xbx-1 encodes the Caenorhabditis elegans cytoplasmic dynein-2 light intermediate chain, the ortholog of mammalian DYNC2LI1/D2LIC. It is a non-catalytic subunit of the dynein-2 (IFT-dynein) motor, which also contains the CHE-3 heavy chain. Dynein-2 is the minus-end-directed microtubule motor that powers retrograde intraflagellar transport (IFT), returning IFT trains and turned-over cargo from the ciliary tip back to the base. XBX-1 binds the dynein heavy chain and is required to build and maintain the sensory cilia of C. elegans: it localizes to the ciliary base and basal body and moves bidirectionally along the axoneme (carried anterogradely as inactive cargo by kinesin-2, then powering the retrograde run). Loss of xbx-1 gives short, malformed cilia with a distal bulb in which IFT proteins accumulate, together with the sensory-behaviour defects typical of ciliary mutants. xbx-1 belongs to the DAF-19 (RFX) X-box-regulated ciliary gene battery, from which it takes its name.
Definition: A cytoplasmic dynein complex, distinct from the cytoplasmic dynein 1 complex, that is specialized for retrograde intraflagellar transport within cilia and flagella. It is built around the dynein-2 heavy chain (DYNC2H1/CHE-3) together with dynein-2-specific light intermediate (DYNC2LI1/D2LIC/XBX-1), intermediate, light and associated chains, and generates minus-end-directed (tip-to-base) movement along the ciliary axoneme.
Justification: GO has a single cellular-component term GO:0005868 "cytoplasmic dynein complex" that conflates the functionally and compositionally distinct dynein-1 (mitotic/ trafficking) and dynein-2 (ciliary/IFT) motors. A dynein-2-specific term would let XBX-1 and its partners be annotated to their actual complex. This benefits dynein-2 subunit annotation across species, not just XBX-1 — e.g. the heavy chain CHE-3/DYNC2H1, the intermediate chains WDR60/WDR34, and the dynein-2 light chains — and would separate IFT-dynein from dynein-1 in enrichment analyses and GO-CAM models.
Parent term: cytoplasmic dynein complex
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0036064 ciliary basal body | IBA GO_REF:0000033 | ACCEPT | Summary: XBX-1 localizes to the ciliary base/basal body, where dynein-2 is loaded onto IFT trains. Phylogenetically inferred and corroborated by multiple IDA calls (PMID:33460640, PMID:22922713). Reason: Consistent with experimentally observed base/basal body localization of XBX-1 and with the known site of dynein-2 loading. A non-core location term but correct. Supporting Evidence: PMID:12802075 XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme |
| GO:0045504 dynein heavy chain binding | IBA GO_REF:0000033 | ACCEPT | Summary: Core molecular function: as the dynein-2 light intermediate chain, XBX-1 binds the dynein-2 heavy chain (CHE-3) within the retrograde IFT motor. Reason: This is the defining, informative molecular function of a dynein light intermediate chain. Supported by orthology to D2LIC and by the functional partnership with CHE-3 in retrograde IFT. Retained as core. Supporting Evidence: PMID:12802075 the DLIC protein XBX-1 functions together with the CHE-3 dynein in retrograde IFT, downstream of the complex A proteins |
| GO:0005868 cytoplasmic dynein complex | IBA GO_REF:0000033 | ACCEPT | Summary: Core complex membership: XBX-1 is a subunit of the cytoplasmic dynein-2 (IFT-dynein) motor complex. Reason: Well supported; XBX-1 is the light intermediate chain of dynein-2. GO:0005868 is the most specific available term (GO lacks a dedicated "cytoplasmic dynein 2 complex" cellular-component class; see knowledge_gaps). Retained as core. Supporting Evidence: PMID:12802075 the DLIC protein XBX-1 functions together with the CHE-3 dynein in retrograde IFT, downstream of the complex A proteins |
| GO:0035721 intraciliary retrograde transport | IBA GO_REF:0000033 | ACCEPT | Summary: Core biological process: XBX-1 is part of the dynein-2 motor that drives retrograde IFT (tip-to-base) in cilia. Reason: Directly supported by IMP evidence (PMID:12802075) and orthology; this is the central process for the gene. Retained as core. Supporting Evidence: PMID:12802075 xbx-1, that is required for retrograde IFT and shares homology with a mammalian dynein light intermediate chain (D2LIC) |
