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Updated: Jul 14, 2026
09:01
Measurement of Protein Import Capacity of Skeletal Muscle Mitochondria
Published on: January 7, 2022
Multiple 40-kDa heat-shock protein chaperones function in Tom70-dependent mitochondrial import
Melanie K Bhangoo1, Stefan Tzankov, Anna C Y Fan
1Department of Biochemistry, McGill University, Montreal, QC, H3G 1Y6, Canada.
Molecular Biology of the Cell
|June 29, 2007
Summary
Cytosolic chaperones, including Hsp70 and Hsp90, assist mitochondrial preprotein import. Specific J-domain proteins (DJAs) and cochaperones show specialized roles in protein folding and mitochondrial targeting.
Area of Science:
- Mitochondrial biology
- Protein import and folding
- Molecular chaperones
Background:
- Mitochondrial preproteins require cytosolic chaperones for import via the TOM70 receptor.
- The adenine nucleotide transporter (ANT) utilizes this import pathway.
Purpose of the Study:
- To identify cytosolic chaperones and cochaperones involved in ANT preprotein import.
- To investigate the specific roles of Hsp40-related J-domain proteins (DJAs) in this process.
Main Methods:
- Reconstitution of purified ANT with chaperones in reticulocyte lysate.
- Mass spectrometry to identify bound proteins.
- Analysis of dominant-negative DJA mutants in vitro and in HeLa cells.
Main Results:
- Identified Hsc70, Hsp90, and a subset of cochaperones, including three DJAs (DJA1, DJA2, DJA4).
- DJAs bind preproteins, and their J-domains are crucial for mitochondrial import and Hsc70 binding.
- DJAs exhibit differential activation of Hsc70 ATPase and protein refolding, with specialized functions in cells.
Conclusions:
- Multiple cochaperones, including DJAs and Hsp90 cochaperones (p23, Aha1), cooperate in forming optimal chaperone-preprotein complexes.
- These cochaperones possess partially specialized properties, contributing to efficient mitochondrial protein import and folding.
Related Concept Videos
01:39Mitochondrial Precursor Proteins
Mitochondrial precursors are partially unfolded or loosely folded polypeptide chains. Newly synthesized precursors are inhibited from spontaneously folding into their native conformation by the cytosolic chaperones, heat shock proteins 70 (Hsp70), and mitochondrial import stimulation factors (MSFs). Precursors bound to MSFs are guided to the TOM70-TOM37 receptors, while precursors bound to Hsp70 chaperones are targetted to TOM20-TOM22 receptor complexes.
Most of the mitochondrial precursors...
Most of the mitochondrial precursors...
01:19Translocation of Proteins into the Mitochondria
Mitochondrial precursors are translocated to the internal subcompartments via independent mechanisms involving distinct protein machineries called translocases.
Sorting of outer membrane proteins:
Mitochondrial outer membrane proteins are of two types: the transmembrane, beta-barrel porins, and the membrane-anchored, alpha-helical proteins. Beta-barrel porin precursors are translocated by the TOM complex and inserted into the outer mitochondrial membrane by the SAM complex. In contrast,...
Sorting of outer membrane proteins:
Mitochondrial outer membrane proteins are of two types: the transmembrane, beta-barrel porins, and the membrane-anchored, alpha-helical proteins. Beta-barrel porin precursors are translocated by the TOM complex and inserted into the outer mitochondrial membrane by the SAM complex. In contrast,...
01:39Mitochondrial Protein Sorting
Mitochondria are double-membrane organelles of the eukaryotes involved in cellular metabolism, signaling, ATP synthesis, and programmed cell death. Each of these processes requires specific proteins and enzymes that must be correctly sorted to the right mitochondrial subcompartment for the proper functioning of the organelle.
Most of these mitochondrial proteins are encoded by the nucleus and imported to the mitochondria as unfolded or loosely folded precursors. Mitochondrial precursors...
Most of these mitochondrial proteins are encoded by the nucleus and imported to the mitochondria as unfolded or loosely folded precursors. Mitochondrial precursors...
01:37Energy to Drive Translocation
Mitochondrial protein import is powered by two distinct energy sources: ATP hydrolysis and electrochemical potential across the inner membrane. Newly synthesized precursors are bound by cytosolic chaperones of the Hsp70 family, which guide them to the import receptors on the mitochondrial surface. Utilizing the energy of ATP hydrolysis, Hsp70 chaperones transfer these precursors to the TOM receptors on the mitochondrial outer membrane.
Generally, polypeptides are unfolded by two distinct...
Generally, polypeptides are unfolded by two distinct...
01:34Protein Transport into the Inner Mitochondrial Membrane
Nuclear encoded mitochondrial precursors are imported to the inner membrane in a multistep process involving two separate translocons, TIM22 and TIM23. TIM23 is a cation-selective pore that remains closed by the N terminal segment of the protein. Negative charges on the TIM23 act as a receptor for the incoming precursor, pulling the positively charged matrix-targeting sequence for peptide insertion and translocation.
Transport of mitochondrial precursors across the TIM23 channel is driven by...
Transport of mitochondrial precursors across the TIM23 channel is driven by...
03:00Molecular Chaperones and Protein Folding
The native conformation of a protein is formed by interactions between the side chains of its constituent amino acids. When the amino acids cannot form these interactions, the protein cannot fold by itself and needs chaperones. Notably, chaperones do not relay any additional information required for the folding of polypeptides; the native conformation of a protein is determined solely by its amino acid sequence. Chaperones catalyze protein folding without being a part of the folded protein.
The...
The...