id: Q9VCT4 gene_symbol: klg product_type: PROTEIN status: DRAFT taxon: id: NCBITaxon:7227 label: Drosophila melanogaster description: >- Klingon (klg) is a Drosophila melanogaster cell-surface glycoprotein of the immunoglobulin superfamily (IgSF). It has three C2-type immunoglobulin-like domains followed by a fibronectin type III repeat and is attached to the outer leaflet of the plasma membrane through a glycosylphosphatidylinositol (GPI) anchor, i.e. it is an extrinsic, lipid-anchored membrane protein rather than a transmembrane one. In cultured cells Klingon mediates homophilic cell-cell adhesion, which is its core molecular activity. It is expressed in restricted subsets of neurons during embryonic neurogenesis and in the developing eye, where it is required in the R7 photoreceptor precursor for correct R7 cell-fate commitment and differentiation, functioning in the context of the sevenless/Ras signalling pathway and binding other IgSF ectodomains (e.g. cDIP). In the adult brain Klingon protein accumulates at neuropil-glia interfaces, including around the lobes and calyces of the mushroom bodies, and is acutely required for protein-synthesis-dependent long-term memory (but not anesthesia-resistant memory); its levels are up-regulated by Notch signalling upon memory induction, placing it downstream of Notch as an effector that links Notch activity to memory consolidation. klingon mutants (also recovered as the ruslan/rus allele) additionally show altered behavioural responses to ethanol. Neuronal expression and IgSF family relationships have further led to its inclusion among candidate axon-guidance/adhesion molecules. existing_annotations: - term: id: GO:0005886 label: plasma membrane evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: is_active_in review: summary: >- Phylogenetic (IBA) localization of this GPI-anchored IgSF adhesion protein to the plasma membrane. This is the correct and core subcellular localization: Klingon is displayed on the cell surface where it engages in homophilic adhesion. action: ACCEPT reason: >- Directly supported by the experimental demonstration that Klingon expressed in S2 cells is associated with the cell membrane through a GPI linkage, consistent with a cell-surface CAM. supported_by: - reference_id: PMID:9043060 supporting_text: >- it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion - term: id: GO:0050808 label: synapse organization evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: involved_in review: summary: >- Phylogenetic (IBA) inference of a synapse-organization role, based on membership in an IgSF adhesion family whose members organize synaptic/neurite contacts. Klingon protein accumulates at neuropil boundaries and mushroom-body lobe/calyx-glia junctures, consistent with an adhesive role at neurite interfaces, but no direct klg loss-of-function synapse-organization assay exists in the cited literature. action: KEEP_AS_NON_CORE reason: >- Plausible family-level inference and consistent with the observed neuropil localization, but not experimentally demonstrated for klg and peripheral to the gene's demonstrated adhesion and R7 developmental functions; retain as a non-core process. supported_by: - reference_id: PMID:19104051 supporting_text: >- immunohistochemical experiments demonstrate extensive localization of Klg protein at the junctures between the neuropil and neuropil glia, including the junctures between the lobes and calyces of the mushroom bodies and the surrounding glial cells - term: id: GO:0030424 label: axon evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: is_active_in review: summary: >- Phylogenetic (IBA) localization to the axon, consistent with Klingon being a neuronal cell-surface adhesion molecule expressed in restricted neuronal populations and concentrated in neuropil. action: ACCEPT reason: >- Consistent with the neuronal expression of klg and localization of Klg protein at neuropil-glia junctures; a reasonable subcellular localization for a neuronal surface CAM, though non-core relative to the plasma-membrane localization. supported_by: - reference_id: PMID:19104051 supporting_text: >- immunohistochemical experiments demonstrate extensive localization of Klg protein at the junctures between the neuropil and neuropil glia - term: id: GO:0007156 label: homophilic cell-cell adhesion evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: involved_in review: summary: >- Phylogenetic (IBA) assignment of homophilic cell-cell adhesion, the core biological process for Klingon. This matches the direct experimental demonstration of homophilic adhesion in S2 cells. action: ACCEPT reason: >- Core function; concordant with the IDA-backed homophilic adhesion assay and with independent description of Klingon as a homophilic cell adhesion molecule. supported_by: - reference_id: PMID:9043060 supporting_text: >- it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion - reference_id: PMID:19104051 supporting_text: >- rus is a new allele of klingon (klg), which encodes a homophilic cell adhesion molecule - term: id: GO:0008046 label: