EFHD1

EFHD1 (Swiprosin-2) is a mitochondrial EF-hand protein that integrates calcium and zinc signaling with mitochondrial function, energy metabolism, and organelle contact site dynamics. Its crystal structure reveals two EF-hand domains capable of coordinating both Ca²⁺ and Zn²⁺, conferring Ca²⁺/Zn²⁺-independent actin-binding and Ca²⁺/Zn²⁺-dependent actin-bundling activities (PMID:33537316, PMID:36862489). At the inner mitochondrial membrane, EFHD1 physically interacts with MCU to suppress mitochondrial Ca²⁺ uptake and with ANT3 to inhibit mPTP opening, thereby regulating mitoflash generation, ROS production, and cell survival under stress (PMID:36747492, PMID:38795203, PMID:35304170). Loss of EFHD1 shifts cellular metabolism from oxidative phosphorylation toward glycolysis, activates AMPK-Ulk1 signaling, impairs axonal morphogenesis, and confers resistance to ischemic cardiac injury, while its downstream effects in cancer cells operate through the Hippo/YAP and SIK3 pathways (PMID:28524857, PMID:32414840, PMID:39895792).

1. 2011 Low

   Initial computational and expression analyses established EFHD1 as a predicted two-EF-hand, coiled-coil domain protein associated with mitochondria, setting the stage for mechanistic studies of its role in apoptosis and differentiation.

   Evidence Structure prediction and expression profiling (review/computational)

   PMID:21244694

   Open questions at the time

   • No direct biochemical or structural validation of domain architecture
   • Functional roles inferred from expression patterns, not genetic perturbation
   • Mitochondrial localization not confirmed by suborganellar fractionation
2. 2017 High

   Genetic loss-of-function and transgenic gain-of-function experiments revealed that EFHD1 is required at the inner mitochondrial membrane for Ca²⁺-induced mitoflashes and controls the metabolic switch between oxidative phosphorylation and glycolysis during B cell development.

   Evidence shRNA knockdown, CRISPR KO, transgenic mouse overexpression, Seahorse metabolic flux analysis in pro/pre B cells

   PMID:28524857

   Open questions at the time

   • Direct molecular target(s) of EFHD1 at the inner membrane not identified
   • Mechanism by which EFHD1 promotes mitoflashes not defined
   • Whether metabolic regulation is Ca²⁺-binding dependent was not tested
3. 2020 High

   Efhd1 knockout in mice established EFHD1 as an upstream regulator of mitochondrial ATP supply in axons, linking its loss to AMPK-Ulk1 activation, increased autophagy, and impaired axonal growth and branching.

   Evidence Efhd1 KO mouse, ATP measurement, live mitochondrial imaging, AMPK/Ulk1 phosphorylation, autophagic flux assays in neurons

   PMID:32414840

   Open questions at the time

   • Whether EFHD1 directly maintains ATP production or acts indirectly through Ca²⁺ regulation was unclear
   • Mitochondrial shortening mechanism at growth cones not resolved
   • Phenotype not rescued to confirm specificity
4. 2021 High

   The crystal structure of the EFHD1 core domain was solved, revealing EF-hand fold similarity to EFhd2 and AIF-1, Zn²⁺-mediated crystal contacts suggesting multimerization, and demonstrating that EFHD1 binds β-actin independently of Ca²⁺ but bundles actin in a Ca²⁺-dependent manner.

   Evidence X-ray crystallography, in vitro actin cosedimentation and bundling assays

   PMID:33537316

   Open questions at the time

   • Physiological relevance of actin-bundling activity not tested in cells
   • Zn²⁺-mediated multimerization inferred from crystal contacts only
   • Which actin pools EFHD1 engages in mitochondria-associated contexts unknown
5. 2022 High

   Efhd1 KO in cardiomyocytes demonstrated that EFHD1 is required for basal ROS levels and mitoflash generation, and its absence protects against hypoxia-induced cardiac injury, identifying EFHD1 as a pathophysiologically relevant driver of ischemic cardiomyocyte death.

