id: Q8IYK4 gene_symbol: COLGALT2 product_type: PROTEIN status: COMPLETE taxon: id: NCBITaxon:9606 label: Homo sapiens description: COLGALT2 (collagen beta(1-O)galactosyltransferase 2; also known as GLT25D2, glycosyltransferase 25 family member 2) is an endoplasmic reticulum-luminal, manganese-dependent glycosyltransferase that catalyzes the transfer of beta-galactose from UDP-galactose to hydroxylysine residues of collagens, forming galactosyl-O-hydroxylysine (Gal-O-Hyl). This is the first step of collagen O-glycosylation, which precedes glucosylation to generate the glucosylgalactosyl-hydroxylysine (Glc-Gal-O-Hyl) disaccharide found on collagens and other proteins with collagenous domains. It is a member of the glycosyltransferase 25 (GT25) family and a paralog of COLGALT1, with which it shares procollagen galactosyltransferase activity (EC 2.4.1.50) but no glucosyltransferase activity. The mature protein is a soluble ER-luminal enzyme retained in the ER by a C-terminal SRDEL (KDEL-like) retention signal. Compared with the ubiquitously expressed COLGALT1, COLGALT2 has a more restricted tissue distribution, with expression enriched in brain and skeletal muscle. references: - id: GO_REF:0000033 title: Annotation inferences using phylogenetic trees findings: [] - id: GO_REF:0000044 title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt findings: [] - id: GO_REF:0000120 title: Combined Automated Annotation using Multiple IEA Methods findings: [] - id: PMID:25416956 title: A proteome-scale map of the human interactome network. findings: [] - id: PMID:28514442 title: Architecture of the human interactome defines protein communities and disease networks. findings: [] - id: PMID:32296183 title: A reference map of the human binary protein interactome. findings: [] - id: PMID:33961781 title: Dual proteome-scale networks reveal cell-specific remodeling of the human interactome. findings: [] - id: PMID:19075007 title: Core glycosylation of collagen is initiated by two beta(1-O)galactosyltransferases. findings: - statement: COLGALT2 (GLT25D2) is a beta(1-O)galactosyltransferase that transfers galactose to hydroxylysine residues of collagen, initiating the core glycosylation of collagen; it has no glucosyltransferase activity and is expressed in brain and skeletal muscle. reference_section_type: ABSTRACT - id: Reactome:R-HSA-1650814 title: Collagen biosynthesis and modifying enzymes findings: [] - id: Reactome:R-HSA-1981120 title: Galactosylation of collagen propeptide hydroxylysines by procollagen galactosyltransferases 1, 2 findings: [] - id: Reactome:R-HSA-8948228 title: COLGALT1,COLGALT2 bind Lysyl hydroxylated collagen propeptides findings: [] - id: Reactome:R-HSA-8948231 title: COLGALT1,COLGALT2:Galactosyl-hydroxylysyl collagen propeptides dissociates findings: [] existing_annotations: - term: id: GO:0050211 label: procollagen galactosyltransferase activity evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: enables review: summary: Core molecular function. COLGALT2 is a procollagen galactosyltransferase that transfers beta-galactose to collagen hydroxylysine residues. This phylogenetic (IBA) inference is concordant with direct biochemical characterization of the human enzyme. action: ACCEPT reason: This is the well-established core activity of COLGALT2, directly demonstrated biochemically (EC 2.4.1.50) and consistent across IBA, IEA and TAS evidence. The IBA transfer within the GT25 procollagen galactosyltransferase family is appropriate. supported_by: - reference_id: file:human/COLGALT2/COLGALT2-uniprot.txt supporting_text: Beta-galactosyltransferase that transfers beta-galactose to hydroxylysine residues of collagen. - term: id: GO:0005788 label: endoplasmic reticulum lumen evidence_type: IEA original_reference_id: GO_REF:0000044 qualifier: located_in review: summary: Core cellular component. COLGALT2 is a soluble ER-luminal enzyme bearing a C-terminal SRDEL ER-retention motif, consistent with action on collagens during their transit through the ER. action: ACCEPT reason: ER lumen localization is supported by the UniProt subcellular location and the C-terminal KDEL-like (...SRDEL) retention signal that prevents secretion. This annotation correctly localizes the enzyme to its site of action. supported_by: - reference_id: file:human/COLGALT2/COLGALT2-uniprot.txt supporting_text: "SUBCELLULAR LOCATION: Endoplasmic reticulum lumen" - term: id: GO:0050211 label: procollagen galactosyltransferase activity evidence_type: IEA original_reference_id: GO_REF:0000120 qualifier: enables review: summary: Core molecular function inferred by automated mapping from the RHEA