Biogenesis of the dicarboxylate carrier (DIC): Translocation across the mitochondrial outer membrane and subsequent release from the TOM channel are membrane potential-independent

The mitochondrial inner membrane of Saccharomyces cerevisiae contains a gro up of homologous carrier proteins that mediate the exchange of several meta bolites. The members of this protein family are synthesized in the cytosol and reach their final topology after translocation across the mitochondrial outer membrane. Using the ADP/ATP carrier (AAC) as a model protein, previo us studies have established four distinct steps of the import pathway (stag es I-IV). In the absence of the mitochondrial membrane potential (Delta psi ), the AAC accumulates at the inner surface of the outer membrane (stage II Ia) and remains bound to the outer membrane import channel. Only in the pre sence of the membrane potential, can a complex of small Tim proteins mediat e transfer of the AAC to the inner membrane. In this study, we characterize d the import pathway of the dicarboxylate carrier (DIC). Different from the AAC, the DIC showed complete Delta psi -independent translocation across t he outer membrane, release from the import pore, and mainly accumulated in a soluble state in the intermembrane space, thus defining a new translocati on intermediate (stage III*). The DIC should be a suitable model protein fo r the characterization of Delta psi -independent functions of the intermemb rane space Tim proteins. (C) 2001 Academic Press.