EFHB

EFHB is a cytosolic EF-hand Ca2+ sensor that gates store-operated Ca2+ entry (SOCE) by controlling the assembly of the STIM1-Orai1 signaling complex following ER Ca2+ store depletion (PMID:30481768). Upon store depletion EFHB associates with STIM1 and then dissociates in a Ca2+-dependent manner, and this interaction is required for productive STIM1-Orai1 coupling, since RNAi silencing of EFHB impairs SOCE activation (PMID:30481768). EFHB acts on the regulatory architecture of STIM1 by enabling the dissociation of the negative regulator SARAF from STIM1, the step relevant for Orai1 channel activation and slow Ca2+-dependent inactivation (PMID:30481768). Through its control of SOCE, EFHB lies upstream of NFAT nuclear translocation and the associated transcriptional responses (PMID:30481768), and its expression is elevated in luminal and triple-negative breast cancer cells where it sustains SOCE and supports proliferation, migration, and viability (PMID:34439314). Beyond this STIM1/SARAF/SOCE axis, no further mechanistic detail has been characterized in the available corpus.

1. 2018 Medium

   Established EFHB as a previously uncharacterized cytosolic Ca2+ sensor that physically engages STIM1 in a store- and Ca2+-dependent fashion, defining a new node in SOCE regulation.

   Evidence Reciprocal co-immunoprecipitation with Western blotting and fura-2 Ca2+ imaging in cells subjected to ER store depletion

   PMID:30481768

   Open questions at the time

   • Direct Ca2+ binding by the EF-hand domains not demonstrated biochemically
   • Structural basis of the EFHB-STIM1 interaction unknown
   • Single lab, single cell system
2. 2018 Medium

   Showed EFHB is functionally required for STIM1-Orai1 coupling and SOCE activation, moving it from a binding partner to a necessary effector of the pathway.

   Evidence RNAi knockdown with fura-2 Ca2+ imaging and STIM1-Orai1 co-immunoprecipitation

   PMID:30481768

   Open questions at the time

   • Mechanism by which EFHB promotes STIM1-Orai1 coupling not resolved
   • No rescue with RNAi-resistant construct reported
3. 2018 Medium

   Identified the SARAF-STIM1 dissociation step as the regulatory event controlled by EFHB, linking it to Orai1 activation and slow Ca2+-dependent inactivation.

   Evidence RNAi knockdown with co-immunoprecipitation of the STIM1-SARAF complex

   PMID:30481768

   Open questions at the time

   • Whether EFHB acts directly on SARAF or indirectly via STIM1 conformation is unknown
   • No reconstitution of the EFHB-STIM1-SARAF system
4. 2018 Medium

   Connected EFHB-dependent SOCE to a downstream transcriptional output by showing both loss and gain of EFHB alter NFAT nuclear translocation.

   Evidence RNAi knockdown and overexpression with confocal imaging of NFAT translocation

   PMID:30481768

   Open questions at the time

   • Downstream NFAT target genes not defined
   • Physiological context of the NFAT response not established
5. 2021 Medium

   Placed EFHB-driven SOCE in a disease context by showing its upregulation in breast cancer cells and its requirement for proliferation, migration, and viability.

   Evidence RNAi knockdown with Ca2+ imaging plus proliferation and migration assays in luminal and triple-negative breast cancer cells

   PMID:34439314

   Open questions at the time

   • Causal contribution in vivo not tested
   • Whether the proliferative effect is solely SOCE-dependent is unresolved
   • Single lab

• How EFHB physically couples Ca2+ sensing to STIM1 conformational change and SARAF release at the molecular and structural level remains unresolved.
• No structure of EFHB or its complexes
• Direct EF-hand Ca2+ binding affinity unmeasured
• Tissue and organismal physiology of EFHB uncharacterized