D5SHU8 is the 299-residue ttv-PC product of the Drosophila tout-velu (ttv) gene, encoded by the alternative transcript ttv-RC. Its sequence is identical to residues 462-760 of full-length Tout-velu (Ttv, EXT1 ortholog, Q9V730, 760 aa), so it consists only of the C-terminal GT64 (GT-A fold) glycosyltransferase domain. It lacks the N-terminal signal-anchor helix and the GT47 (GT-B) domain. Full-length Ttv is a type II ER/Golgi membrane protein. With Sotv (EXT2) it forms the heparan sulfate co-polymerase, which adds alternating GlcA and GlcNAc residues to proteoglycan chains and is needed for Hedgehog, Wingless and Dpp morphogen distribution. ttv-PC keeps the GT-A DxD motif and several donor-binding arginines. At the uridine and phosphate contact positions it carries the substitutions that mark the weakly active human EXT1 GT-A domain. It has no signal or transmembrane segment that would deliver it to the secretory pathway, where heparan sulfate is polymerised. The protein is supported only at the transcript level. Whether it is made in vivo, which compartment it occupies, and whether it has any catalytic or regulatory activity have not been studied.
Summary: ER membrane localization for D5SHU8 comes from an ARBA rule that assigns family-level "single-pass type II membrane protein" topology. That topology cannot apply to ttv-PC, which lacks the signal-anchor helix.
Reason: In full-length Ttv, ER/Golgi membrane residence depends on the N-terminal signal-anchor helix (Q9V730 TRANSMEM 7-25, type II). D5SHU8 begins at Ttv residue 462. It contains no hydrophobic segment comparable to that helix, and UniProt's sequence-analysis features predict no transmembrane or signal region for this entry. A protein without a signal sequence or signal anchor is not inserted into the ER membrane. The annotation is a family-level electronic projection whose stated topology is contradicted by this product's own sequence. Removing it here does not affect the gene's experimentally supported ER and Golgi localization, which belongs to the full-length PA/PB products (Q9V730).
D5SHU8 residue 1 aligns to Ttv residue 462, and identity over the whole of D5SHU8 is 100.0%. D5SHU8 is an N-terminally truncated product of the same gene. It has no residues from Ttv 1-461, which include the signal-anchor helix (7-25) and the N-terminal GT47 (GT-B) domain.
D5SHU8 has no segment comparable to the control helix. Its most hydrophobic window falls inside the GT-A fold, and UniProt's own sequence-analysis tools annotate no transmembrane or signal feature on D5SHU8.
file:DROME/ttv/ttv-reference-Q9V730-uniprot.txt
Helical; Signal-anchor for type II membrane protein
Summary: The gene takes part in heparan sulfate proteoglycan biosynthesis through the full-length Ttv-Sotv co-polymerase. That role is unlikely to transfer to the 299-residue ttv-PC product.
Reason: Heparan sulfate chain elongation happens in the lumen of the ER and Golgi. In the human EXT1-EXT2 complex, the GlcA-transferase step is carried out by the N-terminal EXT1 GT47 domain, which ttv-PC lacks, and the GlcNAc-transferase step mainly by the EXT2 GT-A domain. The EXT1 GT-A domain, the only domain present in ttv-PC, is described as weakly active. When it was expressed on its own, as ttv-PC would be, no protein was produced. ttv-PC also has no signal sequence and so would not reach the lumen where its acceptor chains are. The InterPro exostosin match correctly classifies the sequence, but participation in glycoprotein biosynthesis by this product has never been shown and is biologically implausible. The process is supported for the gene through full-length Ttv (Q9V730), not through this isoform.
Expression of EXT2 alone, single GT-A or GT-B domains from EXT1 or co-expression of GT-A or GT-B domains from EXT1 and EXT2 led to no protein secretion
D5SHU8 residue 1 aligns to Ttv residue 462, and identity over the whole of D5SHU8 is 100.0%. D5SHU8 is an N-terminally truncated product of the same gene. It has no residues from Ttv 1-461, which include the signal-anchor helix (7-25) and the N-terminal GT47 (GT-B) domain.
