Cytochromes c1 and b2 are sorted to the intermembrane space of yeast mitochondria by a stop-transfer mechanism.

The pathway by which cytochromes c1 and b2 reach the mitochondrial intermembrane space has been controversial. According to the "conservative sorting" hypothesis, these proteins are first imported across both outer and inner membranes into the matrix, and then are retranslocated across the inner membrane. Our data argue against this model: import intermediates of cytochromes c1 and b2 were found only outside the inner membrane; maturation of these proteins was independent of the matrix-localized hsp60 chaperone; and dihydrofolate reductase linked to the presequence of either cytochrome was imported to the intermembrane space in the absence of ATP. We conclude that cytochromes c1 and b2 are sorted by a mechanism in which translocation through the inner membrane is arrested by a "stop-transfer" signal in the presequence. The arrested intermediates may be associated with a proteinaceous channel in the inner membrane.