Functional characterization of ACN9 in Drosophila mitochondria. Wendou Yu¹, Daniel K. Bricker¹, James E. Cox², Dennis R. Winge³, Jared Rutter³, Carl S. Thummel¹. 1) Department of Human Genetics, University of Utah School of Medicine, Salt Lake City, UT; 2) Metabolomics Core Research Facility, University of Utah School of Medicine, Salt Lake City, UT; 3) Department of Biochemistry, University of Utah School of Medicine, Salt Lake City, UT.

   Mitochondria have a wide range of cellular functions, including metabolism, signal transduction and cell death. Consistent with these roles, mitochondrial dysfunction is central to many human diseases, including neurodegenerative disorders, type 2 diabetes, and cancer. Accordingly, extensive efforts have focused on functional analysis of the mitochondrial proteome. In spite of this work, however, about one fifth of the nuclear-encoded mitochondrial proteins remain largely uncharacterized. Among these are many proteins that are conserved through evolution, from yeast to humans. We are characterizing these proteins with the goal of gaining new insights into mitochondrial physiology and function. Here we describe our work on a mitochondrial intermembrane space protein ACN9, which is required for efficient succinate dehydrogenase activity in yeast. Drosophila mutants lacking ACN9 are sensitive to a variety of stresses, including starvation and exposure to paraquot or ethanol. Metabolomic analysis shows that ACN9 mutants accumulate succinate and have decreased fumarate and malate, consistent with a defect in succinate dehydrogenase activity. In addition, we see reduced levels of metabolites that are involved in gluconeogenesis, including phosphoenolpyruvate and 3-phosphoglycerate. Interestingly, homocysteine accumulates in ACN9 mutants when compared to controls under normal conditions. Elevated homocysteine is associated with exercise or alcohol consumption in humans as well as cardiovascular and neurodegenerative diseases. This is consistent with genomewide studies that have associated ACN9 polymorphisms with racehorse performance and alcohol dependence in humans. Current efforts are focused on defining the physiological and biochemical functions of ACN9 in flies and yeast.