EXT1 is a type II membrane glycosyltransferase that forms the EXT1–EXT2 complex in the Golgi and elongates the heparan sulfate chains of proteoglycans. Its N-terminal GT-B domain supplies the principal beta-1,4-glucuronyl-transfer activity, adding glucuronic acid from UDP-glucuronate to a terminal N-acetylglucosamine. EXT1 also has alpha-1,4-N-acetylglucosaminyl-transfer capacity; EXT2 supplies the dominant GlcNAc-transfer activity in the assembled enzyme. Alternating sugar addition by the complex builds the backbone subsequently modified into heparan sulfate, which supports extracellular signaling and tissue organization. Loss of EXT1 function disrupts heparan sulfate synthesis and is associated with hereditary multiple exostoses.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000139 Golgi membrane | EXP PMID:10639137 The putative tumor suppressors EXT1 and EXT2 form a stable c... | ACCEPT | Summary: EXT1 localizes to Golgi membranes in the active EXT1–EXT2 assembly. Reason: PMID:10639137 Methods and Figure 2 describe human EXT1 expressed with mouse EXT2 in BHK cells: co-expression redistributes the proteins from a predominantly ER pattern to a compartment positive for the Golgi 58K marker. The full-length membrane-protein context supports Golgi membrane localization. This is a human EXT1 experiment in a hamster host with a mouse partner, not endogenous imaging of an all-human complex. Independent human constructs in PMID:35137078 Figure 7A colocalize with GM130. |
| GO:0000139 Golgi membrane | EXP PMID:10679296 Association of EXT1 and EXT2, hereditary multiple exostoses ... | ACCEPT | Summary: Independent original studies support the Golgi membrane assertion. Reason: The PMID:10679296 abstract positively reports Golgi localization and oligomerization in COS-7 cells, but its complete Methods and figures were not recovered. Retain the source assertion because independently read PMID:10639137 Figure 2 and PMID:35137078 Figure 7A establish Golgi localization of full-length human EXT1 constructs, combined with its membrane-anchor context. Those experiments corroborate the location; they do not verify the unread paper’s exact insert, antibody or imaging protocol. |
| GO:0000139 Golgi membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Retain the exact Golgi-membrane subcellular mapping. Reason: SL-0134 maps exactly to Golgi membrane. Native human Reactome route 2022851 →2022845 →2022878 →2022897 identifies Q16394 residues 1–746 with its own Golgi-membrane compartment. This supports the location independently of event-level lumen labels or soluble structural constructs lacking the membrane anchor. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB-SubCell:SL-0134 SUPPORTS TRANSFER Current exact mapping corroborated by the native human Reactome participant, separately from soluble recombinant structural constructs. Supporting Evidence: GO_REF:0000044 Reactome:R-HSA-2022851 |
| GO:0000139 Golgi membrane | TAS PMID:12907669 In vitro polymerization of heparan sulfate backbone by the E... | ACCEPT | Summary: Golgi membrane is the cellular location of the membrane-anchored EXT1 polymerase. Reason: PMID:12907669 Results identifies EXT proteins as Golgi type-II membrane proteins, while its operative enzyme preparations deliberately lack their N-terminal anchors and are secreted by engineered leader sequences. The TAS location is corroborated by actual human EXT1 localization in PMID:10639137 and PMID:35137078. Secretion of the soluble 29–746 construct is an expression strategy and is not evidence that endogenous EXT1 normally acts extracellularly. |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-2022851 | ACCEPT | Summary: EXT1 has a supported Golgi-membrane location in this explicitly traced Reactome context. Reason: Actual native catalyst route 2022851 → 2022845 → 2022878 → 2022897 reaches human Q16394 residues 1–746. The protein itself has Golgi-membrane compartment, independently of event-level lumen labels. This is a curated participant model, not a new assay of membrane topology or autonomous subunit chemistry. Supporting Evidence: Reactome:R-HSA-2022851 |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-2022856 | ACCEPT | Summary: EXT1 has a supported Golgi-membrane location in this explicitly traced Reactome context. Reason: Actual native catalyst route 2022856 → 2022849 → 2022878 → 2022897 reaches human Q16394 residues 1–746. The protein itself has Golgi-membrane compartment, independently of event-level lumen labels. This is a curated participant model, not a new assay of membrane topology or autonomous subunit chemistry. Supporting Evidence: Reactome:R-HSA-2022856 |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-2076392 | ACCEPT | Summary: EXT1 has a supported Golgi-membrane location in this explicitly traced Reactome context. Reason: Actual native catalyst route 2076392 → 2022849 → 2022878 → 2022897 reaches human Q16394 residues 1–746. The protein itself has Golgi-membrane compartment, independently of event-level lumen labels. This is a curated participant model, not a new assay of membrane topology or autonomous subunit chemistry. Supporting Evidence: Reactome:R-HSA-2076392 |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-3656254 | ACCEPT | Summary: EXT1 has a supported Golgi-membrane location in this explicitly traced Reactome context. Reason: Although this event models defective EXT2, route 3656254 → 9631891 → 9036278 → 2022897 retains native EXT1/Q16394 residues 1–746 with its own Golgi-membrane compartment. Retain the location and disease context without interpreting failed complex transfer as absence of native EXT1 activity. Supporting Evidence: Reactome:R-HSA-3656254 Reactome:R-HSA-2022851 |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-3656257 | ACCEPT | Summary: EXT1 has a supported Golgi-membrane location in this explicitly traced Reactome context. Reason: Route 3656257 → 9631805 → 9036279 → 3730938 → 3730975 reaches a defective-EXT1 set, not native-only protein. Its frameshift endpoints 250/497 and literal R339L label remain source descriptions; current Q16394 has G339/R340, so that variant nomenclature is unresolved and is not remapped. Native 2022851 →2022845 →2022878 →2022897 independently supports Golgi membrane; the original disease citation is preserved. Supporting Evidence: Reactome:R-HSA-3656257 Reactome:R-HSA-2022851 |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-3656261 | ACCEPT | Summary: EXT1 has a supported Golgi-membrane location in this explicitly traced Reactome context. Reason: Route 3656261 → 9631718 → 9036279 → 3730938 → 3730975 reaches a defective-EXT1 set, not native-only protein. Its frameshift endpoints 250/497 and literal R339L label remain source descriptions; current Q16394 has G339/R340, so that variant nomenclature is unresolved and is not remapped. Native 2022851 →2022845 →2022878 →2022897 independently supports