Import of proteins into mitochondria. In vitro studies on the biogenesis of the outer membrane.

The yeast mitochondrial outer membrane contains a major 29-kilodalton protein which is encoded in the nucleus. When this polypeptide is synthesized in vitro and then incubated with isolated yeast mitochondria, it binds to the organelles and becomes resistant to externally added trypsin. Post-translational insertion of the 29-kilodalton protein in vitro is not accompanied by NH2-terminal processing, appears to require neither ATP nor a transmembrane electrochemical potential, and is also observed with purified outer membranes, but not with yeast microsomes. In vitro insertion of the 29-kilodalton polypeptide is thus in several respects different from that of polypeptides destined for the internal compartments of the mitochondrion.