Successive translocation into and out of the mitochondrial matrix

We investigated the import and sorting pathways of cytochrome b2
and cytochrome c1, which are functionally located in the
intermembrane space of mitochondria. Both proteins are synthesized
on cytoplasmic ribosomes as larger precursors and are processed in
mitochondria in two steps upon import. The precursors are first
translocated across both mitochondrial membranes via contact sites
into the matrix. Processing by the matrix peptidase leads to
intermediate-sized forms, which are subsequently redirected across
the inner membrane. The second proteolytic processing occurs in the
intermembrane space. We conclude that the hydrophobic stretches in
the presequences of the intermediate-sized forms do not stop
transfer across the inner membrane, but rather act as transport
signals to direct export from the matrix into the intermembrane
space.