Alg3 is a multi-pass ER membrane mannosyltransferase of the GT-C superfamily (glycosyltransferase family 58). It catalyses the first lumenal step of dolichol-linked oligosaccharide assembly. After the flippase-dependent translocation of Man5GlcNAc2-PP-dolichol into the ER lumen, Alg3 transfers mannose from dolichyl-phosphate-mannose onto the Man5 intermediate in Ξ±1,3 linkage, giving Man6GlcNAc2-PP-dolichol (EC 2.4.1.258). This begins the Dol-P-Man-dependent extension that continues with Alg9, Alg12 and again Alg9. Alg3 has a C-terminal KKXX ER-retrieval signal. ALG3 is not essential: alg3 cells accumulate Man5GlcNAc2-PP-dolichol, transfer truncated Endo H-resistant glycans to proteins and under-glycosylate secretory proteins, and the mutation is synthetically lethal with defects in oligosaccharyltransferase.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000030 mannosyltransferase activity | IEA GO_REF:0000002 | MODIFY | Summary: mannosyltransferase activity, IEA from InterPro IPR007873 (Glycosyltransferase_ALG3). Correct but general. Alg3 has one well-defined reaction: Dol-P-Man-dependent Ξ±1,3-mannosylation of Man5GlcNAc2-PP-Dol. Reason: Make specific with the reaction-level term. Proposed replacements: dol-P-Man:Man(5)GlcNAc(2)-PP-Dol alpha-1,3-mannosyltransferase activity Supporting Evidence: PMID:11308030 This demonstrates that Alg3p represents the mannosyltransferase itself and not an accessory protein involved in the reaction. |
| GO:0005783 endoplasmic reticulum | HDA PMID:26928762 One library to make them all: streamlining the creation of y... | ACCEPT | Summary: endoplasmic reticulum, HDA from the SWAp-Tag GFP library (Yofe et al. 2016). Consistent with Alg3 being an ER multi-pass membrane protein. Supporting Evidence: PMID:26928762 we found that most new localization assignments were to the ER, punctate structures and vacuole |
| GO:0005783 endoplasmic reticulum | IBA GO_REF:0000033 | ACCEPT | Summary: is_active_in endoplasmic reticulum, IBA from PANTHER PTN000291297 (ALG3 family; Arabidopsis, Drosophila, human, trypanosome and yeast donors). ALG3 orthologues act in the ER throughout eukaryotes. Correct. Supporting Evidence: PMID:8842708 The ALG3 locus encodes a potential ER-transmembrane protein of 458 amino acids (53 kDa) with a C-terminal KKXX-retrieval sequence. |
| GO:0005783 endoplasmic reticulum | IMP PMID:8842708 Cloning and characterization of the ALG3 gene of Saccharomyc... | ACCEPT | Summary: endoplasmic reticulum, IMP from Aebi et al. 1996. The location is inferred from the ER LLO-assembly defect and the predicted ER transmembrane protein with a C-terminal KKXX ER-retrieval signal. It is consistent with all other data. Supporting Evidence: PMID:8842708 The ALG3 locus encodes a potential ER-transmembrane protein of 458 amino acids (53 kDa) with a C-terminal KKXX-retrieval sequence. |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: ER membrane, IEA from UniProt subcellular-location mapping (SL-0097). Correct: UniProt predicts 10 transmembrane segments for Alg3. Supporting Evidence: file:yeast/ALG3/ALG3-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005829 cytosol | RCA GO_REF:0000123 | REMOVE | Summary: is_active_in cytosol, RCA from the YeastPathways LLO pathway (GLUCOSE-MANNOSYL-CHITO-DOLICHOL), which gives one compartment to the whole pathway. That compartment is wrong for Alg3. Alg3 uses Dol-P-Man and acts on Man5GlcNAc2-PP-Dol after it has been flipped into the ER lumen. Its active site faces the lumen. No part of the Alg3 reaction takes place in the cytosol. Reason: Pathway-level compartment over-propagation. Alg3 catalyses the first lumenal mannosylation step. Supporting Evidence: PMID:11308030 the final reactions from Man5GlcNAc2-PP-Dol to Man9GlcNAc2-PP-Dol on the lumenal side use Dol-P-Man PMID:32103179 The final seven steps occur in the lumen of the endoplasmic reticulum (ER) and require dolichylphosphate-activated mannose and glucose as donor substrates |
| GO:0006488 dolichol-linked oligosaccharide biosynthetic process | IDA PMID:11308030 Biosynthesis of lipid-linked oligosaccharides in yeast: the ... | ACCEPT | Summary: dolichol-linked oligosaccharide biosynthetic process, IDA from Sharma et al. 2001. Immunoprecipitated HA-Alg3 carries the Dol-P-Man:Man5GlcNAc2-PP-Dol mannosyltransferase activity. Core process. Supporting Evidence: PMID:11308030 succeeded to selectively immunoprecipitate the Dol-P-Man:Man5GlcNAc2-PP-Dol mannosyltransferase activity from the other enzymes of the detergent extract |
| GO:0006488 dolichol-linked oligosaccharide biosynthetic process | IMP PMID:8842708 Cloning and characterization of the ALG3 gene of Saccharomyc... | ACCEPT | Summary: dolichol-linked oligosaccharide biosynthetic process, IMP from Aebi et al. 1996. alg3 deletion causes lipid-linked Man5GlcNAc2 to accumulate. Core process. Supporting Evidence: PMID:8842708 The inactivation of the nonessential ALG3 gene results in the accumulation of lipid-linked Man5GlcNac2 and protein-bound carbohydrates which are completely Endo H resistant. |