reaction and EC number (EC 2.4.1.50). Concordant with the direct biochemical characterization of COLGALT2. action: ACCEPT reason: The EC 2.4.1.50 / RHEA:12637 mapping accurately reflects the experimentally demonstrated galactosyltransferase activity of COLGALT2 on collagen hydroxylysine. supported_by: - reference_id: file:human/COLGALT2/COLGALT2-uniprot.txt supporting_text: EC=2.4.1.50 - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:25416956 qualifier: enables review: summary: Bare 'protein binding' from a high-throughput interactome screen. This term is uninformative and does not identify a specific, functionally meaningful partner relevant to COLGALT2's galactosyltransferase function. action: MARK_AS_OVER_ANNOTATED reason: GO:0005515 protein binding conveys no specific functional information. The interaction (with UBQLN1) derives from a proteome-scale binary interactome map and has no validated functional relationship to collagen galactosylation. - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:28514442 qualifier: enables review: summary: Bare 'protein binding' from a high-throughput affinity-purification interactome study. Uninformative about COLGALT2's actual function. action: MARK_AS_OVER_ANNOTATED reason: GO:0005515 protein binding is non-informative and the interactors (e.g. SEMG1, C1QTNF3) come from a large-scale interactome screen with no demonstrated functional role in collagen glycosylation. - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:32296183 qualifier: enables review: summary: Bare 'protein binding' from the HuRI reference binary interactome map. Does not specify an informative function. action: MARK_AS_OVER_ANNOTATED reason: GO:0005515 protein binding is uninformative; the reported interactors (HPCAL1, UBQLN2) are from a systematic interactome screen and are not validated functional partners relevant to COLGALT2's enzymatic role. - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:33961781 qualifier: enables review: summary: Bare 'protein binding' from a proteome-scale interactome remodeling study. Uninformative regarding COLGALT2 function. action: MARK_AS_OVER_ANNOTATED reason: GO:0005515 protein binding adds no specific functional information; the underlying interactions are high-throughput and lack a demonstrated link to collagen galactosylation. - term: id: GO:0030199 label: collagen fibril organization evidence_type: TAS original_reference_id: Reactome:R-HSA-1650814 qualifier: involved_in review: summary: Pathway-level biological process annotation placing COLGALT2 within collagen biosynthesis and modification. COLGALT2 contributes to collagen post-translational modification, but its direct, evolved role is the galactosylation reaction rather than fibril organization per se, which is a downstream consequence of properly modified collagen. action: KEEP_AS_NON_CORE reason: COLGALT2 acts upstream of fibril assembly by glycosylating collagen hydroxylysines; its effect on collagen fibril organization is indirect/downstream. Retain as a non-core contextual annotation rather than a core function. supported_by: - reference_id: file:human/COLGALT2/COLGALT2-uniprot.txt supporting_text: Beta-galactosyltransferase that transfers beta-galactose to hydroxylysine residues of collagen. - term: id: GO:0050211 label: procollagen galactosyltransferase activity evidence_type: TAS original_reference_id: Reactome:R-HSA-1981120 qualifier: enables review: summary: Core molecular function captured from Reactome (galactosylation of collagen propeptide hydroxylysines). Concordant with the direct biochemical evidence. action: ACCEPT reason: This TAS annotation correctly reflects COLGALT2's experimentally established galactosyltransferase activity on collagen hydroxylysine residues. supported_by: - reference_id: file:human/COLGALT2/COLGALT2-uniprot.txt supporting_text: Beta-galactosyltransferase that transfers beta-galactose to hydroxylysine residues of collagen. - term: id: GO:0005788 label: endoplasmic reticulum lumen evidence_type: TAS original_reference_id: Reactome:R-HSA-1981120 qualifier: located_in review: summary: ER lumen localization from Reactome, consistent with the UniProt subcellular location and the C-terminal ER-retention signal. action: ACCEPT reason: Correct localization of COLGALT2 to the ER lumen, its site of action on collagen during biosynthesis. supported_by: - reference_id: file:human/COLGALT2/COLGALT2-uniprot.txt supporting_text: "SUBCELLULAR LOCATION: Endoplasmic reticulum lumen" - term: id: GO:0005788 label: endoplasmic reticulum lumen evidence_type: TAS original_reference_id: Reactome:R-HSA-8948228 qualifier: located_in