| GO:0035735 intraciliary transport involved in cilium assembly | IBA GO_REF:0000033 | ACCEPT | Summary: IFT (including the retrograde arm powered by dynein-2/XBX-1) is required for cilium assembly and maintenance. Reason: Consistent with the xbx-1 loss-of-function cilium-assembly phenotype and with the established role of IFT in ciliogenesis. Broader BP framing complementary to the more specific retrograde-transport and non-motile-cilium-assembly terms. Supporting Evidence: PMID:12802075 they are shortened and have a bulb like structure in which IFT proteins accumulate |
| GO:0005930 axoneme | IBA GO_REF:0000033 | ACCEPT | Summary: XBX-1 moves along the ciliary axoneme as part of IFT, where the dynein-2 motor operates during retrograde transport. Reason: Phylogenetically inferred and corroborated by IDA axoneme localization (PMID:12802075, PMID:27930654, PMID:25335890). Correct location. Supporting Evidence: PMID:12802075 XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme |
| GO:0005813 centrosome | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Electronic annotation transferred from the UniProt Subcellular Location "microtubule organizing center, centrosome" mapping. In C. elegans ciliated sensory neurons the relevant MTOC is the centriole-derived ciliary basal body, already captured by IDA basal body annotations. Reason: Dynein-2 is the ciliary/IFT dynein, not the mitotic-spindle dynein; there is no direct evidence for a canonical mitotic centrosome role for XBX-1. The "centrosome" call over-generalizes the basal-body/MTOC localization. Kept (basal bodies are centriole-derived MTOCs) but flagged non-core. |
| GO:0005868 cytoplasmic dynein complex | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based electronic support for dynein-2 complex membership, redundant with the IBA part_of annotation. Reason: Consistent electronic corroboration of the experimentally/phylogenetically supported dynein complex membership. |
| GO:0005930 axoneme | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic (Subcellular Location) support for axoneme localization, redundant with IDA axoneme annotations. Reason: Consistent with experimentally observed axoneme localization. |
| GO:0035721 intraciliary retrograde transport | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based electronic support for the core retrograde IFT process, redundant with IMP/IBA annotations. Reason: Consistent electronic corroboration of the central biological process. |
| GO:0035735 intraciliary transport involved in cilium assembly | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based electronic support for the role of IFT in cilium assembly, redundant with the IBA annotation. Reason: Consistent with the gene's established role in ciliogenesis via IFT. |
| GO:0036064 ciliary basal body | IDA PMID:33460640 GRDN-1/Girdin regulates dendrite morphogenesis and cilium po... | ACCEPT | Summary: Direct observation that XBX-1 localizes to the basal body and moves bidirectionally along the axoneme. Reason: Experimentally supported basal body localization (curator read full text); the site where dynein-2 is loaded onto IFT trains. Corroborated by the primary characterization (PMID:12802075). Correct non-core location. Supporting Evidence: PMID:12802075 XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme |
| GO:0005930 axoneme | IDA PMID:27930654 Whole-Organism Developmental Expression Profiling Identifies... | ACCEPT | Summary: XBX-1::tdTomato was used as an IFT marker localizing to the ciliary base and axoneme. Reason: Experimentally supported axoneme localization (curator read full text), corroborated by the primary characterization (PMID:12802075). Consistent with IFT movement of the dynein-2 motor. Supporting Evidence: PMID:12802075 XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme |
| GO:0035869 ciliary transition zone | IDA PMID:27930654 Whole-Organism Developmental Expression Profiling Identifies... | ACCEPT | Summary: XBX-1 traverses the ciliary base/transition zone during IFT, where the dynein-2 motor enters and exits the ciliary compartment. Reason: IDA annotation from a study using XBX-1 as a ciliary marker; the curator read the full text. XBX-1 passes through the transition zone as part of IFT, so the localization is biologically consistent. Non-core location. |