Summary: The InterPro GT64-domain mapping to membrane reflects the type II membrane topology of full-length exostosins. ttv-PC has no membrane anchor.
Reason: The GT64 domain is luminal and soluble. Exostosins are held in membranes by their N-terminal signal-anchor helix, which ttv-PC lacks. The general membrane term is less tightly bound to a specific topology than the ER-membrane row. Membrane association through partners such as Sotv can be imagined, but for a protein without a signal sequence it would require a pathway that has not been demonstrated. This is therefore marked as an over-annotation for this isoform rather than accepted. Full-length Ttv (Q9V730) is a membrane protein.
D5SHU8 has no segment comparable to the control helix. Its most hydrophobic window falls inside the GT-A fold, and UniProt's own sequence-analysis tools annotate no transmembrane or signal feature on D5SHU8.
Summary: ttv-PC keeps a GT-A (GT64) fold with its DxD motif. It corresponds, however, to the EXT1 GT-A domain, which is weakly active in the human co-polymerase, could not be expressed on its own, and in ttv-PC is not linked to a secretory-pathway signal.
Reason: Human EXT1 is placed in the same PANTHER subfamily as Ttv (PTHR48261:SF3), which justifies using the human EXT1-EXT2 structures to interpret ttv-PC. Li et al. found that only two of the four potential active sites in EXT1-EXT2 bind donor and acceptor (EXT1 GT-B and EXT2 GT-A). The EXT1 GT-A domain is "weakly active", and mutating it produced only minor changes in activity. Leisico et al. instead observed reduced GlcNAc-T activity when the EXT1 GT-A DxD was mutated, so this domain may contribute some activity within the intact heterodimer. In ttv-PC the DxD and R595/R701 equivalents are conserved. The uridine-stacking aromatic and phosphate-binding basic residues of the active EXT2 GT-A site are not; as in human EXT1, the aligned positions carry other residues. Any activity reported for EXT1 GT-A was measured inside the full-length heterodimer. A free GT-A domain translated without a signal sequence would not encounter Sotv or the luminal acceptors. Glycosyltransferase capability is therefore an over-reach for this product. It is a reasonable family-level statement but not an activity this isoform is expected to perform. PAINT gives the full-length Ttv entry IBA terms for acetylglucosaminyltransferase and glucuronosyltransferase activity but no IBA rows to D5SHU8. This most likely reflects isoform coverage rather than a curator's decision, and is not used as evidence either way.
In contrast, mutations in the EXT1 GT-A or EXT2 GT-B domains resulted in wild-type activity levels (R325A), minor reductions (D565A, D567A, R595A, W612A, W612A and Y308A) or slightly enhanced (Q328A) activities
A decrease in GlcNAc glycosyl transferase activity in the EXT1 D565N/D567N and EXT2 D538N/D540N mutants confirms that the C-terminal GT-A domains of EXT1 and EXT2 harbor GlcNAc-T activity
Interpretation: ttv-PC keeps the conserved metal- and donor-binding core of the GT-A fold (DxD, R595, R701). At the uridine/phosphate positions, however, it has the same substitutions that distinguish the "weakly active" human EXT1 GT-A domain from the active EXT2 GT-A domain.
GO:1901135 carbohydrate derivative metabolic process
IEA GO_REF:0000117
MARK AS OVER ANNOTATED
Summary: This is a very broad ARBA-derived process term. For the gene it reflects heparan sulfate synthesis by full-length Ttv, and that role is unlikely to transfer to the 299-residue ttv-PC product.
Reason: The same reasoning applies as for glycoprotein biosynthetic process. ttv-PC lacks the GT47 GlcA-transferase domain and the luminal targeting signal, and keeps only the weakly active EXT1-type GT-A domain. No experiment links this product to carbohydrate-derivative metabolism. The ARBA rule is a family-level association and is not evidence about this isoform.
