Antifolding activity of hsp60 couples protein import into the mitochondrial matrix with export to the intermembrane space

Cytochrome b2 reaches the intermembrane space of mitochondria by
transport into the matrix followed by export across the inner
membrane. While in the matrix, the protein interacts with hsp60,
which arrests its folding prior to export. The bacterial-type
export sequence in pre-cytochrome b2 functions by inhibiting the
ATP-dependent release of the protein from hsp60. Release for export
apparently requires, in addition to ATP, the interaction of the
signal sequence with a component of the export machinery in the
inner membrane. Export can occur before import is complete provided
that a critical length of the polypeptide chain has been
translocated into the matrix. Thus, hsp60 combines two activities:
catalysis of folding of proteins destined for the matrix, and
maintaining proteins in an unfolded state to facilitate their
channeling between the machineries for import and export across the
inner membrane. Antifolding signals such as the hydrophobic export
sequence in cytochrome b2 may act as switches between these two
activities.