GRPEL1

GRPEL1 is a mitochondrial matrix nucleotide exchange factor (NEF) that drives the ADP-for-ATP exchange cycle of mitochondrial Hsp70 (mtHsp70), thereby sustaining mtHsp70-dependent preprotein import and Fe-S cluster biogenesis (PMID:28848044, PMID:9694873). It binds Hsp70-family chaperones in an ATP-sensitive manner—co-purifying with DnaK and being released by Mg-ATP—and stimulates mtHsp70 ATPase activity, the biochemical hallmark of GrpE-type NEF function (PMID:11311562, PMID:9694873). GRPEL1 acts together with its paralog GRPEL2 as a hetero-oligomeric subcomplex whose assembly is required for NEF stability, with GRPEL1 serving as the constitutive stress modulator while GRPEL2 preferentially preserves chaperone activity under stress (PMID:28848044). In solution the free protein is an elongated symmetric dimer resembling DnaK-bound E. coli GrpE in shape (PMID:12840016). By maintaining the GRPEL1-mtHsp70 complex, GRPEL1 supports mitochondrial protein quality control and homeostasis, limiting protein aggresome formation and preserving membrane potential and ATP content (PMID:35093469). GRPEL1 is a direct substrate of dengue virus NS3 protease, which is imported into the mitochondrial matrix and cleaves GRPEL1 at defined sites, depleting it during infection (PMID:32581108).

1. 1998 Medium

   Established that mammalian mitochondria encode GrpE-like proteins that function as nucleotide exchange factors, answering whether the bacterial DnaK/GrpE chaperone logic extends to the mitochondrial Hsp70 system.

   Evidence DnaK-affinity binding with ATP-sensitive release and mt-Hsp70 ATPase stimulation assay, with cDNA cloning of two isoforms

   PMID:9694873

   Open questions at the time

   • Did not resolve the distinct roles of the two isoforms
   • Activity shown against bacterial DnaK and mt-Hsp70 ATPase, not full import reconstitution
2. 2001 Medium

   Confirmed mitochondrial localization and conserved ATP-sensitive Hsp70 binding, and unexpectedly linked the protein to cytosolic Hsc70/DnaJ chaperone regulation.

   Evidence GST pulldown/co-purification with DnaK plus ATP elution, subcellular fractionation, immunocytochemistry, and ATPase assays with HSJ1b/Hsc70

   PMID:11311562

   Open questions at the time

   • Cytosolic Hsc70/HSJ1b interactions not shown to be physiologically relevant in vivo
   • Single lab, single study
3. 2003 High

   Defined the quaternary structure of the free protein, showing it adopts an elongated symmetric dimer distinct from the asymmetric DnaK-bound bacterial GrpE conformation.

   Evidence Analytical ultracentrifugation, small-angle X-ray scattering, circular dichroism, differential scanning calorimetry

   PMID:12840016

   Open questions at the time

   • No high-resolution structure of the GRPEL1-mtHsp70 complex
   • Conformational change upon chaperone binding not directly visualized
4. 2006 Medium

   Compared the two human mitochondrial isoforms structurally, finding similar dimeric architecture but divergent stability profiles, hinting at differential stress responsiveness.

   Evidence Small-angle X-ray scattering with chemical and thermal denaturation profiling

   PMID:16579957

   Open questions at the time

   • Functional consequence of differing denaturation profiles not tested
   • No cellular validation of stress-dependent behavior
5. 2017 High

   Resolved how the two paralogs cooperate, showing GRPEL1 and GRPEL2 form a hetero-oligomeric subcomplex required for NEF stability and linking them functionally to preprotein import and Fe-S cluster biogenesis.

   Evidence Reciprocal Co-immunoprecipitation, protein import assays, Fe-S cluster biogenesis assays, and loss-of-function knockdown in human cells

   PMID:28848044

   Open questions at the time

   • Stoichiometry of the hetero-oligomer not defined
   • Mechanism distinguishing GRPEL1 stress-modulator from GRPEL2 stress-maintenance roles not detailed
6. 2020 High

   Identified GRPEL1 as a direct viral protease target, establishing a mechanism by which dengue virus disrupts mitochondrial chaperone function.

   Evidence In vitro cleavage with purified NS3 and GRPEL1, site-mapping mutagenesis, protein import assay, and Western blot in transfected cells, infected cells, and clinical samples

   PMID:32581108

   Open questions at the time

   • Downstream physiological consequences of GRPEL1 depletion during infection not fully quantified
   • Whether GRPEL2 compensates after GRPEL1 cleavage untested
7. 2022 Medium

   Connected GRPEL1-mtHsp70 complex integrity to neuronal mitochondrial homeostasis in a disease-relevant injury context.

   Evidence Co-immunoprecipitation, GRPEL1 overexpression in primary neurons, aggresome staining, JC-1 membrane potential assay, and an in vivo subarachnoid hemorrhage model

   PMID:35093469

   Open questions at the time

   • Causality between complex restoration and neuroprotection inferred from overexpression only
   • Single lab, single injury model

• How GRPEL1's conformational cycle is mechanistically coupled to mtHsp70 nucleotide exchange and how the GRPEL1/GRPEL2 ratio is regulated to tune the chaperone cycle under stress remain unresolved.
• No structure of the GRPEL1-mtHsp70 complex
• Regulation of GRPEL1 vs GRPEL2 abundance/activity under stress undefined
• No direct demonstration of ADP-for-ATP exchange kinetics on human mtHsp70