Protein import into mitochondria: two systems acting in tandem?

Mitochondria can import proteins from the cytoplasm at sites where the two mitochondrial membranes are closely apposed. However, each of these membranes contains a distinct protein translocation channel. Recent evidence suggests that the two types of channel are not permanently coupled, but may dissociate in a reversible manner. This reversible interaction is probably essential for intramitochondrial sorting proteins.