DNAAF6

DNAAF6 (PIH1D3) is a cytoplasmic PIH1-domain protein that functions as a component of an R2TP-like HSP90 co-chaperone complex—together with DNAAF2 and DNAAF4—to mediate the preassembly of axonemal outer dynein arm (ODA) and inner dynein arm (IDA) complexes before their transport into cilia and flagella (PMID:24421334, PMID:28041644, PMID:28176794). DNAAF6 physically interacts with Hsp70, Hsp90, and dynein intermediate chains during this cytoplasmic assembly process, and additionally associates with LARP6 in biomolecular condensates (dynein axonemal particles) in multiciliated cells, where the DNAAF6–LARP6 interaction is required for apical α-tubulin expression during ciliogenesis (PMID:24421334, PMID:38762183). Loss-of-function mutations in DNAAF6 cause X-linked primary ciliary dyskinesia characterized by absent ODA and reduced/absent IDA, situs inversus, hydrocephalus, and male infertility, with the severity in heterozygous female carriers determined by the degree of skewed X-chromosome inactivation (PMID:28041644, PMID:28176794, PMID:38408845).

1. 2014 High

   The first functional characterization of Pih1d3 established that it is a cytoplasmic chaperone cofactor required for axonemal dynein arm preassembly, resolving the question of where and how dynein complexes are stabilized before ciliary transport.

   Evidence Pih1d3-knockout mice analyzed by TEM, immunofluorescence, and co-immunoprecipitation with ODA intermediate chain, Hsp70, and Hsp90

   PMID:24421334

   Open questions at the time

   • Identity of the broader chaperone complex containing Pih1d3 was not defined
   • Relevance to human disease was not established
   • Mechanism by which Pih1d3 stabilizes dynein subunits was unclear
2. 2016 High

   Human genetic studies demonstrated that DNAAF6 loss-of-function mutations cause primary ciliary dyskinesia and identified DNAAF2 and DNAAF4 as direct interaction partners, establishing DNAAF6 as part of a defined cytoplasmic dynein assembly module.

   Evidence Hemizygous patient mutations with TEM of cilia, co-immunoprecipitation of DNAAF6 with DNAAF2 and DNAAF4

   PMID:28041644

   Open questions at the time

   • Whether the DNAAF2–DNAAF4–DNAAF6 module constitutes a formal R2TP-like complex was not resolved
   • Specific contribution of DNAAF6 versus its partners to IDA vs ODA assembly was unclear
3. 2017 High

   Placement of DNAAF6 within a conserved R2TP-like HSP90 co-chaperone complex resolved the mechanistic framework linking PIH1-domain proteins to HSP90-mediated dynein arm assembly, and confirmed X-linked inheritance of PCD caused by DNAAF6 mutations.

   Evidence Genomic deletion and mutation analysis across multiple PCD families with functional pathway contextualization

   PMID:28176794

   Open questions at the time

   • Structural basis of the R2TP-like complex containing DNAAF6 was not determined
   • The full catalog of dynein clients handled by this specific R2TP-like module was incomplete
4. 2020 Medium

   Demonstration that pathogenic DNAAF6 missense variants lead to protein degradation provided mechanistic insight into how hypomorphic alleles cause dynein arm loss, extending the mutation spectrum.

   Evidence HEK293T overexpression-based protein stability assay and TEM/immunostaining of patient sperm

   PMID:32170493

   Open questions at the time

   • Degradation pathway (proteasomal vs autophagic) for unstable variants was not identified
   • Whether partial protein stability allows residual dynein assembly was untested
5. 2022 Medium

   Conservation of the Dnaaf6–Dnaaf4 R2TP-like complex in Drosophila confirmed that this module operates specifically in cells bearing motile cilia, ruling out a general chaperone function.

   Evidence Drosophila Dnaaf6 knockout with motility assays in chordotonal neurons and sperm, protein co-association studies

   PMID:35873488

   Open questions at the time

   • Direct biochemical reconstitution of the Drosophila complex was not performed
   • Precise step at which Dnaaf6 acts (folding, multimerization, or handoff to IFT) was unresolved
6. 2023 Medium

   A knockout rat model broadened the phenotypic spectrum of DNAAF6 deficiency to include situs inversus, hydrocephalus, and spermatocyte death, and identified additional interacting partners involved in dynein uploading to axonemes.

   Evidence TALEN-mediated Pih1d3 knockout rat with Co-IP, histology, electron microscopy, and behavioral analysis

   PMID:37900281

   Open questions at the time

   • Identity of the specific uploading/transport factors that interact with DNAAF6 was not fully resolved
   • Whether spermatocyte death is a direct consequence of unassembled dynein or secondary stress was unclear
7. 2024 Medium

   Discovery that DNAAF6 interacts with LARP6 in biomolecular condensates (dynein axonemal particles) and that this interaction is required for apical α-tubulin expression revealed an unexpected role in translational or post-translational tubulin regulation during ciliogenesis.

   Evidence Xenopus morpholino knockdown, LARP6-binding-deficient DNAAF6 mutant rescue, co-localization imaging in multiciliated cells

   PMID:38762183

   Open questions at the time

   • Whether DNAAF6–LARP6 condensates regulate mRNA translation or protein stability of tubulin is unresolved
   • Relevance of LARP6 interaction to the canonical dynein preassembly function of DNAAF6 is unclear
   • Whether this condensate function is conserved in mammalian multiciliated cells is untested
8. 2024 Medium

   Quantification of the relationship between X-inactivation skewing and ciliary phenotype in heterozygous DNAAF6 carriers established that ~20–50% of DNAAF6-expressing cells suffice to prevent clinical PCD, defining a threshold for cell-autonomous dynein assembly.

   Evidence DNA methylation-based XCI analysis, immunofluorescence, and high-speed videomicroscopy of carrier airway epithelial cells

   PMID:38408845

   Open questions at the time

   • Whether dynein complexes can be transferred between DNAAF6-positive and DNAAF6-negative cells was not addressed
   • Threshold may differ across ciliated tissues (airway vs ependymal vs reproductive)

• The structural basis for DNAAF6 within the R2TP-like complex, the precise step it catalyzes during dynein folding and multimerization, and how its LARP6-dependent condensate function integrates with its canonical chaperone role remain unresolved.
• No high-resolution structure of DNAAF6 or the DNAAF2–DNAAF4–DNAAF6–HSP90 complex exists
• The relationship between the dynein preassembly and tubulin regulation functions is undefined
• Whether DNAAF6 has additional non-ciliary roles has not been systematically tested