Mitochondrial protein import

The precursors of the mitochondrial proteins ADP/ATP carrier (AAC)
and F1-ATPase subunit β (F1β) were accumulated at the stages of
binding to receptor sites on the mitochondrial outer membrane, or
in contact sites between outer and inner membranes. Specific
antibodies raised against the mature proteins were added to the
isolated mitochondria and efficiently bound to these translocation
intermediates. Further movement of the precursors to consecutive
steps along their import pathway was thereby inhibited. Controls
showed that precursor proteins which were inserted into or
translocated across the outer membrane were not recognized by the
antibodies unless the mitochondrial membranes were disrupted. We
conclude that the trapped translocation intermediates have
antigenic sites exposed to the outside of the outer membrane.