ALG6 is a multipass endoplasmic reticulum membrane glucosyltransferase that catalyzes the first of three glucose additions during assembly of the dolichol-linked N-glycan precursor. Its lumen-facing reaction transfers glucose from dolichyl-phosphate-glucose to Man9GlcNAc2-PP-dolichol, forming an alpha-1,3 linkage and producing Glc1Man9GlcNAc2-PP-dolichol for subsequent glucosylation. This catalytic step supplies the mature glycan donor used by oligosaccharyltransferase in protein N-linked glycosylation. ALG6 belongs to the ALG6/ALG8 glucosyltransferase family (CAZy GT57). Biallelic pathogenic variants cause ALG6-congenital disorder of glycosylation, characterized biochemically by accumulation of unglucosylated lipid-linked precursor and protein hypoglycosylation.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005789 endoplasmic reticulum membrane | IBA GO_REF:0000033 | ACCEPT | Summary: ALG6 is an ER membrane enzyme with a lumen-facing glucosyltransferase reaction. Reason: The PAINT ER-membrane assertion agrees with the human protein's hydrophobic architecture, curated localization and reaction compartment. Retain this source's ER-membrane resolution; the separate lumenal-side IC annotation supplies finer topology. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN000275691 SUPPORTS TRANSFER The local PTHR12413 PAINT record places the ER-membrane IBD at this ancestral node. Human ALG6 membership and membrane biology support inheritance; no target-specific loss or compartment divergence was found. Supporting Evidence: file:human/ALG6/ALG6-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0006488 dolichol-linked oligosaccharide biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: ALG6 catalyzes the first glucosylation step in dolichol-linked oligosaccharide assembly. Reason: Transfer of glucose onto Man9GlcNAc2-PP-Dol is chemical work in this biosynthetic process. Human complementation and ALG6-deficient fibroblasts support the inherited pathway role represented by PAINT. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN000275691 SUPPORTS TRANSFER The cached PAINT IBD is for LLO biosynthesis. Human ALG6 experimental evidence is among the descendants supporting node placement; this self-evidence is appropriate, not circular. Supporting Evidence: PMID:10359825 Expression of the human ALG5 and ALG6 cDNA could partially complement the respective S. cerevisiae alg5 and alg6 deficiency. PMID:25792706 We monitored glycosylation of proteins in asparagine-linked glycosylation 6 (ALG6) deficient cell lines that assemble Dol-PP-GlcNAc(2)Man(9) as the largest oligosaccharide donor. |
| GO:0042281 dolichyl pyrophosphate Man9GlcNAc2 alpha-1,3-glucosyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: The ALG6-specific PAINT node carries the first-glucose, Man9GlcNAc2-PP-Dol glucosyltransferase activity. Reason: The substrate-defined term matches human ALG6 function, EC 2.4.1.267 and RHEA:30635. The separate ALG6-specific ancestral node preserves the distinction from ALG8, which uses the monoglucosylated acceptor. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN000275751 SUPPORTS TRANSFER The cached PAINT record assigns the exact activity to this node, with human ALG6 among its experimental descendants. No evidence challenges inheritance on the human ALG6 lineage. Supporting Evidence: PMID:10359825 Expression of the human ALG5 and ALG6 cDNA could partially complement the respective S. cerevisiae alg5 and alg6 deficiency. file:human/ALG6/ALG6-uniprot.txt Man(9)GlcNAc(2)-PP-Dol to produce Glc(1)Man(9)GlcNAc(2)-PP-Dol. Reactome:R-HSA-446202 The donor is a dolichol-phosphate-glucose (synthesized by ALG5). |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: The UniProt subcellular-location mapping places ALG6 in the ER membrane. Reason: UniProt SL-0097 maps the curated ER-membrane location. This is a true core location at the source's stated resolution; finer lumenal-side evidence does not invalidate it. