TBC1D32

TBC1D32 (BROMI) is a ciliary protein that regulates intraflagellar transport (IFT) turnaround at the ciliary tip and is required for proper ciliogenesis and Hedgehog signaling across multiple tissues (PMID:35609210, PMID:34624068). It functions within a complex containing the kinase CCRK/CDK20, FAM149B1/JBTS36, and the conserved ciliary protein CFAP20, where BROMI mutants defective in binding either CCRK or CFAP20 fail to rescue the abnormally long cilia and tip accumulation of IFT machinery seen in BROMI-knockout cells, placing the complex under the control of ICK/CILK1 (PMID:35609210). The CCRK–BROMI interaction is specifically required for retrograde ciliary trafficking and IFT turnaround, since a CCRK mutant unable to bind BROMI cannot reverse the overaccumulation of IFT proteins at bulged ciliary tips or the abnormal enrichment of GPR161 and Smoothened on the ciliary membrane (PMID:34624068). In vivo, TBC1D32 acts redundantly with Dzip1l in ciliogenesis, cilia morphogenesis, and neural tube patterning (PMID:29487109), and is required for retinal pigment epithelium ciliogenesis, epithelial integrity, retinoid cycling, and photoreceptor connecting cilium function (PMID:37768732). Beyond these roles, the biochemical activity of the TBC domain itself has not been characterized in the available corpus.

1. 2018 Medium

   Established that Bromi/Tbc1d32 operates in the same biological program as Dzip1l, resolving whether it acts alone or shares function in cilium formation and patterning.

   Evidence Double-mutant mouse genetic epistasis with ciliary morphology and neural tube readouts

   PMID:29487109

   Open questions at the time

   • Molecular basis of the redundancy not defined
   • No direct physical interaction with Dzip1l shown
   • Does not identify the biochemical activity of Tbc1d32
2. 2020 Medium

   Defined the interaction landscape of TBC1D32 and its expression domains, linking it to cilium assembly, Shh signaling, and brain development.

   Evidence Affinity pulldown-mass spectrometry from HEK cells plus in situ hybridization expression profiling

   PMID:32060556

   Open questions at the time

   • Interactions identified by single pulldown-MS without reciprocal validation
   • Limited functional follow-up
   • Functional consequences of individual interactions not tested
3. 2021 High

   Showed that the CCRK–BROMI interaction is functionally required for retrograde ciliary trafficking and IFT turnaround, distinguishing the interaction from CCRK kinase activity per se.

   Evidence CCRK-knockout cell lines rescued with wild-type, kinase-dead, and BROMI-binding-defective CCRK mutants; immunofluorescence of IFT proteins, GPR161, and Smoothened

   PMID:34624068

   Open questions at the time

   • Does not resolve how BROMI mechanistically promotes IFT turnaround
   • TBC domain catalytic role unaddressed
   • Direct vs. indirect effect on receptor enrichment not separated
4. 2022 High

   Placed BROMI in a defined multiprotein complex with CCRK, FAM149B1/JBTS36, and CFAP20, and showed both CCRK- and CFAP20-binding are required for ciliary length control and IFT turnaround under ICK/CILK1 control.

   Evidence Co-immunoprecipitation and BROMI-knockout cell rescue with binding-defective mutants, with IFT immunofluorescence and cilia length measurements

   PMID:35609210

   Open questions at the time

   • Stoichiometry and architecture of the complex unknown
   • No structural model of the assembly
   • How ICK/CILK1 signaling is transmitted to the complex not defined
5. 2023 High

   Extended TBC1D32 function to retinal tissue, establishing its requirement for RPE ciliogenesis, epithelial integrity, retinoid cycling, and photoreceptor connecting cilium integrity.

   Evidence Xenopus in vivo loss-of-function and human iPSC-derived RPE/retinal organoid models with ciliary markers and functional assays

   PMID:37768732

   Open questions at the time

   • Whether retinal phenotypes depend on the CCRK/CFAP20 complex not tested
   • Mechanism linking ciliary defect to EMT-like phenotype unclear
   • No causative human disease mutation established in this study

• The intrinsic biochemical activity of the TBC1D32 TBC domain and how the BROMI complex molecularly executes IFT turnaround remain unresolved.
• No demonstrated GAP or catalytic activity for the TBC domain
• No structure of the BROMI-containing complex
• Mechanistic coupling between ICK/CILK1 phosphorylation and IFT turnaround undefined