FAM20B (glycosaminoglycan xylosylkinase) is a Golgi-resident, single-pass type II membrane kinase of the FAM20 family of atypical secretory-pathway kinases. Its lumenal kinase domain transfers phosphate from ATP to the C2 hydroxyl of the xylose that links glycosaminoglycan (GAG) chains to serine residues of proteoglycan core proteins. Its preferred substrate is the Galbeta1-4Xylbeta1 disaccharide intermediate, recognised through a dedicated active-site pocket. FAM20B acts as a monomeric enzyme, unlike the FAM20C protein kinase and its pseudokinase partner FAM20A. Xylose 2-O-phosphorylation stimulates the galactosyltransferase B3GALT6 and the glucuronyltransferase B3GAT3, which complete the GlcA-Gal-Gal-Xyl linkage tetrasaccharide. The phosphate is transient: the phosphatase PXYLP1 removes it before chondroitin or heparan sulfate polymerisation begins. FAM20B is therefore required to assemble both chondroitin/dermatan sulfate and heparan sulfate chains. Cells lacking it make a truncated, sialylated Gal-Xyl linker that cannot be elongated, while imbalance between FAM20B and PXYLP1 also lowers GAG output. In mouse and zebrafish, loss of Fam20b disrupts cartilage, bone, tooth and craniofacial development. Biallelic loss-of-function variants in humans cause a lethal neonatal short-limb skeletal dysplasia resembling Desbuquois dysplasia.
Definition: Catalysis of the reaction: 3-O-(beta-D-galactosyl-(1->3)-beta-D-galactosyl-(1->4)-beta-D-xylosyl)-L-seryl-[protein] + ATP = 3-O-(beta-D-galactosyl-(1->3)-beta-D-galactosyl-(1->4)-beta-D-2-O-phosphoxylosyl)-L-seryl-[protein] + ADP + H+, and the equivalent phosphorylation of the Gal-Xyl disaccharide linkage intermediate (RHEA:19461).
Justification: FAM20B and its SF14 orthologs have a well-defined glycan kinase activity on linkage-region xylose. The only existing GO term is the general GO:0016773. GO:0019200 carbohydrate kinase activity is not an is_a ancestor of GO:0016773 and mainly covers free sugars. A specific term would separate the FAM20B glycan-kinase branch from the FAM20C protein kinases.
Parent term: phosphotransferase activity, alcohol group as acceptor
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000139 Golgi membrane | EXP PMID:19473117 FAM20B is a kinase that phosphorylates xylose in the glycosa... | ACCEPT | Summary: FAM20B is a single-pass type II membrane protein of the Golgi, with its kinase domain in the lumen. Reason: Koike et al. 2009 localised FAM20B to the Golgi, and UniProt records it as a Golgi apparatus membrane, signal-anchor type II protein on that experimental basis (ECO:0000269 PubMed:19473117). This fits a lumenal enzyme acting on linkage-region glycans during their synthesis in the Golgi. Golgi membrane is the most precise location the evidence supports. Supporting Evidence: file:human/FAM20B/FAM20B-uniprot.txt SUBCELLULAR LOCATION: Golgi apparatus membrane |
| GO:0000139 Golgi membrane | IEA GO_REF:0000044 | ACCEPT | Summary: FAM20B is a single-pass type II membrane protein of the Golgi, with its kinase domain in the lumen. Reason: Koike et al. 2009 localised FAM20B to the Golgi, and UniProt records it as a Golgi apparatus membrane, signal-anchor type II protein on that experimental basis (ECO:0000269 PubMed:19473117). This fits a lumenal enzyme acting on linkage-region glycans during their synthesis in the Golgi. Golgi membrane is the most precise location the evidence supports. Supporting Evidence: file:human/FAM20B/FAM20B-uniprot.txt SUBCELLULAR LOCATION: Golgi apparatus membrane |
| GO:0000139 Golgi membrane | TAS Reactome:R-HSA-9941039 | ACCEPT | Summary: FAM20B is a single-pass type II membrane protein of the Golgi, with its kinase domain in the lumen. Reason: Koike et al. 2009 localised FAM20B to the Golgi, and UniProt records it as a Golgi apparatus membrane, signal-anchor type II protein on that experimental basis (ECO:0000269 PubMed:19473117). This fits a lumenal enzyme acting on linkage-region glycans during their synthesis in the Golgi. Golgi membrane is the most precise location the evidence supports. Supporting Evidence: file:human/FAM20B/FAM20B-uniprot.txt SUBCELLULAR LOCATION: Golgi apparatus membrane |
