Transport of proteins across mitochondrial membranes

The vast majority of proteins comprising the mitochondrion are encoded by nuclear genes, synthesized on ribosomes in the cytosol, and translocated into the various mitochondrial subcompartments. During this process proteins must cross the lipid membranes of the mitochondrion without interfering with the integrity or functions of the organelle. In recent years an approach combining biochemical, molecular, genetic, and morphological methodology has provided insights into various aspects of this complex process of intracellular protein sorting. In particular, a greater understanding of the molecular specificity and mechanism of targeting of mitochondrial preproteins has been reached, as a protein complex of the outer membrane which facilitates recognition and initial membrane insertion has been identified and characterized. Furthermore, pathways and components involved in the translocation of preproteins across the two mitochondrial membranes are being dissected and defined. The energetics of translocation and the processes of unfolding and folding of proteins during transmembrane transfer are closely linked to the function of a host of proteins known as heat-shock proteins or molecular chaperones, present both outside and inside the mitochondrion. In addition, the analysis of the process of folding of polypeptides in the mitochondrial matrix has allowed novel and unexpected insights into general pathways of protein folding assisted by folding factors. Pathways of sorting of proteins to the four different mitochondrial subcompartments — the outer membrane (OM), intermembrane space, inner membrane (IM) and matrix — are only partly understood and reveal an amazing complexity and variation. Many additional protein factors are involved in these latter processes, a few of which have been analyzed, such as cytochrome c heme lyase and cytochrome c 1 heme lyase, enzymes that catalyze the covalent addition of the heme group to cytochrome c and c 1 preproteins, and the mitochondrial processing peptidase which cleaves signal sequence after import of preproteins into the matrix. Thus, the study of transport of polypeptides through the mitochondrial membranes does not only contribute to the understanding of how biological membranes facilitate the penetration of macromolecules but also provides novel insights into the structure and function of this organelle. are being dissected and defined. The energetics of translocation and the processes of unfolding and folding of proteins during transmembrane transfer are closely linked to the function of a host of proteins known as heat-shock proteins or molecular chaperones, present both outside and inside the mitochondrion. In addition, the analysis of the process of folding of polypeptides in the mitochondrial matrix has allowed novel and unexpected insights into general pathways of protein folding assisted by folding factors. Pathways of sorting of proteins to the four different mitochondrial subcompartments — the outer membrane (OM), intermembrane space, inner membrane (IM) and matrix — are only partly understood and reveal an amazing complexity and variation. Many additional protein factors are involved in these latter processes, a few of which have been analyzed, such as cytochrome c heme lyase and cytochrome c 1 heme lyase, enzymes that catalyze the covalent addition of the heme group to cytochrome c and c 1 preproteins, and the mitochondrial processing peptidase which cleaves signal sequences after import of preproteins into the matrix. Thus, the study of transport of polypeptides through the mitochondrial membranes does not only contribute to the understanding of how biological membranes facilitate the penetration of macromolecules but also provides novel insights into the structure and function of this organelle

Distinct steps in the import of ADP/ATP carrier into mitochondria

Transport of the precursor to the ADP/ATP carrier from the cytosol into the mitochondrial inner membrane was resolved into several consecutive steps. The precursor protein was trapped at distinct stages of the import pathway and subsequently chased to the mature form. In a first reaction, the precursor interacts with a protease-sensitive component on the mitochondrial surface. It then reaches intermediate sites in the outer membrane which are saturable and where it is protected against proteases. This translocation intermediate can be extracted at alkaline pH. We suggest that it is anchored to the membrane by a so far unknown proteinaceous component. The membrane potential delta psi-dependent entrance of the ADP/ATP carrier into the inner membrane takes place at contact sites between outer and inner membranes. Completion of translocation into the inner membrane can occur in the absence of delta psi. A cytosolic component which is present in reticulocyte lysate and which interacts with isolated mitochondria is required for the specific binding of the precursor to mitochondria