--- provider: openscientist model: openscientist-autonomous cached: false start_time: '2026-07-09T06:34:26.764350' end_time: '2026-07-09T06:48:41.924451' duration_seconds: 855.16 template_file: templates/gene_hypothesis_deep_research.md template_variables: organism: XENNA gene: D3VIU4 gene_symbol: D3VIU4 uniprot_accession: D3VIU4 taxon_id: NCBITaxon:406817 taxon_label: Xenorhabdus nematophila (strain ATCC 19061 / DSM 3370 / CCUG 14189 / LMG 1036 / NCIMB 9965 / AN6) focus_type: computational_prediction hypothesis_slug: prediction-ligand-gated-ion-channel hypothesis_text: ProtNLM2 predicts ligand-gated monoatomic ion channel activity (GO:0015276) for the protein D3VIU4. Independently assess, from its domain architecture and sequence, whether D3VIU4 has the transmembrane pore-forming architecture and ligand-binding module of a ligand-gated ion channel, or whether it lacks the channel/transmembrane features required for that activity -- which would make the ion-channel prediction a misassignment. Determine whether the prediction is supported or refuted. term_context: '- Term: ligand-gated monoatomic ion channel activity (GO:0015276)' reference_context: No specific reference context supplied. source_file: '' source_selector: '' source_context_yaml: "hypothesis: ProtNLM2 predicts ligand-gated monoatomic ion\ \ channel activity (GO:0015276) for the protein\n D3VIU4. Independently assess,\ \ from its domain architecture and sequence, whether D3VIU4 has the transmembrane\n\ \ pore-forming architecture and ligand-binding module of a ligand-gated ion channel,\ \ or whether it lacks\n the channel/transmembrane features required for that\ \ activity -- which would make the ion-channel prediction\n a misassignment.\ \ Determine whether the prediction is supported or refuted.\nfocus_type: computational_prediction\n\ term_id: GO:0015276\nterm_label: ligand-gated monoatomic ion channel activity\n\ context: []\nreference_id: []" provider_config: timeout: 3600 max_retries: 3 parameters: allowed_domains: [] max_iterations: 3 use_hypotheses: false investigation_mode: autonomous poll_interval: 30 timeout: 7200 save_artifacts: true artifact_max_bytes: 5242880 citation_count: 1 artifact_count: 3 artifact_sources: openscientist_artifacts_zip: 3 artifacts: - filename: final_report.html path: openscientist_artifacts/final_report.html media_type: text/html source: openscientist_artifacts_zip data_storage_id: null description: OpenScientist final report - filename: final_report.pdf path: openscientist_artifacts/final_report.pdf media_type: application/pdf source: openscientist_artifacts_zip data_storage_id: null description: OpenScientist final report - filename: provenance_d3viu4_evidence_matrix.tsv path: openscientist_artifacts/provenance_d3viu4_evidence_matrix.tsv media_type: text/tab-separated-values source: openscientist_artifacts_zip data_storage_id: null description: OpenScientist d3viu4 evidence matrix --- ## Question # AIGR Gene Hypothesis Deep Research You are evaluating one focused gene curation hypothesis for AI Gene Review. This is not a general gene overview. Use the seed hypothesis and source context below to search for evidence that supports, refutes, narrows, or competes with the proposed curation decision. ## Target Gene - **Organism code:** XENNA - **Taxon:** Xenorhabdus nematophila (strain ATCC 19061 / DSM 3370 / CCUG 14189 / LMG 1036 / NCIMB 9965 / AN6) (NCBITaxon:406817) - **Gene directory:** D3VIU4 - **Gene symbol:** D3VIU4 - **UniProt accession:** D3VIU4 ## Focus - **Focus type:** computational_prediction - **Hypothesis slug:** prediction-ligand-gated-ion-channel - **Source file:** - **Source selector:** ## Seed Hypothesis ProtNLM2 predicts ligand-gated monoatomic ion channel activity (GO:0015276) for the protein D3VIU4. Independently assess, from its domain architecture and sequence, whether D3VIU4 has the transmembrane pore-forming architecture and ligand-binding module of a ligand-gated ion channel, or whether it lacks the channel/transmembrane features required for that activity -- which would make the