CG3631 encodes the single Drosophila member of the FAM20B (glycosaminoglycan xylosylkinase) subfamily of the FAM20 family of secretory-pathway kinases. The 421-residue precursor has an N-terminal signal sequence followed by the conserved FAM20 C-terminal kinase domain, with the ATP-, Mn2+- and catalytic-site residues of the family. By orthology with vertebrate FAM20B it is expected to transfer phosphate from ATP to the 2-hydroxyl of the xylose that links glycosaminoglycan chains to proteoglycan core proteins, a transient modification that promotes completion of the common tetrasaccharide linker and initiation of heparan sulfate and chondroitin sulfate chains; 2-phosphoxylose has been detected on fly proteoglycan linkers. Tagged CG3631 resides in the endoplasmic reticulum and Golgi of S2 cells and is not secreted, unlike the second fly FAM20 protein CG31145, a FAM20C-like protein kinase. No enzyme assay or loss-of-function phenotype has been reported for CG3631 itself.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004674 protein serine/threonine kinase activity | ISS PMID:22900076 The Raine syndrome protein FAM20C is a Golgi kinase that pho... | REMOVE | Summary: Protein serine/threonine kinase activity, transferred by sequence similarity from Four-jointed, is not supported for the FAM20B-subfamily protein CG3631. Reason: The WITH/FROM donor is Four-jointed (fj, FBgn0000658), a Golgi protein kinase that phosphorylates Fat and Dachsous cadherin domains. Fj is classified in a different PANTHER family (PTHR13147) from CG3631 (PTHR12450, subfamily SF14 glycosaminoglycan xylosylkinase), and FAM20B shows only weak similarity to Fj. Within PTHR12450 the substrate class is subfamily-specific. FAM20C-type proteins (SF11) and the fly FAM20C-like CG31145 phosphorylate proteins; the cited paper reports casein kinase activity for CG31145, not CG3631. FAM20B, the SF14 subfamily member with experimental data, phosphorylates glycan-linked xylose rather than amino-acid side chains. PAINT propagated only the broad alcohol-group phosphotransferase term to CG3631 and not a protein-kinase term. A cross-family similarity transfer therefore assigns the substrate class of a different branch and should not be retained. The broad phosphotransferase activity is captured by the IBA row. Supporting Evidence: PMID:22900076 Drosophila CG31145 also has Casein kinase activity (not shown). PMID:22900076 CG3631, which is most closely related to FAM20B PMID:19473117 FAM20B, a protein of unknown function that shows weak similarity to a Golgi kinase encoded by four-jointed PMID:29572475 Fam20B phosphorylates a xylose residue to regulate proteoglycan synthesis. |
| GO:0005783 endoplasmic reticulum | IDA PMID:22900076 The Raine syndrome protein FAM20C is a Golgi kinase that pho... | ACCEPT | Summary: V5-tagged CG3631 overlaps an ER marker in S2 cells. Reason: Direct target-specific imaging in the cited primary study. CG3631 was retained in cells (lysate only) and overlapped both ER and Golgi markers, consistent with a luminal secretory-pathway enzyme that acts on nascent proteoglycans. The evidence is from overexpressed tagged protein in cultured cells; endogenous localization in fly tissues has not been tested. Supporting Evidence: PMID:22900076 CG3631 overlapped both ER and Golgi markers, and was not detectably secreted from S2 cells |
| GO:0005783 endoplasmic reticulum | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping of the same S2-cell ER localization. Reason: The UniProt subcellular location line is itself grounded in the primary localization study (ECO:0000269, PubMed:22900076), so this electronic row restates the experimentally supported ER localization and is consistent with the IDA row. Supporting Evidence: PMID:22900076 CG3631 overlapped both ER and Golgi markers, and was not detectably secreted from S2 cells file:DROME/CG3631/CG3631-uniprot.txt Endoplasmic reticulum {ECO:0000269|PubMed:22900076}. |
| GO:0005794 Golgi apparatus | IDA PMID:22900076 The Raine syndrome protein FAM20C is a Golgi kinase that pho... | ACCEPT | Summary: V5-tagged CG3631 overlaps a Golgi marker in S2 cells. Reason: The same target-specific imaging experiment shows overlap with a Golgi marker. Golgi residence fits the expected site of action, since the FAM20B reaction follows xylosylation and the first galactose addition in the linker pathway. ER and Golgi residence are compatible rather than mutually exclusive annotations. Supporting Evidence: PMID:22900076 CG3631 overlapped both ER and Golgi markers, and was not detectably secreted from S2 cells |
