Affinage

IFT81

Intraflagellar transport protein 81 homolog · UniProt Q8WYA0

Length
676 aa
Mass
79.7 kDa
Annotated
2026-04-28
13 papers in source corpus 8 papers cited in narrative 8 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

IFT81 is a core subunit of the intraflagellar transport complex B (IFT-B) that functions as a structural scaffold, enzymatic regulator, and coupling factor essential for ciliogenesis, flagellar assembly, and ciliary protein trafficking. IFT81 forms a direct heterodimeric and tetrameric complex with IFT74/72 that serves as the backbone of IFT-B assembly; loss of IFT81 destabilizes multiple IFT-B subunits, disrupts anterograde transport, and abolishes axoneme formation in both somatic cilia and sperm flagella (PMID:15955805, PMID:32233951). The IFT81–IFT74 coiled-coil region acts as an unconventional GTPase-activating protein (GAP) for RabL2, triggering GTP hydrolysis that releases RabL2 from IFT trains after departure from the ciliary base, a mechanism conserved from Chlamydomonas to humans (PMID:37606072). The C-terminal region of the IFT81–IFT74 dimer binds the IFT25–IFT27 module, and disruption of this interface—by patient-derived IFT81 mutations or engineered deletions—produces Bardet–Biedl syndrome-like ciliary membrane protein export defects, linking IFT81 to BBSome-dependent cargo retrieval and to skeletal ciliopathies and retinal dystrophy (PMID:34888642, PMID:37427975).

Mechanistic history

Synthesis pass · year-by-year structured walk · 5 steps
  1. 2005 High

    Establishing IFT81 as a core IFT-B scaffold subunit resolved how IFT complex B is organized: IFT81 directly binds IFT74/72 to form a (IFT81)₂(IFT74/72)₂ tetramer within a ~500 kDa core complex.

    Evidence Salt extraction, chemical cross-linking, yeast two-hybrid and three-hybrid in Chlamydomonas with vertebrate conservation

    PMID:15955805

    Open questions at the time
    • Atomic-resolution structure of the IFT81–IFT74 tetramer not determined
    • Functional consequences of disrupting the IFT81–IFT74 interaction in vivo not yet tested
    • How the tetramer integrates peripheral IFT-B subunits remained undefined
  2. 2016 Medium

    Patient and cell-based studies demonstrated that IFT81 is required for ciliogenesis, IFT-B complex stability, and Hedgehog signaling in human cells, connecting IFT81 loss to skeletal ciliopathy.

    Evidence Patient-derived chondrocytes and fibroblasts, Western blot for IFT-B subunits, cilia length/frequency, GLI2 expression

    PMID:26275418 PMID:27666822 PMID:28460050

    Open questions at the time
    • No isogenic knockout controls to isolate IFT81-specific effects from patient genetic background
    • Precise structural basis of IFT-B destabilization upon IFT81 loss unknown
    • Hedgehog signaling defects not mapped to specific IFT81 domains
  3. 2020 High

    Conditional knockout of IFT81 in mouse spermatids proved that IFT81 is essential for flagellar axoneme and para-axonemal structure assembly, with specific destabilization of IFT-B but not IFT-A subunits.

    Evidence Conditional knockout mouse (germ cell-specific Cre), transmission EM, Western blot of IFT subunits

    PMID:32233951

    Open questions at the time
    • Whether IFT81 plays additional roles in manchette-mediated transport during spermiogenesis not resolved
    • Mechanism by which IFT81 loss selectively destabilizes IFT-B but spares IFT-A unclear
  4. 2022 High

    Mapping the IFT25–IFT27 binding site to the C-terminal IFT74–IFT81 dimer, and showing that BBS-associated variants in IFT27 or IFT74 that disrupt this interface cause BBS-like ciliary trafficking defects, revealed how IFT81 couples IFT-B to BBSome-dependent membrane protein export.

    Evidence KO cell lines, co-immunoprecipitation, rescue with BBS-variant constructs, ciliary trafficking assays

    PMID:34888642

    Open questions at the time
    • Direct structural visualization of the IFT81–IFT74/IFT25–IFT27 quaternary interface not available
    • Identity of specific ciliary membrane cargo affected by this disruption only partially characterized
  5. 2023 High

    Two advances completed the mechanistic picture: (1) the IFT81–IFT74 coiled-coil was identified as an unconventional GAP for RabL2, explaining how IFT trains release RabL2 after ciliary base departure; (2) patient IFT81 deletions removing the IFT25–IFT27 binding domain phenocopy BBS in isogenic KO cells, confirming this domain is the critical link to BBSome-dependent export.

