The Protein Import System of Mitochondria

Mitochondria import virtually all of their proteins from the cytoplasm. This import process faces two unique challenges; the proteins must be routed to their correct submitochondrial compartment, and those destined for the innermost compartment (the matrix) must be transported across two membranes. These challenges are met by the joint action of two distinct protein transport systems, one in the outer membrane and the other in the inner membrane. The molecular composition of these two systems is now quite well known, and we are beginning to understand how they work (1, 2, 3, 4).