CC2D2A (coiled-coil and C2 domain-containing protein 2A, also called MKS6) is a large (1,620 residues) non-catalytic structural protein of the ciliary base. It carries an N-terminal disordered region, central coiled coils, a CC2D2AN-C2 module, a transglutaminase-like (TGL) peptidase fold whose catalytic residues are lost throughout the CC2D2A clade, and a C2 domain. CC2D2A is a core subunit of the MKS/Tectonic module of the ciliary transition zone, together with TCTN1-3, MKS1, B9D1, B9D2, TMEM216, TMEM231 and TMEM67, and also concentrates at the basal body and the subdistal appendages of the mother centriole. Its localization at the transition zone and that of B9D1 and TMEM231 are mutually dependent. Loss of the protein impairs subdistal-appendage assembly, microtubule anchoring and docking of ciliary vesicles at the mother centriole, causing tissue-dependent failure of axoneme extension. Where cilia still form, it is required for the ciliary membrane to retain its distinctive composition (for example Arl13b, adenylyl cyclase 3, Pkd2 and Smoothened) and for normal Hedgehog signalling. In photoreceptors it sits at the connecting cilium and is needed for fusion of opsin-carrier vesicles at the periciliary membrane. It interacts with CEP290 and with NINL, which links it to RAB8-MICAL3 vesicle docking. Biallelic loss-of-function variants cause Meckel syndrome 6, Joubert syndrome 9, COACH syndrome 2 and retinitis pigmentosa 93.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Broad but true compartment; the protein sits at the cytoplasmic ciliary base (basal body, mother-centriole appendages). Reason: The Swiss-Prot SubCell mapping is correct at the level of the whole cell. It cannot be refined within the cytoplasm branch, because GO places the informative locations (ciliary transition zone, ciliary basal body, centriolar subdistal appendage) under cilium rather than under cytoplasm. Those locations are therefore captured as separate rows and as NEW annotations rather than as a MODIFY of this row. Retained as a non-core broad location. Supporting Evidence: PMID:24947469 Immunolabeling in ciliated IMCD3 cells revealed the localization of CC2D2A protein to the mother centriole. PMID:26485645 When expressed alone, eCFP-tagged CC2D2A localizes to the ciliary base (basal body, accessory centriole) and also (partly) to the ciliary transition zone |
| GO:0005829 cytosol | TAS Reactome:R-HSA-5617816 | MARK AS OVER ANNOTATED | Summary: Reactome compartment convention; there is no evidence for a soluble cytosolic pool of CC2D2A. Reason: Reactome assigns the cytosol compartment to every participant of this basal-body event (RAB3IP exchange on RAB8A). Every localization experiment, in human RPE1 cells, mouse IMCD3 cells, macaque retina and zebrafish, places CC2D2A in an insoluble ciliary-base structure (basal body, subdistal appendages, transition zone/connecting cilium), not in the cytosol fraction. The event does not make CC2D2A a GEF or a RAB8A regulator. The row is kept as an over-annotation rather than removed, because the protein is cytoplasmic in the broad sense, but it should not be read as a soluble cytosolic location. Supporting Evidence: Reactome:R-HSA-5617816 PMID:24947469 Immunogold labeling for CC2D2A was clearly visible on the SDA both in IMCD3 cells and in monkey retina PMID:26485645 When expressed alone, eCFP-tagged CC2D2A localizes to the ciliary base (basal body, accessory centriole) and also (partly) to the ciliary transition zone |