Golgi membrane; the original disease citation is preserved. Supporting Evidence: Reactome:R-HSA-3656261 Reactome:R-HSA-2022851 |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-3656267 | ACCEPT | Summary: EXT1 has a supported Golgi-membrane location in this explicitly traced Reactome context. Reason: Although this event models defective EXT2, route 3656267 → 9631865 → 9036278 → 2022897 retains native EXT1/Q16394 residues 1–746 with its own Golgi-membrane compartment. Retain the location and disease context without interpreting failed complex transfer as absence of native EXT1 activity. Supporting Evidence: Reactome:R-HSA-3656267 Reactome:R-HSA-2022851 |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-9036285 | ACCEPT | Summary: EXT1 has a supported Golgi-membrane location in this explicitly traced Reactome context. Reason: Route 9036285 → 9631886 → 9036279 → 3730938 → 3730975 reaches a defective-EXT1 set, not native-only protein. Its frameshift endpoints 250/497 and literal R339L label remain source descriptions; current Q16394 has G339/R340, so that variant nomenclature is unresolved and is not remapped. Native 2022851 →2022845 →2022878 →2022897 independently supports Golgi membrane; the original disease citation is preserved. Supporting Evidence: Reactome:R-HSA-9036285 Reactome:R-HSA-2022851 |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-9036289 | ACCEPT | Summary: EXT1 has a supported Golgi-membrane location in this explicitly traced Reactome context. Reason: Although this event models defective EXT2, route 9036289 → 9631922 → 9036278 → 2022897 retains native EXT1/Q16394 residues 1–746 with its own Golgi-membrane compartment. Retain the location and disease context without interpreting failed complex transfer as absence of native EXT1 activity. Supporting Evidence: Reactome:R-HSA-9036289 Reactome:R-HSA-2022851 |
| GO:0000271 polysaccharide biosynthetic process | IDA PMID:12907669 In vitro polymerization of heparan sulfate backbone by the E... | MODIFY | Summary: The experimentally established polymerization belongs to heparan-sulfate biosynthesis. Reason: PMID:12907669 Methods and Figures 2–5 demonstrate extension of K5-derived heparan-sulfate backbone acceptors by soluble human EXT1 preparations, with linkage-sensitive product digestion. Replace the broad polysaccharide process with the specific heparan-sulfate proteoglycan process already represented for this gene. This is an evidence-based lateral specificity correction, not a claimed current ontology descendant: the 2003 acceptors lack a core protein, whereas independent PMID:36593275 directly assays glypican-1-associated chain elongation. EXT1 catalyzes chain elongation; it does not synthesize the core protein or every linker residue. Proposed replacements: heparan sulfate proteoglycan biosynthetic process |
| GO:0001503 ossification | IEA GO_REF:0000117 | UNDECIDED | Summary: The ossification assertion remains unresolved despite a matching rule and positive skeletal-disease evidence. Reason: ARBA00090120 disjunct 15 matches the target FunFam and vertebrate lineage. PMID:7550340 links human EXT1 disruption to familial exostoses, but only its abstract/subscription preview was recovered. GO:0001503 denotes bone formation; this evidence does not resolve what work EXT1 performs in that process or verify the source-specific experimental assertion. The rule match is real, but it does not supply the missing biological interpretation. Retain UNDECIDED consistently with the imported IMP row, without asserting that either annotation is false. Reciprocal mouse transfer from human is not an independent experiment. Propagation Review Root cause: UNRESOLVED Sources checked: ARBA:ARBA00090120 UNRESOLVED The current target-matching rule and its ossification output are verified; available clinical evidence does not resolve participation in bone formation. This is an evidence/interpretation limit, not a demonstrated rule mismatch. |
| GO:0001503 ossification | IMP PMID:7550340 Cloning of the putative tumour suppressor gene for hereditar... | UNDECIDED | Summary: The source-specific ossification assertion remains unresolved. Reason: PMID:7550340 identifies human EXT1 through patient breakpoints and familial frameshift mutations associated with multiple exostoses. The publisher exposes an abstract and subscription preview, and the complete operative original was not recovered after bounded alternate-route searches. This establishes a strong skeletal-disease connection but does not let this review verify the particular ossification experiment or its interpretation. GO:0001503 concerns bone formation; do not remove the curator’s IMP assertion from an abstract-only assessment or equate disease association alone with a directly assayed bone-forming role. Supporting Evidence: |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | MODIFY | Summary: Refine the supported EXT1–EXT2 association to protein heterodimerization. Reason: The released BioPlex 293T interaction table records EXT2 Q93063-2 with EXT1 Q16394 (pInt 0.99473832262035). Its human ORFeome FLAG–HA/anti-HA cellular co-purification supports association, not a purified binary interface or stoichiometry. A/B ordering does not identify bait direction, and absent HCT116 target baits limit that comparison. Independently, PMID:36402845 resolves a purified human EXT1–EXT2 1:1 heterodimer by cryo-EM and mass photometry. Refine the generic association to protein heterodimerization using that independent structural evidence. The BioPlex historical isoform label and unverified target-insert sequence remain distinct from the engineered structural constructs. Proposed replacements: protein heterodimerization activity |
| GO:0005515 protein binding | IPI PMID:35137078 A dominant negative splice variant of the heparan sulfate bi... | KEEP AS NON CORE | Summary: Human EXT1 associates with NDST1 in a cellular FRET assay. Reason: PMID:35137078 Methods specifies full-length human EXT1 and human NDST1A/NDST1B fused to fluorescent proteins and expressed in COS-7 cells. Figure 7B shows positive EXT1/NDST1A and EXT1/NDST1B FRET, and Figure 7C localizes signal to cells expressing both partners. This supports the imported P52848 association without assigning EXT1 the NDST catalytic reactions, proving a purified binary interface or establishing a fixed multienzyme stoichiometry. Retain supported generic binding as non-core. Supporting Evidence: |
| GO:0005515 protein binding | IPI PMID:35137078 A dominant negative splice variant of the heparan sulfate bi... | MODIFY | Summary: Refine the supported EXT1–EXT2 association to protein heterodimerization. Reason: PMID:35137078 Figure 7B shows FRET between full-length human EXT1 and EXT2 expressed as fluorescent fusions in COS-7 cells; Figure 7A shows Golgi-marker overlap. FRET establishes cellular proximity, not fixed stoichiometry. Independent purified human complex structure and mass photometry in PMID:36402845 establish the specific heterodimer. Refine generic binding to protein heterodimerization without attributing that stoichiometric measurement to the FRET assay. Proposed replacements: protein heterodimerization activity |
