Tartan (Trn) is a single-pass type I transmembrane protein of the leucine-rich-repeat (LRR) cell-surface family, closely related to and largely redundant with its paralog Capricious (Caps). Its extracellular region is an LRR horseshoe (roughly ten LRR units capped by an LRR C-terminal cysteine-rich domain) displayed at the plasma membrane, followed by a single transmembrane helix and a short cytoplasmic tail. Trn functions as a cell-surface adhesion/affinity molecule that mediates cell-cell recognition and helps establish differences in affinity between neighbouring cell populations, and it acts largely through its extracellular LRR domain. Through this activity it contributes to the formation of affinity boundaries between cell groups in imaginal discs, to selective recognition of targets during motor and photoreceptor axon guidance, and to epithelial and tracheal morphogenesis, where it provides a permissive substrate for migrating cells. The Trn and Caps LRR domains are interchangeable and are thought to engage a common partner/receptor on apposed cells; the precise molecular binding partners of Trn remain incompletely defined.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0038023 signaling receptor activity | IBA GO_REF:0000033 | MODIFY | Summary: Phylogenetic (IBA) call from GO_Central propagating a signaling-receptor molecular function across an LRR family that also contains bona fide signaling receptors (e.g. human TLR3 and CD180). For tartan there is no evidence of transmembrane signal transduction; the protein acts as a cell-surface LRR adhesion/affinity molecule and exerts its function through its extracellular domain, so a more informative and better supported molecular function is cell-cell adhesion mediator activity. Reason: Trn's molecular function is that of a cell-surface LRR adhesion molecule rather than a classical signal-transducing receptor. Its role maps to the extracellular LRR domain, and it is repeatedly described as a candidate homophilic/heterophilic adhesion receptor whose LRR domain is interchangeable with that of Caps and engages a common partner. The IBA 'signaling receptor activity' is over-broad (inherited from a TLR-containing family); cell-cell adhesion mediator activity captures the evidence more precisely. Propagation Review Root cause: PROPAGATION BAD Failure modes: FUNCTIONAL DIVERGENCE GRANULARITY MISMATCH Sources checked: UniProtKB:O15455 · TLR3 (human) SUPPORTS SOURCE BUT NOT TARGET Bona fide LRR transmembrane signaling receptor; the signaling-receptor activity does not transfer to the divergent Drosophila adhesion protein trn. UniProtKB:Q99467 · CD180/RP105 (human) SUPPORTS SOURCE BUT NOT TARGET LRR cell-surface receptor of the innate-immune Toll-like family; not evidence for signal-transduction activity in trn. Proposed replacements: cell-cell adhesion mediator activity Supporting Evidence: PMID:19064711 It has been suggested that Caps and Trn act as homophilic or heterophilic adhesion receptors PMID:16764850 mesodermal cells and exerts its role in tracheal branch outgrowth through its PMID:18817735 The LRR domains of Trn and Caps are interchangeable, suggesting that they can interact with a common receptor |
| GO:0007436 larval salivary gland morphogenesis | IMP PMID:19064711 A targeted gain-of-function screen identifies genes affectin... | KEEP AS NON CORE | Summary: Loss-of-function analysis of caps and tartan mutants in a study of salivary gland tubulogenesis supports a role for trn in salivary gland morphogenesis. This is a specific developmental context for a pleiotropic LRR adhesion protein rather than its core molecular activity. Reason: The experimental (IMP) annotation is supported by the cited study, which identified caps/trn as LRR adhesion hits and analysed mutant phenotypes. It is a genuine but context-specific developmental role, so it is retained as non-core rather than as a core function. Supporting Evidence: PMID:19064711 The analysis of caps and tartan mutant phenotypes suggests a role for these genes in salivary gland morphogenesis. PMID:19064711 belong to the family of LRR proteins, suggesting a general role for this class of surface receptors in salivary gland morphogenesis |
| GO:0008045 motor neuron axon guidance | IMP PMID:18817735 A screen of cell-surface molecules identifies leucine-rich r... | ACCEPT | Summary: Well-supported experimental annotation. Trn was recovered in a cell-surface-molecule screen for synaptic target selection, and embryonic loss-of-function analysis shows that Trn and Caps act largely redundantly to guide and target motor axons: trn caps double mutants have stronger motor-axon phenotypes than trn alone, and neuronal Trn expression rescues the phenotype. This is a core neural function of trn. Reason: Direct genetic evidence (loss of function, double-mutant enhancement, and cell-type- specific rescue) supports involvement of trn in motor neuron axon guidance/targeting, acting redundantly with its paralog caps. Supporting Evidence: PMID:18817735 If so, Trn and Caps might function in a redundant manner to regulate axon guidance and to label muscles as axonal targets. PMID:18817735 trn caps double mutant embryos have stronger motor axon phenotypes than trn single mutants PMID:18817735 The trn caps phenotype was rescued to ~20% penetrance for both ISNb and SNa phenotypes |
