NPHP-2 is the Caenorhabditis elegans ortholog of mammalian inversin (INVS/nephrocystin-2, NPHP2), an ankyrin-repeat protein that acts as a non-catalytic structural organizer within ciliated sensory neurons. Rather than localizing to the ciliary transition zone like the nephrocystins NPHP-1 and NPHP-4, NPHP-2 marks the ciliary inversin compartment (InvC), a proximal subregion of the doublet-based ciliary shaft immediately distal to the transition zone in amphid channel and phasmid cilia. There it is required for anchoring the proximal-cilium pool of signaling components, including the cyclic-nucleotide-gated channel subunit TAX-2, and for proper ciliary microtubule ultrastructural patterning, cilium placement and anchoring, and ciliogenesis. NPHP-2 functions redundantly within genetic modules (with nphp-1/nphp-4, and with the doublet-region genes arl-13, klp-11 and unc-119) and interacts genetically with Meckel-syndrome (MKS) transition-zone genes in a sensillum-dependent manner; these activities are modulated upstream of tubulin glutamylation and are antagonized by the HDAC-6 deacetylase. Its calcium-binding EF-hand domain is required for InvC targeting and function. NPHP-2 is not a component of the intraflagellar transport machinery and is dispensable for transition-zone protein localization and IFT.
Definition: The action of a protein that contributes to the structural organization and integrity of the ciliary inversin compartment (InvC), a proximal subdomain of the doublet-based ciliary shaft distal to the transition zone, for example by scaffolding the retention of proximal-ciliary signaling components without itself catalyzing a biochemical reaction.
Parent term: structural molecule activity
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003674 molecular_function | ND GO_REF:0000015 | ACCEPT | Summary: Root molecular_function placeholder (ND = No biological Data). No specific molecular activity has been experimentally assigned to NPHP-2. Reason: NPHP-2 is a non-catalytic ankyrin-repeat protein and no specific molecular function has been experimentally determined. Its role is that of a structural organizer/genetic modifier acting through protein-protein interactions and a calcium-binding EF-hand, but no discrete MF term is supported by direct evidence. The ND placeholder is appropriate; the MF darkness is captured in knowledge_gaps. |
| GO:0097543 ciliary inversin compartment | IDA PMID:25335890 Ciliopathy proteins establish a bipartite signaling compartm... | ACCEPT | Summary: Direct experimental localization of NPHP-2 to the ciliary inversin compartment (InvC), the proximal/middle segment of the doublet-based ciliary shaft just distal to the transition zone. This is the core, defining localization of NPHP-2 and distinguishes it from the transition-zone nephrocystins NPHP-1/NPHP-4. Reason: Core localization, supported by multiple independent C. elegans studies. The InvC is conserved and distinct from the doublet region, and NPHP-2 marks it specifically. This is the most informative cellular-component annotation for the gene. Supporting Evidence: PMID:25501555 We conclude that NPHP-2 marks a region of the cilium distinct from the doublet region, and propose that this region is analogous to the InvC of mammalian cilia. PMID:22393243 the C. elegans inversin ortholog, NPHP-2, localizes to the middle segment of sensory cilia |
| GO:1904108 protein localization to ciliary inversin compartment | IMP PMID:25335890 Ciliopathy proteins establish a bipartite signaling compartm... | ACCEPT | Summary: NPHP-2 is required to anchor/localize signaling components to the proximal ciliary (InvC) region; in AFD thermosensory neurons it anchors the cGMP-gated channel within the proximal cilium. This is a core biological process for NPHP-2 and is directly grounded in the source paper. Reason: Well-supported experimental (IMP) annotation. The source reference explicitly shows NPHP-2 is required to anchor a cGMP-gated ion channel in the proximal ciliary region, which is exactly the InvC subdomain NPHP-2 defines. Represents the best-characterized functional role of the protein. Supporting Evidence: PMID:25335890 requires NPHP-2 (known as inversin in mammals) to anchor a cGMP-gated ion channel within the proximal ciliary region |
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Download this section (compressed HTML)Q: Does the NPHP-2 EF-hand bind calcium biochemically, and does calcium specify InvC localization, modulate NPHP-2 activity, or both?
Q: What are the direct binding partners of NPHP-2 in the ciliary inversin compartment, and do they include NEK8/ANKS6-type proteins?
Q: What upstream cue initially patterns the inversin compartment independently of the transition zone, doublet-region and IFT machinery?
Experiment: Proximity-labeling (TurboID) or immunoprecipitation of endogenously tagged NPHP-2 in ciliated sensory neurons to identify the InvC interactome.
Hypothesis: NPHP-2 scaffolds a defined set of proximal-ciliary proteins that retain signaling components (e.g. TAX-2/TAX-4) in the InvC.
Experiment: In vitro calcium-binding assays on the isolated EF-hand plus binding-dead point mutants assayed in vivo for InvC targeting and cGMP-gated channel anchoring.
Hypothesis: The EF-hand is a functional calcium sensor whose calcium binding is required for InvC targeting and/or NPHP-2 activity.