review: summary: ER lumen localization from a Reactome reaction (COLGALT1,COLGALT2 bind lysyl hydroxylated collagen propeptides). Redundant with the other ER lumen annotations but correct. action: ACCEPT reason: Consistent with the established ER-luminal localization of COLGALT2; redundant with other ER lumen annotations but accurate. supported_by: - reference_id: file:human/COLGALT2/COLGALT2-uniprot.txt supporting_text: "SUBCELLULAR LOCATION: Endoplasmic reticulum lumen" - term: id: GO:0005788 label: endoplasmic reticulum lumen evidence_type: TAS original_reference_id: Reactome:R-HSA-8948231 qualifier: located_in review: summary: ER lumen localization from a Reactome reaction (dissociation of COLGALT enzymes from galactosyl-hydroxylysyl collagen propeptides). Redundant but correct. action: ACCEPT reason: Consistent with the established ER-luminal localization of COLGALT2; redundant with other ER lumen annotations but accurate. supported_by: - reference_id: file:human/COLGALT2/COLGALT2-uniprot.txt supporting_text: "SUBCELLULAR LOCATION: Endoplasmic reticulum lumen" - term: id: GO:0180062 label: protein O-linked glycosylation via galactose evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: involved_in review: summary: Core biological process. COLGALT2 galactosylates collagen hydroxylysine residues (Gal-O-Hyl), which is the protein O-linked glycosylation via galactose step of collagen O-glycosylation. This BP is more directly representative of COLGALT2's evolved function than collagen fibril organization, and is the curated IMP-supported process for its paralog COLGALT1. action: NEW reason: This term precisely captures the biological process effected by COLGALT2's galactosyltransferase activity. It is proposed as a NEW annotation because the existing BP block only contains the more downstream collagen fibril organization term. supported_by: - reference_id: file:human/COLGALT2/COLGALT2-uniprot.txt supporting_text: Beta-galactosyltransferase that transfers beta-galactose to hydroxylysine residues of collagen. core_functions: - description: Manganese-dependent transfer of beta-galactose from UDP-galactose to collagen hydroxylysine residues in the ER lumen, forming galactosyl-O-hydroxylysine, the first committed step of collagen O-glycosylation. molecular_function: id: GO:0050211 label: procollagen galactosyltransferase activity directly_involved_in: - id: GO:0180062 label: protein O-linked glycosylation via galactose locations: - id: GO:0005788 label: endoplasmic reticulum lumen supported_by: - reference_id: file:human/COLGALT2/COLGALT2-uniprot.txt supporting_text: Beta-galactosyltransferase that transfers beta-galactose to hydroxylysine residues of collagen. - reference_id: file:human/COLGALT2/COLGALT2-uniprot.txt supporting_text: EC=2.4.1.50 - reference_id: file:human/COLGALT2/COLGALT2-uniprot.txt supporting_text: "SUBCELLULAR LOCATION: Endoplasmic reticulum lumen" proposed_new_terms: [] suggested_questions: - question: Why does COLGALT2 have a more restricted (brain/skeletal-muscle-enriched) tissue distribution than the ubiquitously expressed COLGALT1, and which collagen types are its preferred substrates in those tissues? - question: Does COLGALT2 have non-redundant, tissue-specific functions in collagen glycosylation that cannot be compensated by COLGALT1, and is there an associated human phenotype? suggested_experiments: - description: In vitro galactosyltransferase assays with purified recombinant COLGALT2 against a panel of hydroxylysine-containing collagen substrates, varying Mn2+ and UDP-galactose, to define substrate preference and metal dependence relative to COLGALT1. experiment_type: enzymology/biochemistry hypothesis: COLGALT2 galactosylates collagen hydroxylysine with Mn2+ dependence and has substrate preferences distinct from COLGALT1. - description: CRISPR knockout of COLGALT2 (alone and combined with COLGALT1) in brain- and muscle-derived cell models, followed by mass-spectrometric quantification of Gal-O-Hyl and Glc-Gal-O-Hyl on collagens to assess its non-redundant contribution. experiment_type: genetic manipulation/glycoproteomics hypothesis: Loss of COLGALT2 reduces collagen hydroxylysine galactosylation in a tissue-restricted manner not fully rescued by COLGALT1. - description: Tissue-resolved expression and localization profiling (RNA-seq plus immunohistochemistry/fractionation) to map where COLGALT2 versus COLGALT1 act and confirm ER-luminal residence in vivo. experiment_type: expression profiling/cell biology hypothesis: COLGALT2 is the predominant collagen galactosyltransferase in specific tissues such as brain and skeletal muscle.