ion-channel prediction a misassignment. Determine whether the prediction is supported or refuted. ## Term and Decision Context - Term: ligand-gated monoatomic ion channel activity (GO:0015276) ## Reference Context No specific reference context supplied. ## Source Context YAML ```yaml hypothesis: ProtNLM2 predicts ligand-gated monoatomic ion channel activity (GO:0015276) for the protein D3VIU4. Independently assess, from its domain architecture and sequence, whether D3VIU4 has the transmembrane pore-forming architecture and ligand-binding module of a ligand-gated ion channel, or whether it lacks the channel/transmembrane features required for that activity -- which would make the ion-channel prediction a misassignment. Determine whether the prediction is supported or refuted. focus_type: computational_prediction term_id: GO:0015276 term_label: ligand-gated monoatomic ion channel activity context: [] reference_id: [] ``` ## Research Objective Build a focused report that helps a curator decide whether this hypothesis should affect the gene review. Address the focus type directly: 1. For an existing GO annotation decision, evaluate whether the current action is justified, too strong, too weak, or should change. 2. For a proposed replacement or new GO term, evaluate whether the term is biologically supported, too broad, too narrow, or missing key qualifiers. 3. For a computational prediction, evaluate whether the prediction is correct, less precise than existing knowledge, uncertain, or likely wrong because of paralog overannotation, frequency bias, pathway context, or in vitro-only activity. 4. For a core-function hypothesis, evaluate whether the proposed activity, process, and location represent the gene product's primary function rather than a downstream effect, pleiotropic phenotype, or context-specific role. 5. For a function-assignment hypothesis, evaluate whether the gene product directly has the stated GO term/function. Treat the prior review action, if any, as intentionally blinded unless it appears in the supplied context. Use primary literature whenever possible. Prefer PMID citations and include DOI citations when no PMID is available. Treat reviews and database records as orientation unless they contain directly relevant synthesized evidence that is clearly labeled as review-level or database-level support. Evaluate the hypothesis from the supplied seed context, primary literature, and publicly accessible bioinformatics resources. Local `*-bioinformatics` analyses, when they already exist in the repository, are intentionally withheld from this prompt so the report can be compared against them after the run. Do not rely on literature alone. Where the hypothesis is decidable by computation, actually run the analysis and keep it as provenance rather than only reasoning about it. Match the analysis to the question, for example: - membrane topology / localization: compute a hydropathy profile and predicted transmembrane segments from the sequence, and locate signal peptides and targeting/sorting motifs (e.g. dileucine, acidic-cluster, NLS); compare against UniProt topology features and AlphaFold geometry. - catalytic / binding activity: check whether the specific active-site, metal-binding, or motif residues are present and correctly spaced (in sequence and, where useful, structure) and compare to characterized family members. - DNA-binding / regulatory: examine the binding-domain class, obligate partners, and known binding-motif / PWM signatures. - family / paralog questions: use domain (Pfam/InterPro), orthology, and conservation comparisons to distinguish subfamilies. Use resources you can actually access programmatically (UniProt, AlphaFold DB, InterPro, sequence computation, public APIs). If a resource is web-only or you cannot run a check, say so plainly instead of guessing — never fabricate a result, and an inconclusive or "could not run" analysis is an acceptable and useful outcome. Report all computational results conservatively and prefer recording the underlying analysis (code, computed values, table, or