   Evidence Efhd1 KO mouse, ROS measurement, mitoflash live imaging, cardiac ischemia-reperfusion model

   PMID:35304170

   Open questions at the time

   • Molecular mechanism linking EFHD1 to mitoflash initiation still unknown
   • Whether protective effect of KO is solely ROS-mediated or involves other pathways not resolved
   • Heart-specific conditional KO not performed
6. 2023 High

   Identification of MCU as a direct binding partner of EFHD1 (via its N-terminal domain) resolved the molecular basis of EFHD1's mitochondrial Ca²⁺ regulatory function, showing that EFHD1 suppresses MCU-mediated Ca²⁺ uptake and downstream Hippo/YAP signaling in renal cancer cells.

   Evidence Reciprocal Co-IP with domain mapping, mitochondrial Ca²⁺ measurement, YAP phosphorylation, rescue with MCU overexpression and STARD13 knockdown, in vivo metastasis model in ccRCC

   PMID:36747492

   Open questions at the time

   • Whether MCU interaction is Ca²⁺-dependent was not tested
   • Stoichiometry and structural basis of the EFHD1–MCU complex unknown
   • Generalizability of the STARD13-YAP axis beyond ccRCC not established
7. 2023 High

   Zinc coordination within the EF-hands of EFHD1 was confirmed by anomalous diffraction crystallography, establishing that Zn²⁺ — in addition to Ca²⁺ — regulates EFHD1's actin-bundling activity, broadening the signaling inputs controlling this protein.

   Evidence X-ray crystallography with Zn K-edge anomalous diffraction, in vitro actin-binding and bundling assays

   PMID:36862489

   Open questions at the time

   • Whether physiological Zn²⁺ concentrations in mitochondria are sufficient to occupy EF-hands is unknown
   • No cellular assay of Zn²⁺-dependent actin bundling performed
   • Relative contributions of Ca²⁺ versus Zn²⁺ in vivo not dissected
8. 2024 High

   Discovery that EFHD1 binds ANT3 and inhibits its conformational change to suppress mPTP opening provided a second direct mitochondrial interactor and explained how EFHD1 promotes cell survival and chemoresistance in osteosarcoma.

   Evidence Co-IP of EFHD1–ANT3, mPTP opening assays, pharmacological rescue with CATR and BKA, drug sensitivity assays in osteosarcoma cells

   PMID:38795203

   Open questions at the time

   • Whether EFHD1–MCU and EFHD1–ANT3 interactions are mutually exclusive or simultaneous is unknown
   • Direct binding interface on ANT3 not mapped
   • Relevance to non-cancer contexts not assessed
9. 2025 Medium

   Epistasis experiments placed EFHD1 upstream of SIK3 in Hippo pathway activation, extending its tumor-suppressive signaling to colorectal cancer EMT and metastasis.

   Evidence EFHD1 overexpression/knockdown with SIK3 epistasis, western blot, orthotopic xenograft and pulmonary metastasis models in CRC

   PMID:39895792

   Open questions at the time

   • Mechanism by which EFHD1 upregulates SIK3 expression is unknown
   • Single-laboratory study not yet independently confirmed
   • Relationship between mitochondrial Ca²⁺ regulation and SIK3 induction not tested

• Key unresolved questions include how EFHD1's actin-bundling and Ca²⁺/Zn²⁺-sensing activities coordinate with its MCU and ANT3 interactions in vivo, whether EFHD1 forms a stable complex bridging ER and mitochondria through actin crosslinking under physiological Ca²⁺ dynamics, and how these molecular activities integrate to control tissue-specific metabolic and stress outcomes.
• No reconstituted system combining MCU binding, ANT3 binding, and actin bundling
• In vivo structure-function analysis with EF-hand mutants not performed
• Tissue-specific conditional knockouts beyond germline KO are lacking