| GO:0005515 protein binding | IPI PMID:36402845 Structure of the human heparan sulfate polymerase complex EX... | MODIFY | Summary: Refine the supported EXT1–EXT2 association to protein heterodimerization. Reason: PMID:36402845 purification Results and Figure 1 describe soluble human EXT1 residues 29–746 and EXT2 residues 47–718 coexpressed in HEK293F cells, comparable subunit abundance and a principal mass-photometry peak consistent with a heterodimer. Cryo-EM resolves a tightly associated 1:1 complex. This directly supports protein heterodimerization, a more informative term for the specified EXT2 partner than generic binding. The engineered soluble constructs do not establish physiological secretion or an additional natural product. Proposed replacements: protein heterodimerization activity Supporting Evidence: |
| GO:0005515 protein binding | IPI PMID:36593275 Structural basis for heparan sulfate co-polymerase action by... | MODIFY | Summary: Refine the supported EXT1–EXT2 association to protein heterodimerization. Reason: PMID:36593275 uses human EXT1 residues 28–746 and EXT2 residues 46–718 coexpressed in HEK293F cells and purified after affinity-tag cleavage. Structural analysis resolves the heterodimer, which extends heparan-sulfate acceptors. Refine generic binding to the directly supported protein heterodimerization activity. Poor recovery and proteolysis of separately expressed EXT1 do not negate this association or prove that self-association never occurs in other preparations. Proposed replacements: protein heterodimerization activity Supporting Evidence: |
| GO:0005515 protein binding | IPI PMID:40205054 Multimodal cell maps as a foundation for structural and func... | MODIFY | Summary: Refine the supported EXT1–EXT2 association to protein heterodimerization. Reason: The released PMID:40205054 NDEx AP–MS network records EXT1 as bait and EXT2 as non-bait in U2OS cells. Human ORFeome C-terminal HA–Flag constructs, anti-HA capture and mass spectrometry establish cellular co-purification; the inspected assembly/AlphaFold tables are not the positive pair source. Independent human structural and mass-photometry evidence in PMID:36402845 establishes a 1:1 EXT1–EXT2 heterodimer, supporting refinement to protein heterodimerization. The AP–MS experiment itself does not measure stoichiometry, purified affinity or the exact historical insert sequence. Proposed replacements: protein heterodimerization activity |
| GO:0005783 endoplasmic reticulum | IDA PMID:10679296 Association of EXT1 and EXT2, hereditary multiple exostoses ... | KEEP AS NON CORE | Summary: ER localization is retained with an explicit independent-source boundary. Reason: The full PMID:10679296 original remains unavailable; its abstract does not resolve the imported ER experiment. Independent PMID:9620772 reports an ER pattern for tagged human EXT1 in mouse/CHO hosts, and PMID:10639137 shows EXT2-dependent redistribution toward Golgi. Retain the positive historical ER context as non-core without calling it an artifact or claiming that the unread source protocol has been verified. The source guide distinguishes this context from newer ER imaging. |
| GO:0005783 endoplasmic reticulum | IDA PMID:9620772 The putative tumour suppressor EXT1 alters the expression of... | KEEP AS NON CORE | Summary: The historical ER localization is positive but context-dependent. Reason: Actual PMID:9620772 uses human HeLa-derived EXT1 with C-terminal Myc/His tags in CHO and mouse cell backgrounds. Figure 3 and the operative Results describe an ER pattern; calnexin double staining is reported as data not shown. Deleting the N-terminal 26 residues redistributes the construct, and endoglycosidase-H-sensitive glycosylation supports entry into the secretory pathway. PMID:10639137 shows that co-expression with EXT2 shifts the predominantly ER-localized protein to Golgi. Retain this positive ER context as non-core, without calling it an artifact or the exclusive steady-state location of native EXT1. |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:10679296 Association of EXT1 and EXT2, hereditary multiple exostoses ... | KEEP AS NON CORE | Summary: ER membrane association is supported in the historical expression context. Reason: The complete PMID:10679296 original is unavailable, so its precise membrane-localization assay is not independently verified. Actual PMID:9620772 combines the human EXT1 N-terminal anchor, loss of ER targeting after its deletion, secretory-pathway glycosylation and ER-pattern microscopy; membrane topology is partly inferred rather than a protease-protection measurement. Retain ER membrane as a supported non-core compartment, distinct from the active Golgi complex; do not convert limited source access into evidence against the assertion. |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Retain ER-membrane localization as a supported non-core compartment. Reason: Mouse P97464 has ER-membrane source records for PMID:9620772 and PMID:9703997. The database trace was established from donor metadata; the PMID:9620772 original was subsequently recovered and read in the direct-evidence review, while PMID:9703997 remains abstract-only. Independent PMID:33962942 Figure 6/Methods localizes tagged EXT1 to ER sheets/tubules in Cos7; PMID:27229929 Methods supplies human-ORFeome provenance, not historical insert sequencing. SL-0097 is exact and the Ensembl donor matches P97464. Current ARBA00028685 has no matching target disjunct, a retained version gap rather than evidence against the independently supported location. The positive ER pool is retained as non-core; it does not establish the ER membrane as the principal location of the active EXT1–EXT2 elongation complex, which is supported in the Golgi. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: UniProtKB:P97464 SUPPORTS TRANSFER Actual donor identity, 746-aa MSA/Ensembl identity and experimental source trace are verified. PMID:9620772 was subsequently read in the direct-evidence review; PMID:9703997 remains abstract-only. Independent tagged human-construct imaging supports ER residence. ARBA:ARBA00028685 UNRESOLVED Current endpoint exists but none of its present disjuncts matches the captured target features; historical rule version is unresolved. Other source branches and independent primary evidence support the location. ensembl:ENSMUSP00000076505 SUPPORTS TRANSFER Current mouse Ensembl sequence exactly equals P97464, not an additional independent experiment. UniProtKB-SubCell:SL-0097 SUPPORTS TRANSFER Exact ER-membrane mapping verified. |
