRECEDES description: Man5GlcNAc2 is the acceptor for GlcNAc transferase I. - source: gnt1 target: golgi_mannosidase_ii connection_type: PRECEDES description: Alpha-mannosidase II requires the GlcNAc added by GlcNAc transferase I. - source: golgi_mannosidase_ii target: paucimannose_formation connection_type: PRECEDES description: GlcNAcMan3GlcNAc2 is trimmed by Fused lobes to paucimannose. - source: golgi_mannosidase_ii target: golgi_diversification connection_type: PRECEDES description: GlcNAcMan3GlcNAc2 is alternatively extended to complex glycans. notes: >- Boundary. The module starts after transfer of the lipid-linked oligosaccharide to protein (oligosaccharyltransferase is outside the module) and covers ER trimming, Golgi trimming and Golgi diversification, the three FlyBase N-glycan processing groups plus the glucosidase II complex group. Core fucosylation by FutA and Fut6 needs the GlcNAc added by Mgat1 and can occur before or after mannosidase II trimming, so the order within the diversification part is shown only where it is clear. GlcAT-P and GlcAT-S are glucuronyltransferases that act on N-glycans and glycolipids (GlcAT-P is also in the O-GalNAc branch of dmel_protein_o_glycosylation); GalT1 is a beta-1,3-galactosyltransferase. No molecular function is assigned to the regulatory Gcs2beta subunit or to alpha-Man-IIb, whose in vitro activity differs from that of classical mannosidase II. Molecular functions of Gcs1, alpha-Man-Ib and Mgat1 follow orthology; the others are supported by assays of the fly enzymes in GO.