| Beschreibung: | The integrity of the mitochondrial ultrastructure is provided by a large molecular heterooligomeric complex called mitochondrial contact site and cristae organizing system (MICOS). The evolutionary conserved MICOS complex has been shown to consist of two subcomplexes, the MIC60-subcomplex and the MIC10-subcomplex. The majority of studies have dealt with the core proteins of both subcomplex, MIC60 and MIC10, showing their importance for the formation of CJs as functional barriers to separate cristae from the inner boundary membrane and intermembrane space. This study aimed to define the function, interaction partners, and associated phenotype of the understudied protein MIC13 as part of the MICOS and independent from it. After proving that MIC13 is a part of MICOS we investigated the phenotype upon loss of this protein. We found that loss of MIC13 leads to the loss of the MIC10-subcomplex, reduced respiration, and altered mitochondrial ultrastructure accompanied by loss of CJs. Further, we studied the role of MIC13 in cristae dynamics. We found a dynamic movement of cristae inside the mitochondria, that is accompanied by continuous merging and splitting events. These cristae dynamics is highly dependent on MIC13 and a functional MICOS. These studies suggested a bridging function of MIC13 to assure the proper assembly of MICOS. MICOS is assembled in a hierarchical way with MIC60 as a docking platform. MIC13 is needed to link the MIC10-complex to the MIC60-complex and therefore without MIC13 the MIC10-complex is lost. This role of MIC13 is supported by our latest findings, where we show that two conserved regions in the MIC13 sequence are important for the interaction of MIC13 with other MICOS components. We first performed systematic deletions across the MIC13 sequence to pinpoint regions, which are important for interaction with other MICOS components by co-immunoprecipitation. We found two regions, the N-terminal region, and a middle region between aa 84 and aa 103 to be important for the interaction of MIC13 with MICOS components. With help of multiple sequence alignment, we then found conserved residues in these regions, a GxxxG motif in the N-terminal region and WN in the latter region. We show here, that these conserved regions are important for interaction with other MICOS proteins, therefore they are important for the bridging function of MIC13. Interestingly, we could find also the conserved residues RD (81-82aa), which are dispensable for the bridging function of MIC13 but are important for mitochondrial ultrastructure, showing an additional function of MIC13 independent from the other MICOS components. In additional experiments, I further show, that loss of MIC13 leads to a delayed cytochrome c release upon apoptosis induction, which is most likely due to the alterations in mitochondrial ultrastructure. With a proteomics approach, I found several proteins, which levels are changed upon loss of MIC13. These proteins are potential candidates for MIC13 interaction partners or hint to other functions of MIC13 outside of normal MICOS function. All in all, we showed that MIC13 is not only responsible for the connection between both MICOS subcomplexes but also indispensable for the formation of the small subcomplex and proper mitochondrial ultrastructure independent from other MICOS components.
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