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.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | 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. 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. Supporting Evidence: PMID:9043060 it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion |
| GO:0050808 synapse organization | IBA GO_REF:0000033 | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:19104051 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 |
| GO:0030424 axon | IBA GO_REF:0000033 | ACCEPT | 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. 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. Supporting Evidence: PMID:19104051 immunohistochemical experiments demonstrate extensive localization of Klg protein at the junctures between the neuropil and neuropil glia |
| GO:0007156 homophilic cell-cell adhesion | IBA GO_REF:0000033 | ACCEPT | 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. Reason: Core function; concordant with the IDA-backed homophilic adhesion assay and with independent description of Klingon as a homophilic cell adhesion molecule. Supporting Evidence: PMID:9043060 it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion PMID:19104051 rus is a new allele of klingon (klg), which encodes a homophilic cell adhesion molecule |
| GO:0008046 axon guidance receptor activity | IBA GO_REF:0000033 | MODIFY | 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. 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. Propagation Review Root cause: PROPAGATION BAD Failure modes: FUNCTIONAL DIVERGENCE GRANULARITY MISMATCH Proposed replacements: cell-cell adhesion mediator activity Supporting Evidence: PMID:9043060 it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion |
| GO:0043025 neuronal cell body | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) localization to the neuronal cell body, consistent with klg expression in restricted neuronal populations during neurogenesis and in the developing eye. Reason: Consistent with the documented neuronal expression pattern of klg; a reasonable subcellular localization for a neuronal surface protein. Supporting Evidence: PMID:9043060 expressed in a restricted pattern of neurons during embryonic neurogenesis and in the R7 photoreceptor precursor throughout its development |
| GO:0007411 axon guidance | IEA GO_REF:0000108 | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:9043060 expressed in a restricted pattern of neurons during embryonic neurogenesis |
| GO:0009653 anatomical structure morphogenesis | IEA GO_REF:0000117 | ACCEPT | 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. 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. Supporting Evidence: PMID:9043060 klingon is an essential gene that participates in the development of the R7 neuron |
| GO:0030154 cell differentiation | IEA GO_REF:0000117 | ACCEPT | Summary: Electronic (ARBA) assignment of the broad cell differentiation process, consistent with the established role of klg in R7 photoreceptor differentiation. Reason: Correct but general; the more specific curated term GO:0045466 (R7 cell differentiation) is also annotated. Acceptable as a broad parent. Supporting Evidence: PMID:9043060 klingon is an essential gene that participates in the development of the R7 neuron |
| GO:0160108 animal gross anatomical part developmental process | IEA GO_REF:0000117 | ACCEPT | Summary: Electronic (ARBA) assignment of a broad organ/anatomical-part developmental process. Consistent with klg's role in eye (R7 photoreceptor) development. Reason: Correct but very general; captured more specifically by the R7 development annotations. Acceptable as a broad IEA parent. Supporting Evidence: PMID:9043060 klingon is an essential gene that participates in the development of the R7 neuron |
| GO:0005515 protein binding | IPI PMID:23827685 An extracellular interactome of immunoglobulin and LRR prote... | MODIFY | Summary: IPI annotation from the Γzkan 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. 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 replacements: cell adhesion molecule binding Supporting Evidence: PMID:9043060 it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion |
| GO:0048149 behavioral response to ethanol | IMP PMID:18435628 Ethanol sensitivity and tolerance in long-term memory mutant... | KEEP AS NON CORE | 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. 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.) Supporting Evidence: PMID:18435628 a number of mutants exhibited both increased sensitivity and reduced tolerance |
| GO:0007616 long-term memory | IMP PMID:21490205 The long-term memory trace formed in the Drosophila Ξ±/Ξ² mush... | KEEP AS NON CORE | Summary: IMP annotation: the klg mutant (D0417) abolishes the Ξ±/Ξ² 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. 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. Supporting Evidence: PMID:21490205 which encodes a member of the Ig superfamily of cell adhesion molecules and has also been implicated in photoreceptor development PMID:21490205 Klg has been shown to be acutely required for LTM and regulated by Notch |
| GO:0007615 anesthesia-resistant memory | IMP NOT PMID:19104051 The Drosophila cell adhesion molecule klingon is required fo... | ACCEPT | 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. 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. Supporting Evidence: PMID:19104051 Klg is acutely required for LTM but not anesthesia-resistant memory formation |
| GO:0007616 long-term memory | IMP PMID:19104051 The Drosophila cell adhesion molecule klingon is required fo... | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:19104051 Klg is acutely required for LTM but not anesthesia-resistant memory formation, and Klg expression increases upon LTM induction PMID:19104051 We propose that Klg is a downstream effector of Notch signaling that links Notch activity to memory |
| GO:0007156 homophilic cell-cell adhesion | IDA PMID:9043060 klingon, a novel member of the Drosophila immunoglobulin sup... | ACCEPT | 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. Reason: Core function directly demonstrated in a cell-aggregation assay and corroborated by independent description of klg as a homophilic CAM. Supporting Evidence: PMID:9043060 it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion |
| GO:0007465 R7 cell fate commitment | IGI PMID:9043060 klingon, a novel member of the Drosophila immunoglobulin sup... | ACCEPT | 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. Reason: Core developmental function; the R7 role is the originally characterized and best-established biological process for klg, supported by genetic analysis. Supporting Evidence: PMID:9043060 Ectopic expression of klingon in all neurons in a sevenless background can alter the position of the R8 rhabdomere PMID:9043060 klingon is an essential gene that participates in the development of the R7 neuron |
| GO:0019897 extrinsic component of plasma membrane | ISS PMID:9043060 klingon, a novel member of the Drosophila immunoglobulin sup... | ACCEPT | 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. Reason: Correct and specific localization directly supported by the experimental demonstration of GPI attachment; appropriate for a GPI-anchored cell-surface protein. Supporting Evidence: PMID:9043060 it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage |
| GO:0045466 R7 cell differentiation | IMP PMID:9043060 klingon, a novel member of the Drosophila immunoglobulin sup... | ACCEPT | 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. Reason: Core developmental function, well established from mutant analysis; together with the R7 fate commitment annotation this captures klg's principal in vivo role. Supporting Evidence: PMID:9043060 expressed in a restricted pattern of neurons during embryonic neurogenesis and in the R7 photoreceptor precursor throughout its development PMID:9043060 klingon is an essential gene that participates in the development of the R7 neuron |
| GO:0098632 cell-cell adhesion mediator activity | IDA PMID:9043060 klingon, a novel member of the Drosophila immunoglobulin sup... | NEW | 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. 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. Supporting Evidence: PMID:9043060 it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion PMID:19104051 rus is a new allele of klingon (klg), which encodes a homophilic cell adhesion molecule |
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Download this section (compressed HTML)Q: 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?
Q: 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?
Experiment: 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.
Experiment: 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.
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