axon guidance receptor activity evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: enables review: summary: >- Phylogenetic (IBA) inference of axon guidance receptor activity from the IgSF adhesion/guidance family. There is no direct experimental demonstration that Klingon transduces a guidance signal, and as a GPI-anchored protein it lacks a cytoplasmic domain and would require a co-receptor to signal. The experimentally demonstrated molecular function of Klingon is homophilic cell-cell adhesion. action: MODIFY reason: >- Klingon's demonstrated molecular activity is homophilic cell-cell adhesion, not receptor-mediated guidance signalling. As a GPI-anchored protein it has no cytoplasmic domain and cannot itself transduce a guidance signal, so the family-level 'axon guidance receptor activity' inherited from the IgSF adhesion/guidance family overstates what klg does; cell-cell adhesion mediator activity captures the evidence more precisely. This mirrors the sibling trn review, where the same kind of over-broad IBA molecular function on a cell-surface adhesion protein is modified to GO:0098632. Knock-on: the GO:0007411 axon guidance annotation below is a GO_REF:0000108 inter-ontology link whose WITH/FROM is this GO:0008046 IBA, so it inherits this term's weakness. proposed_replacement_terms: - id: GO:0098632 label: cell-cell adhesion mediator activity propagation_review: root_cause: PROPAGATION_BAD failure_modes: - FUNCTIONAL_DIVERGENCE - GRANULARITY_MISMATCH supported_by: - reference_id: PMID:9043060 supporting_text: >- it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion - term: id: GO:0043025 label: neuronal cell body evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: is_active_in review: summary: >- Phylogenetic (IBA) localization to the neuronal cell body, consistent with klg expression in restricted neuronal populations during neurogenesis and in the developing eye. action: ACCEPT reason: >- Consistent with the documented neuronal expression pattern of klg; a reasonable subcellular localization for a neuronal surface protein. supported_by: - reference_id: PMID:9043060 supporting_text: >- expressed in a restricted pattern of neurons during embryonic neurogenesis and in the R7 photoreceptor precursor throughout its development - term: id: GO:0007411 label: axon guidance evidence_type: IEA original_reference_id: GO_REF:0000108 qualifier: involved_in review: summary: >- Inter-ontology (GO_REF:0000108) assignment of axon guidance derived from a molecular function, not from the domain architecture directly: klg-goa.tsv gives WITH/FROM = GO:0008046, i.e. this BP term is generated from the 'axon guidance receptor activity' IBA reviewed above. Klingon is a neuronal adhesion molecule, so a guidance-related role is plausible, but there is no direct klg experiment demonstrating axon guidance in the cited literature. action: KEEP_AS_NON_CORE reason: >- Plausible inference consistent with neuronal expression, but not experimentally shown for klg and peripheral to its demonstrated homophilic adhesion and R7 developmental roles; retain as non-core. Note the dependency: this term is an inter-ontology product of the GO:0008046 IBA that this review proposes to MODIFY, so if that molecular function is replaced by GO:0098632 the automated basis for this BP annotation no longer holds and it would need independent support to survive. supported_by: - reference_id: PMID:9043060 supporting_text: >- expressed in a restricted pattern of neurons during embryonic neurogenesis - term: id: GO:0009653 label: anatomical structure morphogenesis evidence_type: IEA original_reference_id: GO_REF:0000117 qualifier: involved_in review: summary: >- Electronic (ARBA) assignment of a broad morphogenesis process. This general term is consistent with Klingon's established role in R7 photoreceptor development and eye morphogenesis. action: ACCEPT reason: >- Correct but general; an acceptable high-level parent given the demonstrated requirement of klg in R7 neuron development. More specific curated terms (R7 differentiation/fate commitment) are also present. supported_by: - reference_id: PMID:9043060 supporting_text: >- klingon is an essential gene that participates in the development of the R7 neuron - term: id: GO:0030154 label: cell differentiation evidence_type: IEA original_reference_id: GO_REF:0000117 qualifier: involved_in review: summary: >- Electronic (ARBA) assignment of the broad cell differentiation process, consistent with the established role of klg in R7 photoreceptor differentiation. action: ACCEPT reason: >- Correct but general; the more specific curated term GO:0045466 (R7 cell differentiation) is also annotated. Acceptable as a broad parent. supported_by: - reference_id: PMID:9043060 supporting_text: >- klingon is an essential gene that participates in the development of the R7 neuron - term: id: GO:0160108 label: animal gross anatomical part developmental process evidence_type: IEA original_reference_id: GO_REF:0000117 qualifier: involved_in review: summary: >- Electronic (ARBA) assignment of a broad organ/anatomical-part