| GO:0005515 protein binding | IPI PMID:25331875 Xylose phosphorylation functions as a molecular switch to re... | REMOVE | Summary: Interaction with decorin (DCN), a proteoglycan core protein that FAM20B phosphorylates on its linkage-region xylose. Reason: Decorin is a substrate of FAM20B, and both papers used it in kinase assays, not as a binding partner with a function of its own. FAM20B's activity is already captured by GO:0016773 and by the proposed linkage-region xylose kinase term. Bare protein binding adds nothing, so the generic GO:0005515 policy applies. The enzyme-substrate relationship is real; it is better expressed as has_input on the kinase activity. Supporting Evidence: PMID:25331875 phosphorylates the initiating xylose residue in the proteoglycan tetrasaccharide linkage region, and that this event functions as a molecular switch to regulate subsequent glycosaminoglycan assembly |
| GO:0005515 protein binding | IPI PMID:25789606 A secretory kinase complex regulates extracellular protein p... | REMOVE | Summary: Interaction with decorin (DCN), a proteoglycan core protein that FAM20B phosphorylates on its linkage-region xylose. Reason: Decorin is a substrate of FAM20B, and both papers used it in kinase assays, not as a binding partner with a function of its own. FAM20B's activity is already captured by GO:0016773 and by the proposed linkage-region xylose kinase term. Bare protein binding adds nothing, so the generic GO:0005515 policy applies. The enzyme-substrate relationship is real; it is better expressed as has_input on the kinase activity. Supporting Evidence: PMID:25789606 Fam20B specifically phosphorylated glycosylated decorin PMID:25789606 nor did Fam20A or Fam20C have any effect on Fam20B-catalyzed xylose phosphorylation of the proteoglycan decorin |
| GO:0005515 protein binding | IPI PMID:28514442 Architecture of the human interactome defines protein commun... | REMOVE | Summary: Interaction from a proteome-scale interactome screen (UniProtKB:Q6ICH7). Reason: This IPI row records a binary interaction from a proteome-scale interactome screen (HuRI yeast two-hybrid, BioPlex affinity purification-mass spectrometry, or multimodal cell maps). The partner is not a known component of glycosaminoglycan linkage-region biosynthesis, and the screen supplies no functional context. Under the GO:0005515 policy the bare protein binding term is uninformative and is removed. Removal does not mean the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Interaction from a proteome-scale interactome screen (UniProtKB:J3KQ12). Reason: This IPI row records a binary interaction from a proteome-scale interactome screen (HuRI yeast two-hybrid, BioPlex affinity purification-mass spectrometry, or multimodal cell maps). The partner is not a known component of glycosaminoglycan linkage-region biosynthesis, and the screen supplies no functional context. Under the GO:0005515 policy the bare protein binding term is uninformative and is removed. Removal does not mean the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Interaction from a proteome-scale interactome screen (UniProtKB:O15552). Reason: This IPI row records a binary interaction from a proteome-scale interactome screen (HuRI yeast two-hybrid, BioPlex affinity purification-mass spectrometry, or multimodal cell maps). The partner is not a known component of glycosaminoglycan linkage-region biosynthesis, and the screen supplies no functional context. Under the GO:0005515 policy the bare protein binding term is uninformative and is removed. Removal does not mean the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Interaction from a proteome-scale interactome screen (UniProtKB:P48051). Reason: This IPI row records a binary interaction from a proteome-scale interactome screen (HuRI yeast two-hybrid, BioPlex affinity purification-mass spectrometry, or multimodal cell maps). The partner is not a known component of glycosaminoglycan linkage-region biosynthesis, and the screen supplies no functional context. Under the GO:0005515 policy the bare protein binding term is uninformative and is removed. Removal does not mean the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Interaction from a proteome-scale interactome screen (UniProtKB:Q5JX71). Reason: This IPI row records a binary interaction from a proteome-scale interactome screen (HuRI yeast two-hybrid, BioPlex affinity purification-mass spectrometry, or multimodal cell maps). The partner is not a known component of glycosaminoglycan linkage-region biosynthesis, and the screen supplies no functional context. Under the GO:0005515 policy the bare protein binding term is uninformative and is removed. Removal does not mean the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Interaction from a proteome-scale interactome screen (UniProtKB:Q8TBB6). Reason: This IPI row