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 targeting of proteins to the intermembrane space. Sorting to the intermembrane space represents a particularly interesting situation, as at least three separate targeting pathways to this subcompartment are known to exist.