osm-6 (C. elegans) research notes

Provenance note: just deep-research-falcon worm osm-6 --fallback perplexity-lite
was attempted twice and both runs timed out with no output (SIGTERM, exit 143/144);
no -deep-research-*.md file was produced and none was fabricated. This review is
grounded directly in the UniProt record (G5EDF6), the QuickGO GOA export, and the 11
cached primary publications (all listed below with verbatim provenance). All existing
annotations could be adjudicated from primary literature, so no UNDECIDED calls were
required.

UniProt: G5EDF6 (G5EDF6_CAEEL) · WormBase: WBGene00003886 / R31.3 · ORF R31.3 · Chromosome V
Ortholog: IFT52 (human IFT52 / NGD5; Chlamydomonas IFT52/BLD1). Core structural subunit of the
intraflagellar transport complex B (IFT-B). PANTHER PTHR12969 (NGD5/OSM-6/IFT52), subfamily SF7.
ComplexPortal: CPX-1290 "Intraflagellar transport complex B".

Protein: 472 aa. Domain architecture (InterPro/Pfam): N-terminal IFT52 GIFT domain (PF23355, ~18-257,
a class-I glutamine-amidotransferase-like fold, SSF52317), IFT52 central domain (PF23352, ~274-357),
and IFT52 C-terminal domain (PF21178, ~368-417, CDD cd23683 IFT52_CTD). No catalytic residues / no EC;
the GATase-like fold is used structurally, not enzymatically (consistent with a scaffold subunit).

Summary: what osm-6/IFT52 IS (KNOWN)

• Structural subunit of the IFT-B complex. In Chlamydomonas, the 16S IFT "raft" (IFT particle)
  contains ≥16 polypeptides, two of which are homologues of the C. elegans OSM-1 and OSM-6 proteins
  required for sensory ciliary function PMID:10545497.
  OSM-6 is explicitly "the ortholog of human IFT52" and is used as the canonical GFP-tagged IFT-B
  component reporter PMID:22922713.

• Cargo of the IFT motors / moves by IFT. OSM-6::GFP moves anterogradely (kinesin-II driven) and
  retrogradely along cilia. Kinesin-II and its cargo OSM-1 and OSM-6 all move at ~1.1 µm/s in the
  retrograde direction along cilia and dendrites PMID:10545497. Retrograde
  movement in cilia (but not dendrites) requires the CHE-3 (DHC1b/dynein-2) motor PMID:10545497. Ciliary entry of dynein-2
  itself depends on an intact IFT-B complex PMID:28479320.

• Required for cilium (axoneme) assembly. osm-6 mutants have normal transition zones but severely
  shortened axonemes with ectopic doublet microtubules assembling proximal to the cilium
  PMID:2428682. Cloning +
  transformation rescue + genetic mosaics show osm-6 acts cell-autonomously in ciliated sensory
  neurons PMID:9475731.

• Localization. OSM-6::GFP is expressed exclusively in ciliated sensory neurons; the fusion rescues
  the mutant and accumulates in cytoplasm including processes and dendritic endings where cilia sit
  PMID:9475731. Expression is controlled by the RFX transcription factor
  DAF-19: "in the case of daf-19, reduced OSM-6::GFP accumulation" PMID:9475731. Experimentally
  localized to: non-motile (sensory) cilium and transition zone PMID:10545497, ciliary basal body
  PMID:22922713, and neuronal cell body PMID:9475731. Used as a ciliary IFT marker in RAB-28 work
  PMID:27930654.

What osm-6 loss CAUSES downstream (KNOWN but not the core molecular role)

All of these are sensory consequences of losing functional cilia, not distinct molecular activities
of OSM-6:
- Osmotic avoidance defect — the gene is named for this ("OSMotic avoidance abnormal"); osm-6 was
isolated among the osmotic-avoidance-defective mutants [PMID:730048 osmotic avoidance defective
screen]. (Abstract-only; osm-6 not named in the cached abstract but this is the founding Osm screen
and the IMP is a WormBase curator annotation.)
- Chemotaxis / chemosensation defect and dye-filling defect PMID:2428682.
- Mechanosensation defect — nose-touch response requires intact ciliated sensory endings
PMID:8460126 (osm-6 mutants are among such ciliary-defective animals; WB IMP).
- Dauer formation — osm-6 is one of the cilium-structure genes acting at a single step in the
chemosensory dauer pathway PMID:1732156; IGI with a daf gene.
- Salt-sensing circuit — hypomorphic osm-6 mutants lack functional sensory cilia and therefore
have greatly reduced ASEL/AWC salt responses; cell-autonomous cilium rescue restores sensing
PMID:24013594. The GO:1902075 "cellular response to salt" IMP here reflects this indirect ciliary
requirement, not a salt-signalling activity of OSM-6.

What is NOT known (knowledge gaps)

• No sub-molecular mechanism / no direct-partner map in C. elegans. OSM-6/IFT52 is treated as a
  structural IFT-B subunit, but which IFT-B partners it contacts in the worm (in mammals IFT52 bridges
  IFT88/IFT70/IFT46 and links the IFT-B core to peripheral subunits) and how its GIFT/central/CTD
  domains are used has not been experimentally dissected in C. elegans. GO MF for such a subunit can
  only be expressed as generic "structural molecule activity" (an ontology-shadow gap).
• No independent biochemical/enzymatic activity is known or expected (GATase-like fold appears
  non-catalytic).

Curation plan (actions)

Core (IFT-B structural subunit): intraciliary transport particle B (part_of), intraciliary transport,
cilium/non-motile cilium assembly, cilium/non-motile cilium localization, ciliary basal body,
transition zone — ACCEPT. centriole (IBA), cytoplasm, neuronal cell body — KEEP_AS_NON_CORE (real
but broad / not the functional site). Downstream sensory-phenotype BPs (chemotaxis, response to
osmotic stress, sensory perception of mechanical stimulus, cellular response to salt) —
KEEP_AS_NON_CORE (pleiotropic consequences of ciliary loss). GO:0003674 ND root MF — accept as
placeholder (superseded in spirit by proposed structural MF). Propose MF: structural molecule
activity (GO:0005198).