| GO:0005789 endoplasmic reticulum membrane | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Retain ER-membrane localization as a supported non-core compartment. Reason: The mouse P97464 donor is verified by exact 746-residue MSA/Ensembl identity and ER-membrane source records PMID:9620772/PMID:9703997. The database trace used donor metadata; the PMID:9620772 original was subsequently recovered and read in the direct-evidence review, while PMID:9703997 remains unavailable. Independent PMID:33962942 tagged imaging supports ER residence; PMID:27229929 identifies the human-ORFeome construct source. Cos7 host, tag, optical-resolution and historical insert-sequence limits remain explicit; no endogenous human occupancy measurement is claimed. The positive ER pool is retained as non-core; it does not establish the ER membrane as the principal location of the active EXT1–EXT2 elongation complex, which is supported in the Golgi. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: UniProtKB:P97464 SUPPORTS TRANSFER Actual donor identity, 746-aa MSA/Ensembl identity and experimental source trace are verified. PMID:9620772 was subsequently read in the direct-evidence review; PMID:9703997 remains abstract-only. Independent tagged human-construct imaging supports ER residence. |
| GO:0005789 endoplasmic reticulum membrane | NAS PMID:9620772 The putative tumour suppressor EXT1 alters the expression of... | KEEP AS NON CORE | Summary: The N-terminally anchored EXT1 construct supports the stated ER membrane context. Reason: PMID:9620772 Figure 5 presents a type-II membrane model, supported by an N-terminal hydrophobic anchor, ER localization and N-linked glycosylation of human EXT1 constructs. Removing the first 26 residues disrupts the ER pattern and functional rescue. The topology is not a directly measured protease-protection assay. Retain the NAS membrane statement as non-core, with the later EXT2-dependent Golgi redistribution distinguishing this context from the principal active enzyme location. |
| GO:0005794 Golgi apparatus | IBA GO_REF:0000033 | MODIFY | Summary: Refine the supported ancestral Golgi location to Golgi membrane. Reason: The actual PTHR11062 tree places exact 746-residue human leaf PTN002476064 below the positive Golgi IBD at PTN000115158, with no target-path NOT. Current PTHR48261 classification is recorded separately. Native human Reactome 2022851 →2022845 →2022878 →2022897 supplies the finer Golgi-membrane compartment; the conserved broad PAINT placement is valid, and target self-WITH is not circular. Propagation Review Root cause: TERM SCOPING PROBLEM Sources checked: PANTHER:PTN000115158 SUPPORTS TRANSFER Actual target descends from this positive Golgi IBD; independent native participant supplies the finer membrane localization. Proposed replacements: Golgi membrane Supporting Evidence: GO_REF:0000033 Reactome:R-HSA-2022851 |
| GO:0005794 Golgi apparatus | IDA PMID:10639137 The putative tumor suppressors EXT1 and EXT2 form a stable c... | MODIFY | Summary: Golgi membrane is more precise than the whole Golgi apparatus. Reason: PMID:10639137 Figure 2 resolves Golgi-marker colocalization after co-expression of membrane-anchored human EXT1 and mouse EXT2. Together with the full-length protein’s N-terminal anchor and independent human-construct GM130 localization in PMID:35137078, the evidence supports the already-carried Golgi membrane term. Its current part_of ancestry includes Golgi apparatus. The refinement does not claim that fluorescence alone resolves membrane leaflets. Proposed replacements: Golgi membrane |
| GO:0005794 Golgi apparatus | IDA PMID:10679296 Association of EXT1 and EXT2, hereditary multiple exostoses ... | MODIFY | Summary: Independent evidence supports refining the broad Golgi location to Golgi membrane. Reason: PMID:10679296 has a positive Golgi-localization abstract but its complete original remains unavailable. The actual human EXT1 experiments in PMID:10639137 and PMID:35137078 independently resolve the location in its membrane-protein context, supporting GO:0000139 as a refinement of the broad organelle term. Preserve the original reference and qualifier; this does not claim that the unread experiment’s construct or image has been verified. Proposed replacements: Golgi membrane |
| GO:0005794 Golgi apparatus | IEA GO_REF:0000120 | MODIFY | Summary: Refine the correct broad Golgi mapping to Golgi membrane. Reason: ARBA00028708 disjunct 140 matches the target; SL-0132 correctly maps Golgi apparatus. The mouse donor metadata cites PMID:10639137; its original was subsequently recovered and read in the direct-evidence review, which distinguishes human EXT1 from mouse/bovine EXT2 and host cells. Native human Reactome entity 2022897 independently has its own Golgi-membrane location, justifying this finer compartment without deriving membrane topology from soluble structures. Propagation Review Root cause: TERM SCOPING PROBLEM Sources checked: ARBA:ARBA00028708 SUPPORTS TRANSFER Current target-matching Golgi rule. UniProtKB:P97464 SUPPORTS TRANSFER Current donor IDA PMID:10639137 supports Golgi. Its original was subsequently read in the direct-evidence review with the human-construct, partner and host distinctions retained. ensembl:ENSMUSP00000076505 SUPPORTS TRANSFER Exact mouse donor identity. UniProtKB-SubCell:SL-0132 SUPPORTS TRANSFER Valid broad mapping; native human target compartment supports refinement. Proposed replacements: Golgi membrane Supporting Evidence: GO_REF:0000120 Reactome:R-HSA-2022851 |
| GO:0006024 glycosaminoglycan biosynthetic process | IDA PMID:12907669 In vitro polymerization of heparan sulfate backbone by the E... | MODIFY | Summary: EXT1 elongates heparan-sulfate chains rather than an unspecified glycosaminoglycan. Reason: The purified human preparations in PMID:12907669 extend K5-derived HS-backbone oligosaccharides, and glycosidase digestion verifies the expected alternating products. The more informative existing gene-level process is heparan-sulfate proteoglycan biosynthesis, independently corroborated on glypican-1 by PMID:36593275. This is a lateral evidence-based specificity correction: the current ontology does not place GO:0015012 below the imported glycosaminoglycan biosynthetic process. The 2003 assay alone does not reconstitute core-protein or linker synthesis. Proposed replacements: heparan sulfate proteoglycan biosynthetic process |
| GO:0008375 acetylglucosaminyltransferase activity | IBA GO_REF:0000033 | MODIFY | Summary: Specify the heparan-chain GlcNAc-transfer activity while preserving secondary-activity limits. Reason: Positive acetylglucosaminyltransferase IBD at PTN000869020 lies on the exact human PTHR11062 target path; current PTHR48261 classification is distinguished. Human protein experiments in PMID:36402845 support EXT1 GT-A involvement and PMID:36593275 reports low qualitative EXT1-only GlcNAc activity despite EXT2 dominance. Refine to the existing HS-chain reaction GO:0050508, without assigning the heterocomplex rate or equal subunit contributions to autonomous EXT1. Propagation Review Root cause: TERM SCOPING PROBLEM Sources checked: PANTHER:PTN000869020 SUPPORTS TRANSFER Actual positive acetylglucosaminyltransferase IBD is on target ancestry; independent target biochemistry supports a substrate-specific refinement. |
