Import of apocytochrome c into the mitochondrial intermembrane space along a cytochrome c1 sorting pathway

The question of whether cytochrome c could be functionally sorted
to the mitochondrial intermembrane space along a "conservative
sorting" pathway was investigated using a fusion protein termed
pLc1-c. pLc1-c contains 3-fold targeting information, namely, the
complete bipartite presequence of the cytochrome c1 precursor
joined to the amino terminus of apocytochrome c. pLc1-c could be
selectively imported into the intermembrane space either directly
across the outer membrane along a cytochrome c import route or
along a cytochrome c1 route via the matrix. Thus, apocytochrome c
could be sorted along a conservative sorting pathway; however,
following reexport from the matrix, apo-Lc1-c could not be
converted to its holo counterpart. Despite the apparent similarity
of structure and functional location of the heme lyases and
similarity of the heme binding regions in their respective
apoproteins, cytochrome c heme lyase and cytochrome c1 heme lyase
apparently have different and nonoverlapping substrate
specificities.