    Evidence Reconstituted pentameric IFT complex GTPase assay with domain mapping; IFT81-KO cells expressing patient variants and domain deletions with ciliary trafficking readouts

    PMID:37427975 PMID:37606072

    Open questions at the time
    • High-resolution structure of the RabL2–IFT81/74 GAP complex not solved
    • Whether GAP activity is regulated by post-translational modifications or additional cofactors unknown
    • How IFT81's scaffold, GAP, and BBSome-coupling functions are coordinated during a single IFT cycle not established

Open questions

Synthesis pass · forward-looking unresolved questions
  • Outstanding questions include the atomic structure of IFT81 in the context of full IFT-B trains, the regulatory inputs controlling its GAP activity, and the full repertoire of ciliary cargoes whose trafficking depends on the IFT81–IFT25/IFT27 axis.
  • No high-resolution cryo-EM structure of full-length IFT81 within an assembled IFT train
  • Regulatory mechanisms (phosphorylation, allosteric control) of IFT81 GAP activity uncharacterized
  • Comprehensive identification of BBSome cargo whose export depends on the IFT81–IFT25/IFT27 interface incomplete

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0005198 structural molecule activity 2 GO:0098772 molecular function regulator activity 1
Localization
GO:0005929 cilium 5 GO:0005856 cytoskeleton 1
Pathway
R-HSA-1852241 Organelle biogenesis and maintenance 5 R-HSA-162582 Signal Transduction 2 R-HSA-5653656 Vesicle-mediated transport 2
Partners
IFT25IFT27IFT46IFT52IFT74IFT88RABL2A
Complex memberships
IFT-B complexIFT81-IFT74 heterodimer/tetramer

Evidence

Reading pass · 8 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2005 IFT81 is a core subunit of the IFT complex B in Chlamydomonas reinhardtii. Salt extraction revealed a 500-kDa core complex containing IFT88, IFT81, IFT74/72, IFT52, IFT46, and IFT27. Chemical cross-linking produced multiple IFT81-IFT74/72 products, and yeast two-hybrid and three-hybrid analyses demonstrated that IFT81 directly interacts with IFT74/72 to form a tetrameric complex (IFT81)2(IFT74/72)2, proposed to serve as a scaffold for IFT complex B assembly. This interaction is evolutionarily conserved in vertebrates. Ionic strength fractionation, chemical cross-linking, yeast two-hybrid, yeast three-hybrid The Journal of biological chemistry High 15955805
2022 The IFT25-IFT27 dimer binds the C-terminal region of the IFT74-IFT81 dimer. BBS variants of IFT27 are impaired in IFT74-IFT81 binding and cannot rescue BBS-like phenotypes of IFT27-KO cells. BBS variants of IFT74, which delete the IFT25-IFT27-binding region, rescue ciliogenesis but produce BBS-like ciliary trafficking defects, establishing that impaired IFT74-IFT81/IFT25-IFT27 cooperation causes Bardet-Biedl syndrome-associated ciliary defects. Knockout cell lines, co-immunoprecipitation, rescue assays with BBS-variant constructs, ciliary phenotype analysis Human molecular genetics High 34888642
2023 The IFT81-IFT74 dimer acts as an unconventional GTPase-activating protein (GAP) for RabL2. A reconstituted pentameric IFT complex containing IFT81/74 enhances RabL2 GTP hydrolysis. The GAP activity was mapped to a 70-amino-acid coiled-coil region of IFT81/74. Structural models for RabL2-containing IFT complexes were built and validated in vitro and in cellulo, and Chlamydomonas IFT81/74 also stimulates human RabL2 GTP hydrolysis, indicating ancient evolutionary conservation. This mechanism explains why RabL2 dissociates from anterograde IFT trains shortly after departure from the ciliary base. Protein reconstitution, GTPase activity assay, structural modelling validated in vitro and in cellulo, cross-species (Chlamydomonas/human) biochemistry The EMBO journal High 37606072
2016 Mutations in IFT81 in patient chondrocytes lead to low IFT81 protein levels, elongated cilia, altered Hedgehog signaling, increased post-translational modification of tubulin, and destabilization of other IFT-B anterograde transport complex components (including IFT-B subunits), establishing IFT81 as essential for IFT-B complex stability and anterograde transport in skeletal development. Patient-derived chondrocyte analysis, Western blot, ciliogenesis assay, Hedgehog signaling readout Scientific reports Medium 27666822
2020 Conditional knockout of IFT81 in male mouse germ cells causes complete disorganization of the sperm axoneme and para-axonemal structures (mitochondrial sheath, fibrous sheath, outer dense fibers), with accumulation of vesicles containing unassembled microtubules in developing spermatids. Expression levels of IFT-B subunits IFT20, IFT25, IFT27, IFT57, and IFT74/IFT88, but not IFT140 (IFT-A), were significantly reduced, demonstrating IFT81 is required for IFT-B complex stability and flagellar axoneme assembly during spermatogenesis. Conditional knockout mouse, transmission electron microscopy, Western blot of IFT components, histology American journal of physiology. Cell physiology High 32233951
2015 A loss-of-function IFT81 mutation in human fibroblasts causes a significant decrease in ciliated cell abundance and increased expression of transcription factor GLI2, indicating deranged Sonic Hedgehog signaling, confirming IFT81 is required for ciliogenesis and Hedgehog pathway regulation in human cells. Patient fibroblast ciliogenesis assay, GLI2 expression analysis Journal of medical genetics Medium 26275418
2017 Loss of IFT81 in vitro impairs ciliogenesis, and a missense variant (p.L614P) shows significantly reduced rescue of ciliogenesis in IFT81 knockdown cells, demonstrating that IFT81 is required for ciliogenesis and that this variant is functionally deficient. In ift81 mutant zebrafish, mRNA carrying the missense variant shows dramatically reduced rescue efficiency compared to wild-type mRNA, confirming pathogenicity in vivo. siRNA knockdown with rescue assay (in vitro), ift81 mutant zebrafish mRNA rescue assay Investigative ophthalmology & visual science Medium 28460050
2023 Compound heterozygous IFT81 variants from a skeletal ciliopathy patient cause defects in IFT81's interactions with other IFT-B subunits. An IFT81 deletion (Δ490-519) that removes the IFT25-IFT27 binding site, when expressed in IFT81-KO cells, causes BBS-like ciliary trafficking defects (impaired ciliary membrane protein export), resembling those of BBS cells and cells expressing the BBS variant of IFT74, placing IFT81's IFT25-IFT27 interaction domain as critical for BBSome-coupled membrane protein export from cilia. IFT81-knockout cells, patient variant expression, ciliogenesis and ciliary protein trafficking assays, co-immunoprecipitation of IFT-B subunits Human molecular genetics High 37427975

