IFT74

IFT74 is a core subunit of intraflagellar transport complex B (IFT-B) that scaffolds train assembly and coordinates the transport machinery required to build and maintain cilia (PMID:15955805, PMID:26051893). It heterodimerizes with IFT81 to form a higher-order (IFT81)2(IFT74/72)2 tetramer that nucleates the salt-stable IFT-B core, and its central/C-terminal region (aa 197–641) is sufficient to stabilize IFT-B integrity in vivo; complete loss destabilizes the complex and abolishes ciliogenesis (PMID:15955805, PMID:26051893). The N-terminal coiled-coil (aa 1–196) mediates IFT-A/IFT-B association at the ciliary base and is required for IFT-A import, while the extreme N-terminus (first 40 residues) binds tubulin and is especially critical for tubulin delivery into motile cilia (PMID:26051893, PMID:37315079, PMID:37555648, PMID:36865301). The C-terminal region of the IFT74–IFT81 dimer recruits the IFT25–IFT27 dimer to coordinate BBSome-dependent export of ciliary membrane proteins, and disruption of this interface produces Bardet-Biedl-like membrane-protein export defects (PMID:34888642). A 70-residue coiled-coil segment of IFT81/74 additionally functions as an unconventional GTPase-activating protein for RabL2, accelerating its GTP hydrolysis to drive RabL2 release from anterograde IFT trains after departure from the ciliary base (PMID:37606072). In humans, IFT74 variants cause ciliopathy phenotypes including disrupted ciliary Hedgehog signaling and Bardet-Biedl-spectrum disease, as well as isolated sperm-flagellar defects (MMAF) (PMID:34888642, PMID:33531668, PMID:33689014).

1. 2005 High

   Established that IFT74 is not a peripheral component but a structural backbone of IFT-B, defining which subunits form the cohesive core versus accessory periphery.

   Evidence High ionic strength fractionation, cross-linking, and yeast two-/three-hybrid in Chlamydomonas with vertebrate homologue confirmation

   PMID:15955805

   Open questions at the time

   • Does not resolve atomic architecture of the tetramer
   • Stoichiometry of additional core members within trains not defined
2. 2010 Low

   Raised the possibility of a nuclear transcriptional role for the IFT74 ortholog CMG-1, distinct from cilia.

   Evidence siRNA knockdown, cyclin D2 promoter reporter assays, and FLAG nuclear localization in mouse GC-2 and COS7 cells

   PMID:20545763

   Open questions at the time

   • Single-lab cell-line study not independently confirmed
   • Nuclear transcriptional function inconsistent with the cytoplasmic/ciliary IFT role and may reflect cell-line artifact or alias ambiguity
   • No direct DNA-binding mechanism demonstrated
3. 2015 High

   Dissected which IFT74 domains drive distinct functions, separating IFT-B stabilization from IFT-A recruitment and tubulin entry.

   Evidence Chlamydomonas ift74 null rescue with truncated transgenes, flagellar assembly assays, and IFT injection-frequency measurements

   PMID:26051893

   Open questions at the time

   • Molecular basis of how the N-terminus bridges IFT-A and IFT-B unresolved
   • Tubulin-binding residues not pinpointed in this study
4. 2021 Medium

   Linked IFT74 loss to ciliary membrane-protein mislocalization and Hedgehog pathway disruption, and demonstrated human disease causality.

   Evidence Patient fibroblasts with truncating variants, ciliary marker immunofluorescence, Hedgehog reporters, and zebrafish morphant rescue with a human variant

   PMID:33531668

   Open questions at the time

   • Single-lab patient cohort
   • Mechanistic link between IFT74 and Hedgehog effectors indirect
5. 2021 Medium

   Showed IFT74 is required for ciliary maintenance and cargo (opsin) transport rather than only initial assembly, explaining tissue-specific degenerative phenotypes.

   Evidence Zebrafish ift74 mutant live imaging of connecting cilia, opsin transport assays, and maternal transcript quantification

   PMID:34502236

   Open questions at the time

   • Maternal contribution complicates timing of null phenotype
   • Direct opsin-IFT74 interaction not tested
6. 2021 Medium

   Demonstrated that a splice-affecting IFT74 variant can cause isolated sperm-flagellar disease without systemic ciliopathy, indicating tissue-selective requirements.

   Evidence Patient exome sequencing, RT-PCR splice analysis, and immunofluorescence of mutant IFT74 along sperm flagellum

   PMID:33689014

   Open questions at the time

   • Single patient/lab
   • Functional consequence of mislocalized isoforms on transport not directly measured
7. 2022 High

   Mapped the IFT74-IFT81 C-terminal region as the docking site for IFT25-IFT27 and showed this interface specifically underlies BBSome-dependent membrane export, with separation-of-function disease alleles.

   Evidence Reciprocal Co-IP domain mapping plus IFT27-KO and IFT74-KO rescue with BBS variant constructs and ciliary export readouts

   PMID:34888642

   Open questions at the time

   • How the IFT74/81-IFT25/27 module physically couples to the BBSome not resolved
   • Identity of exported cargoes incompletely defined
8. 2023 High

   Identified an enzymatic function for the IFT81/74 coiled-coil as an unconventional GAP for RabL2, explaining how RabL2 is released from anterograde trains.

   Evidence In vitro reconstitution of a pentameric IFT-RabL2 complex, GTP hydrolysis assays, domain mapping, structural modeling, and cross-species functional comparison

   PMID:37606072

   Open questions at the time

   • Spatial/temporal trigger of GAP activity at the base not fully defined
   • Catalytic residues not pinpointed
9. 2023 High

   Pinpointed the first 40 residues of IFT74 as a tubulin-binding determinant disproportionately required for motile cilia, rationalizing differential cilia-type phenotypes.

   Evidence In vitro tubulin-binding assays with N-terminal deletions, mouse exon 2-deletion knock-in phenotyping, and AP-MS, with parallel human patient data

   PMID:36865301 PMID:37315079 PMID:37555648

   Open questions at the time

   • Structural basis of tubulin binding by the N-terminus unresolved
   • Why primary cilia tolerate reduced tubulin transport not mechanistically explained

• How IFT74's multiple modular activities — tubulin loading, IFT-A coupling, IFT25/27-BBSome coordination, and RabL2 GAP activity — are spatially and temporally integrated within a single train remains open.
• No integrated structural model of an assembling train carrying all activities
• Regulation switching IFT74 between functional states unknown