CYTOPLASMIC CHAPERONES DETERMINE THE TARGETING PATHWAY OF PRECURSOR PROTEINS TO MITOCHONDRIA

Two ATP-dependent cytosolic chaperones, mitochondrial import stimulati on factor (MSF) and hsp70, are known to be involved in the import of p recursor proteins into mitochondria. Hsp70 generally recognizes unfold ed proteins, while MSF specifically recognizes mitochondrial precursor proteins and targets them to mitochondria in a NEM-sensitive manner. Here we analyzed the relative contribution of these chaperones in the import process and confirmed that the precursor proteins are targeted to mitochondria via two distinct pathways: one requiring MSF and the o ther requiring hsp70. Both pathways depend on distinct proteinaceous c omponents of the outer mitochondrial membrane. The MSF-dependent pathw ay is NEM-sensitive and requires the hydrolysis of extra-mitochondrial ATP for the release of MSF from the mitochondrial import receptor, wh ereas the hsp70-dependent pathway is NEM-insensitive and does not requ ire extra-mitochondrial ATP. The NEM-insensitive, hsp70-dependent impo rt became NEM-sensitive depending on the amount of MSF added, The rela tive importance of the two pathways appears to be determined by the af finities of MSF and hsp70 for the precursor proteins.