HGSNAT (heparan-alpha-glucosaminide N-acetyltransferase; TMEM76; EC 2.3.1.78) is a polytopic lysosomal membrane transacetylase that carries out an essential step of heparan sulfate degradation. It transfers an acetyl group from cytosolic acetyl-CoA onto the free amino group of the terminal non-reducing alpha-glucosamine residue of intralysosomal heparan sulfate (and heparin), converting it to N-acetyl-alpha-glucosamine so that the downstream lysosomal exoglycosidase NAGLU (alpha-N-acetylglucosaminidase) can remove it and degradation can proceed. Unlike the other three enzymes of the heparan sulfate degradation pathway, which are soluble luminal hydrolases/sulfatases, HGSNAT is the only membrane-bound member and the only transferase, spanning the lysosomal membrane with 11 transmembrane helices; its active-site nucleophile (His297) is positioned to access cytoplasmic acetyl-CoA and transfer the acetyl group across the membrane to the luminal glucosamine substrate. The enzyme is synthesised as an inactive precursor that is trafficked to lysosomes via adaptor-protein/dileucine- and tyrosine-based signals, proteolytically cleaved into an N-terminal alpha-chain and C-terminal beta-chain, and assembled into active homo-oligomers. Loss-of-function mutations cause mucopolysaccharidosis type IIIC (Sanfilippo syndrome C), an autosomal recessive lysosomal storage disorder with severe progressive neurodegeneration; certain HGSNAT variants also cause non-syndromic retinitis pigmentosa (RP73).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005765 lysosomal membrane | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) assignment that HGSNAT acts in the lysosomal membrane. This is the correct core subcellular location: HGSNAT is a multi-pass lysosomal membrane protein whose enzymatic action occurs at that membrane. Directly supported by experimental evidence in this gene (colocalization with LAMP-2; lysosomal-membrane proteomics) and consistent with the UniProt subcellular-location annotation. Reason: Correct core location, supported experimentally in human HGSNAT and by the phylogenetic model. Supporting Evidence: file:human/HGSNAT/HGSNAT-uniprot.txt Lysosome membrane PMID:19823584 almost completely co-localized with the lysosomal markers LysoTracker Red or LAMP-2 |
| GO:0015019 heparan-alpha-glucosaminide N-acetyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) assignment of the exact-match molecular function: acetyl-CoA:alpha-glucosaminide N-acetyltransferase activity (EC 2.3.1.78; RHEA:15125). This is the core catalytic activity of HGSNAT and is confirmed experimentally by direct N-acetyltransferase assays of the recombinant human enzyme. Reason: Exact-match term for the demonstrated core catalytic activity; well supported by experimental data and the phylogenetic model. Supporting Evidence: file:human/HGSNAT/HGSNAT-uniprot.txt Lysosomal acetyltransferase that acetylates the non-reducing PMID:20650889 catalyzes the transmembrane acetylation of heparan sulfate in lysosomes required |
| GO:0030200 heparan sulfate proteoglycan catabolic process | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) assignment placing HGSNAT in heparan sulfate proteoglycan catabolism. This is the core biological process: HGSNAT's N-acetylation step is an obligatory intermediate in stepwise lysosomal degradation of heparan sulfate, and its deficiency blocks the pathway and causes lysosomal HS storage (MPS IIIC). Reason: Correct core catabolic pathway; the enzyme's step is required for heparan sulfate degradation and its loss causes HS accumulation. Supporting Evidence: PMID:19823584 acetylation of the terminal glucosamine residues of heparan sulfate prior to PMID:20650889 for its further catabolism |
| GO:0005765 lysosomal membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic (UniProt SubCell mapping SL-0157) annotation of lysosomal membrane location, derived from the curated UniProt subcellular-location statement. Correct and redundant with the experimentally supported location. Reason: Consistent with curated UniProt location and experimental evidence for lysosomal membrane localization. Supporting Evidence: file:human/HGSNAT/HGSNAT-uniprot.txt Lysosome membrane |
