Polypeptides traverse the mitochondrial envelope in an extended state

Most mitochondrial proteins are synthesized as precursors in the
cytosol and imported through the contact sites between outer and
inner mitochondrial membranes. The molecular mechanism of membrane
translocation of precursor proteins is largely unclear. For this
report, various hybrid proteins between portions of the precursor
of cytochrome b2 and the entire dihydrofolate reductase (DHFR) were
accumulated in mitochondrial contact sites. We unexpectedly found
that about 30 amino acid residues of the polypeptide chain in
transit were sufficient to span both membranes. This suggests
linear translocation of the polypeptide chain and presents evidence
for a high degree of unfolding of polypeptides traversing the
mitochondrial membranes.