Regulation of mitochondrial protein import by the nucleotide exchange factors GrpEL1 and GrpEL2 in human cells

J Biol Chem. 2017 Nov 3;292(44):18075-18090. doi: 10.1074/jbc.M117.788463. Epub 2017 Aug 28.


Mitochondria are organelles indispensable for maintenance of cellular energy homeostasis. Most mitochondrial proteins are nuclearly encoded and are imported into the matrix compartment where they are properly folded. This process is facilitated by the mitochondrial heat shock protein 70 (mtHsp70), a chaperone contributing to mitochondrial protein quality control. The affinity of mtHsp70 for its protein clients and its chaperone function are regulated by binding of ATP/ADP to mtHsp70's nucleotide-binding domain. Nucleotide exchange factors (NEFs) play a crucial role in exchanging ADP for ATP at mtHsp70's nucleotide-binding domain, thereby modulating mtHsp70's chaperone activity. A single NEF, Mge1, regulates mtHsp70's chaperone activity in lower eukaryotes, but the mammalian orthologs are unknown. Here, we report that two putative NEF orthologs, GrpE-like 1 (GrpEL1) and GrpEL2, modulate mtHsp70's function in human cells. We found that both GrpEL1 and GrpEL2 associate with mtHsp70 as a hetero-oligomeric subcomplex and regulate mtHsp70 function. The formation of this subcomplex was critical for conferring stability to the NEFs, helped fine-tune mitochondrial protein quality control, and regulated crucial mtHsp70 functions, such as import of preproteins and biogenesis of Fe-S clusters. Our results also suggested that GrpEL2 has evolved as a possible stress resistance protein in higher vertebrates to maintain chaperone activity under stress conditions. In conclusion, our findings support the idea that GrpEL1 has a role as a stress modulator in mammalian cells and highlight that multiple NEFs are involved in controlling protein quality in mammalian mitochondria.

Keywords: 70 kilodalton heat shock protein (Hsp70); cell culture; mitochondria; mitochondrial Hsp70; mitochondrial transport; nucleotide exchange factor; protein import; protein translocation; yeast.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Biomarkers / metabolism
  • Genetic Complementation Test
  • Green Fluorescent Proteins / genetics
  • Green Fluorescent Proteins / metabolism
  • HEK293 Cells
  • HSP70 Heat-Shock Proteins / chemistry
  • HSP70 Heat-Shock Proteins / metabolism*
  • HeLa Cells
  • Humans
  • Immunoprecipitation
  • Intracellular Signaling Peptides and Proteins / antagonists & inhibitors
  • Intracellular Signaling Peptides and Proteins / chemistry
  • Intracellular Signaling Peptides and Proteins / genetics
  • Intracellular Signaling Peptides and Proteins / metabolism*
  • Ligands
  • Mitochondria / metabolism*
  • Mitochondrial Proteins / chemistry
  • Mitochondrial Proteins / metabolism*
  • Models, Molecular*
  • Molecular Chaperones / antagonists & inhibitors
  • Molecular Chaperones / chemistry
  • Molecular Chaperones / genetics
  • Molecular Chaperones / metabolism*
  • Oxidative Stress
  • Phylogeny
  • Protein Isoforms / metabolism
  • Protein Multimerization
  • Protein Stability
  • Protein Transport
  • RNA Interference
  • Recombinant Fusion Proteins / chemistry
  • Recombinant Fusion Proteins / metabolism


  • Biomarkers
  • GRPEL1 protein, human
  • GRPEL2 protein, human
  • HSP70 Heat-Shock Proteins
  • HSPA1A protein, human
  • Intracellular Signaling Peptides and Proteins
  • Ligands
  • Mitochondrial Proteins
  • Molecular Chaperones
  • Protein Isoforms
  • Recombinant Fusion Proteins
  • Green Fluorescent Proteins