Source papers

Stage 0 corpus · 13 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2005 Characterization of the intraflagellar transport complex B core: direct interaction of the IFT81 and IFT74/72 subunits. The Journal of biological chemistry 154 15955805
2016 Destabilization of the IFT-B cilia core complex due to mutations in IFT81 causes a Spectrum of Short-Rib Polydactyly Syndrome. Scientific reports 37 27666822
2020 The essential role of intraflagellar transport protein IFT81 in male mice spermiogenesis and fertility. American journal of physiology. Cell physiology 36 32233951
2015 IFT81, encoding an IFT-B core protein, as a very rare cause of a ciliopathy phenotype. Journal of medical genetics 34 26275418
2022 Impaired cooperation between IFT74/BBS22-IFT81 and IFT25-IFT27/BBS19 causes Bardet-Biedl syndrome. Human molecular genetics 31 34888642
2017 IFT81 as a Candidate Gene for Nonsyndromic Retinal Degeneration. Investigative ophthalmology & visual science 21 28460050
2018 Alu-Alu mediated intragenic duplications in IFT81 and MATN3 are associated with skeletal dysplasias. Human mutation 15 30080953
2023 Compound heterozygous IFT81 variations in a skeletal ciliopathy patient cause Bardet-Biedl syndrome-like ciliary defects. Human molecular genetics 9 37427975
2023 The IFT81-IFT74 complex acts as an unconventional RabL2 GTPase-activating protein during intraflagellar transport. The EMBO journal 9 37606072
2002 Identification of human CDV-1R and mouse Cdv-1R, two novel proteins with putative signal peptides, especially highly expressed in testis and increased with the male sex maturation. Molecular biology reports 6 12549821
2020 Expanding the phenotypic spectrum of IFT81: Associated ciliopathy syndrome. American journal of medical genetics. Part A 4 32783357
2000 Genomic organization and mapping of mouse CDV (carnitine deficiency-associated gene expressed in ventricle)-1 and its related CDV-1R gene. Mammalian genome : official journal of the International Mammalian Genome Society 3 11130971
2025 Intron-oriented HTLV-1 integration in an adult T-cell leukemia/lymphoma cell line sustains expression of intact ift81 mRNA. FEBS letters 0 40597293