| GO:0015019 heparan-alpha-glucosaminide N-acetyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic assignment of the exact-match N-acetyltransferase activity based on EC 2.3.1.78 / RHEA:15125 and ortholog evidence. Correct and redundant with the experimentally and phylogenetically supported MF annotation. Reason: Exact-match MF term mapped from EC/RHEA; correct core catalytic activity. Supporting Evidence: file:human/HGSNAT/HGSNAT-uniprot.txt Lysosomal acetyltransferase that acetylates the non-reducing |
| GO:0030200 heparan sulfate proteoglycan catabolic process | IEA GO_REF:0000107 | ACCEPT | Summary: Electronic transfer (Ensembl Compara) of the heparan sulfate proteoglycan catabolic process annotation from the experimentally characterized mouse ortholog. Correct core biological process. Reason: Correct core catabolic pathway, transferred from an experimentally characterized ortholog; consistent with human experimental data. Supporting Evidence: PMID:20650889 for its further catabolism |
| GO:0015019 heparan-alpha-glucosaminide N-acetyltransferase activity | TAS Reactome:R-HSA-2263492 | ACCEPT | Summary: Reactome traceable-author annotation of the exact-match N-acetyltransferase activity, from a reaction representing HGSNAT acetylation of the heparan chain (the "defective" reaction node modelling loss of this activity in MPS IIIC). Supports the core catalytic function. Reason: Exact-match core catalytic activity, curated by Reactome; consistent with all other evidence. Supporting Evidence: file:human/HGSNAT/HGSNAT-uniprot.txt Lysosomal acetyltransferase that acetylates the non-reducing |
| GO:0015019 heparan-alpha-glucosaminide N-acetyltransferase activity | TAS Reactome:R-HSA-9036056 | ACCEPT | Summary: Reactome traceable-author annotation of the exact-match N-acetyltransferase activity (second Reactome reaction node). Redundant with the other MF annotations and supports the core catalytic function. Reason: Exact-match core catalytic activity, curated by Reactome. Supporting Evidence: file:human/HGSNAT/HGSNAT-uniprot.txt Lysosomal acetyltransferase that acetylates the non-reducing |
| GO:0005765 lysosomal membrane | TAS Reactome:R-HSA-2263492 | ACCEPT | Summary: Reactome traceable-author annotation of lysosomal membrane location. Correct core location, redundant with the experimentally supported CC annotations. Reason: Correct core location, curated by Reactome; consistent with experimental data. Supporting Evidence: file:human/HGSNAT/HGSNAT-uniprot.txt Lysosome membrane |
| GO:0005765 lysosomal membrane | TAS Reactome:R-HSA-9036056 | ACCEPT | Summary: Reactome traceable-author annotation of lysosomal membrane location (second reaction node). Correct core location, redundant with other CC annotations. Reason: Correct core location, curated by Reactome. Supporting Evidence: file:human/HGSNAT/HGSNAT-uniprot.txt Lysosome membrane |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-6798747 | KEEP AS NON CORE | Summary: Reactome annotation of plasma membrane location, from the "Exocytosis of tertiary granule membrane proteins" reaction in the neutrophil degranulation pathway. This reflects mobilization of granule-membrane HGSNAT to the cell surface during degranulation rather than the enzyme's core lysosomal site of action. Not wrong in that specific context, but peripheral to HGSNAT's core catalytic role. Reason: Neutrophil-degranulation exocytosis context; a secondary, cell-type-restricted localization, not the core lysosomal membrane location where catalysis occurs. Supporting Evidence: Reactome:R-HSA-6798747 Exocytosis of tertiary granule membrane proteins |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-6799350 | KEEP AS NON CORE | Summary: Reactome annotation of plasma membrane location from the "Exocytosis of specific granule membrane proteins" neutrophil-degranulation reaction. As above, reflects degranulation-associated surface exposure rather than the core lysosomal site of action. Reason: Neutrophil-degranulation exocytosis context; peripheral to the core lysosomal function. Supporting Evidence: Reactome:R-HSA-6799350 Exocytosis of specific granule membrane proteins |