| GO:0009101 glycoprotein biosynthetic process | IEA GO_REF:0000002 | MODIFY | Summary: Specify heparan-sulfate proteoglycan biosynthesis within the broad glycoprotein process. Reason: The target genuinely matches IPR004263 and its broad glycoprotein-biosynthesis mapping. Human protein experiments in PMID:36593275 directly extend glypican-1-associated HS and defined oligosaccharides; PMID:36402845 separately tests catalytic mutants and full-length cell complementation. The existing HS-proteoglycan synthesis term specifies this catalytic work without assigning generic N-glycan initiation or dolichol-linked Alg chemistry. Propagation Review Root cause: TERM SCOPING PROBLEM Sources checked: InterPro:IPR004263 SUPPORTS TRANSFER Actual family/domain match and broad mapping; target substrate assays justify the narrower process. Proposed replacements: heparan sulfate proteoglycan biosynthetic process |
| GO:0015012 heparan sulfate proteoglycan biosynthetic process | IDA PMID:10639137 The putative tumor suppressors EXT1 and EXT2 form a stable c... | ACCEPT | Summary: Human EXT1 performs the chain-elongation step of heparan-sulfate proteoglycan biosynthesis. Reason: PMID:10639137 combines rescue of cell-surface HS in EXT1-defective mouse sog9 cells by human EXT1 with sugar-transfer assays of immunoprecipitated EXT proteins. Table 1 distinguishes human EXT1 from mouse or bovine EXT2 partners and shows strong GlcA transfer in the assembled preparations. These cellular and biochemical results support participation, not merely a disease association. The original immunoprecipitates are not purified EXT1 alone; later human structural and polymerization studies independently resolve the catalytic contribution. |
| GO:0015012 heparan sulfate proteoglycan biosynthetic process | IDA PMID:36593275 Structural basis for heparan sulfate co-polymerase action by... | ACCEPT | Summary: EXT1 directly catalyzes chain elongation during heparan sulfate proteoglycan biosynthesis. Reason: PMID:36593275 Results and Figure 4 show extension of heparan-sulfate chains on glypican-1 and synthetic oligosaccharide acceptors by purified human EXT1–EXT2. The UDP-Glo readout is corroborated by glypican mobility shifts and MALDI product analysis; EXT1 GT-B active-site mutations impair glucuronic-acid transfer and co-polymerization. EXT1 therefore performs a biosynthetic step, beyond merely being necessary in a knockout. The source is represented once in the seeded review although the preserved raw import has matching UniProt and ComplexPortal assertions. This does not imply that EXT1 synthesizes the proteoglycan core protein or the complete linkage region. Supporting Evidence: |
| GO:0015012 heparan sulfate proteoglycan biosynthetic process | IDA PMID:9620772 The putative tumour suppressor EXT1 alters the expression of... | ACCEPT | Summary: Human EXT1 restores heparan-sulfate production in deficient cells. Reason: PMID:9620772 uses a HeLa-derived human EXT1 cDNA in mouse sog9 and related cell backgrounds. Figure 2 and the HPLC Methods show restoration or alteration of metabolically labeled HS, with glycosidase-sensitive identity, alongside the HSV attachment assay. The directly measured product supports the biosynthetic process. The 1998 work does not itself identify an isolated enzyme reaction; independent purified human EXT1–EXT2 studies establish chain elongation as the step performed by EXT1. |
| GO:0015012 heparan sulfate proteoglycan biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: EXT1 performs a catalytic step in heparan-sulfate proteoglycan synthesis. Reason: ARBA00026373 disjuncts 0/7 match the target; mouse P97464 and its exact Ensembl product have experimental PMID:10639137 grounding. Independently, human-protein studies PMID:36402845 and PMID:36593275 directly measure glycan extension and EXT1 catalytic-site contributions, including glypican-1-associated HS. Retain actual synthetic participation, not merely knockout necessity; older donor-body and soluble-construct limits remain separate. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:P97464 SUPPORTS TRANSFER Current experimental IDA/IMP10639137 and later perturbation sources trace the donor process; independent human chemistry is decisive here. ARBA:ARBA00026373 SUPPORTS TRANSFER Current disjuncts 0 and 7 match the target InterPro/FunFam/lineage features. ensembl:ENSMUSP00000076505 SUPPORTS TRANSFER Exact sequence identity to mouse P97464 verified. |
| GO:0015012 heparan sulfate proteoglycan biosynthetic process | IMP PMID:17761672 Contribution of EXT1, EXT2, and EXTL3 to heparan sulfate cha... | ACCEPT | Summary: Human EXT1 depletion shortens the heparan-sulfate chains made by HEK293 cells. Reason: PMID:17761672 Methods specifies human-targeted siRNAs, quantitative transcript checks and metabolic labeling of HEK293 glycosaminoglycans. Results and Figure 2 show shorter HS chains after EXT1 depletion, with a lower HS/chondroitin-sulfate ratio. This agrees with the independently measured EXT1 polymerase reaction. The paper’s full-length overexpression constructs are mouse brain cDNAs and must not be recast as human constructs; the human IMP support is the separate endogenous knockdown experiment. |
| GO:0015012 heparan sulfate proteoglycan biosynthetic process | ISS GO_REF:0000024 | ACCEPT | Summary: EXT1 performs a catalytic step in heparan-sulfate proteoglycan synthesis. Reason: Mouse P97464 has experimental IDA/IMP PMID:10639137 supporting HS synthesis. The database trace was established from donor metadata; that original was subsequently recovered and read in the direct-evidence review. Independent glycan-extension and catalytic-site assays on human proteins in PMID:36402845 and PMID:36593275 establish EXT1 participation, including glypican-1-associated HS extension. This supports the conserved process without equating soluble expression constructs with additional native products. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:P97464 SUPPORTS TRANSFER Current experimental IDA/IMP10639137 and later perturbation sources trace the donor process; independent human chemistry is decisive here. |