records a binary interaction from a proteome-scale interactome screen (HuRI yeast two-hybrid, BioPlex affinity purification-mass spectrometry, or multimodal cell maps). The partner is not a known component of glycosaminoglycan linkage-region biosynthesis, and the screen supplies no functional context. Under the GO:0005515 policy the bare protein binding term is uninformative and is removed. Removal does not mean the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Interaction from a proteome-scale interactome screen (UniProtKB:Q8TDT2). Reason: This IPI row records a binary interaction from a proteome-scale interactome screen (HuRI yeast two-hybrid, BioPlex affinity purification-mass spectrometry, or multimodal cell maps). The partner is not a known component of glycosaminoglycan linkage-region biosynthesis, and the screen supplies no functional context. Under the GO:0005515 policy the bare protein binding term is uninformative and is removed. Removal does not mean the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Interaction from a proteome-scale interactome screen (UniProtKB:Q96HE8). Reason: This IPI row records a binary interaction from a proteome-scale interactome screen (HuRI yeast two-hybrid, BioPlex affinity purification-mass spectrometry, or multimodal cell maps). The partner is not a known component of glycosaminoglycan linkage-region biosynthesis, and the screen supplies no functional context. Under the GO:0005515 policy the bare protein binding term is uninformative and is removed. Removal does not mean the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Interaction from a proteome-scale interactome screen (UniProtKB:Q9NPL8). Reason: This IPI row records a binary interaction from a proteome-scale interactome screen (HuRI yeast two-hybrid, BioPlex affinity purification-mass spectrometry, or multimodal cell maps). The partner is not a known component of glycosaminoglycan linkage-region biosynthesis, and the screen supplies no functional context. Under the GO:0005515 policy the bare protein binding term is uninformative and is removed. Removal does not mean the interaction is false. |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | REMOVE | Summary: Interaction from a proteome-scale interactome screen (UniProtKB:Q6ICH7). Reason: This IPI row records a binary interaction from a proteome-scale interactome screen (HuRI yeast two-hybrid, BioPlex affinity purification-mass spectrometry, or multimodal cell maps). The partner is not a known component of glycosaminoglycan linkage-region biosynthesis, and the screen supplies no functional context. Under the GO:0005515 policy the bare protein binding term is uninformative and is removed. Removal does not mean the interaction is false. |
| GO:0005515 protein binding | IPI PMID:40205054 Multimodal cell maps as a foundation for structural and func... | REMOVE | Summary: Interaction from a proteome-scale interactome screen (UniProtKB:Q6ICH7). Reason: This IPI row records a binary interaction from a proteome-scale interactome screen (HuRI yeast two-hybrid, BioPlex affinity purification-mass spectrometry, or multimodal cell maps). The partner is not a known component of glycosaminoglycan linkage-region biosynthesis, and the screen supplies no functional context. Under the GO:0005515 policy the bare protein binding term is uninformative and is removed. Removal does not mean the interaction is false. |
| GO:0005737 cytoplasm | IEA GO_REF:0000107 | TOO GENERAL | Summary: Cytoplasm, propagated from mouse Fam20b by Ensembl Compara. Reason: FAM20B sits in the Golgi membrane (accepted here as GO:0000139), and that location falls within the cytoplasm. The cytoplasm term is true but carries no information beyond the Golgi membrane annotation. Supporting Evidence: file:human/FAM20B/FAM20B-uniprot.txt SUBCELLULAR LOCATION: Golgi apparatus membrane |
| GO:0005794 Golgi apparatus | IDA GO_REF:0000052 | ACCEPT | Summary: Golgi apparatus localisation from Human Protein Atlas immunofluorescence. Reason: Immunofluorescence resolves FAM20B to the Golgi but cannot place it in the membrane. The result agrees with the experimentally established Golgi membrane localisation, and Golgi apparatus is as precise as this method allows. Supporting Evidence: file:human/FAM20B/FAM20B-uniprot.txt SUBCELLULAR LOCATION: Golgi apparatus membrane |