Experiment: Candidate and forward genetic screens for factors required to establish the InvC, using an InvC marker (NPHP-2 GFP territory) and channel-anchoring readouts.
Hypothesis: A dedicated patterning cue, distinct from TZ/doublet-region/IFT genes, defines the proximal InvC subdomain.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: NPHP-2 has no assigned molecular function and no adequate GO molecular-function term to express one. It is a non-catalytic ankyrin-repeat protein whose role is to organize the ciliary inversin compartment and anchor proximal-ciliary signaling components, but GO has no structural-constituent-of-the-ciliary-inversin-compartment activity, so the gene reads as MF-dark despite a well-defined cellular and process-level role.
OPEN ONTOLOGYCURATION MF_DARK
What is known: It is firmly established that NPHP-2 = inversin localizes to and marks the InvC, is required for microtubule patterning, cilia anchoring/placement and ciliogenesis, and anchors the proximal-cilium cGMP-gated channel. What is missing is a molecular-function representation: the worm GOA record carries only a cellular-component and a biological-process term plus the ND molecular_function root.
Significance: This is the canonical structural/scaffold-subunit ontology gap shared across the ciliary inversin/nephrocystin gene set: a mechanistically well-understood organizer that cannot be annotated with an informative MF term, contributing to apparent molecular-function darkness.
What would resolve it: Develop/adopt a molecular-function term for a structural constituent of the ciliary inversin compartment (analogous to structural constituent of ribosome) and annotate NPHP-2 to it; see proposed_new_terms.
Provenance (the field's own admissions):
Proposed term (ontology gap):
Gap: How the ciliary inversin compartment is initially established and maintained, and which direct molecular partners recruit and retain NPHP-2 there, is unknown. NPHP-2 reaches and is restricted to the InvC independently of transition-zone, doublet-region and other InvC genes, so the factor(s) that pattern the compartment and the NPHP-2 interactome that executes its scaffolding function have not been identified.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Established: NPHP-2 marks the InvC and is required for microtubule patterning and cilia anchoring; its ciliary targeting/restriction does not require the tested TZ, doublet-region or InvC genes; the InvC is set up early in development. Not established: the upstream cue that patterns the InvC and the direct binding partners through which NPHP-2 organizes it.
Significance: Identifying the InvC-patterning cue and the NPHP-2 interactome would convert a marks-a-region, required-for-patterning description into a mechanism, and would explain how nephronophthisis (inversin) mutations disrupt proximal-ciliary signaling.
What would resolve it: Unbiased proximity-labeling/immunoprecipitation of tagged NPHP-2 in ciliated neurons, plus forward/candidate genetic screens for factors required for InvC establishment, with InvC-marker and channel-anchoring readouts.
Provenance (the field's own admissions):
Gap: Whether the NPHP-2 EF-hand actually binds calcium and, if so, whether the calcium signal specifies NPHP-2 localization, modulates its activity, or both, is undetermined. The EF-hand is only a predicted calcium-binding motif; it is required for InvC targeting and function, but the biochemical calcium binding and the origin of any calcium cue are unproven.
OPEN BIOLOGY MF_DARK
What is known: Established: deletion of the EF-hand abolishes NPHP-2 localization to amphid/phasmid cilia and impairs rescue, so the domain is functionally essential in those cells. Not established: direct Ca2+ binding by the domain, and the source/nature of the calcium signal it would sense.
Significance: If the EF-hand is a bona fide calcium sensor, NPHP-2 would gain a concrete molecular activity (calcium ion binding) linking ciliary calcium signaling to InvC organization; resolving this would also inform the conserved requirement for a calcium-binding domain in vertebrate inversin.
What would resolve it: In vitro calcium-binding assays on the isolated EF-hand, and separation-of-function EF-hand point mutants (binding-dead vs structural) tested for InvC targeting and channel anchoring in vivo.
Provenance (the field's own admissions):
Gap: The relationship between the ciliary NPHP-2/Y32G9A.6 and the canonical vertebrate-style Inversin complex is unresolved. In C. elegans the classical Inversin complex is built from MLT-4 (INVS), NEKL-2 (NEK8) and MLT-2 (ANKS6) and functions at epithelial junctions during molting rather than in cilia; whether ciliary NPHP-2 assembles an analogous NEK8/ANKS6-type module in the InvC, or acts independently, is not known.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Established: nphp-2/Y32G9A.6 is the ciliary inversin ortholog in the sensory-cilia literature, while the MLT-4/NEKL-2/MLT-2 complex acts at junctions in molting. Not established: whether the two systems share components or mechanism, and which worm protein is the true functional inversin ortholog in each context.
Significance: Clarifying the ortholog relationship determines how findings about the junctional Inversin complex (e.g. dimerization-dependent activation) transfer to ciliary NPHP-2 function and to human inversin/nephronophthisis biology.
What would resolve it: Systematic tagging/interaction mapping of NPHP-2, MLT-4, NEKL-2 and MLT-2 across ciliated and epithelial tissues, with cross-rescue and complex-composition assays.
Provenance (the field's own admissions):
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