Mitochondrial protein import

The transport of nuclear-encoded proteins from the cytosol into
mitochondria is mediated by targeting (signal) sequences present on
precursor forms. Most precursors of the mitochondrial matrix
possess amino-terminal signals which characteristically contain
hydroxylated and basic amino acids and lack acidic residues. With a
minority of precursor proteins, internal sequence motifs can direct
proteins to the mitochondria (Pfanner, N., Hoeben, P., Tropschug,
M. and Neupert, W. (1987) J. Biol. Chem. 262, 14851–14854). The
presence of a mitochondrial targeting sequence alone, however, is
not sufficient for specific targeting to the organelle and further
to the various subcompartments. There is the need for components
which recognise the targeting sequences and others which keep the
precursor protein in a translocation-competent form. Beyond the
recognition step, components are required which mediate
translocation across the mitochondrial membranes. Mitochondria
possess two translocation machineries, one in the outer membrane
and one in the inner membrane. The matrix space harbors a number of
factors which participate in the import of proteins, in their
unfolding and folding. Energy is required at several steps of these
processes.