Contact sites between the outer and inner membrane of mitochondria-role in protein transport

Many essential functions of mitochondrial metabolism have been studied in the past three decades in considerable depth: oxidative phosphorylation, catabolism of fatty acids, role in nitrogen metabolism, and amino acid metabolism. More recently, other aspects attracted much attention like protein translocation into mitochondria, inheritance of mitochondrial DNA, movement of mitochondria, their fusion and fission, and their involvement in apoptosis, ageing, cancer and other cellular processes. Together with these new views on the function of mitochondria, new ideas on the structure of mitochondria emerged. Here we will discuss the current knowledge about how the membranes of mitochondria are organized and how they interact. Interactions between components of the inner and the outer membrane are necessary for a number of central mitochondrial functions such as the channeling of metabolites, coordinated fusion and fission of mitochondria, and protein transport. Some of these interactions appear stable such as the so-called morphological contact sites; others are quite dynamic. Direct evidence that a certain protein is part of morphologically defined contact sites is lacking. Nevertheless, protein translocase complexes of the outer and the inner membrane exhibit stable interactions between the two membranes when precursor proteins are arrested during import into mitochondria. Finally, we discuss possible roles of cristae junctions, another morphologically defined membrane structure in mitochondria.