D5SHU8 residue 1 aligns to Ttv residue 462, and identity over the whole of D5SHU8 is 100.0%. D5SHU8 is an N-terminally truncated product of the same gene. It has no residues from Ttv 1-461, which include the signal-anchor helix (7-25) and the N-terminal GT47 (GT-B) domain.
Q: Is the ttv-RC transcript translated into a stable ttv-PC protein in vivo, and if so, in which tissues and subcellular compartment does it accumulate?
Q: If ttv-PC is made, does it act as a dominant-negative or regulatory partner of Ttv or Sotv (for example through GT-A domain dimerization), or is it a non-functional product of alternative transcript processing?
Suggested Experiments
Experiment: Detect endogenous ttv-PC with an antibody against the C-terminal domain, or with a C-terminal epitope knock-in, comparing ttv-RC-specific RNAi or isoform-specific deletion to distinguish the 34 kDa product from full-length Ttv. Determine its localization relative to ER and Golgi markers.
Type: protein expression and localization
Experiment: Express ttv-PC in ttv mutant or Sotv-overexpressing backgrounds and measure heparan sulfate chain length and morphogen (Hh, Wg, Dpp) distribution in wing imaginal discs. This tests whether the isoform has rescuing, inert or dominant-negative activity.
Type: genetic rescue and dominant-negative test
External Prediction Reviews
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
ProtNLM2 (UniProt API snapshot 2026-09-08) emitted one GO prediction for D5SHU8, the 299-residue ttv-PC product that corresponds to the C-terminal GT64 (GT-A) domain of Tout-velu (residues 462-760 of Q9V730). Broad glycosyltransferase activity is already present as an InterPro2GO electronic annotation, so it is not novel. Its correctness for this product is unresolved: the DxD motif is retained, but the equivalent human EXT1 GT-A domain is weakly active within the EXT1-EXT2 heterodimer, and ttv-PC lacks the signal anchor and partner domain needed to reach luminal acceptors. The prediction is scored UNC.
Review rationale: D5SHU8 is the 299-residue ttv-PC product, identical to Ttv residues 462-760, so it contains only the C-terminal GT64 (GT-A) domain. It keeps the DxD motif and the active-site Asp equivalent. The same term is already carried by D5SHU8 as an InterPro2GO electronic annotation, so the prediction is not novel; the gene review marks that row over-annotated for this product. Family transfer is justified at the domain level, because Ttv and human EXT1 share a PANTHER subfamily. However, the human EXT1 GT-A domain is described as weakly active in the EXT1-EXT2 co-polymerase, and mutating it gave only minor activity changes in one study. A second study found reduced GlcNAc-T activity after EXT1 GT-A DxD mutation, so the domain may contribute activity inside the intact heterodimer. All of these measurements were made on full-length protein. ttv-PC lacks the signal-anchor helix and the GT47 domain and has no predicted signal or transmembrane segment, so it is not expected to reach the secretory-pathway acceptors or partner Sotv. Neither activity of the isolated domain nor its absence has been tested, and a PubMed search found no study of this isoform; UNC is retained.
Supporting Evidence:
file:DROME/ttv/ttv-bioinformatics/RESULTS.md: "The C-terminal glycosyltransferase-region active-site Asp670 maps to target Asp209, and the source-record nucleotide-sugar/metal-binding residues in this region are retained."
PMID:36593275: "For example, Lys651 in the active EXT2 GT-A domain is replaced by Gln677 in the weakly active EXT1 GT-A domain"
PMID:36402845: "A decrease in GlcNAc glycosyl transferase activity in the EXT1 D565N/D567N and EXT2 D538N/D540N mutants confirms that the C-terminal GT-A domains of EXT1 and EXT2 harbor GlcNAc-T activity"
tout-velu (ttv) encodes a glycosyltransferase that forms a complex with sotv to catalyze the alternating transfer of glucuronic acid (GlcA) and N-acetylglucosamine (GlcNAc) to their respective acceptors, GlcNAc and GlcA, during the elongation of the heparan sulfate glycosaminoglycan chain.