| GO:0035147 branch fusion, open tracheal system | IMP PMID:16764850 Distinct functions of the leucine-rich repeat transmembrane ... | KEEP AS NON CORE | Summary: Supported experimental annotation. Caps and Tartan contribute differently to joining isolated tracheal metameres into a continuous tubular network; Tartan is expressed broadly in the mesoderm and provides a permissive substrate for the outgrowing/fusing tracheal branches. A genuine but context-specific developmental role, retained as non-core. Reason: The IMP annotation is supported by the cited study showing that Tartan acts (via its extracellular domain) in the formation of tracheal branch interconnections. It is a specific morphogenetic context rather than trn's core molecular activity. Supporting Evidence: PMID:16764850 repeat transmembrane proteins Capricious and Tartan contribute differently to PMID:16764850 migrating tracheal cells during the network formation. |
| GO:0007155 cell adhesion | NAS PMID:12717815 Pattern formation in the Drosophila wing: The development of... | ACCEPT | Summary: Cell adhesion is a core biological process for tartan: as an LRR cell-surface molecule it mediates cell-cell interactions and cell affinity in imaginal discs, trachea and the nervous system, acting redundantly with Caps. The annotation is biologically correct and well supported by the broader literature, even though the specific NAS reference (a wing-vein-patterning review) is a weak citation for it. Reason: The essence of the annotation - involvement in cell adhesion - is central to trn's function and supported by multiple studies describing Caps/Trn as adhesion molecules mediating cell-cell interactions. Retained as a core biological process. The original NAS reference is retained but flagged as low relevance in the references section. Supporting Evidence: PMID:18817735 Trn and Caps are involved in cell-cell interactions in tracheae and imaginal discs PMID:19064711 It has been suggested that Caps and Trn act as homophilic or heterophilic adhesion receptors |
| GO:0016477 cell migration | TAS PMID:12508275 Size isn't everything. | KEEP AS NON CORE | Summary: Trn contributes to cell migration indirectly/non-cell-autonomously: in the trachea it is expressed broadly in the mesoderm and provides a permissive substrate for the migrating tracheal branch cells. This is a defensible but peripheral, context-specific role rather than a core function, so it is retained as non-core. Reason: The general 'cell migration' term is supported by the concrete role of Tartan as a permissive mesodermal substrate for migrating tracheal cells; it captures a real but indirect and non-core aspect of trn function. The original TAS reference is a short commentary that does not itself discuss trn, so supporting evidence is drawn from the tracheal study. Supporting Evidence: PMID:16764850 migrating tracheal cells during the network formation. PMID:16764850 In contrast, Tartan is expressed broadly in |
| GO:0005886 plasma membrane | IEA file:DROME/trn/trn-uniprot.txt | NEW | Summary: Trn is a single-pass type I transmembrane protein (N-terminal signal peptide, one predicted transmembrane helix, extracellular LRR ectodomain), consistent with residence in the plasma membrane where it engages neighbouring cell surfaces. UniProt carries an ARBA electronic annotation to the parent term cell periphery (GO:0071944); plasma membrane is the specific compartment for this cell-surface adhesion molecule. Added as a NEW annotation to capture the localization underlying trn's adhesion function. Reason: Localization to the plasma membrane is not present in the GOA set but is strongly implied by the protein architecture and by the existing cell-periphery electronic annotation, and it is required to represent where trn's cell-cell adhesion activity occurs. Supporting Evidence: file:DROME/trn/trn-uniprot.txt C:cell periphery file:DROME/trn/trn-uniprot.txt Transmembrane helix |
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Download this section (compressed HTML)Q: What is the direct molecular binding partner of the Tartan LRR ectodomain - does it bind homophilically to Trn/Caps, heterophilically to a shared receptor, or to a distinct ligand?
Q: Is any part of Tartan's function dependent on its short cytoplasmic tail, or is the extracellular LRR domain fully sufficient across all developmental contexts?
Experiment: Cell aggregation / bead-binding assays with S2 cells expressing Trn (and Caps) to test directly for homophilic vs heterophilic adhesion and to identify the binding partner.
Hypothesis: Trn mediates cell-cell adhesion by binding a partner (Caps, Trn, or a shared receptor) on apposed cell surfaces.
Experiment: Domain-swap and cytoplasmic-tail-truncation rescue experiments across imaginal-disc affinity, axon-targeting, and tracheal contexts to define the minimal functional unit of Trn.
Hypothesis: The extracellular LRR domain is sufficient for Trn function in most or all contexts, with the short cytoplasmic tail largely dispensable.
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