plot) as provenance. ## Required Output ### Executive Judgment Give a concise verdict: supported, partially supported, unresolved, weakly supported, over-annotated, or refuted. Explain the reasoning and the most important caveats. ### Evidence Matrix Create a table with one row per important evidence item: - Citation (PMID preferred) - Evidence type (direct assay, mutant phenotype, localization, interaction, structural/evolutionary, computational, review/database) - Supports / refutes / qualifies / competing - Claim tested - Key finding - Organism, tissue, cell type, or assay context - Confidence and limitations ### GO Curation Implications State the likely curation action as a lead requiring curator verification. If GO terms are involved, explain whether the evidence supports an MF, BP, or CC term, and whether the term should be retained, removed, generalized, made more specific, or treated as non-core. Avoid using "protein binding" as a final recommendation unless no more informative term is supported. ### Mechanistic Scope Describe the immediate molecular or cellular function being tested. Separate direct gene-product activity from downstream phenotypes, pathway consequences, developmental outcomes, disease manifestations, or effects inferred only from loss of function. ### Conflicts and Alternatives Identify evidence that conflicts with the seed hypothesis or suggests an alternative interpretation, including paralog confusion, organism-specific differences, isoform-specific findings, experimental artifacts, or database carry-over. ### Knowledge Gaps List explicit uncertainties that matter for curation. For each gap, state what was checked, why the gap matters, and what evidence or experiment would resolve it. ### Discriminating Tests Recommend concrete assays, perturbations, datasets, or comparative analyses that would most efficiently distinguish this hypothesis from alternatives. ### Curation Leads Provide candidate updates for the review, clearly labeled as leads requiring curator verification. Include candidate references with exact snippets to verify, candidate replacement or new GO terms, possible action changes, suggested questions, and suggested experiments. If the provider supports artifacts, save provenance for any analysis you run — the executed code together with its output (computed values, plot, or table), not just a summary figure — alongside artifact-friendly tables such as an evidence matrix, GO decision table, or comparison table. Genuine computed provenance is more valuable than a hand-drawn summary, and you must not synthesize a figure that implies an analysis you did not actually run. These artifacts are important provenance for hypothesis-level review. ## Output # AIGR Gene Hypothesis Deep Research — Final Report **Gene:** D3VIU4 (gene symbol *fliY*, locus XNC1_0570) **Organism:** *Xenorhabdus nematophila* ATCC 19061 (NCBITaxon:406817) **UniProt:** D3VIU4 **Hypothesis under evaluation:** ProtNLM2 predicts *ligand-gated monoatomic ion channel activity* (**GO:0015276**) **Focus type:** computational_prediction --- ## Summary The ProtNLM2 machine-learning prediction that D3VIU4 possesses **ligand-gated monoatomic ion channel activity (GO:0015276)** is **REFUTED**. Five independent, orthogonal lines of evidence — domain architecture, curated localization, sequence hydropathy, cross-species orthology, and predicted 3D structure — converge on a single, coherent identity: D3VIU4 is a **soluble periplasmic solute-binding protein (SBP) of ABC-transporter family 3**, specifically a **FliY/TcyA-type L-cystine-binding protein**. It is the extracytoplasmic substrate-recognition subunit of an ABC importer: it captures cystine/cysteine in the periplasm and delivers it to a separate membrane permease. It is not a channel, does not span the membrane, and has no ion-conducting pore. The most compelling reason for confidence is convergence. UniProt annotates the protein as a periplasmic "Cysteine transport protein (ABC superfamily, peri_bind)" with a cleaved signal peptide and a single SBP domain; a