Characterization of GTPBPs involved in ribosome assembly and function in human mitochondria

Sammanfattning: Mitochondria are organelles often referred to as the powerhouses of the cell as they provide most of the chemical energy via aerobic respiration. However, in the last half a century, it has become clear that their function extends to other fundamental metabolic tasks. These fascinating organelles have their own DNA and protein synthesis machinery, the mitochondrial ribosome (mitoribosome), the latter made of a set of mitoribosomal proteins (MRPs) and rRNA that all together build a 2.7 MDa complex. The mitoribosome translates for 13 polypeptides which are later incorporated in the respiratory chain. Therefore, it is not surprising that mutations in MRPs or auxiliary factors involved in its assembly can lead to multisystemic human disorders. Here lies the importance of studying the molecular mechanisms of the mitoribosomal assembly process, which has been the focus of my PhD studies. Guanosine triphosphate binding proteins (GTPBPs) involved in ribosome biogenesis in bacteria have extensively been studied and have provided key knowledge for the understanding of the role of their human mitochondrial homologues identified so far. In my work, I have addressed the role of human mitochondrial proteins GTPBP5, GTPBP10 and GTPBP8 and their possible involvement in mitochondrial ribosome biogenesis. In paper I, we have biochemically characterised GTPBP5 by assessing its interactome and the effects of its depletion on mitochondrial functionality, showing the importance of this protein as an assembly factor. These data have later been confirmed in paper II, where we have structurally determined in more details the function of GTPBP5 as well as several other late-stage mitoribosomal assembly factors. GTPBP10 involvement in the assembly process has been determined biochemically in paper III, where we also co-immunoprecipitated GTPBP10 with the mitoribosome in vivo using a knock-in mouse model. Additionally, preliminary results discussed in this thesis regard the investigation of GTPBP8 function in mitochondrial gene expression, demonstrating its significance for mitochondrial viability.