Import of cytochrome c heme lyase into mitochondria

Cytochrome c heme lyase (CCHL) catalyses the covalent attachment of
the heme group to apocytochrome c during its import into
mitochondria. The enzyme is membrane-associated and is located
within the intermembrane space. The precursor of CCHL synthesized
in vitro was efficiently translocated into isolated mitochondria
from Neurospora crassa. The imported CCHL, like the native protein,
was correctly localized to the intermembrane space, where it was
membrane-bound. As with the majority of mitochondrial precursor
proteins, CCHL uses the MOM19-GIP receptor complex in the outer
membrane for import. In contrast to proteins taking the general
import route, CCHL was imported independently of both
ATP-hydrolysis and an electrochemical potential as external energy
sources. CCHL which lacks a cleavable signal sequence apparently
does not traverse the inner membrane to reach the intermembrane
space; rather, it translocates through the outer membrane only.
Thus, CCHL represents an example of a novel, 'non-conservative'
import pathway into the intermembrane space, thereby also showing
that the import apparatus in the outer membrane acts separately
from the import machinery in the inner membrane.