| GO:0015020 glucuronosyltransferase activity | IBA GO_REF:0000033 | MODIFY | Summary: Specify the heparan-chain beta-glucuronosyltransferase reaction. Reason: The positive glucuronosyltransferase IBD at PTN000869020 lies on the exact human PTHR11062 target path. Human protein experiments in PMID:36402845/PMID:36593275 independently assign robust GlcA transfer to EXT1 GT-B. Refine the valid broad activity to GO:0050509, specifying UDP-GlcA addition to terminal GlcNAc in the HS precursor; preserve the separate current-family classification and truncated-construct boundaries. Propagation Review Root cause: TERM SCOPING PROBLEM Sources checked: PANTHER:PTN000869020 SUPPORTS TRANSFER Positive glucuronosyltransferase IBD and exact target descent; independent EXT1 GT-B biochemistry specifies the substrate. Proposed replacements: N-acetylglucosaminyl-proteoglycan 4-beta-glucuronosyltransferase activity |
| GO:0016020 membrane | IEA GO_REF:0000002 | MODIFY | Summary: Specify the supported Golgi and ER membrane compartments. Reason: IPR015338 is a real GT64-domain match, but does not alone locate this protein. Native human Reactome 2022897 establishes modeled Golgi-membrane residence; independent PMID:33962942 tagged imaging with PMID:27229929 human-ORFeome provenance supports ER residence in Cos7 hosts. The positive PMID:9620772 original was subsequently recovered and read in the direct-evidence review; PMID:9703997 remains abstract-only. Specify both supported compartments without claiming optical membrane topology or universal distribution. Propagation Review Root cause: TERM SCOPING PROBLEM Sources checked: InterPro:IPR015338 SUPPORTS TRANSFER Target domain match verified; compartment refinement is independently anchored, not inferred from the domain alone. Proposed replacements: Golgi membrane endoplasmic reticulum membrane Supporting Evidence: GO_REF:0000002 Reactome:R-HSA-2022851 |
| GO:0016757 glycosyltransferase activity | IEA GO_REF:0000120 | MODIFY | Summary: Specify the two supported heparan-chain sugar-transfer activities. Reason: The InterPro family/domain and exact mouse/Ensembl mappings support glycosyltransferase activity. Human protein experiments in PMID:36402845/PMID:36593275 define robust EXT1 GlcA transfer and bounded secondary GlcNAc activity. Refine to existing GO:0050509 and GO:0050508; the latter retains low qualitative EXT1 activity and does not assign EXT2’s dominant rate to isolated EXT1. Propagation Review Root cause: TERM SCOPING PROBLEM Sources checked: UniProtKB:P97464 SUPPORTS TRANSFER Mouse experimental PMID:10639137 grounding traced; independent human structural/biochemical evidence used. ensembl:ENSMUSP00000076505 SUPPORTS TRANSFER Exact mouse donor identity. InterPro:IPR004263 SUPPORTS TRANSFER Target family match verified. InterPro:IPR015338 SUPPORTS TRANSFER Target GT64-domain match verified, interpreted with actual catalytic assays. |
| GO:0042328 heparan sulfate N-acetylglucosaminyltransferase activity | NAS PMID:12907669 In vitro polymerization of heparan sulfate backbone by the E... | MODIFY | Summary: Use the specific alpha-1,4-GlcNAc-transferase reaction demonstrated on HS acceptors. Reason: PMID:12907669 directly measures GlcNAc transfer from UDP-GlcNAc onto GlcA-terminated HS-backbone acceptors and verifies polymer products with alpha-N-acetylglucosaminidase/beta-glucuronidase digestion. GO:0050508 specifies that reaction more precisely than the unspecific linkage in GO:0042328. This is a reaction-specific replacement, not a claimed current is_a relationship. The assay uses soluble human EXT1 preparations; newer work assigns most complex GlcNAc transfer to EXT2 while retaining low qualitative EXT1-alone activity, so no universal or equally efficient autonomous EXT1 reaction is inferred. |
| GO:0042803 protein homodimerization activity | IDA PMID:12907669 In vitro polymerization of heparan sulfate backbone by the E... | KEEP AS NON CORE | Summary: Soluble human EXT1 has positive apparent homodimerization evidence. Reason: PMID:12907669 Results and Figure 6 report that soluble human EXT1 elutes near 160 kDa on Superose-12, whereas EXT2 alone elutes near 80 kDa. The authors use isoelectric focusing to assess endogenous-partner contamination, but that supporting result is not displayed. Together with independent tagged EXT1 self-association in PMID:10639137 Figure 3, this supports the imported homodimerization capacity. It does not establish an obligate native homodimer or replace the strongly characterized EXT1–EXT2 catalytic assembly; retain it as non-core. The modern structures establish a heterodimer for coexpressed, copurified EXT1–EXT2 preparations; their lack of homodimer particles does not exclude the conditional self-association seen in the older separately expressed or tagged preparations. Apparent mass and co-IP are less definitive than a resolved homodimer structure, so this remains a bounded non-core capacity rather than the native catalytic assembly. |
| GO:0043541 UDP-N-acetylglucosamine transferase complex | IPI PMID:36593275 Structural basis for heparan sulfate co-polymerase action by... | MODIFY | Summary: EXT1 belongs to the EXT1–EXT2 heparan sulfate polymerase complex; the imported term names a different biosynthetic assembly. Reason: The current definition of GO:0043541 specifies the Alg7/Alg13/Alg14 assembly that produces dolichol-linked oligosaccharides, rather than any complex using UDP-GlcNAc. PMID:36593275 instead characterizes the human EXT1–EXT2 heterodimer during heparan-sulfate elongation. Live ComplexPortal CPX-8307 confirms the two human subunits but still carries GO:0043541 as its cellular-component cross-reference, matching the ComplexPortal-assigned imported row. Replace it with the current, unrestricted GO:0120504 EXT1-EXT2 complex, whose definition explicitly identifies this heterodimer and cites both structural studies. Preserve the source term and qualifier in the imported object; the proposed correction changes the reviewed term, not the observed association. Proposed replacements: EXT1-EXT2 complex |
| GO:0046982 protein heterodimerization activity | IPI PMID:12907669 In vitro polymerization of heparan sulfate backbone by the E... | ACCEPT | Summary: EXT1 heterodimerizes with EXT2 to form the catalytic assembly. Reason: PMID:12907669 co-expresses soluble human FLAG-EXT1 and Myc/His-EXT2, captures the associated proteins and detects both tags in the approximately 160-kDa gel-filtration fractions. Its membrane-deleted constructs are defined explicitly. Independent human cryo-EM and mass-photometry work resolves the corresponding heterodimer. This is a specific assembly facet of the polymerase, unlike unspecified protein binding; it does not imply that the secreted constructs are additional natural products. |
| GO:0050508 glucuronosyl-N-acetylglucosaminyl-proteoglycan 4-alpha-N-acetylglucosaminyltransferase activity | IDA PMID:12907669 In vitro polymerization of heparan sulfate backbone by the E... | ACCEPT | Summary: Human EXT1 preparations catalyze the specified GlcNAc-transfer reaction. Reason: PMID:12907669 Figures 2–5 and Methods demonstrate GlcNAc transfer from UDP-GlcNAc to GlcA-terminated acceptors by affinity-captured soluble human EXT1, with alternating-chain products and linkage-sensitive digestion. The study tests human residues 29–746, not every possible transcript. Later PMID:36593275 assigns the dominant GlcNAc site in the heterocomplex to EXT2, but also reports low qualitative EXT1-alone activity; PMID:36402845 notes a stability confound for EXT1 GT-A mutants. Retain the positive historical activity without asserting equal autonomous efficiency or that EXT1 supplies all GlcNAc transfer in vivo. |