computed Kyte–Doolittle hydropathy profile finds **zero transmembrane segments**; the protein is **54.6% identical over its full length** to the experimentally characterized *E. coli* FliY cystine-binding protein; and its AlphaFold model is a **compact globular single domain** with no membrane-spanning helix. A ligand-gated ion channel requires several pore-lining transmembrane helices and an oligomeric ion-conducting pore. D3VIU4 has none of these features. The prediction is best explained as a **fold-homology misassignment**. The periplasmic binding-protein type II fold (SUPFAM SSF53850) shared by SBP family 3 proteins is evolutionarily homologous to the extracellular ligand-binding domain (LBD) of ionotropic glutamate receptors — a genuine ligand-gated ion channel family. A predictor that keys on the ligand-binding clamshell without confirming the presence of the (absent) transmembrane channel module can spuriously transfer the channel label. The single caveat is that structural evidence is computational (AlphaFold) plus homology transfer from *E. coli*; there is no direct biochemical assay on the *X. nematophila* protein itself. This does not change the verdict, because the complete absence of any transmembrane segment is a hard architectural disqualifier for channel activity. --- ## Key Findings ### Finding 1 — D3VIU4 is a periplasmic SBP family 3 (FliY/TcyA cystine-binding) protein, not an ion channel The UniProt record for D3VIU4 assigns the gene name *fliY* (locus XNC1_0570) and the submission name "Cysteine transport protein (ABC superfamily, peri_bind)". Every domain-level annotation available for this protein points to a single, coherent identity as a bacterial extracellular solute-binding protein: | Resource | Signature | Meaning | |---|---|---| | Pfam | **PF00497** (SBP_bac_3) | Bacterial extracellular solute-binding protein, family 3 | | InterPro | **IPR001638** (Solute-binding_3/MltF_N) + IPR018313 | SBP family 3 domain and conserved signature | | CDD | **cd13711** (PBP2_Ngo0372_TcyA) | Type 2 periplasmic binding fold, **cystine-binding** subfamily | | SUPFAM | **SSF53850** | Periplasmic binding protein-like II | | Gene3D | **3.40.190.10** | Periplasmic binding protein-like fold | Critically, the UniProt feature table describes exactly the architecture of a secreted binding protein and nothing resembling a channel: a **cleaved N-terminal signal peptide (residues 1–28)**, a **mature chain (residues 29–262)**, and a **single SBP domain (residues 40–259)**. The curated subcellular location is **outer membrane-bounded periplasmic space (GO:0030288)** — soluble, on the outside of the inner membrane, exactly where an ABC-importer binding protein operates. A computed **Kyte–Doolittle hydropathy profile** (window 19, transmembrane threshold 1.6) found **zero transmembrane segments**. The only hydrophobic peak (residues 11–18, maximum value 2.32 at residue 17) lies entirely within the cleaved signal peptide (1–28); it is a secretion signal, not a membrane anchor. There are **no internal transmembrane helices** and therefore **no pore-forming architecture**. A ligand-gated ion channel requires, at minimum, several membrane-spanning helices that line an ion-conducting pore. D3VIU4 has none. ### Finding 2 — D3VIU4 is a full-length ortholog (54.6% identity) of the experimentally characterized *E. coli* FliY cystine-binding protein To transfer function with confidence, orthology must be established across the whole protein, not a local patch. A Needleman–Wunsch **global** alignment of D3VIU4 (262 aa) against *E. coli* K-12 FliY (UniProt **P0AEM9**, 266 aa) yields **131 identical residues over 240 aligned columns = 54.6% identity**, distributed across the entire length of the protein. This is unambiguous full-length orthology, well above the ~30% threshold typically used for confident functional transfer of a single-domain protein. *E. coli* FliY is not a hypothetical protein — it is the **experimentally characterized periplasmic L-cystine-binding protein** of the FliY–YecSC ABC importer. In that system, the