EFCAB9

EFCAB9 is a cytoplasmic, bifunctional EF-hand Ca2+ sensor that serves as a pH and Ca2+ tuning element of the sperm CatSper calcium channel and is required for hyperactivated sperm motility and male fertility (PMID:31056283). It binds the CatSper subunit CATSPERζ directly through a conserved IQ-like motif in a Ca2+-dependent, pH-sensitive manner, dissociating at elevated pH, and this interaction is evolutionarily conserved across divergent species (PMID:31056283, PMID:33946695). Structurally, EFCAB9 assembles with CATSPERζ and ARMH2 into a cytosolic ternary subcomplex resolved by cryo-EM, and it further associates with the C2-domain subunit C2CD6, integrating it into the extended CatSper complex (PMID:34919125, PMID:41271765). Through these interactions EFCAB9 enables pH-dependent and Ca2+-sensitive CatSper channel activation and the stable linear organization of CatSper Ca2+ signaling nanodomains along the flagellum; its loss abolishes channel gating, disrupts nanodomain arrangement, and eliminates the clockwise swim-path chirality of sperm (PMID:31056283, PMID:35438819, PMID:41271765).

1. 2019 High

   Established EFCAB9 as a bifunctional Ca2+ sensor physically coupled to the CatSper channel, answering how channel activity could be jointly tuned by intracellular Ca2+ and pH.

   Evidence Knockout mice, Co-IP/direct binding assays, patch-clamp electrophysiology, and super-resolution imaging of CatSper nanodomains

   PMID:31056283

   Open questions at the time

   • Atomic-level structural basis of the Ca2+/pH switch not resolved
   • Stoichiometry within the channel complex undefined
2. 2019 High

   Demonstrated that EFCAB9 is functionally required, linking its loss to abolished pH/Ca2+-dependent channel gating, disrupted nanodomains, and subfertility.

   Evidence Knockout mouse sperm motility analysis, CatSper current electrophysiology, super-resolution imaging

   PMID:31056283

   Open questions at the time

   • Quantitative contribution of nanodomain disorganization versus channel gating to the motility defect not separated
3. 2021 Medium

   Mapped the EFCAB9 binding determinant to a conserved IQ-like motif in CATSPERζ and showed cross-species interaction, defining the molecular interface and its evolutionary conservation.

   Evidence Recombinant protein interaction assays, motif/sequence analysis, cross-species (opossum/mouse) binding experiments

   PMID:33946695

   Open questions at the time

   • Single lab; in vivo requirement of the IQ-like motif not tested by mutation
   • No structural model of the bound interface at this stage
4. 2022 Medium

   Placed EFCAB9 within an extended CatSper complex by identifying its interaction with the C2-domain subunit C2CD6 required for nanodomain integrity.

   Evidence Co-immunoprecipitation, immunofluorescence, and C2CD6 knockout showing nanodomain depletion

   PMID:34919125

   Open questions at the time

   • Direct versus indirect nature of EFCAB9-C2CD6 interaction not fully resolved
   • Single lab; reciprocal validation limited
5. 2022 Medium

   Connected EFCAB9/CatSper Ca2+ signaling to a specific motility output, clockwise swim-path chirality, expanding the physiological role beyond bulk motility.

   Evidence EFCAB9 knockout sperm tracking with pharmacological CatSper inhibition and ZP2 fragment stimulation

   PMID:35438819

   Open questions at the time

   • Mechanistic link between Ca2+ nanodomain geometry and chirality not defined
   • Single lab
6. 2025 High

   Resolved the cytosolic ternary subcomplex of EFCAB9, ARMH2, and CATSPERζ, providing the structural framework for how EFCAB9 anchors and tunes the channel.

   Evidence Cryo-EM structure, mass spectrometry, AlphaFold/coevolutionary analysis, and ARMH2 knockout mouse with disrupted nanodomains and subfertility

   PMID:41271765

   Open questions at the time

   • Conformational basis of the pH/Ca2+ switch within the resolved structure not fully described
   • Human relevance and fertility variants not addressed

• How EF-hand Ca2+ occupancy and protonation are mechanistically transduced into channel gating and nanodomain assembly remains unresolved.
• No dynamic structural model coupling Ca2+/pH state to gating
• Human physiological and clinical role uncharacterized in the corpus