Reconstitution of the initial steps of mitochondrial protein import.

We have reconstituted the initial steps of mitochondrial protein import with a purified precursor protein, a purified, ATP-dependent, cytosolic chaperone selective for mitochondrial precursors (mitochondrial import stimulating factor; MSF), and either intact mitochondria or intact or solubilized mitochondrial outer membranes. We show that the precursor-MSF complex first binds to the Mas37p/Mas70p subunits of the mitochondrial import receptor. After ATP-dependent release of MSF, the precursor is transferred from Mas37p/Mas70p to the Mas20p/Mas22p subunits of the receptor, and finally delivered to the import channel in the outer membrane. Import in the absence of the MSF bypasses Mas37p/Mas70p. The ATP-mediated transfer of a precursor from MSF to specific subunits of the import receptor is similar to the GTP-mediated transfer of precursors from the signal recognition particle to its receptor on the endoplasmic reticulum.