Transport of proteins into the various subcompartments of mitochondria

The import of proteins into mitochondria is an intricate process
comprised of multiple steps. The first step involves the sorting of
cytosolically synthesized precursor proteins to the mitochondrial
surface. There precursor proteins are recognized by specific
receptors which deliver them to the general import site present in
the outer membrane. The second stage of import involves a series of
complex intraorganelle sorting events which results in the delivery
of the proteins to one of the four possible submitochondrial
destinations, namely the outer and inner membranes, the matrix and
intermembrane space. Here in this review, we discuss the current
knowledge on these intramitochondrial sorting events. We especially
focus on targetting of proteins to the intermembrane space. Sorting
to the intermembrane space represents a particularly interesting
situation, as at least three separate targetting pathways to this
subcompartment are known to exist.