HSP60-INDEPENDENT PROTEIN-FOLDING IN THE MATRIX OF YEAST MITOCHONDRIA

Proteins that are imported from the cytosol into mitochondria cross th e mitochondrial membranes in an unfolded conformation and then fold in the matrix. Some of these proteins require the chaperonin hsp60 for f olding. To test whether hsp60 is required for the folding of all impor ted matrix proteins, we monitored the folding of four monomeric protei ns after import into mitochondria from wild-type yeast or from a mutan t strain in which hsp60 had been inactivated. The four precursors incl uded two authentic matrix proteins (rhodanese and the mitochondrial cy clophilin Cpr3p) and two artificial precursors (matrix-targeted varian ts of dihydrofolate reductase and barnase). Only rhodanese formed a ti ght 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 ea rlier in vitro studies, which had demonstrated that only a subset of p roteins require chaperones for efficient folding.