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB-SubCell:SL-0097 SUPPORTS TRANSFER The source record explicitly describes ALG6 as an ER membrane protein; the mapping preserves that location. Supporting Evidence: file:human/ALG6/ALG6-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0016758 hexosyltransferase activity | IEA GO_REF:0000002 | MODIFY | Summary: The ALG6/ALG8 family mapping correctly identifies hexosyl transfer but leaves the acceptor unspecified. Reason: Human ALG6 transfers the first glucose from Dol-P-Glc to Man9GlcNAc2-PP-Dol. Refine the broad MF to GO:0042281 using target-level functional evidence. The family mapping is sound; this refinement does not claim that all ALG6/ALG8 family members use the ALG6 acceptor. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: InterPro:IPR004856 SUPPORTS TRANSFER The recorded ALG6/ALG8 family mapping supports hexosyl transfer. Human ALG6 evidence supplies the narrower substrate specificity; the original family assertion is not a wrong-function transfer. Proposed replacements: dolichyl pyrophosphate Man9GlcNAc2 alpha-1,3-glucosyltransferase activity Supporting Evidence: PMID:10359825 Expression of the human ALG5 and ALG6 cDNA could partially complement the respective S. cerevisiae alg5 and alg6 deficiency. file:human/ALG6/ALG6-uniprot.txt Man(9)GlcNAc(2)-PP-Dol to produce Glc(1)Man(9)GlcNAc(2)-PP-Dol. Reactome:R-HSA-446202 The donor is a dolichol-phosphate-glucose (synthesized by ALG5). |
| GO:0042281 dolichyl pyrophosphate Man9GlcNAc2 alpha-1,3-glucosyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: The combined EC/RHEA mapping resolves the exact ALG6 glucosyltransferase reaction. Reason: EC 2.4.1.267 and RHEA:30635 encode first-glucose transfer onto the unglucosylated Man9GlcNAc2 lipid precursor. These identifiers agree with the human genetic and cell-based evidence. Propagation Review Root cause: NO FAILURE CORE Sources checked: RHEA:30635 SUPPORTS TRANSFER The proximate RHEA/EC source specifies the ALG6 donor, acceptor and alpha-1,3 product linkage; both mappings converge on GO:0042281. Supporting Evidence: file:human/ALG6/ALG6-uniprot.txt EC=2.4.1.267 file:human/ALG6/ALG6-uniprot.txt Man(9)GlcNAc(2)-PP-Dol to produce Glc(1)Man(9)GlcNAc(2)-PP-Dol. |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | REMOVE | Summary: BioPlex reports an ALG6-associated ALG8 interaction, represented here only as generic protein binding. Reason: The source is AP-MS in human cell lines, and UniProt records the ALG6-ALG8 pair. Retain that association as contextual evidence, but remove the uninformative generic binding MF. Co-purification alone does not establish an adaptor, stable enzyme complex or substrate-channeling function for ALG6. This action does not dispute the interaction. Supporting Evidence: PMID:33961781 Through affinity-purification mass spectrometry, we have created two proteome-scale, cell-line-specific interaction networks. file:human/ALG6/ALG6-uniprot.txt Q9Y672; Q9BVK2: ALG8; NbExp=2 |
| GO:0006488 dolichol-linked oligosaccharide biosynthetic process | TAS Reactome:R-HSA-446193 | ACCEPT | Summary: Reactome places ALG6-mediated first-glucose transfer within LLO biosynthesis. Reason: The parent pathway includes the explicit ALG6-catalyzed reaction R-HSA-446202. ALG6 performs a synthetic step, supporting direct participation rather than an inference from disease necessity alone. Supporting Evidence: Reactome:R-HSA-446202 The first glucose is added to the N-glycan precursor, mediated by ALG6. file:human/ALG6/ALG6-uniprot.txt Man(9)GlcNAc(2)-PP-Dol to produce Glc(1)Man(9)GlcNAc(2)-PP-Dol. |
