GRPEL2

GRPEL2 is a mitochondrial matrix nucleotide exchange factor (NEF) that partners with GRPEL1 and mtHSP70 to regulate mitochondrial protein import and Fe-S cluster biogenesis. GRPEL2 binds mtHSP70 with lower affinity than GRPEL1 and does not stimulate mtHSP70 ATPase activity under basal conditions; instead, it undergoes Cys87-mediated disulfide dimerization under oxidative stress, functioning as a redox-regulated co-chaperone that maintains mitochondrial proteostasis during stress (PMID:9694873, PMID:28848044, PMID:30098457, PMID:39445986). GRPEL2 facilitates import of specific substrates such as DLST and modulates mitochondrial calcium homeostasis by suppressing MCU expression, protecting cardiomyocytes against ischemia-reperfusion injury and diabetic cardiomyopathy (PMID:36927450, PMID:35447394). Loss of GRPEL2 in cancer cells activates MAPK/JNK-dependent apoptosis and disrupts mitochondrial function, and GRPEL2 interacts with TIGAR to support mitochondrial regulation in colorectal cancer (PMID:40269881, PMID:40499524).

1. 1998 High

   The fundamental question of whether GRPEL2 functions as a bona fide nucleotide exchange factor was answered by demonstrating that it binds DnaK in an ATP-dissociable manner and stimulates mtHSP70 ATPase activity, establishing it as a mitochondrial co-chaperone.

   Evidence Co-immunoprecipitation with DnaK and ATPase stimulation assay using recombinant proteins

   PMID:9694873

   Open questions at the time

   • No measurement of GRPEL2-specific affinity for mammalian mtHSP70 versus GRPEL1
   • Functional significance in intact mitochondria not tested
   • No structural information on GRPEL2-mtHSP70 interaction
2. 2017 High

   The question of whether GRPEL1 and GRPEL2 act independently or cooperatively was resolved: they form a hetero-oligomeric subcomplex with mtHSP70 that confers mutual stability and is required for both preprotein import and Fe-S cluster biogenesis, with GRPEL2 specifically evolved for stress resistance.

   Evidence Reciprocal co-immunoprecipitation, knockdown/overexpression with mitochondrial import and Fe-S cluster readouts in human cells

   PMID:28848044

   Open questions at the time

   • Stoichiometry of the GRPEL1-GRPEL2-mtHSP70 complex not determined
   • Mechanism by which GRPEL2 confers stress resistance not established
3. 2018 High

   The mechanism of GRPEL2's stress-specific activation was identified: oxidative stress triggers Cys87-mediated intermolecular disulfide dimerization, acting as a thiol switch that activates GRPEL2 to protect mitochondrial proteostasis.

   Evidence BioID proximity labeling, disulfide crosslinking, Cys87 mutagenesis, and GRPEL2-knockout human cells with import assay

   PMID:30098457

   Open questions at the time

   • Whether the dimer form enhances or alters mtHSP70 binding was not directly measured
   • In vivo relevance of the redox switch not tested in animal models
4. 2022 Medium

   The cardiac-protective role of GRPEL2 was established: GRPEL2 loss increases MCU expression and mitochondrial calcium overload during ischemia-reperfusion, and pharmacological MCU inhibition rescues the phenotype, placing GRPEL2 upstream of mitochondrial calcium homeostasis.

   Evidence Adenoviral cardiac-specific knockdown in mice, ischemia-reperfusion model, Ru360 rescue

   PMID:35447394

   Open questions at the time

   • No direct physical interaction between GRPEL2 and MCU demonstrated
   • Mechanism by which GRPEL2 suppresses MCU expression unknown
   • Single-lab finding
5. 2023 Medium

   GRPEL2 was shown to interact with DLST and facilitate its mitochondrial import under high-glucose conditions, linking GRPEL2 to substrate-specific import in diabetic cardiomyopathy, with transcription regulated by Nr2f6.

   Evidence Co-immunoprecipitation of GRPEL2-DLST, AAV9-mediated cardiac overexpression, ChIP/luciferase for Nr2f6 promoter binding

   PMID:36927450

   Open questions at the time

   • Whether GRPEL2-DLST interaction is direct or mediated through mtHSP70 not clarified
   • Generalizability to other mitochondrial import substrates not tested
6. 2024 High

   A longstanding ambiguity about the relative biochemical roles of GRPEL1 versus GRPEL2 was resolved: GRPEL1 binds ADP-bound mtHSP70 with substantially higher affinity and stimulates its ATPase, whereas GRPEL2 does not, and Cys87 reduces GRPEL2's affinity for mtHSP70 rather than promoting dimerization.

   Evidence Quantitative binding affinity measurements, Pi release ATPase assay, AlphaFold structural modeling, Cys87 site-directed analysis

   PMID:39445986

   Open questions at the time

   • How GRPEL2 contributes to proteostasis if it does not efficiently stimulate mtHSP70 under basal conditions remains unclear
   • No crystal structure of GRPEL2-mtHSP70 complex
7. 2025 Medium

   GRPEL2 was placed upstream of TIGAR and MAPK/JNK signaling in cancer contexts: GRPEL2 interacts with TIGAR to support mitochondrial regulation in CRC, and GRPEL2 loss activates JNK-dependent apoptosis in ESCC, expanding its functional roles beyond import.

   Evidence LC-MS/MS and Co-IP for TIGAR interaction; RNA-Seq, JNK inhibitor rescue, and proliferation assays in cancer cell lines

   PMID:40269881 PMID:40499524

   Open questions at the time

   • TIGAR interaction awaits validation by reciprocal pulldown and endogenous conditions
   • JNK activation mechanism upon GRPEL2 loss not defined at molecular level
   • Cancer-specific findings not validated in non-transformed cells

• How GRPEL2's redox-regulated dimerization mechanistically alters mtHSP70 function in vivo, the structural basis of GRPEL2's substrate selectivity, and whether its roles in calcium homeostasis and cancer signaling are direct or secondary to impaired import remain unresolved.
• No high-resolution structure of GRPEL2 alone or in complex with mtHSP70
• Physiological import substrates preferentially dependent on GRPEL2 versus GRPEL1 not systematically identified
• Relationship between redox switch activation and downstream signaling (MCU, JNK, TIGAR) not established