| GO:0005794 Golgi apparatus | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping of the same S2-cell Golgi localization. Reason: The UniProt subcellular location line cites the primary localization study (ECO:0000269, PubMed:22900076), so this electronic row is consistent with the IDA row. Supporting Evidence: PMID:22900076 CG3631 overlapped both ER and Golgi markers, and was not detectably secreted from S2 cells file:DROME/CG3631/CG3631-uniprot.txt SUBCELLULAR LOCATION: Golgi apparatus {ECO:0000269|PubMed:22900076}. |
| GO:0016773 phosphotransferase activity, alcohol group as acceptor | IBA GO_REF:0000033 | ACCEPT | Summary: Alcohol-group phosphotransferase activity is the activity shared by the whole FAM20 kinase lineage and is correctly inherited by CG3631. Reason: PTN000966557 is a deep node whose descendants include both the FAM20C-type protein kinases (human FAM20C Q8IXL6, mouse/rat Fam20c, worm famk-1) and FAM20B (O75063). The common chemistry is ATP-dependent phosphorylation of a hydroxyl group, on Ser/Thr side chains for FAM20C and on the xylose 2-hydroxyl for FAM20B. CG3631 sits in the FAM20B subfamily (PANTHER PTHR12450:SF14, with human FAM20B, mouse Fam20b and zebrafish fam20b) and retains the family ATP-binding, Mn2+-binding and catalytic residues annotated by UniProt. The PAINT curator placed this broad term at the deep node and did not propagate a protein-kinase term to the FAM20B branch, which is consistent with the subfamily difference in substrate. No more specific GO term for glycan-linked xylose 2-O-kinase activity exists, so the broad term is as precise as GO currently allows. Direct assays of fly CG3631 are lacking. Supporting Evidence: PMID:22900076 CG3631, which is most closely related to FAM20B PMID:19473117 we identified FAM20B as a kinase that phosphorylates the xylose residue in the linkage region PMID:29572475 Fam20B phosphorylates a xylose residue to regulate proteoglycan synthesis. file:DROME/CG3631/CG3631-uniprot.txt PANTHER; PTHR12450:SF14; GLYCOSAMINOGLYCAN XYLOSYLKINASE; 1. |
| GO:0030166 proteoglycan biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: Proteoglycan biosynthesis is the process in which the FAM20B-type xylose kinase acts. Reason: This IBA comes from a FAM20B-branch node (PTN002488224) seeded by zebrafish fam20b, which is in the same PANTHER subfamily (SF14) as CG3631. The enzyme itself performs a step of the process, phosphorylating the linker xylose on the core protein, which promotes linker completion and glycosaminoglycan chain initiation, so this is participation rather than mere requirement. FAM20B knockdown and overexpression alter chondroitin sulfate and heparan sulfate amounts in human cells, and 2-phosphoxylose occurs on Drosophila linkage regions, although no CG3631-specific perturbation has been reported. The broad process term is appropriate because the linker is shared by heparan sulfate and chondroitin sulfate proteoglycans. Supporting Evidence: PMID:19473117 These results suggest that FAM20B regulates the number of glycosaminoglycan chains by phosphorylating the xylose residue in the glycosaminoglycan-protein linkage region of proteoglycans. PMID:29572475 xylose phosphorylation within the tetrasaccharide linker region of CS and HS proteoglycans has also been demonstrated in Drosophila , and this activity can probably be ascribed to Fam20B PMID:22900076 CG3631, which is most closely related to FAM20B |
Loading supporting content…
Download this section (compressed HTML)Q: Does purified CG3631 phosphorylate Gal-Xyl or linker tetrasaccharide acceptors, and is it inactive on protein substrates such as casein (the substrate used for its paralog CG31145)?
Q: Is CG3631 the enzyme responsible for the 2-phosphoxylose detected on Drosophila proteoglycan linkage regions (for example on Windpipe)?
Q: GO lacks a specific molecular function term for glycosaminoglycan-protein linkage region xylose 2-O-kinase activity (RHEA:19461); should one be requested so that FAM20B orthologs need not carry the broad GO:0016773?
Experiment: Recombinant CG3631 kinase assay with Gal-beta1-4-Xyl-benzyl and linker tetrasaccharide acceptors versus casein, including a catalytic-site mutant as negative control.
Type: in vitro enzyme assay
Experiment: CG3631 null or RNAi flies analysed for linker-region 2-phosphoxylose by glycoproteomics and for heparan sulfate and chondroitin sulfate amounts and chain number.
Type: genetic loss of function with glycan analysis
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)