| GO:0098553 lumenal side of endoplasmic reticulum membrane | IC PMID:10359825 A mutation in the human ortholog of the Saccharomyces cerevi... | ACCEPT | Summary: The curator-inferred location captures the lumen-facing side of the ALG6 reaction. Reason: The human complementation study establishes the ER glycan-assembly defect; UniProt and the cached human LLO GO-CAM place ALG6's activity on the lumenal membrane side. Preserve the IC evidence scope: the 1999 study did not image human active-site topology. The term includes proteins embedded in the lumen-facing leaflet and does not require a soluble lumenal enzyme. The yeast ALG6 structural result is ortholog corroboration, not a direct human topology measurement. Supporting Evidence: PMID:32103179 ALG6 forms a large, hydrophilic cavity facing the ER lumen and a groove-shaped cavity facing the lipid bilayer. file:human/ALG6/ALG6-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0006487 protein N-linked glycosylation | IMP PMID:10359825 A mutation in the human ortholog of the Saccharomyces cerevi... | ACCEPT | Summary: Human wild-type ALG6 complements yeast alg6 hypoglycosylation in the foundational CDG-Ic study. Reason: Human ALG6 restores glycosylation of yeast carboxypeptidase Y through its LLO glucosylation step. This chemical step belongs to protein N-linked glycosylation; the conclusion is supported by the complementation assay and pathway mechanism. Supporting Evidence: PMID:10359825 Expression of the human ALG5 and ALG6 cDNA could partially complement the respective S. cerevisiae alg5 and alg6 deficiency. |
| GO:0006487 protein N-linked glycosylation | IMP PMID:25792706 Reduced expression of the oligosaccharyltransferase exacerba... | ACCEPT | Summary: ALG6 deficiency reduces N-glycosylation of protein substrates in human patient fibroblasts and hamster cells. Reason: The study measures skipped glycosylation sites in ALG6-CDG fibroblasts and distinguishes these from hamster MI8-5 cells, which also have low STT3B expression. Human ALG6 constructs improve glycosylation in MI8-5 cells. This supports ALG6-mediated precursor synthesis as part of N-linked glycosylation without attributing all MI8-5 defects to ALG6. Supporting Evidence: PMID:25792706 We monitored glycosylation of proteins in asparagine-linked glycosylation 6 (ALG6) deficient cell lines that assemble Dol-PP-GlcNAc(2)Man(9) as the largest oligosaccharide donor. PMID:25792706 30-40% of STT3A-dependent glycosylation sites and 20% of STT3B-dependent sites are skipped in ALG6-congenital disorders of glycosylation fibroblasts |
| GO:0006488 dolichol-linked oligosaccharide biosynthetic process | IMP PMID:10359825 A mutation in the human ortholog of the Saccharomyces cerevi... | ACCEPT | Summary: The human ALG6 defect causes accumulation of unglucosylated Man9GlcNAc2-PP-Dol. Reason: The paper links patient ALG6 variants, the LLO defect and complementation by human wild-type ALG6 in yeast. ALG6 is the enzyme that extends this intermediate, so LLO biosynthesis is its direct core process. Supporting Evidence: PMID:10359825 Recently, we identified in four related patients a novel type of CDGS characterized by an accumulation of dolichyl pyrophosphate-linked Man9GlcNAc2. PMID:10359825 Expression of the human ALG5 and ALG6 cDNA could partially complement the respective S. cerevisiae alg5 and alg6 deficiency. |
| GO:0006488 dolichol-linked oligosaccharide biosynthetic process | IMP PMID:25792706 Reduced expression of the oligosaccharyltransferase exacerba... | ACCEPT | Summary: ALG6-deficient mammalian cells assemble unglucosylated Man9GlcNAc2 as their largest LLO donor. Reason: The analyzed human fibroblasts and hamster MI8-5 cells demonstrate the assembly defect, while the separately identified human ALG6 construct complements the hamster defect. These data support the ALG6-catalyzed glucosylation step in LLO biosynthesis. Supporting Evidence: PMID:25792706 We monitored glycosylation of proteins in asparagine-linked glycosylation 6 (ALG6) deficient cell lines that assemble Dol-PP-GlcNAc(2)Man(9) as the largest oligosaccharide donor. |