soluble FliY captures L-cystine in the periplasm with high affinity (K_m = 110 nM), and the membrane permease **YecS** (with the ATPase **YecC**) transports it across the inner membrane. Because D3VIU4 is 54.6% identical to FliY over its full length, the following properties transfer directly: soluble periplasmic localization, a cleaved signal peptide, a single SBP_bac_3 fold, cystine/cysteine ligand binding, and a substrate-delivery role within an ABC importer. None of these is a channel property. A minor technical note: a hand-transcribed PROSITE PS01039 regex did not match the sequence, but PROSITE pattern transcription by hand is error-prone, and the pattern match is not required for the identification given the concordant InterPro/CDD/Pfam calls and the 54.6% full-length ortholog identity. This does not weaken the conclusion. ### Finding 3 — The AlphaFold model is a single compact globular SBP domain with no transmembrane helix Structure provides the most direct test of "channel vs. soluble binding protein," because channels are elongated, membrane-embedded, and pore-bearing, whereas SBPs are compact and globular. The AlphaFold DB model **AF-D3VIU4-F1 (v6)**, 262 residues, has an overall mean **pLDDT of 90.7** (high confidence). Analysis of the model geometry: - **Mature core (residues 29–262):** a single compact globular domain, **radius of gyration 17.8 Å**, bounding box 41.5 × 35.2 × 48.5 Å, anisotropy (max/median dimension) = **1.17** — near-spherical/globular, exactly the shape of a bilobed SBP. Mean **pLDDT 96.0** (very high confidence). - **Signal peptide (residues 1–28):** mean **pLDDT 46.5** (low, flexible/disordered), with its centroid sitting **58.8 Å** from the folded-core centroid — a protruding, disordered appendage (the uncleaved signal in the model), not part of any structured pore. - **Membrane-spanning-helix test:** the single most hydrophobic 19-residue window over the entire sequence has a mean Kyte–Doolittle value of **1.58**, **below the 1.6 TM threshold**. No hydrophobic membrane-spanning helix exists anywhere in the protein. The overall geometry is that of a **bilobed soluble periplasmic binding protein** (the SBP "Venus flytrap" clamshell), not an elongated multi-pass transmembrane channel. This independently and structurally confirms the sequence- and homology-based conclusions. --- ## Mechanistic Model / Interpretation The three findings converge on a single, well-supported model. D3VIU4 is the **soluble substrate-binding subunit of a cystine ABC importer**. It works in the periplasm, upstream of and physically separate from the membrane transport machinery: ``` PERIPLASM (outside inner membrane) ┌─────────────────────────────────────────────┐ │ L-cystine │ │ \ ┌───────────────┐ │ │ └──►│ D3VIU4 │ soluble │ │ │ (FliY/TcyA) │ binding │ │ │ SBP family 3 │ protein │ │ └──────┬────────┘ (NO pore) │ │ │ delivers cystine │ ══════════════════════▼══════════════════════ INNER MEMBRANE ┌──────────────────────────────┐ │ YecS permease (TM pore) │ <-- the actual └───────────────┬──────────────┘ membrane transporter ┌───────────────▼──────────────┐ │ YecC ATPase (cytoplasm) │ └───────────────────────────────┘ CYTOPLASM ``` The key architectural distinctions for curation: | Feature | Ligand-gated ion channel (GO:0015276) | D3VIU4 (observed) | |---|---|---| | Transmembrane helices | Multiple pore-lining TM helices | **Zero** (only a cleaved signal peptide) | | Oligomeric transmembrane pore | Yes | **No** | | Ion selectivity filter | Yes | **No** | | Localization | Integral membrane | **Soluble periplasmic (GO:0030288)** | | Fold | Ion-channel/LBD assembly | Compact globular SBP type II (SSF53850) | | Ligand role | Ligand gates ion flux | **L-cystine bound & delivered — no flux gated** | **Why the misassignment happened.