RSPH6A

RSPH6A is a radial spoke head protein essential for axonemal assembly, maintenance, and motility regulation in motile cilia and sperm flagella. It forms part of the RS1 head complex together with RSPH9, RSP1, and RSP4, where direct protein–protein interactions assemble a 7–10S spokehead subcomplex that controls dynein-driven flagellar beating patterns, including the maintenance of planar waveform (PMID:21391306, PMID:9450971, PMID:39849482). In mice, Rsph6a knockout causes male infertility due to short, immotile spermatozoa with disrupted axoneme assembly and loss of RSPH9 from the flagellum, and RSPH6A undergoes capacitation-induced phosphorylation at a eutherian-specific site, linking it to functional sperm maturation (PMID:30185526, PMID:30239614). Mutations in RSPH6A are identified as causative for human male infertility (asthenozoospermia) (PMID:38884051).

1. 1992 Medium

   Molecular cloning of RSP6 in Chlamydomonas established that the radial spoke head contains a proline-rich polypeptide closely related to RSP4, and its mRNA upregulation during flagellar regeneration implied a structural role in spoke assembly.

   Evidence Gene sequencing and mRNA accumulation analysis during flagellar regeneration in Chlamydomonas

   PMID:1508197

   Open questions at the time

   • No loss-of-function phenotype yet established
   • Protein–protein interactions within the spokehead not mapped
   • Mammalian ortholog not yet identified
2. 1998 Medium

   Antibody perturbation of a sea urchin RSP4/6 homolog demonstrated that spokehead proteins actively control flagellar waveform dimensionality, converting beating from planar to three-dimensional when disrupted — the first functional evidence that the spoke head is not merely structural but a waveform regulator.

   Evidence Monoclonal antibody perturbation of demembranated sea urchin sperm, co-immunoprecipitation, and gel filtration of native complex

   PMID:9450971

   Open questions at the time

   • Mechanism by which antibody binding alters waveform unclear
   • Experiments performed in sea urchin ortholog, not mammalian RSPH6A directly
   • Downstream signaling pathway not identified
3. 2006 Medium

   Demonstration that Tetrahymena RSP4/6 homologs bind Ca²⁺/calmodulin in a calcium-dependent manner provided a candidate signaling mechanism through which the spoke head could transduce calcium signals to regulate ciliary waveform.

   Evidence Ca²⁺/CaM-affinity chromatography, immunoelectron microscopy in Tetrahymena cilia

   PMID:16936294

   Open questions at the time

   • Direct Ca²⁺/CaM binding not shown for mammalian RSPH6A
   • Functional consequence of CaM interaction on motility not tested
   • Binding site on RSP6 not mapped
4. 2010 Medium

   Chlamydomonas RSP6 mutant analysis resolved the paradox that RSP6 is dispensable for initial motility but required for helical swimming trajectory and long-term spoke head maintenance, revealing a role in assembly stability rather than core motility.

   Evidence Chlamydomonas RSP6 loss-of-function mutant with motility assays, EM, and immunoblot across growth phases

   PMID:20169531

   Open questions at the time

   • Whether mammalian RSPH6A shares the partial dispensability phenotype unknown
   • Mechanism of progressive spoke head loss not defined
   • Structural basis for helical vs. spinning trajectory not resolved
5. 2011 High

   In vitro reconstitution and in situ crosslinking mapped the direct protein–protein interaction network within the spoke head, showing RSP6 forms a 7–10S subcomplex with RSP1, RSP4, and RSP9 — defining the quaternary organization of the spoke head for the first time.

   Evidence GST pulldown with recombinant Chlamydomonas proteins, chemical crosslinking of axonemes, sucrose density gradient sedimentation

   PMID:21391306

   Open questions at the time

   • Stoichiometry within the complex not determined
   • No high-resolution structure of the assembled spoke head
   • Whether mammalian spoke head has identical interaction topology not confirmed
6. 2018 High

   The mouse knockout established that RSPH6A is absolutely essential for mammalian sperm flagellar assembly — unlike the partial Chlamydomonas phenotype — causing complete male infertility with short immotile sperm, disrupted axoneme elongation, and co-dependent loss of RSPH9.

   Evidence Rsph6a knockout mouse, immunofluorescence, electron microscopy of axoneme ultrastructure, western blot for RSPH9

   PMID:30185526

   Open questions at the time

   • Mechanism by which RSPH6A loss leads to axoneme shortening not defined
   • Whether RSPH6A has functions beyond structural spoke assembly unknown
   • Female fertility and airway cilia not examined in KO
7. 2019 High

   Phosphoproteomic discovery of a capacitation-induced phosphorylation site on a eutherian-specific RSPH6A exon linked spoke head regulation to sperm functional maturation signaling, suggesting an evolutionarily acquired regulatory layer.

   Evidence Tandem mass spectrometry of capacitated vs. uncapacitated mouse sperm, cytoskeletal fractionation, immunofluorescence

   PMID:30239614

   Open questions at the time

   • Kinase responsible for RSPH6A phosphorylation not identified
   • Functional consequence of phosphorylation on motility not tested
   • Whether phosphorylation alters spoke head complex interactions unknown
8. 2023 Medium

   Developmental expression profiling in rat testis showed RSPH6A appears coincident with primary spermatocytes and is sensitive to oxidative stress, placing its expression in the context of spermatogenic differentiation and environmental vulnerability.

   Evidence Western blot and immunofluorescence time-course during rat postnatal spermatogenesis, cadmium/melatonin in vivo model

   PMID:37795044

   Open questions at the time

   • Direct mechanism of cadmium-induced RSPH6A loss unknown
   • Whether melatonin protection is RSPH6A-specific or a general antioxidant effect unclear
   • Functional consequence of reduced RSPH6A on rat sperm motility not directly measured
9. 2024 Low

   Human exome sequencing identified RSPH6A mutations as causative for asthenozoospermia, extending the gene's essential role in male fertility from mouse to human.

   Evidence Whole-exome sequencing and segregation analysis in infertile human males

   PMID:38884051

   Open questions at the time

   • Specific mutations lack direct functional validation (e.g. rescue or protein-level assay)
   • Number of families studied is limited
   • Mechanism by which identified variants disrupt protein function not characterized
10. 2025 Medium

    Proteomic analysis following IQUB knockout placed RSPH6A specifically within the RS1 head (not RS2 or RS3), resolving which of the three mammalian radial spokes incorporates RSPH6A.

    Evidence Mass spectrometry of Iqub-/- mouse sperm, western blotting, structural modeling of RS1

    PMID:39849482

    Open questions at the time

    • Placement based on loss in a single KO model; direct structural confirmation lacking
    • Whether RSPH6A is exclusive to RS1 or present at lower levels in other spokes not fully excluded
    • High-resolution cryo-EM structure of mammalian RS1 head not yet available

• The kinase(s) mediating capacitation-induced RSPH6A phosphorylation, the structural basis of spoke head complex assembly in mammals, and whether RSPH6A contributes to motile cilia function outside the male germline remain unresolved.
• No high-resolution structure of mammalian RS1 head complex
• Kinase for capacitation phosphorylation unidentified
• Role in airway or ependymal cilia not examined