EFCAB9

EFCAB9 is a cytoplasmic EF-hand Ca²⁺-binding protein that functions as a dual pH and Ca²⁺ sensor within the sperm-specific CatSper calcium channel complex, coupling extracellular alkalinization and intracellular calcium signals to channel activation and sperm hyperactivated motility. It binds directly to CATSPERζ via a conserved IQ-like motif in a Ca²⁺-dependent manner and dissociates at elevated pH, thereby gating CatSper activity; together with CATSPERζ and ARMH2 it forms a cytosolic ternary subcomplex required for the stable linear organization of CatSper nanodomains along the flagellum (PMID:31056283, PMID:41271765). Cryo-EM of the native CatSpermasome confirms EFCAB9 as a structural subunit of the multi-protein channel complex alongside CATSPER1–4, CATSPERβ/γ/δ/ε/ζ, and SLCO6C1 (PMID:34225353). Loss of EFCAB9 abolishes pH/Ca²⁺-sensitive CatSper activation, eliminates clockwise sperm path chirality, and causes male infertility in mice (PMID:31056283, PMID:35438819).

1. 2019 High

   Identification of EFCAB9 as the pH-dependent Ca²⁺ sensor of CatSper established how the channel integrates two key physiological signals—intracellular Ca²⁺ and alkaline pH—during sperm capacitation.

   Evidence Knockout mouse, co-immunoprecipitation, direct Ca²⁺-dependent binding assays with CATSPERζ, super-resolution imaging of CatSper nanodomains

   PMID:31056283

   Open questions at the time

   • Structural basis of the EFCAB9–CATSPERζ interface was unresolved
   • Whether additional cytosolic partners stabilize the EFCAB9–CATSPERζ subcomplex was unknown
   • Mechanism by which pH triggers EFCAB9 dissociation from CATSPERζ was not determined
2. 2021 High

   Cryo-EM determination of the CatSpermasome architecture placed EFCAB9 within the native multi-subunit complex and defined its spatial relationship to the pore-forming and auxiliary subunits.

   Evidence Cryo-EM of the CatSper complex isolated from mouse sperm at near-atomic resolution

   PMID:34225353

   Open questions at the time

   • Resolution did not fully resolve the EFCAB9–CATSPERζ interface at atomic detail
   • Conformational changes upon Ca²⁺ or pH shift were not captured
3. 2021 Medium

   Demonstration that a conserved IQ-like motif in CATSPERζ mediates the EFCAB9 interaction—and that this interface is functionally conserved across therians—defined the molecular determinant of binding and explained evolutionary constraint on both proteins.

   Evidence Recombinant protein interaction assays and site-directed mutagenesis of the IQ-like motif; cross-species (opossum/mouse) binding test

   PMID:33946695

   Open questions at the time

   • Binding assay performed with recombinant proteins only; not validated in sperm from IQ-motif-mutant animals
   • Quantitative affinity measurements under varying pH were not reported
4. 2022 High

   Discovery that C2CD6 interacts with EFCAB9 within the CatSper complex and is required for nanodomain incorporation into the flagellar membrane expanded the functional network around EFCAB9 beyond CATSPERζ.

   Evidence Co-immunoprecipitation, C2CD6-knockout mouse, patch-clamp electrophysiology, immunofluorescence

   PMID:34919125

   Open questions at the time

   • Whether C2CD6 contacts EFCAB9 directly or through CATSPERζ was not distinguished
   • How C2CD6 Ca²⁺-dependent C2 domain cooperates with EFCAB9 EF-hands in signal integration is unknown
5. 2022 Medium

   Linking EFCAB9-dependent CatSper regulation to sperm path chirality revealed a previously unrecognized biomechanical output of Ca²⁺ signaling: CatSper activity breaks flagellar symmetry to produce clockwise swimming.

   Evidence EFCAB9-knockout and CatSper-knockout sperm motility tracking, pharmacological CatSper inhibition, zona pellucida stimulation assays

   PMID:35438819

   Open questions at the time

   • Findings from a single laboratory; independent replication in other species is absent
   • Downstream effectors linking CatSper Ca²⁺ influx to asymmetric dynein activity were not identified
6. 2025 High

   Identification of ARMH2 as a third member of the cytosolic EFCAB9–CATSPERζ subcomplex explained how linear CatSper nanodomain organization is stabilized and provided an additional regulatory node for pH/Ca²⁺ sensitivity.

   Evidence Cryo-EM, mass spectrometry, AlphaFold structural prediction, coevolutionary analysis, ARMH2-knockout mouse

   PMID:41271765

   Open questions at the time

   • Stoichiometry of ARMH2 within the ternary subcomplex is not definitively established
   • Whether ARMH2 directly senses pH or relays the EFCAB9 conformational change is unresolved

• The atomic-level conformational cycle of EFCAB9 upon sequential Ca²⁺ binding and pH-induced dissociation from CATSPERζ has not been captured, leaving the gating mechanism incompletely understood at the structural level.
• No structure of EFCAB9 in Ca²⁺-free or high-pH states
• No reconstituted electrophysiology of CatSper with mutant EFCAB9 EF-hands to assign individual Ca²⁺-binding sites to gating
• Human relevance: no human loss-of-function variants or male infertility cases linked to EFCAB9 mutations have been reported