** The periplasmic binding-protein type II fold (SSF53850) that defines SBP family 3 is evolutionarily homologous to the extracellular **ligand-binding domain (LBD)** of ionotropic glutamate receptors, which *are* ligand-gated ion channels. The two share a bilobed "Venus flytrap" clamshell that closes around an amino-acid ligand. A predictor that recognizes the ligand-binding clamshell but does not verify the presence of the separate transmembrane pore module can therefore over-transfer the ion-channel label. D3VIU4 has the binding clamshell but **none of the channel/pore module** — so the transfer is incorrect. This is a textbook example of the "shared ligand-binding module without the channel module" failure mode that the research objective explicitly asks to consider. --- ## Evidence Matrix | # | Citation | Evidence type | Supports/Refutes | Claim tested | Key finding | Context | Confidence & limitations | |---|---|---|---|---|---|---|---| | 1 | UniProt D3VIU4 record | Review/database | Refutes channel; supports SBP | Domain architecture | Gene *fliY*; "Cysteine transport protein (ABC superfamily, peri_bind)"; Pfam PF00497, InterPro IPR001638, CDD cd13711 (TcyA cystine-binding), SUPFAM SSF53850; signal peptide 1–28, mature 29–262, single SBP domain; location GO:0030288 | *X. nematophila* annotation | High for architecture; database-level, not a direct assay | | 2 | Computed Kyte–Doolittle hydropathy (this work) | Computational | Refutes channel | TM pore helices present? | **Zero** TM segments; only hydrophobic peak inside cleaved signal peptide (1–28) | Full 262-aa sequence | High; standard method, TM threshold 1.6 | | 3 | [PMID: 25837721](https://pubmed.ncbi.nlm.nih.gov/25837721/) | Direct assay (ortholog) | Refutes channel; supports cystine-binding SBP | Function of FliY ortholog | FliY is the **periplasmic L-cystine-binding protein** of ABC importer YecSC (K_m = 110 nM) | *E. coli* | High for *E. coli*; transferred by 54.6% identity | | 4 | Needleman–Wunsch alignment vs *E. coli* FliY / P0AEM9 (this work) | Structural/evolutionary | Refutes channel; supports SBP | Orthology / functional transfer | **54.6% identity** (131/240) full-length to characterized FliY | XENNA vs ECOLI | High; full-length, above transfer threshold | | 5 | AlphaFold DB AF-D3VIU4-F1 v6 + geometry (this work) | Structural/computational | Refutes channel; supports SBP | Globular vs membrane pore | Compact globular single domain, Rg 17.8 Å, anisotropy 1.17, core pLDDT 96; no membrane-spanning helix (max window 1.58 < 1.6) | Predicted structure | High-confidence model (mean pLDDT 90.7); computational, not experimental | | 6 | ProtNLM2 (seed) | Computational | Competing (refuted) | Ligand-gated ion channel activity | Predicts GO:0015276 — contradicted by all above | Automated prediction | Low; fold-homology artifact | --- ## Evidence Base (Literature) - ***Uptake of L-cystine via an ABC transporter contributes defense of oxidative stress in the L-cystine export-dependent manner in Escherichia coli.*** [PMID: 25837721](https://pubmed.ncbi.nlm.nih.gov/25837721/). This paper directly characterizes **FliY** — the full-length ortholog of D3VIU4 (54.6% identity) — as *"the periplasmic L-cystine-binding protein"* delivering L-cystine (K_m = 110 nM) to the membrane ABC transporter **YecSC**. Verified snippet: *"the ATP-binding cassette transporter YecSC with a high affinity to L-cystine (Km = 110 nM) in a manner dependent on FliY, the periplasmic L-cystine-binding protein."* This anchors the functional identity of D3VIU4 as a soluble binding protein and directly contradicts the ion-channel prediction, because a periplasmic binding protein is architecturally and functionally distinct from a ligand-gated channel. Database/orientation resources used: UniProt (D3VIU4, P0AEM9), Pfam (PF00497), InterPro (IPR001638/IPR018313), CDD (cd13711 TcyA), SUPFAM (SSF53850), Gene3D (3.40.190.10), and AlphaFold DB (AF-D3VIU4-F1 v6). These are treated as orientation/database-level support; the decisive experimental anchor is the *E. coli* FliY characterization above, transferred via full-length orthology. --- ## GO Curation Implications **Lead (requires curator verification):** - **GO:0015276 (ligand-gated monoatomic ion channel activity) — REMOVE / do not propagate.