| GO:0005829 cytosol | TAS Reactome:R-HSA-5626681 | MARK AS OVER ANNOTATED | Summary: Reactome cytosol label on the transition-zone recruitment event; the informative location is the transition zone itself. Reason: The Reactome event describes recruitment of transition-zone proteins to the ciliary base, which directly supports the ciliary transition zone location (accepted on other rows). The cytosol compartment is a Reactome modelling convention for the participants before recruitment and is not supported by any CC2D2A localization data. Kept consistent with the other two Reactome cytosol rows. Supporting Evidence: Reactome:R-HSA-5626681 PMID:21725307 The Tctn1 interactors Tctn2, Tctn3, Mks1, Tmem67, Cep290, Cc2d2a and B9d1 also localized to the transition zone |
| GO:0005829 cytosol | TAS Reactome:R-HSA-5638009 | MARK AS OVER ANNOTATED | Summary: Reactome cytosol label on the CEP164 vesicle-recruitment event; CC2D2A is a ciliary-base structural protein. Reason: The event concerns CEP164-dependent recruitment of RAB3IP-carrying vesicles to the basal body. CC2D2A is not the vesicle cargo, the GEF or the CEP164 binding partner. Its relevant location is the basal body/appendage region (proposed as NEW below), not the cytosol. Supporting Evidence: Reactome:R-HSA-5638009 PMID:24947469 Immunogold labeling for CC2D2A was clearly visible on the SDA both in IMCD3 cells and in monkey retina |
| GO:0007224 smoothened signaling pathway | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: CC2D2A is needed for Hedgehog signalling because it gates Smoothened entry into the cilium, not because it transduces the signal. Reason: Mouse Cc2d2a is the 1:1 ortholog in the same PANTHER subfamily (PTHR20837:SF7). Cc2d2a-/- MEFs show strongly reduced ciliary Smoothened after SAG stimulation, and null embryos lose Shh-dependent ventral neural-tube fates. The contribution operates through transition-zone control of ciliary membrane composition. Gating Smoothened access is a real input into the pathway, so the term is not wrong, but CC2D2A has no ligand, receptor, kinase or transcription-factor role. Kept as non-core; it is not part of the core function. Supporting Evidence: PMID:21725307 Smo displayed reduced ciliary localization in Tctn1−/− and Tctn2−/− MEFs treated with the Hh pathway agonist SAG, an effect that was even stronger in Cc2d2a−/− MEFs PMID:24947469 Thus, the neural tube patterning defect of Cc2d2a-/- embryos mostly affects ventral cell fates, consistent with a perturbation of Shh signaling. |
| GO:0007224 smoothened signaling pathway | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Same claim as the Ensembl row; the manual mouse-to-human transfer is sound but peripheral. Reason: Consistent with the IEA row. The UniProt curator transfer from mouse Cc2d2a (same PANTHER subfamily SF7) is phylogenetically justified. The biology is indirect, through ciliary gating of Smoothened, so it remains non-core. Supporting Evidence: PMID:21725307 Smo displayed reduced ciliary localization in Tctn1−/− and Tctn2−/− MEFs treated with the Hh pathway agonist SAG, an effect that was even stronger in Cc2d2a−/− MEFs |
| GO:0035869 ciliary transition zone | IBA GO_REF:0000033 | ACCEPT | Summary: Core location, inherited from a eukaryote-level PAINT node and confirmed directly for human CC2D2A. Reason: PAINT placed the IBD at PTN000464650 (taxon Eukaryota) in PTHR20837, seeded by fly, mouse, trypanosome, worm and zebrafish CC2D2A orthologs. Human CC2D2A (SF7) descends from that node together with the invertebrate CC2D2A/MKS-6 orthologs (SF0). The CC2D2B paralog (SF2) does not carry the term. Garcia-Gonzalo et al. report CC2D2A orthologs in all Tectonic-possessing eukaryotes, which fits the ancestral placement. Target-specific evidence agrees with it, because tagged human CC2D2A localizes partly to the RPGRIP1L-marked transition zone in hTERT-RPE1 cells. Supporting Evidence: PMID:26485645 eCFP-tagged CC2D2A (green signal; b) also (partly) localizes to the ciliary transition zone, which was visualized using anti-RPGRIP1L as a marker PMID:21725307 Accordingly, Tmem67 and Cc2d2a orthologs are found in all Tectonic-possessing eukaryotes for which genomic information exists PMID:21422230 MKS-5::tdTomato and MKS-6::GFP (both osm-5 promoter driven) localize to the TZ |