| GO:0035579 specific granule membrane | TAS Reactome:R-HSA-6799350 | KEEP AS NON CORE | Summary: Reactome annotation placing HGSNAT in the neutrophil specific granule membrane (neutrophil degranulation pathway). A real cell-type-specific granule localization but peripheral to the core lysosomal catabolic function of the enzyme. Reason: Cell-type-restricted neutrophil granule localization; not the core lysosomal site of action. Supporting Evidence: Reactome:R-HSA-6799350 Exocytosis of specific granule membrane proteins |
| GO:0070821 tertiary granule membrane | TAS Reactome:R-HSA-6798747 | KEEP AS NON CORE | Summary: Reactome annotation placing HGSNAT in the neutrophil tertiary granule membrane (neutrophil degranulation pathway). A real cell-type-specific granule localization but peripheral to the core lysosomal catabolic function. Reason: Cell-type-restricted neutrophil granule localization; not the core lysosomal site of action. Supporting Evidence: Reactome:R-HSA-6798747 Exocytosis of tertiary granule membrane proteins |
| GO:0005765 lysosomal membrane | HDA PMID:17897319 Integral and associated lysosomal membrane proteins. | ACCEPT | Summary: High-throughput direct assay (lysosomal-membrane proteomics of purified placental lysosomes) identifying HGSNAT among proteins residing in the lysosomal membrane. Independent experimental support for the core lysosomal membrane location. Reason: Experimental proteomic support for the core lysosomal membrane location. Supporting Evidence: PMID:17897319 proteins, known to reside at least partially in the lysosomal membrane PMID:17897319 In membranes purified from placental lysosomes |
| GO:0005765 lysosomal membrane | TAS Reactome:R-HSA-1678660 | ACCEPT | Summary: Reactome traceable-author annotation of lysosomal membrane location (from the "HGSNAT oligomer acetylates Heparan sulfate chain" reaction). Correct core location. Reason: Correct core location, curated by Reactome. Supporting Evidence: file:human/HGSNAT/HGSNAT-uniprot.txt Lysosome membrane |
| GO:0005765 lysosomal membrane | TAS Reactome:R-HSA-2090085 | ACCEPT | Summary: Reactome traceable-author annotation of lysosomal membrane location (from the "HGSNAT oligomer acetylates Heparan chain" reaction). Correct core location. Reason: Correct core location, curated by Reactome. Supporting Evidence: file:human/HGSNAT/HGSNAT-uniprot.txt Lysosome membrane |
| GO:0005765 lysosomal membrane | IDA PMID:19823584 Protein misfolding as an underlying molecular defect in muco... | ACCEPT | Summary: Direct assay: confocal immunofluorescence of recombinant HGSNAT in human fibroblasts showed near-complete colocalization with the lysosomal markers LysoTracker Red and LAMP-2. Strong experimental support for the core lysosomal membrane location. Reason: Direct experimental colocalization with lysosomal markers; core location. Supporting Evidence: PMID:19823584 almost completely co-localized with the lysosomal markers LysoTracker Red or LAMP-2 |
| GO:0005765 lysosomal membrane | IDA PMID:20650889 Analysis of the biogenesis of heparan sulfate acetyl-CoA:alp... | ACCEPT | Summary: Direct assay from the biogenesis/trafficking study: HGSNAT is a lysosomal membrane enzyme trafficked to and residing in lysosomes. Independent experimental support for the core location. Reason: Experimental support for the core lysosomal membrane location. Supporting Evidence: PMID:20650889 transported to the lysosomes via an adaptor protein-mediated pathway |
| GO:0007041 lysosomal transport | IDA PMID:20650889 Analysis of the biogenesis of heparan sulfate acetyl-CoA:alp... | MARK AS OVER ANNOTATED | Summary: IDA annotation to "lysosomal transport" derived from the biogenesis study, which characterized the adaptor-protein/tyrosine- and dileucine-signal-mediated trafficking of the HGSNAT precursor to lysosomes. That work describes how HGSNAT itself is delivered to the lysosome, not a role for HGSNAT in transporting cargo (the sense of GO:0007041, transport of substances into/within the lysosome). HGSNAT is an acetyltransferase, not a transporter, so this is an over-interpretation of the trafficking/targeting observation. Reason: The supporting paper documents targeting/biogenesis of HGSNAT to the lysosome, not HGSNAT-mediated lysosomal cargo transport; annotating the enzyme to "lysosomal transport" over-extends the trafficking observation. Not removed (it is an experimental annotation whose full text was read by the curator), but flagged as an over-annotation relative to HGSNAT's actual transferase function. Supporting Evidence: PMID:20650889 transported to the lysosomes via an adaptor protein-mediated pathway |
