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Schreiner, Bernadette (2013): Role of the AAA protease Yme1 in folding of proteins in the mitochondrial intermembrane space. Dissertation, LMU München: Faculty of Biology



The vast majority of mitochondrial proteins are encoded in the nucleus and synthesized as precursor proteins on cytosolic ribosomes. After translation, these precursor proteins are imported in a largely, if not completely, unfolded state into one of the four mitochondrial subcompartments, the outer membrane, the intermembrane space, the inner membrane or the matrix. Once the precursor proteins reach their compartment of destination, they can fold into the functionally active three-dimensional native structure. Therefore, internal mitochondrial folding systems are needed in each subcompartment to assist folding of these precursor proteins upon import. Members of several “classical” chaperone families are present in the mitochondrial matrix and have been shown to support import and folding of newly imported polypeptides. However, folding of proteins in the mitochondrial intermembrane space is only poorly understood. Recently, a disulfide relay system in the intermembrane space that mediates import and folding was described, but this system is limited to proteins that form disulfide bonds. For the majority of intermembrane proteins, folding helpers that promote folding have not yet been discovered. In order to identify general folding helpers of the intermembrane space, the well studied model substrate mouse dihydrofolate reductase (DHFR) was targeted to the mitochondrial intermembrane space of S. cerevisiae and its folding analyzed. DHFR assumes its mature fold in the intermembrane space and heat shock induces DHFR aggregation. Interestingly, aggregation is counteracted by an ATP-dependent process. The i-AAA protease Yme1 that is anchored in the inner mitochondrial membrane and exposes its functional domains to the intermembrane space was able to prevent the aggregation of DHFR. A number of proteins of diverse structural and functional classes were found in the aggregate fractions of mitochondria lacking Yme1. Amongst them were factors that are involved in the establishment and maintenance of the mitochondrial ultrastructure, lipid metabolism, protein translocation and respiratory growth. Considering the diversity of the proteins affected in the absence of Yme1 and their function in mitochondria, the pleiotropic effects of the deletion of Yme1 can be readily explained. The findings of the present in vivo study confirm previous hints to a chaperone-like function of Yme1 resulting from in vitro experiments. Yme1 thus has a dual role as protease and as chaperone and occupies a key position in the protein quality control system of the mitochondrial intermembrane space.