IMPORT OF CYTOCHROME-B(2) TO THE MITOCHONDRIAL INTERMEMBRANE SPACE - THE TIGHTLY FOLDED HEME-BINDING DOMAIN MAKES IMPORT DEPENDENT UPON MATRIX ATP

Cytochrome b2 is synthesized as a precursor in the cytoplasm and impor ted to the intermembrane space of yeast mitochondria. We show here tha t the precursor contains a tightly folded heme-binding domain and that translocation of this domain across the outer membrane requires ATP. Surprisingly, it is ATP in the mitochondrial matrix rather than extern al ATP that drives import of the heme-binding domain. When the folded structure of the heme-binding domain is disrupted by mutation or by ur ea denaturation, import and correct processing take place in ATP-deple ted mitochondria. These results indicate that (1) cytochrome b2 reache s the intermembrane space without completely crossing the inner membra ne, and (2) some precursors fold outside the mitochondria but remain t ranslocation-competent, and import of these precursors in vitro does n ot require ATP-dependent cytosolic chaperone proteins.