HEBP2

HEBP2 (SOUL) is a heme-binding protein that couples heme sensing to cell-death and microtubule regulatory functions, and has emerged as a driver of chemoresistance and immune evasion in cancer (PMID:15518569, PMID:39345749, PMID:40992373). Biochemically, SOUL purifies as a dimer in the absence of heme and assembles into a hexamer upon heme binding, coordinating one Fe(III)-heme per monomer through His42 as the axial ligand; its nanomolar heme affinity distinguishes it from its paralog p22HBP (PMID:15518569). Structurally, the SOUL monomer is a single-domain distorted β-barrel flanked by α-helices, and it carries a BH3 domain (residues 147–172) that engages the anti-apoptotic protein Bcl-xL with an accompanying conformational change (PMID:21639858). Functionally, SOUL sensitizes cells to necrotic death by provoking cyclosporine A-inhibitable mitochondrial permeability transition under sub-threshold calcium, acting via necrosis rather than apoptosis (PMID:17098234). SOUL also binds ALG-2 in a Ca²⁺-dependent, 1:1, high-affinity manner that requires SOUL Phe100 and ALG-2 Trp57; through this axis it controls cytoplasmic ALG-2 distribution, microtubule dynamics, and mitotic spindle orientation in cancer cells (PMID:28004381, PMID:30501057). In triple-negative breast cancer, HEBP2 mediates resistance to DNA crosslinking agents (cisplatin and BCNU) as shown by overexpression, CRISPR knockout, and rescue (PMID:39345749), and disrupts cytoplasmic FOXA1 phase separation to promote FOXA1 nuclear translocation and GSTP1 upregulation, driving glutamine competition that triggers macrophage ferroptosis and impairs antitumor immunity (PMID:40992373).

1. 1999 Medium

   Established the existence of a distinct heme-binding protein family by identifying SOUL as a tissue-restricted gene related to p22HBP, framing the question of whether SOUL is a genuine heme binder.

   Evidence Suppression subtractive hybridization and sequence analysis in chicken retina and pineal gland

   PMID:10640688

   Open questions at the time

   • No direct biochemical demonstration of heme binding in this work
   • Function of the protein unknown
   • Restricted to identification by homology
2. 2004 High

   Resolved how SOUL binds heme and how it differs from its paralog, defining His42 as the axial ligand and demonstrating heme-induced oligomerization.

   Evidence Recombinant mouse SOUL purification with optical/resonance Raman spectroscopy, His42Ala mutagenesis, and binding kinetics

   PMID:15518569

   Open questions at the time

   • Physiological consequence of dimer-to-hexamer transition not established
   • Cellular heme source/sensing role undefined
3. 2006 High

   Provided the structural framework for the family via the p22HBP fold and tetrapyrrole-binding cleft, distinguishing it from SOUL's His42-coordinated mode.

   Evidence NMR structure of murine p22HBP with chemical shift mapping and fluorescence Kd determination

   PMID:16905545

   Open questions at the time

   • Structure is of the paralog, not SOUL itself
   • No axial coordination, unlike SOUL
4. 2006 High

   Connected SOUL to cell-death regulation by showing it promotes necrosis through calcium-dependent mitochondrial permeability transition rather than apoptosis.

   Evidence Cell viability assays in NIH3T3, in vitro mPT reconstitution with recombinant SOUL, and cyclosporine A inhibition

   PMID:17098234

   Open questions at the time

   • Molecular target of SOUL at the mitochondrion not identified
   • Link between heme binding and mPT induction unresolved
5. 2009 Medium

   Advanced human SOUL toward structural characterization, confirming its fold relates to p22HBP.

   Evidence Recombinant human SOUL crystallization and molecular replacement at 3.5 Å

   PMID:19574650

   Open questions at the time

   • Preliminary low-resolution structure
   • No functional readout
6. 2011 High

   Identified a BH3 domain in SOUL and its physical interaction with Bcl-xL, placing SOUL within the apoptotic machinery via direct structural evidence.

   Evidence NMR, surface plasmon resonance, and crystal structures of SOUL alone and the BH3 peptide–Bcl-xL complex

   PMID:21639858

   Open questions at the time

   • Cellular consequence of Bcl-xL engagement not demonstrated
   • Reconciliation with necrosis-promoting role unclear
7. 2017 Medium

   Linked HEBP2 to cytoskeletal control by establishing the ALG-2/HEBP2 axis as a regulator of spindle orientation and microtubule dynamics.

   Evidence Co-expression, localization analysis, and spindle/microtubule dynamics assays in cancer cells

   PMID:28004381

   Open questions at the time

   • Biochemical detail of interaction limited in this study
   • Mechanism linking ALG-2 binding to microtubule effects unresolved
8. 2018 High

   Defined the thermodynamics and interface of the HEBP2–ALG-2 interaction, pinpointing Phe100 and Trp57 and revealing a second binding mode.

   Evidence Pulldown, isothermal titration calorimetry, and site-directed mutagenesis of SOUL and ALG-2

   PMID:30501057

   Open questions at the time

   • Functional role of the endothermic second binding mode unknown
   • How Ca²⁺-dependent binding controls downstream microtubule effects not shown
9. 2024 Medium

   Established HEBP2 as a causal mediator of resistance to DNA crosslinking chemotherapy with agent specificity.

   Evidence Overexpression, CRISPR/Cas9 knockout, and rescue with cisplatin, BCNU, MMS, and paclitaxel in MDA-MB-231 cells

   PMID:39345749

   Open questions at the time

   • Molecular mechanism of crosslink resistance not defined
   • Connection to heme binding or known interactors unexplored
10. 2025 Medium

    Placed HEBP2 in a tumor-immune metabolic pathway, showing it disrupts FOXA1 phase separation to upregulate GSTP1 and drive glutamine competition that causes macrophage ferroptosis.

    Evidence Single-cell RNA-seq, spatial transcriptomics, and in vitro/in vivo functional studies including GSTP1 inhibitor treatment in TNBC

    PMID:40992373

    Open questions at the time

    • Direct biochemical mechanism by which HEBP2 disrupts FOXA1 condensates unresolved
    • Relationship to heme binding and other HEBP2 functions not connected

• How heme binding mechanistically integrates with HEBP2's diverse roles in necrosis, Bcl-xL engagement, microtubule regulation, chemoresistance, and FOXA1/GSTP1 signaling remains unresolved.
• No unifying model linking the heme sensor function to downstream phenotypes
• Whether the BH3/Bcl-xL, ALG-2, and FOXA1 activities operate in the same cells or contexts is unknown
• Physiological (non-cancer) function in retina/pineal tissue uncharacterized in the corpus