Recognition of mitochondria-targeting signals by a cytosolic import stimulation factor, MSF.

MSF, a mitochondrial import stimulation factor purified from rat liver cytosol, is an ATP-dependent precursor protein conformational modulator. As a step toward understanding the specificity of substrate recognition by MSF, various synthetic peptides were examined for their ability to induce MSF ATPase activity. The peptides corresponding to various mitochondria-targeting signal sequences elicited significant ATPase activity. MSF bound the synthetic mitochondrial signal peptides, and ATP hydrolysis caused dissociation of the peptides from MSF. Basic amino acid residues in the signal peptides seemed to be essential for recognition. Thus, MSF is a member of the polypeptide chain-binding protein family with unique recognition specificity and is distinct from the hsp70 family of proteins.