Cooperation of protein machineries in mitochondrial protein sorting.

The function of mitochondria depends on the import of proteins, which are synthesized as precursors on cytosolic ribosomes. The majority of the precursor proteins are sorted into the mitochondrial subcompartments via five distinct routes. Recent studies revealed that molecular cooperation between protein machineries is a central feature of mitochondrial protein biogenesis. First, coupling to various partner proteins affects the substrate specificity of translocases and single translocation steps. Second, there is a substantial cooperation between different protein translocases in the import of specific precursor proteins. Third, protein transport is intimately linked to processing, folding and assembly reactions. Fourth, sorting of precursor proteins is functionally and physically connected to protein machineries, which fulfill central functions for respiration, maintenance of membrane architecture and form contacts to the endoplasmic reticulum. Therefore, we propose that the protein transport systems are part of a complicated protein network for mitochondrial biogenesis.