| GO:0035869 ciliary transition zone | IEA GO_REF:0000107 | ACCEPT | Summary: Mouse ortholog localizes to the transition zone; consistent with human data. Reason: Mouse Cc2d2a (same subfamily SF7) localizes to the transition zone in MEFs, including in Tctn1-/- cells, and partially overlaps the transition-zone marker RPGR in IMCD3 cells. The Compara transfer is consistent with direct human RPE1 data. Supporting Evidence: PMID:21725307 Similarly, Cc2d2a was at the transition zone of Tctn1−/− MEFs PMID:24947469 partially overlapped with RPGR |
| GO:0035869 ciliary transition zone | ISS GO_REF:0000024 | ACCEPT | Summary: Manual mouse-to-human transfer of transition-zone localization; correct. Reason: Same claim as the IBA and IEA rows. Supported by mouse transition-zone localization and by tagged human CC2D2A in RPE1 cells. Supporting Evidence: PMID:21725307 The Tctn1 interactors Tctn2, Tctn3, Mks1, Tmem67, Cep290, Cc2d2a and B9d1 also localized to the transition zone PMID:26485645 eCFP-tagged CC2D2A (green signal; b) also (partly) localizes to the ciliary transition zone, which was visualized using anti-RPGRIP1L as a marker |
| GO:0035869 ciliary transition zone | NAS PMID:22179047 A ciliopathy complex at the transition zone protects the cil... | ACCEPT | Summary: ComplexPortal statement for the MKS transition-zone complex (CPX-2531); the cited paper reports CC2D2A at the transition zone. Reason: The abstract states directly that CC2D2A localization at the transition zone depends on TMEM231, B9D1 and Sept2. The cache is abstract-only, but the claim is stated there and is independently confirmed for the human protein in RPE1 cells. Supporting Evidence: PMID:22179047 at the transition zone PMID:26485645 eCFP-tagged CC2D2A (green signal; b) also (partly) localizes to the ciliary transition zone, which was visualized using anti-RPGRIP1L as a marker |
| GO:0036038 MKS complex | IEA GO_REF:0000107 | ACCEPT | Summary: Core complex membership; mouse Cc2d2a co-purifies and co-immunoprecipitates with Tctn1 and other MKS-module subunits. Reason: Tandem-affinity purification of Tctn1 from mouse cells recovered Cc2d2a among the most abundant co-purifying proteins, and co-immunoprecipitation confirmed that Tctn1 interacts with Cc2d2a. Chih et al. independently identified CC2D2A in the TMEM231/B9D1 complex. Mouse is the same-subfamily (SF7) ortholog, so the transfer is justified. Membership does not imply direct contact with every subunit. Supporting Evidence: PMID:21725307 Tctn1 interacted with Tctn2, Tctn3, Mks1, Cc2d2a and B9d1 (Figure 2c). |
| GO:0036038 MKS complex | ISS GO_REF:0000024 | ACCEPT | Summary: Manual transfer of MKS-complex membership from mouse; correct. Reason: Same claim as the IEA row; supported by the mouse Tectonic complex purification. Supporting Evidence: PMID:21725307 we have identified a complex containing Tctn1, Tctn3 and seven MKS proteins (Mks1, Tmem216, Tmem67, Cep290, Cc2d2a, B9d1 and Tctn2) |
| GO:0036038 MKS complex | NAS PMID:22179047 A ciliopathy complex at the transition zone protects the cil... | ACCEPT | Summary: ComplexPortal (CPX-2531) membership statement; supported by the cited complex study. Reason: The abstract describes a nine-protein complex whose transition-zone localization is mutually dependent, and names CC2D2A among its members. The Tectonic complex purification gives independent biochemical support. Supporting Evidence: PMID:22179047 dependent on one another and on Sept2 PMID:21725307 Tctn1 interacted with Tctn2, Tctn3, Mks1, Cc2d2a and B9d1 (Figure 2c). |