| GO:0042281 dolichyl pyrophosphate Man9GlcNAc2 alpha-1,3-glucosyltransferase activity | IMP PMID:25792706 Reduced expression of the oligosaccharyltransferase exacerba... | ACCEPT | Summary: The mutant-cell study supports the substrate-specific ALG6 glucosyltransferase assignment. Reason: This IMP assertion is grounded in LLO profiles and cell complementation, not a claim that this paper purified the human enzyme. The loss of glucosylated LLO and complementation by human ALG6 agree with the established first-glucose reaction and exact GO:0042281 chemistry. Supporting Evidence: PMID:25792706 We monitored glycosylation of proteins in asparagine-linked glycosylation 6 (ALG6) deficient cell lines that assemble Dol-PP-GlcNAc(2)Man(9) as the largest oligosaccharide donor. file:human/ALG6/ALG6-uniprot.txt Man(9)GlcNAc(2)-PP-Dol to produce Glc(1)Man(9)GlcNAc(2)-PP-Dol. Reactome:R-HSA-446202 The donor is a dolichol-phosphate-glucose (synthesized by ALG5). |
| GO:0042281 dolichyl pyrophosphate Man9GlcNAc2 alpha-1,3-glucosyltransferase activity | IGI PMID:10359825 A mutation in the human ortholog of the Saccharomyces cerevi... | ACCEPT | Summary: Human ALG6 complements the yeast alg6 defect and the synthetic alg6/wbp1-2 growth phenotype. Reason: The original full study assays human ALG6 in Saccharomyces cerevisiae alg6 and alg6/wbp1-2 strains. Q12001 identifies yeast ALG6, not ALG8; the genetic rescue supports the conserved first-glucose activity. Human A333V impairs CPY glycosylation rescue but retains partial growth rescue, so it is not universally catalytically null. Supporting Evidence: PMID:10359825 Expression of the human ALG5 and ALG6 cDNA could partially complement the respective S. cerevisiae alg5 and alg6 deficiency. PMID:10359825 While the humanALG6cDNA was able to correct the growth defect ofalg6wbp1–2yeasts, theALG6[A333V] cDNA was only capable of partially restoring growth of the yeastalg6wbp1–2strain (Fig.5b). |
| GO:0004583 dolichyl-phosphate-glucose-glycolipid alpha-glucosyltransferase activity | TAS Reactome:R-HSA-446202 | MODIFY | Summary: The normal Reactome event explicitly models first-glucose transfer by ALG6. Reason: The broad donor-defined MF is true, but the event's Man9GlcNAc2 acceptor and monoglucosylated product resolve the exact ALG6 activity. GO:0042281 is the verified substrate-specific child of GO:0004583. Proposed replacements: dolichyl pyrophosphate Man9GlcNAc2 alpha-1,3-glucosyltransferase activity Supporting Evidence: Reactome:R-HSA-446202 The first glucose is added to the N-glycan precursor, mediated by ALG6. Reactome:R-HSA-446202 The donor is a dolichol-phosphate-glucose (synthesized by ALG5). file:human/ALG6/ALG6-uniprot.txt Man(9)GlcNAc(2)-PP-Dol to produce Glc(1)Man(9)GlcNAc(2)-PP-Dol. |
| GO:0004583 dolichyl-phosphate-glucose-glycolipid alpha-glucosyltransferase activity | TAS Reactome:R-HSA-4724291 | MODIFY | Summary: The disease Reactome event describes altered ALG6 activity in the same first-glucose reaction. Reason: The event explicitly contrasts normal ALG6 first-glucose transfer with disease variants. Refine the broad reaction MF to GO:0042281. The unnegated source row is preserved; the disease event does not establish that every human ALG6 protein or every disease variant lacks activity. Proposed replacements: dolichyl pyrophosphate Man9GlcNAc2 alpha-1,3-glucosyltransferase activity Supporting Evidence: Reactome:R-HSA-4724291 These mutations result in altered activity of ALG6 but don't completely abolish its activity file:human/ALG6/ALG6-uniprot.txt Man(9)GlcNAc(2)-PP-Dol to produce Glc(1)Man(9)GlcNAc(2)-PP-Dol. |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-4724291 | ACCEPT | Summary: The disease-context Reactome catalyst is assigned to the ER membrane. Reason: This compartment is compatible with human ALG6's established membrane function and the corresponding normal event. Retain the location without inferring that all individual disease variants have experimentally verified normal trafficking. Supporting Evidence: file:human/ALG6/ALG6-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0016020 membrane | HDA PMID:19946888 Defining the membrane proteome of NK cells. | ACCEPT | Summary: The YTS membrane-proteome survey supplies a