| GO:0050508 glucuronosyl-N-acetylglucosaminyl-proteoglycan 4-alpha-N-acetylglucosaminyltransferase activity | IDA PMID:36593275 Structural basis for heparan sulfate co-polymerase action by... | ACCEPT | Summary: EXT1 contributes to the complex that transfers N-acetylglucosamine during heparan-sulfate chain elongation. Reason: The imported qualifier is contributes_to, which is consistent with PMID:36593275: purified human EXT1–EXT2 adds GlcNAc to a GlcA-terminated acceptor, while mutations assign the dominant GlcNAc-transferase site to EXT2 GT-A. EXT1 remains part of that active assembly. The same study reports low but detectable GlcNAc transfer and co-polymerization by EXT1 expressed alone, with extensive proteolysis making those measurements qualitative. PMID:36402845 instead finds reduced GlcNAc transfer after EXT1 GT-A mutation and explicitly notes a complex-stability confound. These differences concern the magnitude and context of EXT1 catalytic contribution; neither justifies deleting this qualified complex-level assertion or claiming categorical absence of EXT1 GlcNAc-transferase activity. |
| GO:0050508 glucuronosyl-N-acetylglucosaminyl-proteoglycan 4-alpha-N-acetylglucosaminyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Retain the supported HS-chain GlcNAc-transfer activity with quantitative limits. Reason: ARBA00090157 disjunct 1 matches the target; the exact mouse/Ensembl donor has IDA/IGI PMID:10639137 grounding. Human protein experiments in PMID:36402845 support EXT1 GT-A involvement, while PMID:36593275 reports low qualitative activity from a low-yield/proteolyzed EXT1-only preparation despite dominant EXT2 GlcNAc activity. Retain the reaction with those quantitative/construct limits, not an equal-subunit or autonomous native-rate claim. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:P97464 SUPPORTS TRANSFER Current donor IDA/IGI PMID:10639137 reaction traced from metadata; the original was subsequently recovered and read in the direct-evidence review. Independent recent human activity supports transfer. ARBA:ARBA00090157 SUPPORTS TRANSFER Current FunFam/Metazoa disjunct 1 matches target. ensembl:ENSMUSP00000076505 SUPPORTS TRANSFER Exact mouse donor identity. |
| GO:0050508 glucuronosyl-N-acetylglucosaminyl-proteoglycan 4-alpha-N-acetylglucosaminyltransferase activity | ISS GO_REF:0000024 | ACCEPT | Summary: Retain the supported HS-chain GlcNAc-transfer activity with quantitative limits. Reason: Mouse P97464 has IDA/IGI PMID:10639137 for this reaction. The database trace was established from donor metadata; the original was subsequently recovered and read in the direct-evidence review. Independent studies of human proteins, PMID:36402845 and PMID:36593275, retain positive EXT1 GlcNAc evidence, including low qualitative EXT1-only activity in the latter study. This supports transfer without claiming that native isolated EXT1 supplies the dominant heterocomplex GlcNAc rate. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:P97464 SUPPORTS TRANSFER Current donor IDA/IGI PMID:10639137 reaction traced from metadata; the original was subsequently recovered and read in the direct-evidence review. Independent recent human activity supports transfer. |
| GO:0050508 glucuronosyl-N-acetylglucosaminyl-proteoglycan 4-alpha-N-acetylglucosaminyltransferase activity | NAS PMID:12907669 In vitro polymerization of heparan sulfate backbone by the E... | ACCEPT | Summary: The specific GlcNAc-transferase assertion has actual original experimental support. Reason: PMID:12907669 directly measures GlcNAc addition to GlcA-terminated HS acceptors using soluble human EXT1 residues 29–746, with linkage-sensitive product digestion. Retain this measured activity while distinguishing its historical preparation from the dominant EXT2 GlcNAc-transfer site in the modern heterocomplex; PMID:36593275 also reports low qualitative EXT1-alone activity. This is not an equal-efficiency or all-product claim. |
| GO:0050509 N-acetylglucosaminyl-proteoglycan 4-beta-glucuronosyltransferase activity | EXP PMID:10639137 The putative tumor suppressors EXT1 and EXT2 form a stable c... | ACCEPT | Summary: Human EXT1 contributes the GlcA-transfer reaction directly resolved in later work. Reason: PMID:10639137 Table 1 measures GlcA incorporation into HS oligosaccharide acceptors in immunoprecipitated human EXT1 preparations, with enhanced activity after mouse or bovine EXT2 co-expression and loss after EXT1 G339D/R340C substitutions. These are mixed cellular preparations, not an isolated human binary enzyme. Independent human structural/biochemical PMID:36593275 specifically assigns beta-1,4-GlcA transfer to EXT1 GT-B, corroborating the imported activity. |
| GO:0050509 N-acetylglucosaminyl-proteoglycan 4-beta-glucuronosyltransferase activity | IDA PMID:12907669 In vitro polymerization of heparan sulfate backbone by the E... | ACCEPT | Summary: Human EXT1 catalyzes beta-1,4-glucuronic-acid addition to HS acceptors. Reason: PMID:12907669 uses UDP-GlcA and GlcNAc-terminated K5-derived acceptors with affinity-captured human EXT1, and product digestion verifies the expected beta-linked GlcA addition. Figures 2–5 distinguish single-sugar transfer from alternating polymerization. Independent PMID:36593275 identifies EXT1 GT-B as the principal GlcA-transfer site in the purified human heterodimer. This supports the specific reaction without claiming that EXT1 alone carries out every step of proteoglycan synthesis. |
| GO:0050509 N-acetylglucosaminyl-proteoglycan 4-beta-glucuronosyltransferase activity | IDA PMID:36593275 Structural basis for heparan sulfate co-polymerase action by... | ACCEPT | Summary: EXT1 supplies the principal beta-1,4-glucuronosyltransferase activity of the heparan sulfate polymerase. Reason: PMID:36593275 assigns glucuronic-acid transfer to the EXT1 GT-B domain through substrate-bound structures and mutations that eliminate or strongly reduce transfer to GlcNAc-terminated acceptors. UDP-Glo activity is corroborated by MALDI detection of the expected extension products; co-polymerization extends glypican-1-associated heparan sulfate. The independent PMID:36402845 FACE and HeLa complementation experiments agree that the glucuronic-acid transfer step depends on EXT1. Retain the specific enzyme activity, distinguishing catalysis by EXT1 within the assembled complex from claims that the complete biosynthetic pathway or both sugar-transfer activities are equally autonomous in isolated EXT1. |
