klingon (klg / CG6669 / FBgn0017590), Drosophila melanogaster — review notes

UniProt: Q9VCT4 (Q9VCT4_DROME, TrEMBL, PE=1 evidence at protein level). 545 aa.
Synonyms: CT20712, Dmel\CG6669, h214, Klg, l(3)rN712. Also recovered as the
memory/ethanol mutant allele ruslan (rus) PMID:19104051.

Protein architecture / molecular features

• IgSF cell-surface protein. UniProt features: three Ig-like domains (101–194,
  197–279, 288–376) plus an FN3 module (InterPro FN3_dom; CDD cd00063 FN3, cd00096 Ig x2;
  SMART IG x3). Consistent with the original description of "three C2-type
  Immunoglobulin-like domains followed by one fibronectin type III repeat"
  PMID:9043060.
• Membrane attachment is via a GPI anchor (not a transmembrane domain), i.e. an
  extrinsic/lipid-anchored component of the plasma membrane:
  PMID:9043060.
• KEGG/Reactome cross-references map it to the L1CAM/basigin-type IgSF adhesion module
  (Reactome R-DME-373760 "L1CAM interactions", etc.) — supportive of an adhesion-molecule role,
  not independent experimental evidence for klg.

Molecular function

• Homophilic cell-cell adhesion (IDA). Direct S2 cell-aggregation assay shows Klingon
  mediates homophilic adhesion PMID:9043060; reaffirmed
  as "a homophilic cell adhesion molecule" PMID:19104051.
  → Best MF representation: cell-cell adhesion mediator activity (GO:0098632) /
  cell adhesion molecule binding (GO:0050839). This is the gene's core molecular function.
• Protein interaction (IPI, PMID:23827685, Özkan et al. 2013 Cell IgSF/LRR extracellular interactome).
  UniProt IntAct records Q9VCT4 (klg) binding Q9VDD5 = cDIP (a "common Dpr-interacting protein",
  an IgSF ectodomain), NbExp=2 (IntAct EBI-91271/EBI-6881617). The cached full text of PMID:23827685
  does not name klg/cDIP (the pairwise hits are in supplementary tables), so I cannot quote a verbatim
  klg-specific sentence from that paper; the interaction is captured in UniProt. The bare "protein binding"
  (GO:0005515) is uninformative and should be sharpened to an adhesion-molecule binding MF.
• "Axon guidance receptor activity" (GO:0008046, IBA). Phylogenetic (GO_Central) inference from the
  IgSF adhesion/guidance family. No direct experimental demonstration that klg transduces a guidance
  signal, and as a GPI-anchored protein it lacks a cytoplasmic signaling domain (would require a
  co-receptor). The family-level term therefore overstates what klg does: MODIFY to
  cell-cell adhesion mediator activity (GO:0098632), which matches the demonstrated homophilic
  adhesion. Knock-on: the GO:0007411 axon guidance annotation is a GO_REF:0000108 inter-ontology
  link whose WITH/FROM is this GO:0008046 IBA, so it inherits this term's weakness.

Biological process

• R7 photoreceptor development (core). klg is expressed in the R7 precursor throughout its
  development and is an essential gene required for R7 neuron development
  PMID:9043060;
  PMID:9043060.
  Genetic interaction with the sevenless pathway: PMID:9043060.
  Curated as R7 cell fate commitment (IGI), R7 cell differentiation (IMP) PMID:9043060.
• Axon guidance / synapse organization (IBA/IEA). Neuronal-expression + family-based inferences.
  Klg protein accumulates at neuropil boundaries and mushroom-body lobe/calyx–glia junctures
  PMID:19104051,
  consistent with an adhesive role at neurite/synaptic interfaces, but no direct klg loss-of-function
  axon-guidance or synapse-organization assay is in the cached literature. Non-core.
• Long-term memory (IMP) — a genuine, acute requirement, but a distal/behavioral role.
  klg (=rus) is required for protein-synthesis-dependent LTM and its protein rises on LTM induction
  PMID:19104051;
  it is a downstream effector of Notch PMID:19104051.
  Independently confirmed as an LTM mutant abolishing the α/β mushroom-body LTM calcium trace
  PMID:21490205;
  PMID:21490205.
• NOT anesthesia-resistant memory (negated IMP, valid negative finding). The same study shows Klg is
  specifically required for LTM but not ARM
  PMID:19104051.
  The negated GO:0007615 annotation correctly records this specificity — keep negated:true, ACCEPT.
• Behavioral response to ethanol (IMP, PMID:18435628). The klg allele (ruslan) is among LTM mutants
  scored for ethanol behavior; ruslan shows increased ethanol sensitivity and reduced tolerance
  PMID:18435628.
  (The paper refers to mutants by original names; ruslan = klg per PMID:19104051.) Experimental but a
  distal/pleiotropic behavioral phenotype → non-core.

Localization

• Plasma membrane (GPI-anchored, extrinsic component) [PMID:9043060 GPI linkage quote above].
• Neuronal cell body / axon / neuropil (IBA + immunostaining at neuropil-glia junctures) PMID:19104051.

Curation decisions (summary)

• Core MF: cell-cell adhesion mediator activity (GO:0098632) — homophilic Ig adhesion (IDA-backed).
• Core BP: homophilic cell-cell adhesion (GO:0007156); R7 cell differentiation / fate commitment
  (GO:0045466 / GO:0007465).
• Core CC: plasma membrane / extrinsic component of plasma membrane (GO:0005886 / GO:0019897).
• MODIFY bare protein binding (GO:0005515) → cell adhesion molecule binding (GO:0050839).
• MODIFY axon guidance receptor activity (GO:0008046) → cell-cell adhesion mediator activity (GO:0098632).
• KEEP_AS_NON_CORE: axon guidance (GO:0007411), synapse organization (GO:0050808),
  long-term memory (GO:0007616), behavioral response to ethanol (GO:0048149).
• ACCEPT (negated) the NOT anesthesia-resistant memory annotation (GO:0007615).
• Broad ARBA/IEA parents (GO:0009653, GO:0030154, GO:0160108) accepted as correct-but-general.