| GO:0006488 dolichol-linked oligosaccharide biosynthetic process | RCA GO_REF:0000123 | ACCEPT | Summary: dolichol-linked oligosaccharide biosynthetic process, RCA from YeastPathways. Correct, and redundant with the experimental rows. Supporting Evidence: PMID:11308030 This demonstrates that Alg3p represents the mannosyltransferase itself and not an accessory protein involved in the reaction. |
| GO:0009101 glycoprotein biosynthetic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: glycoprotein biosynthetic process, IBA from PANTHER PTN000291297. True in a broad sense, because ALG3 builds the LLO donor for N-glycoproteins. Alg3's direct role is better captured by GO:0006488, which the gene already carries. Reason: Pathway-level process beside the specific LLO-assembly step (GO:0006488, part_of this process); kept under the pathway-beside-step carve-out in docs/action_consistency.md. Supporting Evidence: PMID:8842708 The isolated ALG3 gene complements both the defect in the biosynthesis of lipid-linked oligosaccharides of the alg3-mutant and the under-glycosylation of secretory proteins. |
| GO:0009101 glycoprotein biosynthetic process | IMP PMID:8842708 Cloning and characterization of the ALG3 gene of Saccharomyc... | KEEP AS NON CORE | Summary: glycoprotein biosynthetic process, IMP from Aebi et al. 1996. Secretory proteins are under-glycosylated in alg3 mutants. Correct, but the effect is downstream of Alg3's direct LLO step. Supporting Evidence: PMID:8842708 The isolated ALG3 gene complements both the defect in the biosynthesis of lipid-linked oligosaccharides of the alg3-mutant and the under-glycosylation of secretory proteins. |
| GO:0052925 dol-P-Man:Man(5)GlcNAc(2)-PP-Dol alpha-1,3-mannosyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Dol-P-Man:Man5GlcNAc2-PP-Dol Ξ±1,3-mannosyltransferase activity, IBA from PANTHER PTN000291297 (ALG3 family). The function is conserved across eukaryotes and the target has its own IDA support. Core. Supporting Evidence: PMID:11308030 This demonstrates that Alg3p represents the mannosyltransferase itself and not an accessory protein involved in the reaction. |
| GO:0052925 dol-P-Man:Man(5)GlcNAc(2)-PP-Dol alpha-1,3-mannosyltransferase activity | IDA PMID:11308030 Biosynthesis of lipid-linked oligosaccharides in yeast: the ... | ACCEPT | Summary: GO:0052925, IDA from Sharma et al. 2001. HA-Alg3 immunoprecipitates catalyse Dol-P-Man-dependent conversion of Man5GlcNAc2-PP-Dol to Man6GlcNAc2-PP-Dol, so Alg3 is the transferase itself. Core. Supporting Evidence: PMID:11308030 This demonstrates that Alg3p represents the mannosyltransferase itself and not an accessory protein involved in the reaction. PMID:11308030 succeeded to selectively immunoprecipitate the Dol-P-Man:Man5GlcNAc2-PP-Dol mannosyltransferase activity from the other enzymes of the detergent extract |
| GO:0052925 dol-P-Man:Man(5)GlcNAc(2)-PP-Dol alpha-1,3-mannosyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: GO:0052925, IEA from RHEA:29527 / EC 2.4.1.258 mapping. Correct. Supporting Evidence: file:yeast/ALG3/ALG3-uniprot.txt alpha-1,3 linkage to Man(5)GlcNAc(2)-PP-dolichol to produce |
| GO:0052925 dol-P-Man:Man(5)GlcNAc(2)-PP-Dol alpha-1,3-mannosyltransferase activity | RCA GO_REF:0000123 | ACCEPT | Summary: GO:0052925, RCA from YeastPathways. YeastCyc lists the reaction under the generic EC 2.4.1.130, but the reaction itself (Dol-P-Man + Man5GlcNAc2-PP-Dol) is correct for Alg3. Supporting Evidence: PMID:11308030 This demonstrates that Alg3p represents the mannosyltransferase itself and not an accessory protein involved in the reaction. |
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Download this section (compressed HTML)Q: Alg3 adds the first lumenal mannose. Is it rate-limiting for lumenal LLO extension, and does its activity respond to Dol-P-Man supply?
Experiment: Solve an Alg3 structure in complex with a Dol-P-Man analogue and Man5GlcNAc2-PP-Dol to identify the catalytic base, by analogy with ALG6 Asp69.
Dolichol-P-Man dependent alpha(1-3) mannosyltransferase; involved in synthesis of dolichol-linked oligosaccharide donor for N-linked glycosylation of proteins; G353A missense mutation in human ortholog ALG3 implicated in carbohydrate deficient glycoprotein syndrome type IV, which is characterized by microcephaly, severe epilepsy, minimal psychomotor development, partial deficiency of sialic acids in serum glycoproteins; wild-type human ALG3 can complement yeast alg3 mutant
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