developmental process. Consistent with klg's role in eye (R7 photoreceptor) development. action: ACCEPT reason: >- Correct but very general; captured more specifically by the R7 development annotations. Acceptable as a broad IEA parent. supported_by: - reference_id: PMID:9043060 supporting_text: >- klingon is an essential gene that participates in the development of the R7 neuron - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:23827685 qualifier: enables review: summary: >- IPI annotation from the Özkan et al. (2013) extracellular IgSF/LRR interactome, in which Klingon's ectodomain was tested for pairwise binding (UniProt IntAct records Klg/Q9VCT4 binding the IgSF ectodomain cDIP/Q9VDD5). Bare "protein binding" is uninformative; the interaction is an adhesion-molecule interaction and Klingon is itself a homophilic cell adhesion molecule, so a more specific cell-adhesion-molecule binding term is warranted. action: MODIFY reason: >- Replace the uninformative GO:0005515 with cell adhesion molecule binding (GO:0050839): Klingon binds other IgSF cell-adhesion ectodomains and mediates homophilic adhesion (i.e. binds a cell adhesion molecule). The cached full text of PMID:23827685 does not name klg (pairwise hits are in supplementary tables), so the adhesion nature is anchored to the direct homophilic-adhesion evidence. proposed_replacement_terms: - id: GO:0050839 label: cell adhesion molecule binding supported_by: - reference_id: PMID:9043060 supporting_text: >- it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion - term: id: GO:0048149 label: behavioral response to ethanol evidence_type: IMP original_reference_id: PMID:18435628 qualifier: involved_in review: summary: >- IMP annotation: the klg mutant (recovered as the memory mutant ruslan) shows altered behavioural responses to ethanol, specifically increased sensitivity and reduced tolerance. This is an experimentally observed phenotype but a distal, pleiotropic behavioural role rather than a core molecular/developmental function of klg. action: KEEP_AS_NON_CORE reason: >- Experimental phenotype (do not remove), but ethanol behaviour is a downstream/pleiotropic consequence likely secondary to klg's roles in neural adhesion and memory circuitry; retain as non-core. (The paper refers to mutants by original names; ruslan is an allele of klg per PMID:19104051.) supported_by: - reference_id: PMID:18435628 supporting_text: >- a number of mutants exhibited both increased sensitivity and reduced tolerance additional_reference_ids: - PMID:19104051 - term: id: GO:0007616 label: long-term memory evidence_type: IMP original_reference_id: PMID:21490205 qualifier: involved_in review: summary: >- IMP annotation: the klg mutant (D0417) abolishes the α/β mushroom-body long-term-memory calcium trace and is impaired in long-term behavioural memory, confirming an acute requirement for klg in LTM. This is a genuine but distal behavioural role. action: KEEP_AS_NON_CORE reason: >- Well-supported experimental role in LTM, but memory is a downstream, circuit-level function of this adhesion molecule rather than its core molecular activity; retain as non-core. supported_by: - reference_id: PMID:21490205 supporting_text: >- which encodes a member of the Ig superfamily of cell adhesion molecules and has also been implicated in photoreceptor development - reference_id: PMID:21490205 supporting_text: >- Klg has been shown to be acutely required for LTM and regulated by Notch - term: id: GO:0007615 label: anesthesia-resistant memory evidence_type: IMP original_reference_id: PMID:19104051 qualifier: involved_in negated: true review: summary: >- NOT annotation (negated): Matsuno et al. (2009) show that Klingon is acutely required for protein-synthesis-dependent long-term memory but specifically NOT for anesthesia-resistant memory (ARM). This negative finding correctly records that klg does not function in the ARM branch of consolidated memory. action: ACCEPT reason: >- Valid, experimentally supported negative annotation that captures the specificity of klg for LTM over ARM; the negated qualifier is appropriate and should be retained. supported_by: - reference_id: PMID:19104051 supporting_text: >- Klg is acutely required for LTM but not anesthesia-resistant memory formation - term: id: GO:0007616 label: long-term memory evidence_type: IMP original_reference_id: PMID:19104051 qualifier: involved_in review: summary: >- IMP annotation: Klingon is acutely required for long-term memory formation, shown by inducible klg rescue and acute klg RNAi, with Klg protein rising upon LTM induction; klg acts as a downstream effector of Notch linking Notch activity to memory. A genuine but distal behavioural role. action: KEEP_AS_NON_CORE reason: >- Strongly supported experimental role in LTM, but this circuit-/behaviour-level function is downstream of klg's core adhesion activity; retain as non-core. supported_by: - reference_id: PMID:19104051 supporting_text: >- Klg is acutely required for LTM but not anesthesia-resistant memory formation, and Klg expression increases upon LTM induction - reference_id: PMID:19104051 supporting_text: >- We propose that Klg is a downstream effector of Notch signaling that links Notch activity to memory - term: id: GO:0007156 label: homophilic cell-cell adhesion evidence_type: IDA original_reference_id: PMID:9043060 qualifier: involved_in review: summary: >- Direct assay (IDA): Klingon expressed in S2 tissue-culture cells mediates homophilic cell-cell adhesion. This is the primary experimental evidence establishing Klingon's core molecular/biological function as a homophilic cell adhesion molecule. action: ACCEPT reason: >- Core function directly demonstrated in a cell-aggregation assay and corroborated by independent description of klg as a homophilic CAM. supported_by: - reference_id: PMID:9043060 supporting_text: >- it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion - term: id: GO:0007465 label: R7 cell fate commitment evidence_type: IGI original_reference_id: PMID:9043060 qualifier: involved_in review: summary: >- Genetic-interaction (IGI) evidence that klg participates in R7 photoreceptor fate/development, including interaction with the sevenless pathway (ectopic klg in a sevenless background can alter R8 rhabdomere position). klg is an essential gene required for R7 neuron development. action: ACCEPT reason: >- Core developmental function; the R7 role is the originally characterized and best-established biological process for klg, supported by genetic analysis. supported_by: - reference_id: PMID:9043060 supporting_text: >- Ectopic expression of klingon in all neurons in a sevenless background can alter the position of the R8 rhabdomere - reference_id: PMID:9043060 supporting_text: >- klingon is an essential gene that participates in the development of the R7 neuron - term: id: GO:0019897 label: extrinsic component of plasma membrane evidence_type: ISS original_reference_id: PMID:9043060 qualifier: located_in review: summary: >- Localization as an extrinsic (lipid-anchored) component of the plasma membrane, based on the GPI linkage: Klingon is attached to the outer leaflet via a glycosyl-phosphatidylinositol anchor rather than a transmembrane domain, consistent with the absence of a TM region in the sequence. action: ACCEPT reason: >- Correct and specific localization directly supported by the experimental demonstration of GPI attachment; appropriate for a GPI-anchored cell-surface protein. supported_by: - reference_id: PMID:9043060 supporting_text: >- it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage - term: id: GO:0045466 label: R7 cell differentiation evidence_type: IMP original_reference_id: PMID:9043060 qualifier: involved_in review: summary: >- IMP evidence that klg is required for differentiation/development of the R7 photoreceptor neuron; klg is expressed in the R7 precursor throughout development and is an essential gene participating in R7 neuron development. action: ACCEPT reason: >- Core developmental function, well established from mutant analysis; together with the R7 fate commitment annotation this captures klg's principal in vivo role. supported_by: - reference_id: PMID:9043060 supporting_text: >- expressed in a restricted pattern of neurons during embryonic neurogenesis and in the R7 photoreceptor precursor throughout its development - reference_id: PMID:9043060 supporting_text: >- klingon is an essential gene that participates in the development of the R7 neuron - term: id: GO:0098632 label: cell-cell adhesion mediator activity evidence_type: IDA original_reference_id: PMID:9043060 qualifier: enables review: summary: >- Proposed molecular-function annotation capturing Klingon's demonstrated activity as a homophilic cell-cell adhesion molecule. The bare GO:0005515 (protein binding) IPI annotation does not convey this adhesive activity; cell-cell adhesion mediator activity is the informative MF supported by the direct S2-cell homophilic adhesion assay. action: NEW reason: >- Klingon expressed in S2 cells mediates homophilic adhesion, i.e. it directly enables cell-cell adhesion; this MF is currently absent from GOA and is the core molecular function of the gene. Note this is the same term proposed as the replacement for the GO:0008046 IBA above: the two requests collapse to a single GO:0098632 annotation on klg, which the IDA evidence here supports directly rather than phylogenetically. supported_by: - reference_id: PMID:9043060 supporting_text: >- it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion - reference_id: PMID:19104051 supporting_text: >- rus is a new allele of klingon (klg), which encodes a homophilic cell adhesion molecule core_functions: - description: >- GPI-anchored immunoglobulin-superfamily cell-surface protein that mediates homophilic cell-cell adhesion at the plasma membrane. This adhesive activity is the core evolved molecular function of Klingon, demonstrated directly by cell-aggregation assays of Klingon-expressing S2 cells. supported_by: - reference_id: PMID:9043060 supporting_text: >- it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion - reference_id: PMID:19104051 supporting_text: >- rus is a