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. Anti-folding signals such as the hydrophobic export sequence in cytochrome b2 may act as switches between these two activities.