| GO:0097546 ciliary base | IDA PMID:27930654 Whole-Organism Developmental Expression Profiling Identifies... | ACCEPT | Summary: Direct observation of XBX-1 at the ciliary base, the site of dynein-2 loading and IFT-train assembly/turnaround. Reason: Experimentally supported and central to dynein-2 function (loading/unloading); corroborated by the primary characterization (PMID:12802075). Correct location. Supporting Evidence: PMID:12802075 XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme |
| GO:0005929 cilium | IDA PMID:27623382 A Conserved Role for Girdin in Basal Body Positioning and Ci... | ACCEPT | Summary: XBX-1 localizes to cilia (general ciliary compartment), consistent with its role in IFT. Reason: Experimentally supported general ciliary localization (curator read full text); a parent of the more specific non-motile cilium term also annotated. Corroborated by the primary characterization (PMID:12802075). Correct but non-core. Supporting Evidence: PMID:12802075 XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme |
| GO:0097730 non-motile cilium | IDA PMID:17420466 Mutation of the MAP kinase DYF-5 affects docking and undocki... | ACCEPT | Summary: XBX-1 localizes to the non-motile sensory cilia of C. elegans, where it operates in IFT. Reason: IDA from a study using XBX-1 as a ciliary/IFT marker (curator read full text); C. elegans sensory cilia are non-motile. Corroborated by the primary characterization (PMID:12802075). Correct location for the cell type. Non-core. Supporting Evidence: PMID:12802075 XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme |
| GO:0005930 axoneme | IDA PMID:25335890 Ciliopathy proteins establish a bipartite signaling compartm... | ACCEPT | Summary: XBX-1 used as an axoneme/ciliary marker in AFD sensory neurons. Reason: Experimentally supported axoneme localization (curator read full text), redundant with other IDA axoneme calls and corroborated by the primary characterization (PMID:12802075). Correct location. Supporting Evidence: PMID:12802075 XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme |
| GO:0035721 intraciliary retrograde transport | IMP PMID:12802075 XBX-1 encodes a dynein light intermediate chain required for... | ACCEPT | Summary: Loss of xbx-1 disrupts retrograde IFT, causing IFT proteins to accumulate in a distal ciliary bulb; XBX-1 functions with the CHE-3 dynein downstream of IFT-A. Reason: Primary experimental (IMP) evidence for the gene's central biological process. Retained as core. Supporting Evidence: PMID:12802075 the DLIC protein XBX-1 functions together with the CHE-3 dynein in retrograde IFT, downstream of the complex A proteins PMID:12802075 they are shortened and have a bulb like structure in which IFT proteins accumulate |
| GO:0045504 dynein heavy chain binding | ISS PMID:12802075 XBX-1 encodes a dynein light intermediate chain required for... | ACCEPT | Summary: By sequence similarity to mammalian D2LIC, XBX-1 binds the dynein-2 heavy chain within the retrograde IFT motor. Reason: Informative, specific molecular function (not generic protein binding). Supported by orthology to D2LIC (with/from rat D2LIC) and by the functional partnership with CHE-3. Retained as core. Supporting Evidence: PMID:12802075 xbx-1, that is required for retrograde IFT and shares homology with a mammalian dynein light intermediate chain (D2LIC) |
| GO:0097546 ciliary base | IDA PMID:25335890 Ciliopathy proteins establish a bipartite signaling compartm... | ACCEPT | Summary: XBX-1 localizes to the ciliary base in AFD sensory neurons. Reason: Experimentally supported ciliary base localization (curator read full text), redundant with other IDA base calls and corroborated by the primary characterization (PMID:12802075). Correct location. Supporting Evidence: PMID:12802075 XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme |
| GO:1905515 non-motile cilium assembly | IMP PMID:12802075 XBX-1 encodes a dynein light intermediate chain required for... | ACCEPT | Summary: xbx-1 mutants have short, malformed sensory cilia with a distal IFT-protein bulb, showing XBX-1 is required for assembly of non-motile cilia. Reason: Primary experimental (IMP) evidence for the cilium-assembly requirement; C. elegans sensory cilia are non-motile. Retained as core. Supporting Evidence: PMID:12802075 they are shortened and have a bulb like structure in which IFT proteins accumulate |