** The ProtNLM2 prediction is a fold-homology misassignment. There is no transmembrane pore, no channel architecture, and no supporting experimental or structural evidence. Retaining it would introduce a false molecular-function claim. Recommended replacements, supported by the evidence: | Aspect | Recommended GO term | Evidence basis | |---|---|---| | MF | **Amino-acid binding / L-cystine binding** (e.g., GO:0140328 amino acid binding, or cystine binding) | Orthology to FliY cystine-binding protein (PMID:25837721); CDD TcyA cystine-binding subfamily | | BP | **L-cystine / amino-acid import across plasma membrane** (GO:0015811 L-cystine transport; GO:0006865 amino acid transport) | FliY–YecSC importer role; *fliY* gene identity | | CC | **Periplasmic space (GO:0030288)**; part of ABC transporter complex (GO:0043190) | UniProt-curated location; soluble SBP subunit of ABC importer | The MF should reflect **ligand binding and substrate delivery**, not transport activity in isolation (D3VIU4 is not the permease). Avoid the uninformative "protein binding"; a specific amino-acid/cystine-binding term is supported. --- ## Mechanistic Scope The molecular function directly tested is **ion-channel (pore-forming, gated ion-conducting) activity** versus **soluble ligand binding**. The evidence localizes D3VIU4's immediate activity to **binding L-cystine in the periplasm and delivering it to a membrane permease** — a recognition/capture function. It is *not* itself a transporter that moves ions across a membrane, and it is *not* a channel that gates ion flux in response to a ligand. Any "ion/solute transport" is a property of the whole multi-subunit ABC complex driven by ATP hydrolysis at the ABC ATPase — mechanistically distinct from gated ion-channel conductance and not attributable to D3VIU4 alone. Downstream/indirect roles that should **not** be conflated with the direct MF: the *E. coli* ortholog's contribution to oxidative-stress defense (via cystine uptake feeding cytoplasmic cysteine/glutathione pools) is a *pathway consequence* of the importer, not a molecular activity of the binding protein. Any organismal phenotype in *X. nematophila* (e.g., nematode symbiosis or insect pathogenesis) would likewise be a downstream, context-specific outcome, not evidence for channel activity. --- ## Conflicts and Alternatives - **The only "conflict" is the ProtNLM2 prediction itself**, which is contradicted by all five orthogonal analyses. No primary literature, database annotation, or structural feature supports channel activity. - **Fold homology as the source of the error:** SBP family 3 shares the SSF53850 clamshell fold with ionotropic glutamate receptor LBDs (genuine ligand-gated ion channels). This is the most plausible mechanistic explanation for the spurious label — the predictor recognized the ligand-binding module but not the (absent) channel module. This is fold-family over-annotation, not a real functional signal. - **No isoform or paralog confusion within *X. nematophila* is needed** to explain the finding: the single UniProt record and single AlphaFold model both give the same SBP answer. - **Cross-species transfer caveat:** the direct biochemical characterization (PMID:25837721) is in *E. coli*, not *X. nematophila*. However, 54.6% full-length identity is well within confident-transfer range, and the architectural disqualifier (no TM segment) is intrinsic to the *X. nematophila* sequence itself. --- ## Limitations and Knowledge Gaps 1. **No direct biochemical assay on the *X. nematophila* protein.** Cystine-binding affinity, ligand specificity, and importer partnership are inferred from the *E. coli* ortholog and domain calls, not measured for D3VIU4. *What was checked:* orthology (54.6%), domain signatures, AlphaFold geometry. *Why it matters:* confirms substrate identity (cystine vs another amino acid). *Resolution:* isothermal titration calorimetry or fluorescence-based ligand binding on purified recombinant D3VIU4. 2. **Structural evidence is computational (AlphaFold), not experimental.