GRPEL2

GRPEL2 is a mitochondrial nucleotide exchange factor (NEF) for mitochondrial Hsp70 (mtHSP70/mt-Hsp70) that participates in chaperone-dependent mitochondrial proteostasis (PMID:9694873, PMID:28848044). It binds mt-Hsp70 (and bacterial DnaK) in a salt-stable, ATP-dissociable manner consistent with NEF activity, and stimulates Hsp70 ATPase activity (PMID:9694873). In human cells GRPEL2 forms a hetero-oligomeric subcomplex with GRPEL1 and mtHSP70 that governs mtHSP70 nucleotide exchange, supports preprotein import, and is required for Fe-S cluster biogenesis (PMID:28848044). Functional and structural analyses establish a division of labor between the two NEFs: ADP-bound mtHSP70 binds GRPEL1 with markedly higher affinity than GRPEL2, and GRPEL1 — but not GRPEL2 — opens the mtHSP70 nucleotide-binding cleft and enhances its ATPase activity, indicating GRPEL2 has lower intrinsic exchange-promoting activity (PMID:39445986). GRPEL2 is distinctively redox-regulated through Cys87, which forms intermolecular disulfide-linked dimers under oxidative stress and lowers GRPEL2 affinity for mtHSP70, consistent with a specialized stress-response role rather than constitutive import; GRPEL2 is dispensable for baseline import in cultured cells (PMID:30098457, PMID:39445986). In disease contexts GRPEL2 mediates mitochondrial protection through several downstream effectors: it promotes import of DLST to preserve mitochondrial function in diabetic cardiomyopathy (PMID:36927450), acts upstream of MCU to limit mitochondrial calcium overload and cell death during cardiac ischemia/reperfusion (PMID:35447394), and supports cancer-cell survival via TIGAR and suppression of MAPK/JNK signaling (PMID:40269881, PMID:40499524). The GRPEL2 locus additionally encodes the distinct primate-specific small protein NERCLIN, which regulates cardiolipin homeostasis and is functionally separate from GRPEL2 (PMID:37463214).

1. 1998 High

   Established GRPEL2's core biochemical identity by showing it binds mtHsp70 and acts as a nucleotide exchange factor, answering what this mitochondrial protein does molecularly.

   Evidence Co-IP/pulldown with E. coli DnaK and mammalian mt-Hsp70, ATPase assay, and subcellular fractionation

   PMID:9694873

   Open questions at the time

   • Did not resolve a distinct functional role relative to other GrpE homologs
   • No structural basis for nucleotide exchange defined
2. 2017 High

   Defined GRPEL2's place in the human mtHsp70 machinery as part of a GRPEL1/GRPEL2 hetero-oligomeric subcomplex linked to import and Fe-S biogenesis, answering how the two NEFs are organized.

   Evidence Reciprocal Co-IP, knockdown with protein import and Fe-S cluster assays, stress assays in human cells

   PMID:28848044

   Open questions at the time

   • Quantitative stoichiometry of the GRPEL1/GRPEL2/mtHsp70 complex unresolved
   • Did not separate GRPEL2-specific from GRPEL1-specific contributions
3. 2018 High

   Identified Cys87 as a redox thiol switch driving oxidative-stress dimerization, distinguishing GRPEL2 as a stress-responsive NEF rather than a constitutive import factor.

   Evidence BioID proximity labeling, redox gel-shift, Cys87 mutagenesis, siRNA with import assays

   PMID:30098457

   Open questions at the time

   • Physiological oxidant trigger in vivo not established
   • Functional consequence of disulfide dimers for chaperone cycling not directly measured
4. 2024 High

   Resolved the functional asymmetry between the two NEFs, showing GRPEL1 drives ADP release and ATPase stimulation while GRPEL2 binds mtHSP70 more weakly and lacks cleft-opening activity, explaining their non-redundant roles.

   Evidence ADP-bound vs apo affinity assays, Pi-based ATPase assay, AlphaFold modeling, Cys87 mutagenesis

   PMID:39445986

   Open questions at the time

   • No experimental high-resolution structure of GRPEL2-mtHSP70
   • Conditions under which GRPEL2 contributes exchange activity not defined
5. 2023 Medium

   Connected GRPEL2 to disease-relevant mitochondrial protection by identifying DLST as an import client and downstream effector in diabetic cardiomyopathy, and Nr2f6 as a transcriptional activator.

   Evidence Co-IP, AAV9 cardiac overexpression, import assays, siDLST epistasis, ChIP for Nr2f6

   PMID:36927450

   Open questions at the time

   • Single lab study
   • Whether DLST import depends on canonical mtHsp70 NEF activity not shown
6. 2022 Medium

   Placed GRPEL2 upstream of MCU-mediated calcium overload in cardiac ischemia/reperfusion, linking its loss to mitochondrial fission and cardiomyocyte death.

   Evidence In vivo cardiac-specific adenoviral knockdown, mitochondrial calcium measurement, Ru360 pharmacological rescue

   PMID:35447394

   Open questions at the time

   • Single lab study
   • Molecular mechanism linking GRPEL2 to MCU expression unresolved
7. 2025 Medium

   Extended GRPEL2's pro-survival role to cancer, identifying TIGAR as a downstream effector in colorectal cancer (with E2F8 driving GRPEL2 transcription) and JNK suppression in esophageal squamous cell carcinoma.

   Evidence LC-MS/MS interactome, Co-IP, TIGAR/JNK epistasis (overexpression and SP600125 rescue), ChIP/luciferase for E2F8

   PMID:40269881 PMID:40499524

   Open questions at the time

   • Single lab studies in distinct cancer types
   • Whether these effects depend on GRPEL2's NEF/chaperone function not established

• How GRPEL2's redox-regulated, lower-affinity NEF activity is mechanistically coupled to its protective roles across DLST, MCU, TIGAR, and JNK in different tissues remains unresolved.
• No unifying mechanism linking chaperone NEF activity to disease-context effectors
• No structural data defining GRPEL2-specific binding modes
• In vivo oxidant trigger and stress-condition relevance of Cys87 switch undefined