| GO:0016746 acyltransferase activity | IDA PMID:19823584 Protein misfolding as an underlying molecular defect in muco... | MODIFY | Summary: Direct assay of N-acetyltransferase activity of recombinant HGSNAT (activity of wild-type vs. MPS IIIC mutants was measured). The annotation captures this as the general parent term "acyltransferase activity", but the specific, exact-match child term GO:0015019 (heparan-alpha-glucosaminide N-acetyltransferase activity) is warranted because the assay measured precisely this activity. Reason: Correct in essence but too general; the measured activity is specifically heparan-alpha-glucosaminide N-acetyltransferase activity. Replace with the exact-match MF term. Proposed replacements: heparan-alpha-glucosaminide N-acetyltransferase activity Supporting Evidence: PMID:19823584 assayed for N-acetyltransferase activity file:human/HGSNAT/HGSNAT-uniprot.txt Lysosomal acetyltransferase that acetylates the non-reducing |
| GO:0051259 protein complex oligomerization | IDA PMID:20650889 Analysis of the biogenesis of heparan sulfate acetyl-CoA:alp... | KEEP AS NON CORE | Summary: Direct experimental demonstration that HGSNAT assembles into ~440-kDa homo-oligomers and that homo-oligomerization is required for enzymatic activity. This is a genuine, experimentally supported property, but it describes a biogenesis/assembly step rather than HGSNAT's core molecular function (acetyltransferase) or its core catabolic pathway role. Reason: Real and experimentally supported (homo-oligomerization needed for activity), but an assembly/biogenesis process peripheral to the core MF/BP; retain as non-core. Supporting Evidence: PMID:20650889 assembled into active ∼440-kDa oligomers file:human/HGSNAT/HGSNAT-uniprot.txt Homooligomerization is necessary for enzyme |
| GO:0016746 acyltransferase activity | IDA PMID:20650889 Analysis of the biogenesis of heparan sulfate acetyl-CoA:alp... | MODIFY | Summary: Direct assay of N-acetyltransferase activity in the biogenesis study (N-acetyltransferase activity was measured to track functional maturation of HGSNAT). As with the other acyltransferase IDA, the general parent term should be refined to the exact-match child GO:0015019, which precisely describes the activity assayed. Reason: Correct but too general; the assayed activity is specifically heparan-alpha-glucosaminide N-acetyltransferase activity. Replace with the exact-match MF term. Proposed replacements: heparan-alpha-glucosaminide N-acetyltransferase activity Supporting Evidence: PMID:20650889 of the acetyl group to the terminal glucosamine on heparan sulfate file:human/HGSNAT/HGSNAT-uniprot.txt Lysosomal acetyltransferase that acetylates the non-reducing |
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Download this section (compressed HTML)Q: What is the precise mechanism by which the active-site nucleophile accesses cytosolic acetyl-CoA and transfers the acetyl group across the lysosomal membrane to the luminal glucosamine substrate (ping-pong vs. direct transfer), and how do the recent cryo-EM structures explain this transmembrane catalysis?
Q: How does the requirement for intralysosomal proteolytic cleavage and homo-oligomer assembly relate to catalytic activation, and could these steps be therapeutic targets?
Experiment: Structure-guided mutagenesis of candidate acetyl-CoA-binding and catalytic residues in the cytosolic loops, combined with transmembrane-acetylation assays, to define the acetyl-transfer path across the membrane.
Experiment: Pharmacological-chaperone (e.g. glucosamine-analog) screens on a panel of misfolding MPS IIIC missense mutants to identify variants amenable to enzyme-enhancement therapy.
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