Figure 4.

Cytoplasmic targeting factors for yeast mitochondrial precursor proteins. For most mitochondrial precursor proteins synthesized in the cytosol, the amino terminus has the potential to form a basic, amphipathic helix. NAC (Egd1p and Egd2p) protects the nascent chain from all cytosolic proteins until 30–50 amino acids have been translated. Subsequently, the targeting sequence becomes accessible to Mft52p and the molecular chaperones Hsp70 (Ssa1p and Ssa2p) and DnaJ (Ydj1p). Other yeast factors, including Mft2p and a homolog of the mammalian protein mitochondrial import stimulating factor (MSF), also may be involved in delivering the precursor protein to the outer membrane translocase (TOM) complex (29) on the mitochondrial surface.