Abstract
Oxidative phosphorylation (OXPHOS) is the final biochemical pathway of energy (ATP) production in the cell. Defects of the OXPHOS system are amongst the most frequent inborn errors of metabolism and result in many, often devastating diseases affecting different organs and tissues. The OXPHOS system consists of five multiprotein complexes that are built out of 89 protein components, which are encoded by both the mitochondrial and nuclear DNA. Correct biogenesis and functioning of the OXPHOS system is dependent on the finely tuned interaction between the nuclear and the mitochondrial genomes. Disturbances of the system can be caused by numerous genetic defects and can manifest in a variety of metabolic alterations that result in a plethora of clinical symptoms. The most common cause of OXPHOS disease is complex I deficiency. Complex I is the largest and most complicated of five complexes and represents the main entrance of the OXPHOS pathway. The biogenesis of complex I is an intricate process of assembly of 45 individual subunits into a membrane-bound multiprotein structure. This process is suspected of being facilitated by the action of several assembly factors. In this thesis we show that mutations in nuclear DNA-encoded subunits affect the integrity of complex I, which correlates with the lowered spectrophotometrically measured complex I enzyme activities. We report the identification of the first human complex I assembly factor, NDUFAF1, we characterize its function in the assembly process, and investigate its involvement in complex I deficiency. By means of a baculoviral complementation assay, we show that lack of a second complex I assembly factor, NDUFAF2, results in disturbance of complex I assembly and alterations in mitochondrial physiology, and is the cause of complex I deficiency in a unique case of a contiguous gene deletion. The proposed roles of assembly factors NDUFAF1 and NDUFAF2 are integrated in a revised model of the complex I assembly pathway.
