ALG14 is the membrane-anchoring, non-catalytic subunit of the bipartite ALG14-ALG13 UDP-N-acetylglucosamine (UDP-GlcNAc) transferase. ALG14 is a single-pass endoplasmic reticulum membrane protein that recruits and anchors the otherwise-soluble catalytic subunit ALG13 to the cytoplasmic face of the ER membrane; the resulting heterodimer catalyses the second step of dolichol-linked oligosaccharide (LLO) assembly, transferring a second, beta-1,4-linked GlcNAc from UDP-GlcNAc onto GlcNAc-PP-dolichol to form the chitobiose core GlcNAc2-PP-dolichol. This activity is essential for the biosynthesis of the lipid-linked oligosaccharide precursor used in protein N-linked glycosylation. Neither ALG14 nor ALG13 is active alone: ALG13 provides catalysis but lacks a membrane domain, whereas ALG14 provides the membrane anchor but lacks catalytic residues. In humans, biallelic loss-of-function variants cause ALG14-CDG, a spectrum encompassing congenital myasthenic syndrome (CMS15), a myasthenic/myopathic neurodegeneration with progressive cerebral atrophy (MEPCA), and an intellectual developmental disorder with epilepsy and coarse facies (IDDEBF).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0043495 protein-membrane adaptor activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetically-inferred molecular function. ALG14 is the non-catalytic subunit that spans the ER membrane and recruits the soluble catalytic subunit ALG13, i.e. it acts as a protein-membrane adaptor/anchor. This is the correct, informative molecular function for ALG14 (catalysis proper is contributed by ALG13). Reason: Directly supported by experimental work in yeast and human orthologues: ALG14 spans the membrane, lacks catalytic residues, and functions to recruit/anchor ALG13 to the cytosolic face of the ER. This is a core molecular function of ALG14. Supporting Evidence: PMID:16100110 Alg14 spans the membrane but lacks any sequences PMID:16100110 as a membrane anchor that recruits Alg13 to the cytosolic face of the |
| GO:0006488 dolichol-linked oligosaccharide biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetically-inferred involvement in dolichol-linked oligosaccharide (LLO/DLO) biosynthesis. As the anchoring subunit of the ALG13/14 GnTase, ALG14 is required for the second step of LLO assembly. Reason: Core biological process for ALG14. The ALG13/14 complex catalyses addition of the second GlcNAc to GlcNAc-PP-dolichol, an essential step in assembly of the 14-sugar dolichol-linked oligosaccharide precursor. Supported by experimental data in the same paper series. Supporting Evidence: PMID:36200043 complex catalyzes the addition of a Ξ²1,4-linked GlcNAc to Gn-PDol to produce Gn2-PDol in the DLO synthetic pathway |
| GO:0043541 UDP-N-acetylglucosamine transferase complex | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetically-inferred complex membership. ALG14 is part of the heterodimeric UDP-N-acetylglucosamine transferase complex it forms with ALG13. Reason: Core cellular-component/complex assignment, corroborated experimentally by co-immunoprecipitation of the human ALG13/14 complex. Supporting Evidence: PMID:36200043 Co-immunoprecipitation assays confirmed the formation of the ALG13/14 complex |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic annotation from the UniProt Swiss-Prot subcellular location vocabulary mapping (SL-0097). ALG14 is an ER membrane protein. Reason: Consistent with the experimental EXP and TAS ER-membrane annotations and with the single-pass transmembrane topology in UniProt. Supporting Evidence: file:human/ALG14/ALG14-uniprot.txt Single-pass membrane protein |
| GO:0006488 dolichol-linked oligosaccharide biosynthetic process | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO electronic annotation (IPR013969, Oligosacch_biosynth_Alg14). Duplicate of the IBA and IDA involvement in LLO biosynthesis. Reason: Correct and consistent with experimental data; the ALG14 InterPro domain is diagnostic of the LLO-biosynthesis subunit. Supporting Evidence: PMID:36200043 complex catalyzes the addition of a Ξ²1,4-linked GlcNAc to Gn-PDol to produce Gn2-PDol in the DLO synthetic pathway |
| GO:0005515 protein binding | IPI PMID:36200043 An in vitro assay for enzymatic studies on human ALG13/14 he... | MARK AS OVER ANNOTATED | Summary: IPI annotation recording physical interaction of ALG14 with ALG13 isoform 2 (WITH/FROM UniProtKB:Q9NP73-2). The biologically meaningful content of this interaction (recruiting/anchoring the catalytic ALG13 subunit) is already captured by the more informative protein-membrane adaptor activity (GO:0043495) and by complex membership (GO:0043541). Reason: Bare "protein binding" is uninformative and per curation guidelines should not stand as a molecular function when a more specific term applies. The interaction with ALG13 is real and important, but is better represented by the adaptor-activity MF and the UDP-GlcNAc transferase complex CC. Retained but flagged as over-annotated rather than removed (it is an experimentally supported IPI). Supporting Evidence: PMID:36200043 Co-immunoprecipitation assays confirmed the formation of the ALG13/14 complex |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:16100110 Alg14 recruits Alg13 to the cytoplasmic face of the endoplas... | ACCEPT | Summary: Experimental localization of ALG14 to the ER membrane. ALG14 spans the ER membrane and, together with ALG13, is associated with the ER membrane. Reason: Core cellular-component annotation with direct experimental support; ALG14 is a single-pass ER membrane protein. Supporting Evidence: PMID:16100110 Alg13 and Alg14 physically interact and file:human/ALG14/ALG14-uniprot.txt Single-pass membrane protein |
