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Hsp60-independent protein folding in the matrix of yeast mitochondria
S Rospert1, R Looser, Y Dubaquie
1Department of Biochemistry, Biozentrum der Universitat Basel, Switzerland.
Abstract:
Proteins that are imported from the cytosol into mitochondria cross the mitochondrial membranes in an unfolded conformation and then fold in the matrix. Some of these proteins require the chaperonin hsp60 for folding. To test whether hsp60 is required for the folding of all imported matrix proteins, we monitored the folding of four monomeric proteins after import into mitochondria from wild-type yeast or from a mutant strain in which hsp60 had been inactivated. The four precursors included two authentic matrix proteins (rhodanese and the mitochondrial cyclophilin Cpr3p) and two artificial precursors (matrix-targeted variants of dihydrofolate reductase and barnase). Only rhodanese formed a tight complex with hsp60 and required hsp60 for folding. The three other proteins folded efficiently without, and showed no detectable binding to, hsp60. Thus, the mitochondrial chaperonin system is not essential for the folding of all matrix proteins. These data agree well with earlier in vitro studies, which had demonstrated that only a subset of proteins require chaperones for efficient folding.
Insights
Mitochondrial chaperonin hsp60 is not essential for all protein folding within the mitochondrial matrix. Only some imported proteins, like rhodanese, require hsp60 for proper folding.
Area of Science:
- Mitochondrial biology
- Protein folding
- Molecular chaperones
Background:
- Proteins imported into the mitochondrial matrix must unfold for translocation.
- Protein folding within the matrix is crucial for function.
- The chaperonin hsp60 is known to assist in folding for some matrix proteins.
Purpose of the Study:
- To determine if the chaperonin hsp60 is universally required for the folding of all proteins imported into the mitochondrial matrix.
- To investigate the specific requirement of hsp60 for different types of matrix proteins.
Main Methods:
- Import and monitor folding of four distinct monomeric proteins into mitochondria from wild-type and hsp60-inactivated yeast strains.
- Proteins studied included authentic matrix proteins (rhodanese, Cpr3p) and artificial precursors (dihydrofolate reductase, barnase).
- Assessed protein binding to hsp60 and folding efficiency.
Main Results:
- Only rhodanese formed a complex with hsp60 and required it for folding.
- The other three proteins (Cpr3p, dihydrofolate reductase, barnase) folded efficiently without hsp60.
- No detectable binding to hsp60 was observed for these three proteins.
Conclusions:
- The mitochondrial chaperonin system, specifically hsp60, is not essential for the folding of all imported matrix proteins.
- These findings support in vitro studies indicating that only a subset of proteins necessitate chaperone assistance for folding.
- Protein folding requirements within the mitochondrial matrix are diverse.