Biochemical characterisation of the Tom40 mutant mouse

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Institute for Molecular Sciences, Department of Biochemistry, School of Molecular Sciences, Faculty of Science, Technology and Engineering, La Trobe University, Melbourne.

About 99 percent of mitochondrial proteins are encoded by nuclear genes, synthesised in the cytosol and are imported into the organelle. Tom40, the core component of the TOM (Translocase of the outer membrane) complex forms a central channel for precursor translocation across the mitochondrial outer membrane and it is essential for cell viability. Given the indispensable role of Tom40 in protein import into mitochondria, mutations in the mammalian Tom40 gene are likely to have pathological consequences. The work outlined in this thesis mainly focusses on characterizing mitochondria from stiliom (Tom40 mutant) mouse strain obtained from collaborators. None of the stiliom homozygous mice have lived for more than 7 weeks of age, the post mortem analyses revealing pulmonary vascular congestion. Genome sequencing of the mouse revealed a mutation in the TOMM40 gene resulting in a substitution of an alanine for a valine at amino acid 326 (Tom40A326V). The alteration in Tom40 protein causes a change in its electrophoretic mobility on an SDS-PAGE and the total Tom40 protein levels in the stiliom mouse tissues were reduced. Biochemical studies demonstrated that the single mutation in Tom40 leads to the destabilization of the TOM complex. Interestingly, 2DPAGE analyses show that theTom40A326V can stably associate with small TOM proteins, Tom5, Tom6 and Tom7, however, its interaction with Tom22, a subunit of the TOM complex, was significantly reduced. Based on the mitochondrial in vitro import assays, the import pathway of proteins VDAC1 (β-barrel outer membrane protein), Bak (single transmembrane domain outer membrane protein), ANT1 (inner membrane protein) and pOTC (matrix protein) remained unaffected in both stiliom fibroblasts and tissue mitochondria. Taken together, these observations indicate that the A326V mutation affects the stability of Tom40. The residue altered in Tom40 is important for interactions between components of the TOM complex. The majority of the mutagenesis studies on Tom40 performed previously were based on fungal models and stiliom is the first mouse model unveiling the correlation between alterations in Tom40 to the pathogenicity of a disease.