Spatial distribution of mitochondrial translation and OXPHOS complex assembly Stefan Stoldt1, Dirk Wenzel2, Kirsten Kehrein3, Dietmar Riedel2, Martin Ott3, Stefan Jakobs1,4* 1Department of NanoBiophotonics, MPI for Biophysical Chemistry, Göttingen, Germany; 2Laboratory of Electron Microscopy, MPI for Biophysical Chemistry, Göttingen, Germany; 3Department of Biochemistry and Biophysics, Stockholm University, Sweden; 4Department of Neurology, University Medical Center Göttingen, Germany
Oxidative phosphorylation (OXPHOS) is carried out by large multi-subunit protein complexes in the mitochondrial inner membrane. The OXPHOS complexes are preferentially located in the cristae membrane (CM). They are composed of a mix of subunits encoded in both the nuclear and the mitochondrial genomes. For this reason, the assembly of these dual-origin complexes represents an enormous logistical challenge for the cell. How OXPHOS complex assembly is spatially coordinated within the highly convoluted mitochondrial inner membrane is an enigma. In this study we used various proteins mediating specific steps in the assembly of complexes III to V as proxies to determine the spatial distribution of mitochondrial translation and OXPHOS assembly. Using super-resolution microscopy (nanoscopy) and quantitative cryo-immunogold electron microscopy, we determined where specific transcripts are translated and where distinct assembly steps of the dual-origin complexes in the yeast Saccharomyces cerevisiae occur. Our data suggest that the mitochondrially encoded proteins of complexes III and IV are preferentially inserted in different sites of the inner membrane than those of complex V. We further demonstrate that early, but not late, assembly steps of complexes III and IV occur preferentially in the inner boundary membrane (IBM). In contrast, all steps of complex V assembly occur in the CM. Our data suggest that the assembly of the OXPHOS super-complexes is well organized spatially within the mitochondrial inner membrane.
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