Most proteins that are to be imported into the mitochondrial matrix are synthesized as precursors, each composed of an N-terminal targeting sequence followed by a mature domain. Precursors are recognized through their targeting sequences by receptors at the mitochondrial surface and are then threaded through import channels into the matrix. Both the targeting sequence and the mature domain contribute to the efficiency with which proteins are imported into mitochondria. Precursors must be in an unfolded conformation during translocation. Mitochondria can unfold some proteins by changing their unfolding pathways. The effectiveness of this unfolding mechanism depends on the local structure of the mature domain adjacent to the targeting sequence. This local structure determines the extent to which the unfolding pathway can be changed and, therefore, the unfolding rate increased. Atomic force microscopy studies find that the local structures of proteins near their N and C termini also influence their resistance to mechanical unfolding. Thus, protein unfolding during import resembles mechanical unfolding, and the specificity of import is determined by the resistance of the mature domain to unfolding as well as by the properties of the targeting sequence. © 2005 by The National Academy of Sciences of the USA.