| GO:0036064 ciliary basal body | IDA PMID:22922713 The BBSome controls IFT assembly and turnaround in cilia. | ACCEPT | Summary: XBX-1 (dynein-2 light chain, D2LIC ortholog) used as a retrograde IFT motor marker localizing to cilia/basal body. Reason: Experimentally supported basal body localization (curator read full text), redundant with the PMID:33460640 IDA call and corroborated by the primary characterization (PMID:12802075). Correct location. Supporting Evidence: PMID:12802075 XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme |
| GO:0097730 non-motile cilium | IDA PMID:12802075 XBX-1 encodes a dynein light intermediate chain required for... | ACCEPT | Summary: XBX-1 localizes within the non-motile sensory cilia, moving between the base and the axoneme. Reason: Experimentally supported localization to C. elegans (non-motile) sensory cilia. Correct location. Supporting Evidence: PMID:12802075 XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme |
| GO:0005930 axoneme | IDA PMID:12802075 XBX-1 encodes a dynein light intermediate chain required for... | ACCEPT | Summary: XBX-1 undergoes bidirectional movement along the ciliary axoneme. Reason: Primary experimental evidence for axoneme localization/movement of the dynein-2 motor. Correct location. Supporting Evidence: PMID:12802075 XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme |
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Download this section (compressed HTML)Q: Does XBX-1 act mainly to stabilize/assemble the dynein-2 motor, to couple it to IFT trains, or to regulate its activation during ciliary turnaround?
Suggested experts: Intraflagellar transport / dynein-2 motor biologists
Q: Does XBX-1 have any dynein-2-independent role at the basal body/centriole?
Suggested experts: C. elegans ciliary cell biologists
Experiment: Structure-function analysis of XBX-1 (targeted deletions/point mutations of the DLIC fold) combined with co-immunoprecipitation of CHE-3 and live imaging of IFT-train velocities and turnaround in the mutant alleles.
Hypothesis: XBX-1 is required for dynein-2 complex stability and for coupling the motor to IFT trains.
Type: structure-function / live imaging
Experiment: Dual-color live imaging of anterograde (kinesin-2/IFT-B) and retrograde (XBX-1/CHE-3) markers in xbx-1 mutants to quantify which transport arm and which ciliary segment are affected.
Hypothesis: XBX-1 loss selectively impairs the retrograde (dynein-2) IFT arm.
Type: live imaging
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The specific molecular contribution of the XBX-1 light intermediate chain to dynein-2 function in C. elegans is undetermined: whether it is chiefly required for dynein-2 complex assembly/stability, for coupling the motor to IFT trains as a cargo adaptor, or for regulating motor activation/autoinhibition at the ciliary tip.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: It is firmly established that XBX-1 is the dynein-2 LIC (D2LIC ortholog), binds the CHE-3 heavy chain, localizes to the ciliary base/axoneme, and is required for retrograde IFT and cilium assembly. The light intermediate chain itself has no demonstrated enzymatic activity; the ATP-dependent minus-end-directed motor activity is a property of the CHE-3 heavy chain/dynein-2 complex.
Significance: DYNC2LI1 mutations cause skeletal ciliopathies (short-rib thoracic dysplasia); understanding the LIC's mechanistic role would clarify how dynein-2 assembly and IFT-train coupling are controlled and how they fail in disease.
What would resolve it: Structure-function dissection of XBX-1 (domain/point mutants) with biochemical assays of dynein-2 assembly and heavy-chain binding, plus live IFT imaging of train coupling and velocities in xbx-1 alleles.
Provenance (the field's own admissions):
Gap: No GO cellular-component term expresses the cytoplasmic dynein-2 (IFT-dynein) complex distinctly from cytoplasmic dynein-1, so XBX-1's actual complex membership can only be annotated with the conflated parent term GO:0005868.
OPEN ONTOLOGY CC_DARK
What is known: XBX-1 is unambiguously a subunit of the ciliary retrograde IFT dynein-2 motor (with CHE-3/DYNC2H1); the only available GO CC term, GO:0005868 "cytoplasmic dynein complex", also covers dynein-1.
Significance: A dynein-2-specific complex term would let IFT-dynein subunits be annotated to their true complex and separated from dynein-1 in enrichment and model building.
What would resolve it: Add a "cytoplasmic dynein 2 complex" cellular-component term as a child of GO:0005868 (see proposed_new_terms).
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