new allele of klingon (klg), which encodes a homophilic cell adhesion molecule molecular_function: id: GO:0098632 label: cell-cell adhesion mediator activity directly_involved_in: - id: GO:0007156 label: homophilic cell-cell adhesion locations: - id: GO:0005886 label: plasma membrane - id: GO:0019897 label: extrinsic component of plasma membrane - description: >- Ig-superfamily homophilic adhesion molecule acting in R7 photoreceptor development: expressed in the R7 precursor throughout its development, klg is an essential gene required for R7 cell-fate commitment and differentiation, functioning in the context of the sevenless/Ras pathway. supported_by: - reference_id: PMID:9043060 supporting_text: >- klingon is an essential gene that participates in the development of the R7 neuron - reference_id: PMID:9043060 supporting_text: >- Ectopic expression of klingon in all neurons in a sevenless background can alter the position of the R8 rhabdomere molecular_function: id: GO:0098632 label: cell-cell adhesion mediator activity directly_involved_in: - id: GO:0045466 label: R7 cell differentiation - id: GO:0007465 label: R7 cell fate commitment locations: - id: GO:0005886 label: plasma membrane proposed_new_terms: [] suggested_questions: - question: >- Does Klingon function as a signalling receptor for axon guidance, or purely as an adhesive ligand that requires a transmembrane co-receptor given its GPI anchor? - question: >- What is the identity and functional relevance of Klingon's heterophilic ectodomain partners (e.g. cDIP) in R7 development and in mushroom-body long-term memory? suggested_experiments: - description: >- Tissue-/time-restricted klg knockdown and rescue with cell-adhesion-dead vs GPI-anchor-deleted variants to separate homophilic adhesion from any receptor/co-receptor signalling in R7 development and LTM. - description: >- Quantitative bead-binding / SPR of the Klingon ectodomain against candidate IgSF partners (cDIP, Dpr/DIP family) to map its heterophilic interaction network and test its contribution to neuron-glia adhesion at mushroom-body neuropil boundaries. references: - id: GO_REF:0000033 title: Annotation inferences using phylogenetic trees findings: [] - id: GO_REF:0000108 title: Automatic assignment of GO terms using logical inference, based on on inter-ontology links findings: [] - id: GO_REF:0000117 title: Electronic Gene Ontology annotations created by ARBA machine learning models findings: [] - id: PMID:18435628 title: Ethanol sensitivity and tolerance in long-term memory mutants of Drosophila melanogaster. findings: [] reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Full text available and read. The klg allele (ruslan) is among the LTM mutants scored; ruslan shows increased ethanol sensitivity and reduced (rapid+chronic) tolerance. Supports the IMP behavioral response to ethanol annotation (a non-core, pleiotropic phenotype). Paper uses original mutant names; ruslan=klg per PMID:19104051. - id: PMID:19104051 title: The Drosophila cell adhesion molecule klingon is required for long-term memory formation and is regulated by Notch. findings: [] reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Full text available and read. Establishes that rus is a klg allele encoding a homophilic CAM; Klg is acutely required for LTM but not ARM, its levels rise on LTM induction, and it acts downstream of Notch. Directly supports the homophilic adhesion, LTM (positive) and ARM (negated) annotations. - id: PMID:21490205 title: The long-term memory trace formed in the Drosophila α/β mushroom body neurons is abolished in long-term memory mutants. findings: [] reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Full text available and read. Confirms the klg mutant (D0417) as an LTM mutant that abolishes the α/β mushroom-body LTM calcium trace; corroborates the acute LTM requirement of klg. Klg is one of 26 diverse LTM mutants, so this is supporting rather than mechanistic for klg specifically. - id: PMID:23827685 title: An extracellular interactome of immunoglobulin and LRR proteins reveals receptor-ligand networks. findings: [] reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Source of the IPI protein-binding annotation (Drosophila IgSF/LRR extracellular interactome; UniProt IntAct records Klg/Q9VCT4 binding cDIP/Q9VDD5). The cached full text does not name klg/cDIP (pairwise hits are in supplementary tables), so no klg-specific verbatim quote is available here; the annotation is sharpened from bare protein binding to cell adhesion molecule binding. - id: PMID:9043060 title: klingon, a novel member of the Drosophila immunoglobulin superfamily, is required for the development of the R7 photoreceptor neuron. findings: [] reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Abstract-only in cache (full_text_available:false) but the abstract itself establishes the core findings: three C2-type Ig domains + one FN3 repeat, GPI membrane attachment, homophilic adhesion in S2 cells, and an essential role in R7 photoreceptor development. Primary reference for the core adhesion and R7 annotations.