broad membrane HDA annotation. Reason: The study analyzes isolated membranes, and human ALG6 is independently established as a multipass ER membrane enzyme. Retain the curator's membrane assignment at its original resolution. The target peptide table was not independently recovered, so no peptide-level confirmation or ER-specific localization is claimed from this survey. The retained membrane assertion is a positive core location. Its assay resolution differs from the broad InterPro MF because human complementation and reaction evidence resolve ALG6 substrate chemistry, whereas this proteomic experiment does not distinguish membrane compartments. Supporting Evidence: PMID:19946888 The present study was initiated to define the composition of the membrane proteome of the Natural Killer (NK) like cell line YTS. file:human/ALG6/ALG6-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-446202 | ACCEPT | Summary: The normal Reactome first-glucose reaction places its ALG6 catalyst in the ER membrane. Reason: The live event identifies ALG6 as catalyst, with its lipid-linked donor, acceptor and product on the lumenal membrane side. This supports the source ER-membrane assignment at its stated granularity. Supporting Evidence: file:human/ALG6/ALG6-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane Reactome:R-HSA-446202 The first glucose is added to the N-glycan precursor, mediated by ALG6. |
| GO:0006487 protein N-linked glycosylation | IDA PMID:10924277 Analysis of multiple mutations in the hALG6 gene in a patien... | ACCEPT | Summary: Patient hALG6 alleles are tested for rescue of N-glycosylation in an ALG6-deficient yeast strain. Reason: The accessible primary abstract reports human allele constructs in yeast and links their activity or expression to the glycosylation defect. Retain the curated IDA annotation with that scope and the independent human evidence: ALG6 catalyzes the LLO step within the N-glycosylation pathway. The full original paper was not independently recovered. Supporting Evidence: PMID:10924277 This enzyme is required for the addition of the first glucose residue to the lipid-linked oligosaccharide precursor for N-linked glycosylation. |
| GO:0046527 glucosyltransferase activity | IDA PMID:10359825 A mutation in the human ortholog of the Saccharomyces cerevi... | MODIFY | Summary: The human ALG6 cloning and complementation study supports a defined glucosyltransferase function. Reason: The specific activity is first-glucose addition to Man9GlcNAc2-PP-Dol. Refine the valid broad glucosyltransferase MF to GO:0042281 using the source's human ALG6 rescue and established substrate assignment. Preserve the original evidence code; the full study's complementation should not be retold as purified-human-enzyme kinetics. Proposed replacements: dolichyl pyrophosphate Man9GlcNAc2 alpha-1,3-glucosyltransferase activity Supporting Evidence: PMID:10359825 Expression of the human ALG5 and ALG6 cDNA could partially complement the respective S. cerevisiae alg5 and alg6 deficiency. file:human/ALG6/ALG6-uniprot.txt Man(9)GlcNAc(2)-PP-Dol to produce Glc(1)Man(9)GlcNAc(2)-PP-Dol. |
Loading supporting content…
Download this section (compressed HTML)Q: Which features of the yeast ALG6 donor-bound structure and catalytic mechanism extend to purified human ALG6, and how do human disease variants alter activity, stability or expression?
Q: Does the reported human ALG6-ALG8 association alter glycan flux or substrate transfer, beyond co-purification of sequential ER enzymes?
Experiment: Compare purified human wild-type and disease-variant ALG6 using defined lipid-linked donor and acceptor substrates, adapting the established yeast assay while measuring protein abundance and stability separately.
Experiment: Test ALG6 and ALG8 individually and together in a defined membrane system to distinguish independent sequential reactions from an interaction-dependent change in catalytic flux.
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)