| GO:0050509 N-acetylglucosaminyl-proteoglycan 4-beta-glucuronosyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Retain the experimentally supported HS-chain beta-glucuronosyltransferase activity. Reason: ARBA00090155 disjunct 0 and the exact mouse/Ensembl donor support the mapping. Human protein experiments in PMID:36402845/PMID:36593275 independently identify EXT1 GT-B GlcA chemistry. Official GO:0050509 and ENZYME 2.4.1.225 give the same beta-(1→4) addition and RHEA:20908 correspondence. Direct Rhea access failed; no Rhea body read or substitution of GlcNAc chemistry is claimed. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:P97464 SUPPORTS TRANSFER Current experimental PMID:10639137 GlcA activity supports donor grounding, with independent human active-site assays. ARBA:ARBA00090155 SUPPORTS TRANSFER Current FunFam/Eukaryota disjunct 0 matches target. ensembl:ENSMUSP00000076505 SUPPORTS TRANSFER Exact mouse donor identity. RHEA:20908 UNRESOLVED Official GO definition and ENZYME 2.4.1.225 reaction agree; direct Rhea endpoints returned 403 and are not claimed read. EC:2.4.1.225 SUPPORTS TRANSFER Actual official ENZYME reaction and human Q16394 cross-reference verified. |
| GO:0050509 N-acetylglucosaminyl-proteoglycan 4-beta-glucuronosyltransferase activity | ISS GO_REF:0000024 | ACCEPT | Summary: Retain the experimentally supported HS-chain beta-glucuronosyltransferase activity. Reason: Current mouse P97464 has experimental PMID:10639137 GlcA-transfer grounding. Its original was subsequently recovered and read in the direct-evidence review, with the cellular immunoprecipitate and species distinctions retained. Independent catalytic-site assays on human proteins in PMID:36402845 and PMID:36593275 establish the same specific EXT1 reaction. Retain this activity with the soluble human construct boundaries documented. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:P97464 SUPPORTS TRANSFER Current experimental PMID:10639137 GlcA activity supports donor grounding, with independent human active-site assays. |
| GO:0050509 N-acetylglucosaminyl-proteoglycan 4-beta-glucuronosyltransferase activity | NAS PMID:12907669 In vitro polymerization of heparan sulfate backbone by the E... | ACCEPT | Summary: The stated GlcA-transferase function is supported by the actual original assays. Reason: The UDP-GlcA transfer and linkage-sensitive digestion in PMID:12907669 directly support beta-1,4-GlcA addition by soluble human EXT1 preparations. The independent human heterodimer experiments in PMID:36593275 identify EXT1 GT-B as the principal site for this reaction. Neither result assigns EXT1 every step of proteoglycan synthesis. |
| GO:1902494 catalytic complex | IDA PMID:10679296 Association of EXT1 and EXT2, hereditary multiple exostoses ... | MODIFY | Summary: The catalytic complex is specifically the EXT1–EXT2 heterodimer. Reason: The PMID:10679296 abstract reports EXT oligomerization, but its complete operative original remains unavailable. Independently read human studies PMID:12907669, PMID:36402845 and PMID:36593275 purify, assay and resolve the specific EXT1–EXT2 assembly. Replace the broad catalytic complex with current GO:0120504, whose ancestor closure includes GO:1902494. This preserves the original source and does not pretend that all of its experimental details were recovered. Proposed replacements: EXT1-EXT2 complex |
| GO:1902494 catalytic complex | IDA PMID:36593275 Structural basis for heparan sulfate co-polymerase action by... | MODIFY | Summary: The active catalytic assembly is specifically the EXT1–EXT2 complex. Reason: PMID:36593275 directly resolves and assays the human heterodimer, so the broad catalytic-complex term is correct but unnecessarily general. Current GO:0120504 EXT1-EXT2 complex describes the observed assembly and is a descendant of GO:1902494. Use this specific component term; preserve the imported is_active_in qualifier rather than treating it as a source defect. The N-terminal truncations and purification tags are experimental constructs, and the soluble assay does not establish extracellular localization of endogenous EXT1. Proposed replacements: EXT1-EXT2 complex Supporting Evidence: |
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Download this section (compressed HTML)Q: How much GlcNAc transfer does the EXT1 GT-A domain supply in an intact, membrane-associated human EXT1–EXT2 complex, and under which acceptor or donor conditions does that contribution become appreciable relative to EXT2?
Q: Which differences between the older separately expressed EXT1 preparations and the modern heterocomplex assays arise from acceptor chemistry, expression and purification, protein integrity, or catalytic versus allosteric effects of the GT-A substitutions?
Q: How are alternating GlcA and GlcNAc additions coordinated between catalytic sites in the native Golgi complex, and does the growing chain dissociate between transfers under physiological conditions?
Experiment: Compare sequence-verified human wild-type complexes with EXT1 GT-A, EXT2 GT-A and EXT1 GT-B substitutions under matched expression and purification conditions. Quantify intact protein abundance, complex assembly and stability before measuring single-sugar transfer with defined acceptors and product-resolved chromatography or mass spectrometry. Include both soluble catalytic constructs and membrane-reconstituted full-length proteins, with appropriate inactive and donor-omission controls.
Hypothesis: EXT1 GT-A supplies a condition-dependent GlcNAc-transfer component that can be separated from effects on heterocomplex stability.
Type: Controlled enzyme kinetics and product analysis
Experiment: Reproduce the earlier K5-derived oligosaccharide and modern short-acceptor or glypican assays side by side using matched human constructs. Measure both transfer reactions, chain-length distributions and enzyme integrity; test for copurifying endogenous EXT2 with sequence-specific proteomics. Normalize rates to intact active preparations rather than total tag signal, and distinguish qualitative activity from quantitative equivalence.
Hypothesis: Preparation integrity and acceptor conditions explain part of the different activity assignments in separately expressed EXT1 and assembled EXT1–EXT2 assays.
Type: Comparative biochemical reconstitution
Experiment: Use pulse-chase labeled donors and competing acceptors with purified human heterocomplexes to test whether partially elongated chains remain enzyme-bound between additions. Combine time-resolved product distributions with binding and turnover measurements, and compare the behavior of membrane-associated and soluble complexes without assuming that structural proximity alone establishes channeling.
Hypothesis: Alternating extension can be coordinated without obligatory direct substrate channeling.
Type: Time-resolved chain-extension and substrate-exchange assays
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