| GO:0060271 cilium assembly | IEA GO_REF:0000107 | ACCEPT | Summary: CC2D2A does structural work in early ciliogenesis at the mother centriole; core process. Reason: CC2D2A passes the participation test, not only the necessity test. The endogenous protein sits on the subdistal appendages it is needed to build (immunogold), loss abolishes subdistal appendage assembly, microtubule anchoring, vesicle docking and transition-zone elaboration while the mother centriole persists, and a Cc2d2a transgene rescues axoneme assembly. The requirement is tissue-dependent. Some Cc2d2a-/- MEFs in one study, and zebrafish photoreceptors, still form cilia. That is why the general term (which covers both motile nodal/tracheal and primary cilia) is correct here, alongside the more specific IBA non-motile cilium assembly. Supporting Evidence: PMID:24947469 rescue the axoneme assembly defect PMID:24947469 We therefore conclude that CC2D2A is needed for the assembly of SDA at mother centriole, for stable MT anchoring and docking of transport vesicles, and for further elaboration of the transition zone. PMID:21725307 Thus, Cc2d2a shares tissue-specific ciliogenic functions with Tctn1 and Tctn2. |
| GO:0060271 cilium assembly | ISS GO_REF:0000024 | ACCEPT | Summary: Manual mouse-to-human transfer of cilium assembly; correct. Reason: Same claim as the IEA row. Note the incomplete penetrance in MEFs and the tissue-specificity reported by Garcia-Gonzalo et al. (Cc2d2a-/- MEFs can generate cilia in their hands). Supporting Evidence: PMID:24947469 ∼10% of Cc2d2a-/- MEFs escaped PMID:21725307 Furthermore, like Tctn1−/− MEFs, Cc2d2a−/− MEFs can generate cilia (Figure 5d). |
| GO:1904491 protein localization to ciliary transition zone | IBA GO_REF:0000033 | ACCEPT | Summary: Inherited role in assembling the transition-zone MKS module, from the eukaryote-level PAINT node. Reason: The PAINT IBD at PTN000464650 is seeded by mouse and worm experimental annotations. In worms, MKS-6 is part of the MKS/MKSR module required to build the transition zone. In mammals, CC2D2A, B9D1 and TMEM231 depend on one another for transition-zone localization. CC2D2A is itself a structural subunit of the assembly being localized, so it participates in the process rather than merely being required for it. Human CC2D2A lies inside the clade below the node. Supporting Evidence: PMID:22179047 dependent on one another and on Sept2 PMID:21422230 We find that MKS-5/RPGRIP1L interacts with two distinct TZ functional modules, MKS/MKSR and NPHP, consisting of MKS-1/MKSR-1/MKSR-2/MKS-3/MKS-6 and NPHP-1/NPHP-4 proteins, respectively. |
| GO:1904491 protein localization to ciliary transition zone | IEA GO_REF:0000107 | ACCEPT | Summary: Mouse ortholog transfer; same claim as the IBA row. Reason: Consistent with the IBA row. The mouse study additionally shows that RAB8A becomes diffuse at normal total RAB8A levels in Cc2d2a-/- MEFs, and that the transition zone fails to elaborate. These are ciliary-base organization defects; the transition-zone claim rests mainly on the mutual-dependence data. Supporting Evidence: PMID:22179047 dependent on one another and on Sept2 PMID:24947469 similar levels of Rab8a protein |
| GO:1904491 protein localization to ciliary transition zone | NAS PMID:22179047 A ciliopathy complex at the transition zone protects the cil... | ACCEPT | Summary: ComplexPortal process statement for the MKS complex; directly supported by the cited abstract. Reason: The cited abstract reports that TMEM231, B9D1 and CC2D2A depend on one another for transition-zone localization. Dependence on Sept2 does not imply direct Sept2 binding. Supporting Evidence: PMID:22179047 dependent on one another and on Sept2 |