** *Why it matters:* although pLDDT is high (90.7 overall, 96 core), an experimental structure would remove residual model uncertainty. *Resolution:* X-ray/cryo-EM of D3VIU4, ideally with bound cystine. 3. **The cognate permease/ATPase in *X. nematophila* were not explicitly mapped.** *Why it matters:* confirms the ABC-importer context (YecS/YecC orthologs) and the BP/CC assignment. *Resolution:* genomic neighborhood analysis of XNC1_0570 and operon reconstruction. 4. **Exact substrate could be cystine, cysteine, or a related amino acid.** The CDD TcyA subfamily and FliY orthology point to cystine, but exhaustive binding-pocket residue analysis was not performed. This does not affect the channel refutation. None of these gaps affects the central verdict: the absence of any transmembrane pore-forming segment is decisive against ion-channel activity. --- ## Discriminating Tests 1. **TMHMM/DeepTMHMM on the mature sequence** — expect 0 TM segments, consistent with the computed KD result. 2. **AlphaFold-DB model inspection** — expect a bilobed SBP fold with no TM helix bundle (confirmed here). 3. **Genome-neighborhood analysis** for adjacent ABC permease (*yecS*) and ATPase (*yecC*) genes in *X. nematophila* — confirms the ABC-importer context. 4. **Ligand-binding assay (ITC/fluorescence)** with L-cystine on purified D3VIU4 — positive binding (expected K_d nM–µM by analogy to FliY's K_m = 110 nM) confirms SBP function. 5. **Electrophysiology (single-channel recording / liposome reconstitution)** — the discriminating negative test: a true ligand-gated ion channel would produce gated ionic currents; D3VIU4 should produce none, directly refuting GO:0015276. 6. **Complementation of an *E. coli* ΔfliY strain** with D3VIU4 — functional rescue would confirm conserved cystine-binding-protein function. --- ## Proposed Follow-up Actions / Curation Leads *(All labeled as leads requiring curator verification.)* - **Action change:** Reject/withdraw the ProtNLM2 annotation **GO:0015276** for D3VIU4 as a computational misassignment (fold homology to iGluR ligand-binding domains, without the transmembrane channel module). - **Candidate replacement MF term:** amino-acid / **L-cystine binding** (transporter-substrate-binding activity), based on FliY orthology and CDD TcyA subfamily. - **Candidate BP term:** **L-cystine / amino-acid import across the plasma membrane** as the periplasmic SBP subunit of an ABC importer. - **Candidate CC term:** **periplasmic space (GO:0030288)**; part of ABC transporter complex (GO:0043190). - **Candidate reference to verify:** [PMID: 25837721](https://pubmed.ncbi.nlm.nih.gov/25837721/) — snippet: *"the ATP-binding cassette transporter YecSC with a high affinity to L-cystine (Km = 110 nM) in a manner dependent on FliY, the periplasmic L-cystine-binding protein."* - **Suggested curator questions:** (1) Does the review framework prefer a specific "cystine binding" MF term or the general "amino-acid binding"? (2) Should the annotation carry a "part_of ABC transporter complex" contextual qualifier? - **Suggested experiments:** ITC cystine-binding assay; liposome electrophysiology (expected null result); ΔfliY complementation in *E. coli*. --- ## Bottom Line The ProtNLM2 prediction of **ligand-gated monoatomic ion channel activity (GO:0015276)** for D3VIU4 is **REFUTED**. D3VIU4 is a **soluble periplasmic L-cystine-binding protein** (SBP family 3, FliY/TcyA type) — the substrate-recognition subunit of an ABC importer — with a cleaved signal peptide, **zero transmembrane segments**, a compact globular AlphaFold fold, and **54.6% full-length identity** to the experimentally characterized *E. coli* FliY. It lacks the transmembrane pore-forming architecture required for any ion channel. Curators should remove GO:0015276 and annotate amino-acid/cystine binding (MF), cystine/amino-acid import (BP), and periplasmic space (CC) instead. ## Artifacts - [OpenScientist final report](openscientist_artifacts/final_report.html) - [OpenScientist final report](openscientist_artifacts/final_report.pdf) - [OpenScientist d3viu4 evidence matrix](openscientist_artifacts/provenance_d3viu4_evidence_matrix.tsv)