IQCE

IQCE is a primary cilium scaffold protein that functions as a positive regulator of Hedgehog signaling by organizing a signaling microdomain at the ciliary base (PMID:24582806). It forms a stable complex with EFCAB7 at the base of primary cilia (the EvC zone), and this IQCE-EFCAB7 module anchors the EVC-EVC2 transmembrane heterodimer at this site; depletion of IQCE mislocalizes EVC-EVC2 within cilia and impairs activation of the GLI2/GLI1 transcription factors that execute the Hedgehog response (PMID:24582806, PMID:37684519). Proteomic characterization of the endogenous EVC interactome confirms IQCE and EFCAB7 as primary EVC interactors and shows that EvC zone targeting depends on two separate EFCAB7-binding motifs in EVC2, with SUMO3 modification of the EVC-EVC2 tails enhancing accumulation at the EvC zone via this complex (PMID:37576597). Loss of IQCE function disrupts cilia structure and function in vivo, producing a full ciliopathy spectrum in zebrafish including body curvature, kidney cysts, left-right asymmetry defects, and retinal defects (PMID:31549751). Biallelic loss-of-function variants in IQCE cause autosomal recessive postaxial polydactyly type A in humans, and pathogenic variants dysregulate Hedgehog-pathway gene expression in patient fibroblasts (PMID:31549751, PMID:28488682).

1. 2014 High

   Established the core molecular function of IQCE: how the Hedgehog-promoting EVC-EVC2 heterodimer is retained in a discrete ciliary microdomain was unknown, and this work showed IQCE forms a complex with EFCAB7 that anchors EVC-EVC2 at the EvC zone and is required for GLI2 activation.

   Evidence Reciprocal Co-IP, siRNA depletion with ciliary immunofluorescence and GLI2 activation assays in cells

   PMID:24582806

   Open questions at the time

   • No structural model of the IQCE-EFCAB7 interface
   • Direct IQCE-EFCAB7 binding determinants on IQCE not mapped
   • Mechanism by which the complex restricts EVC-EVC2 to the EvC zone not resolved at atomic detail
2. 2017 Medium

   Connected IQCE loss-of-function to human disease by demonstrating that a homozygous splice variant produces a frameshift and premature stop, establishing IQCE as a cause of autosomal recessive postaxial polydactyly type A.

   Evidence Exome/Sanger sequencing with segregation and a mini-gene splicing assay

   PMID:28488682

   Open questions at the time

   • Single variant in a single family; allelic spectrum not defined
   • Does not directly link the truncation to ciliary or Hedgehog defects in patient tissue
3. 2019 High

   Demonstrated the in vivo requirement for IQCE in cilia, showing that knockdown produces a full ciliopathy spectrum and that patient variants dysregulate Hedgehog-pathway gene expression, extending the cell-based model to organismal phenotypes.

   Evidence Zebrafish morpholino knockdown with multiple ciliopathy readouts and RNA expression analysis in patient fibroblasts

   PMID:31549751

   Open questions at the time

   • Morpholino knockdown lacks genetic mutant confirmation
   • Which Hedgehog target genes are most directly affected not specified
   • Tissue-specific basis of the polydactyly versus broader ciliopathy phenotypes unresolved
4. 2023 Medium

   Refined the assembly logic of the EvC zone by confirming IQCE/EFCAB7 as the primary endogenous EVC interactors, mapping a second EFCAB7-binding motif on EVC2, and implicating SUMO3 modification in enhancing complex accumulation.

   Evidence Endogenous EVC interactome by mass spectrometry in control and Evc-null cells, plus ubiquitination/SUMOylation and immunofluorescence assays

   PMID:37576597

   Open questions at the time

   • SUMO-dependent enhancement of binding to the IQCE complex shown in a single lab
   • Direct demonstration that SUMO3 increases IQCE-EFCAB7 binding not established
   • Role of ubiquitination in the complex not clarified
5. 2023 Medium

   Reinforced the IQCE-EFCAB7 module's role in Hedgehog-dependent limb development by showing that mutations in the direct partner EFCAB7 also cause autosomal recessive postaxial polydactyly, with depletion of either protein inhibiting GLI1 induction.

   Evidence Exome/Sanger sequencing in multiple families with synthesis of prior GLI1 induction data

   PMID:37684519

   Open questions at the time

   • GLI1 inhibition data drawn from prior studies rather than newly generated here
   • Does not separate the individual contributions of IQCE versus EFCAB7 to complex function

• How IQCE's IQ-motif/calmodulin-binding architecture contributes mechanistically to EVC-EVC2 anchoring and whether IQCE has Hedgehog-independent cellular roles remains unresolved.
• No structural or biochemical dissection of IQCE domains in the corpus
• Broad constitutive expression noted but no non-ciliary function characterized
• No high-resolution structure of the IQCE-EFCAB7-EVC-EVC2 assembly