RTTN

RTTN (Rotatin) is a centriolar structural protein that functions in centriole elongation, mitotic spindle integrity, and primary cilium-dependent signaling. It localizes to the inner luminal walls of centrioles, where it directly binds STIL and acts downstream of STIL-mediated centriole assembly to promote distal centriole elongation through recruitment of CEP295 (which loads POC1B and POC5) and through complex formation with the downstream effector PPP1R35 (PMID:28811500, PMID:30168418). RTTN is also required for normal mitotic spindle organization, correct spindle positioning via regulation of pericentriolar γ-tubulin, NuMA, and astral microtubule dynamics (PMID:34207628), and for primary cilium structure and BMP/WNT signaling (PMID:22939636). Biallelic RTTN mutations cause microcephaly with abnormal cortical development, consistent with its requirement for neural stem cell apico-basal polarization, rosette formation, and cell cycle fidelity in cortical organoids (PMID:22939636, PMID:39680576).

1. 2012 Medium

   Establishing that RTTN is a cilium-associated protein whose loss disrupts cilia structure and downstream BMP/WNT signaling resolved the initial question of where and at what developmental process RTTN acts, linking it to cortical patterning defects in microcephaly patients.

   Evidence Immunofluorescence colocalization with basal bodies, patient fibroblast and NSC analysis with gene expression profiling

   PMID:22939636

   Open questions at the time

   • Mechanism by which RTTN maintains cilium structure was not defined
   • Direct protein-protein interactions were not identified
   • Whether cilia defect is secondary to a centriolar role was untested
2. 2017 High

   Pinpointing RTTN to the inner centriolar lumen and demonstrating its direct binding to STIL, its epistatic position upstream of CEP295 in distal centriole assembly, and the functional impact of a disease-associated missense mutation answered how RTTN contributes to centriole biogenesis at a molecular level.

   Evidence Super-resolution and electron microscopy, co-immunoprecipitation, CRISPR knockout and rescue, mutant binding assays in human cells

   PMID:28811500

   Open questions at the time

   • Structural basis of the RTTN–STIL interaction is unknown
   • How RTTN activates CEP295 loading mechanistically was not resolved
   • Whether RTTN has additional partners beyond STIL at the procentriole was not tested
3. 2018 High

   Identifying PPP1R35 as a downstream effector that forms a complex with RTTN extended the centriole elongation pathway and clarified that RTTN operates through at least two downstream branches (CEP295 and PPP1R35).

   Evidence Quantitative super-resolution microscopy, BioID proximity mapping, loss-of-function epistasis experiments

   PMID:30168418

   Open questions at the time

   • Whether the RTTN–PPP1R35 complex is direct or bridged by additional subunits is unresolved
   • Stoichiometry and regulation of the complex during the cell cycle are unknown
4. 2021 Medium

   Demonstrating that RTTN depletion causes γ-tubulin dispersal, spindle multipolarity, mislocalized NuMA/p150Glued, and reduced astral microtubules established a second major function for RTTN in mitotic spindle organization and positioning, beyond centriole assembly.

   Evidence RNAi depletion with immunofluorescence and spindle positioning assays in human cells

   PMID:34207628

   Open questions at the time

   • Single-lab RNAi study; independent confirmation with orthogonal depletion strategies is lacking
   • Whether spindle defects are a direct consequence of centriole structural defects or reflect a separable RTTN function is untested
   • Molecular mechanism linking RTTN to NuMA cortical recruitment is unknown
5. 2024 Medium

   Showing that RTTN-mutant iPSC-derived neural stem cells fail to polarize apico-basally, delay rosette self-organization, and exhibit aneuploidy and cell death in cortical organoids connected RTTN's centriolar and mitotic functions to a tissue-level developmental phenotype relevant to human microcephaly.

   Evidence CRISPR/Cas9-edited iPSC-derived cortical organoids with live imaging and immunostaining

   PMID:39680576

   Open questions at the time

   • Single-lab organoid model; replication in animal models or independent iPSC lines would strengthen claims
   • Relative contributions of centriole elongation defects versus spindle positioning defects to the cortical phenotype are not delineated
   • Whether RTTN's role in rosette formation depends on primary cilia signaling or is purely centrosomal is unresolved

• A structural understanding of RTTN—including its domain architecture, how it bridges STIL binding to CEP295 and PPP1R35 activation, and whether its spindle-positioning function is mechanistically separable from its centriole elongation role—remains an open question.
• No high-resolution structure of RTTN or its complexes exists
• Separation-of-function mutations distinguishing centriole elongation from spindle positioning roles have not been generated
• Regulatory inputs (phosphorylation, cell-cycle-dependent degradation) are unknown