| GO:0043495 protein-membrane adaptor activity | IGI PMID:16100110 Alg14 recruits Alg13 to the cytoplasmic face of the endoplas... | ACCEPT | Summary: Experimental (genetic-interaction) support that ALG14 functions as a protein-membrane adaptor/anchor recruiting ALG13 to the cytosolic face of the ER. WITH/FROM the yeast ALG14 (P38242) genetic system. Reason: This is the core molecular function of ALG14 and is directly demonstrated: ALG14 is necessary and sufficient for ER localization of the catalytic ALG13 subunit. Supporting Evidence: PMID:16100110 as a membrane anchor that recruits Alg13 to the cytosolic face of the PMID:16100110 Alg14 is both necessary and sufficient for the ER |
| GO:0098554 cytoplasmic side of endoplasmic reticulum membrane | IGI PMID:16100110 Alg14 recruits Alg13 to the cytoplasmic face of the endoplas... | ACCEPT | Summary: ALG14 acts at the cytoplasmic (cytosolic) face of the ER membrane, where it recruits ALG13 and the complex catalyses formation of GlcNAc2-PP-dolichol. Reason: More precise cellular-component location than "endoplasmic reticulum membrane" and consistent with the cytoplasmic topology of the large ALG14 domain and the site of second-GlcNAc addition. Supporting Evidence: PMID:16100110 recruits Alg13 to the cytosolic face of the |
| GO:0006487 protein N-linked glycosylation | IDA PMID:36200043 An in vitro assay for enzymatic studies on human ALG13/14 he... | ACCEPT | Summary: ALG14 acts upstream of and positively affects protein N-linked glycosylation: by supporting synthesis of the LLO precursor, ALG14 activity is required for downstream transfer of the N-glycan to nascent proteins. Loss-of-function ALG14-CDG variants reduce GnTase activity. Reason: Appropriate acts_upstream_of_positive_effect relation between LLO assembly (ALG14 function) and the N-glycosylation process. Experimentally supported by demonstration that ALG14-CDG variants have reduced GnTase activity. Supporting Evidence: PMID:36200043 Notably, all of the ALG13/ALG14-CDG mutants that were tested displayed reduced GnTase activity |
| GO:0006488 dolichol-linked oligosaccharide biosynthetic process | IDA PMID:36200043 An in vitro assay for enzymatic studies on human ALG13/14 he... | ACCEPT | Summary: Direct experimental support for ALG14 involvement in dolichol-linked oligosaccharide biosynthesis: the purified human ALG13/14 complex catalyses addition of the second, beta-1,4-linked GlcNAc to GlcNAc-PP-dolichol to form GlcNAc2-PP-dolichol. Reason: Core biological process, directly demonstrated in vitro with purified human enzyme. Duplicate term to the IBA/IEA involvement annotations. Supporting Evidence: PMID:36200043 complex catalyzes the addition of a Ξ²1,4-linked GlcNAc to Gn-PDol to produce Gn2-PDol in the DLO synthetic pathway |
| GO:0043541 UDP-N-acetylglucosamine transferase complex | IDA PMID:36200043 An in vitro assay for enzymatic studies on human ALG13/14 he... | ACCEPT | Summary: Direct experimental demonstration that ALG14 is part of the heterodimeric ALG13/14 UDP-N-acetylglucosamine transferase complex (confirmed by co-immunoprecipitation and by reconstitution of the active complex). Reason: Core complex-membership annotation, experimentally verified for the human protein. Supporting Evidence: PMID:36200043 Co-immunoprecipitation assays confirmed the formation of the ALG13/14 complex |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-5633241 | ACCEPT | Summary: Reactome traceable-author annotation placing ALG14 at the ER membrane (reaction: defective ALG14 does not transfer GlcNAc from UDP-GlcNAc to GlcNAcDOLP). Consistent with all other localization evidence. Reason: Correct ER-membrane location; duplicate of the EXP and IEA annotations. Supporting Evidence: file:human/ALG14/ALG14-uniprot.txt Single-pass membrane protein |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-446207 | ACCEPT | Summary: Reactome traceable-author annotation placing ALG14 at the ER membrane (reaction: ALG13:ALG14 transfers GlcNAc from UDP-GlcNAc to GlcNAcDOLP). Consistent with all other localization evidence. Reason: Correct ER-membrane location; duplicate of the EXP and IEA annotations. Supporting Evidence: file:human/ALG14/ALG14-uniprot.txt Single-pass membrane protein |
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Download this section (compressed HTML)Q: Why do most ALG14-CDG patients show normal serum transferrin glycosylation despite reduced ALG13/14 GnTase activity in vitro, and does ALG14 have a tissue- or substrate-specific requirement (e.g. at the neuromuscular junction) that explains the myasthenic phenotype?
Q: Does ALG14 scaffold or influence the first LLO step (DPAGT1) in addition to anchoring ALG13, and if so how is the cytosolic-face LLO machinery organized?
Experiment: Structure of the human ALG14-ALG13 heterodimer in the ER membrane (e.g. by cryo-EM) to define the ALG14 residues that mediate ALG13 recruitment and how CDG variants (P65L, D74N, R109Q, V141G) destabilize the complex.
Experiment: Tissue-specific / neuromuscular-junction glycoproteomics in ALG14-deficient models to explain why a general LLO-assembly defect manifests predominantly as a myasthenic/neurodegenerative phenotype.
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