The role of RNA-binding proteins in mitochondrial gene expression

[Truncated] Mitochondria are ubiquitous organelles of eukaryotic cells that contain a circular double-stranded genome encoding 13 polypeptide subunits of the mitochondrial respiratory chain. The genes for these polypeptides are transcribed and translated in the mitochondrial matrix, using 22 tRNAs and 2 rRNAs encoded by the compact mitochondrial DNA. Mammalian mitochondrial mRNAs generally begin at the start codon and lack conventional 5′ untranslated regions or Shine-Dalgarno sequences. In addition, mitochondrial encoded proteins are hydrophobic and are likely to be co-translationally inserted into the membrane-embedded complexes providing unique constraints for their translation. Mitochondrial ribosomes must have evolved an alternative way to regulate translation initiation and elongation to ensure accurate start codon recognition and facilitate protein complex assembly. Mammalian mitochondrial ribosomes are unique from bacterial and cytoplasmic ribosomes of eukaryotes because their ribosomal RNA has been reduced considerably and has been replaced by additional proteins. In addition to the core ribosomal proteins that share homology with prokaryotic ribosomes, mammalian mitochondrial ribosomes have many extra proteins, some of which may have new functions in mitochondrial translation and recognition of mitochondrial mRNAs within the small and large ribosomal subunits.

Mitochondrial gene expression is predominantly regulated at the post-transcriptional level and mitochondrial RNA-binding proteins play a key role in RNA metabolism and protein synthesis. In this thesis the role of the RNA-binding protein AU-binding homolog of enoyl CoA hydratase (AUH) in mitochondrial gene expression was investigated. It was discovered that AUH localizes to the mitochondrial matrix and associates with the inner membrane and mitochondrial ribosomes. AUH plays an important role in protein synthesis and overexpression and knock down of the protein were found to cause morphological changes, decreased mitochondrial RNA stability, biogenesis and respiratory function. In addition study of a catalytically inactive form of AUH as well as AUH containing a pathogenic mutation revealed that the catalytic activity of AUH is necessary for its role in mitochondrial protein synthesis. Finally, it is shown that the catalytic activity of AUH acts as a sensor to regulate mitochondrial translation and biogenesis in response to leucine.