| GO:0005794 Golgi apparatus | IDA PMID:19473117 FAM20B is a kinase that phosphorylates xylose in the glycosa... | ACCEPT | Summary: Golgi localisation of FAM20B, reported in the paper that identified its xylose kinase activity. Reason: Koike et al. 2009 showed FAM20B in the Golgi, and the same paper supports the Golgi membrane row. Golgi apparatus is correct and consistent with the lumenal enzyme acting on linkage-region intermediates. Supporting Evidence: file:human/FAM20B/FAM20B-uniprot.txt SUBCELLULAR LOCATION: Golgi apparatus membrane |
| GO:0005794 Golgi apparatus | IEA GO_REF:0000107 | ACCEPT | Summary: Golgi apparatus, propagated from mouse Fam20b by Ensembl Compara. Reason: Mouse Fam20b is a one-to-one ortholog in the same PANTHER subfamily (SF14). The Golgi location is experimentally established for human FAM20B, so this electronic transfer agrees with direct evidence. Supporting Evidence: file:human/FAM20B/FAM20B-uniprot.txt SUBCELLULAR LOCATION: Golgi apparatus membrane |
| GO:0016773 phosphotransferase activity, alcohol group as acceptor | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic propagation from PANTHER node PTN000966557, a deep node in PTHR12450 whose descendants include both FAM20B (SF14) and FAM20C (SF11). Their phosphotransferase activities have been characterised experimentally. Reason: The node placement is sound. FAM20B and FAM20C are catalytically active members whose activities transfer a phosphate to a hydroxyl: on xylose for FAM20B, and on Ser/Thr for FAM20C. FAM20B appears among the sources because it has its own IDA for this term, and that is expected. The term is general, but GO has no child for the linkage-region xylose 2-O-kinase reaction (RHEA:19461). It is therefore the most precise existing term; a new term is proposed below. Supporting Evidence: PMID:19473117 we identified FAM20B as a kinase that phosphorylates the xylose residue in the linkage region PMID:29572475 Similarly to human Fam20B, aqFam20 and hmFam20 are monomers file:interpro/panther/PTHR12450/PTHR12450-entries.csv O75063,Glycosaminoglycan xylosylkinase,protein,9606,Homo sapiens,Homo sapiens (Human),FAM20B,409,PTHR12450:SF14,GLYCOSAMINOGLYCAN XYLOSYLKINASE |
| GO:0016773 phosphotransferase activity, alcohol group as acceptor | IDA PMID:19473117 FAM20B is a kinase that phosphorylates xylose in the glycosa... | ACCEPT | Summary: Direct biochemical demonstration that human FAM20B transfers phosphate from ATP to the 2-OH of xylose in the GAG-protein linkage region. Reason: This is the defining activity of FAM20B. Koike et al. 2009 identified it, and later studies confirmed it with recombinant human protein, including the Gal-Xyl disaccharide specificity and catalytic-residue mutants (Zhang et al. 2018). The acceptor is an alcohol group on a glycan-linked sugar, so GO:0016773 is accurate. It is the most specific existing GO term. Supporting Evidence: PMID:19473117 we identified FAM20B as a kinase that phosphorylates the xylose residue in the linkage region PMID:29572475 Fam20B has a unique active site for recognizing GalΞ²1-4XylΞ²1, the initiator disaccharide within the tetrasaccharide linker region of proteoglycans PMID:29572475 mutations of the equivalent residues in the human Fam20B protein significantly diminish kinase activity |
| GO:0016773 phosphotransferase activity, alcohol group as acceptor | TAS Reactome:R-HSA-9941039 | ACCEPT | Summary: Reactome event R-HSA-9941039: FAM20B phosphorylates the xylose moiety of the GAG linker. Reason: The Reactome event correctly records the catalytic activity, which is directly established for human FAM20B (PMID:19473117). Supporting Evidence: Reactome:R-HSA-9941039 Glycosaminoglycan xylosylkinase FAM20B phosphorylates the xylose (Xyl) moiety of the GAG linker glycan chain of proteoglycans PMID:19473117 we identified FAM20B as a kinase that phosphorylates the xylose residue in the linkage region |
| GO:0030166 proteoglycan biosynthetic process | IBA GO_REF:0000033 | MODIFY | Summary: Phylogenetic propagation of proteoglycan biosynthetic process from PTN002488224 on the FAM20B branch of PTHR12450 (SF14). Zebrafish fam20b is the experimental donor. Reason: The claim is sound, and FAM20B lies inside the clade that carries it. Human FAM20B catalyses a covalent step in building the GAG-protein linkage tetrasaccharide. It 2-O-phosphorylates Xyl in the Gal-Xyl and Gal-Gal-Xyl intermediates, which raises B3GALT6 and B3GAT3 activity. Knockout cells cannot complete the linker. FAM20B is one of the five enzymes in a reconstituted one-pot linker synthesis, so it does part of the work of the process and passes the participation test. GO:0120532 glycosaminoglycan-protein linkage region biosynthetic process is part_of chondroitin, dermatan and heparan sulfate proteoglycan biosynthesis, and therefore lies below GO:0030166. Its current human annotations are the other linker enzymes: XYLT1, XYLT2, B4GALT7 and B3GALT6. The more specific term is recommended. GO:0030166 remains true for every SF14 member, so the IBA itself is not wrong. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN002488224 SUPPORTS TRANSFER FAM20B-branch (SF14) node; human FAM20B lies inside it and has direct evidence for a linkage-region biosynthetic step. The propagated term is correct but broader than GO:0120532. ZFIN:ZDB-GENE-040724-125 Β· fam20b SUPPORTS TRANSFER Zebrafish experimental donor in the same SF14 subfamily. Proposed replacements: glycosaminoglycan-protein linkage region biosynthetic process Supporting Evidence: PMID:25331875 Proteoglycans from FAM20B knockout cells contain a truncated tetrasaccharide linkage region consisting of a disaccharide capped with sialic acid PMID:37828045 co-incubation of the peptide with all the GAG linker enzymes (XT1, B4GALT7, B3GALT6, B3GAT3 and FAM20B), their cognate UDP-sugars and ATP PMID:37828045 Next, the galactosyltransferase B4GALT7 adds Gal and the kinase FAM20B phosphorylates Xyl to form Gal-Xyl-2-phosphate PMID:24425863 The phosphorylation of Xyl residues by FAM20B enhances the formation of the linkage region. |
| GO:1902729 negative regulation of proteoglycan biosynthetic process | TAS Reactome:R-HSA-9941039 | REMOVE | Summary: Reactome assigns FAM20B to negative regulation of proteoglycan biosynthesis, based on the statement that xylose phosphorylation makes the linker chain unavailable for GAG biosynthesis. Reason: FAM20B is a biosynthetic enzyme of the linkage region, not a negative regulator of proteoglycan synthesis. Loss of FAM20B leaves a truncated linker that cannot be elongated and reduces CS and HS (PMID:25331875, PMID:19473117, PMID:40422215). Its phosphorylation accelerates linker synthesis and the initiation of both HS and CS (PMID:37828045). The fact Reactome cites is real but narrow: polymerases do not extend the phosphorylated tetrasaccharide. This is a transient intermediate that PXYLP1 dephosphorylates during normal synthesis (PMID:24425863). Excess phosphorylation can lower GAG output when FAM20B is overexpressed or PXYLP1 is lost, and EXTL2 can cap the phosphorylated linker (PMID:40422215, PMID:23395820). Those are imbalance effects of a pathway enzyme, and the authors describe them as a phosphorylation/dephosphorylation balance. The gene's normal role is positive and required, so the negative-regulation assertion is contradicted by the loss-of-function data. Supporting Evidence: PMID:25331875 Proteoglycans from FAM20B knockout cells contain a truncated tetrasaccharide linkage region consisting of a disaccharide capped with sialic acid PMID:40422215 In line with previous reports, we found that FAM20B loss-of-function reduced GAG chain synthesis. PMID:40422215 suggesting that a balance between phosphorylation and dephosphorylation of the xylose by FAM20B and XYLP, respectively, is probably an essential factor for the regulation of the rate of PG synthesis Reactome:R-HSA-9941039 making the chain unavailable for the biosynthesis of heparan/chondoitin/dermatan |
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Download this section (compressed HTML)Q: Should FAM20B be annotated to GO:0120532 glycosaminoglycan-protein linkage region biosynthetic process? Its phosphate does not remain in the mature linker, but FAM20B catalyses a covalent step on the linker intermediate during its assembly.
Q: Is the Reactome event R-HSA-9941039 summary ("making the chain unavailable") meant to describe the transient phosphorylated intermediate? If so, should its GO mapping to negative regulation of proteoglycan biosynthesis be withdrawn?
Q: Is FAM20B activity itself regulated in vivo, for example by GlcAT-I/B3GAT3 complex formation or by core-protein context? Or is the extent of xylose phosphorylation set mainly by PXYLP1?
Experiment: Quantitative kinetic comparison of human FAM20B on Xyl-, Gal-Xyl- and Gal-Gal-Xyl-glycopeptides, with and without B3GALT6/B3GAT3, to define the physiological acceptor and test for complex-dependent activation.
Experiment: Express Drosophila CG3631 and test it in the reconstituted linker-synthesis system, to confirm experimentally that SF14 xylosylkinase activity is conserved in insects.
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