| GO:1905515 non-motile cilium assembly | IBA GO_REF:0000033 | ACCEPT | Summary: Conserved role in assembling primary/sensory cilia; inherited from the eukaryote-level node. Reason: The IBD is seeded by mouse (primary-cilium axoneme loss in MEFs, kidney and neural tube) and worm (MKS-6 contributes to sensory-cilium transition-zone assembly). Human CC2D2A is in the descendant clade, and nothing indicates loss of function. The term does not exclude a role in motile cilia, which the mouse node and trachea phenotypes also suggest; the general cilium assembly rows cover that. Supporting Evidence: PMID:24947469 These data suggest that ciliary axoneme biogenesis was not initiated from the mother centriole in the absence of Cc2d2a. |
| GO:0036064 ciliary basal body | IDA PMID:26485645 The Ciliopathy Protein CC2D2A Associates with NINL and Funct... | NEW | Summary: Human CC2D2A localizes to the basal body in ciliated human cells. Reason: Tagged human CC2D2A localizes to the ciliary base (basal body and accessory centriole) in hTERT-RPE1 cells, and recombinant CC2D2A localizes to the basal body with CEP290. UniProt curates "cilium basal body" as a subcellular location. GOA records only the transition zone and broad cytoplasm/cytosol, so the basal body location, which is distinct in GO and is where the protein's appendage-assembly role is exercised, is missing. Supporting Evidence: PMID:26485645 When expressed alone, eCFP-tagged CC2D2A localizes to the ciliary base (basal body, accessory centriole) and also (partly) to the ciliary transition zone PMID:18950740 localization of recombinant CC2D2A at the basal body and colocalization with CEP290 |
| GO:0120103 centriolar subdistal appendage | ISS PMID:24947469 Ciliopathy-associated gene Cc2d2a promotes assembly of subdi... | NEW | Summary: Endogenous CC2D2A is on the subdistal appendages of the mother centriole (mouse IMCD3 cells and macaque retina, immunogold EM). Reason: Immunogold EM with an anti-CC2D2A antibody labels the subdistal appendages in mouse IMCD3 cells and in rhesus macaque photoreceptors. The macaque is a primate, and mouse Cc2d2a belongs to the same PANTHER subfamily (SF7) as human CC2D2A. The antibody specificity is backed by loss of signal in the null. This location is where CC2D2A's structural role in SDA assembly is exercised. Transferred by sequence similarity to human, which has no ultrastructural localization of its own. Supporting Evidence: PMID:24947469 Immunogold labeling for CC2D2A was clearly visible on the SDA both in IMCD3 cells and in monkey retina |
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Download this section (compressed HTML)Q: Which CC2D2A surfaces (coiled coils, CC2D2AN-C2, inactive TGL fold, C-terminal C2) contact MKS-module partners, CEP290, NINL and subdistal-appendage proteins? Does the C2 domain bind membrane lipids?
Q: Why do Cc2d2a-null MEFs lack cilia in one study but form cilia in another, and why do zebrafish photoreceptors keep cilia? Is the ciliogenesis requirement set by cell type, genetic background or allele?
Experiment: Test whether recombinant C2 and CC2D2AN-C2 domains of human CC2D2A bind phosphoinositide-containing or curved membranes in a calcium-dependent or calcium-independent manner, and whether Joubert missense variants in these domains abolish binding.
Type: in vitro lipid/liposome binding assay
Experiment: Map CC2D2A-dependent subdistal appendage and transition-zone architecture in CC2D2A-knockout human RPE1 cells by expansion or super-resolution microscopy, with domain-deletion rescue constructs.
Type: super-resolution microscopy with genetic rescue
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