The Effects of Copper Availability on Mitochondrial Function in Saccharomyces cerevisiae

Abstract

Copper (Cu) is crucial for cell function and survival and plays important roles in enzyme-catalyzed metabolic processes, transport of cargo through cellular membranes, and protein breakdown. The mitochondria are home to two important Cu-containing enzymes: cytochrome c oxidase (COX) and superoxide dismutase 1 (Sod1). Delivery of Cu to COX, the terminal electron acceptor in the respiratory chain, is suggested to happen in a bucket brigade fashion via the proteins Cox17, Sco1, and Cox11. Sod1, the other major mitochondrial cuproprotein, is thought to obtain Cu from its chaperone, Ccs1. Because the mitochondria are a central player in Cu storage and regulation, our goal in this study is to analyze how changes in extracellular Cu concentration impact mitochondrial function in Saccharomyces cerevisiae. Our results showed that Cox17, Sco1, and Cox11 responded differently to excess and limiting Cu, suggesting alternate functions for these proteins in mitochondria unrelated to COX assembly. During Cu deficient conditions we noticed an interesting redistribution of mitochondrial Cu: retention of Cu in mitochondrial Sod1 is accompanied by a complete loss of COX activity. This phenotype suggests a regulatory mechanism where Cu is preferentially donated to mitochondrial Sod1 instead of COX when Cu is limited. Interestingly, we also find that Cu redistribution in the mitochondria is affected when we change the primary carbon source from glucose to galactose. We propose that Cu distribution in mitochondria is controlled by an affinity gradient. Analysis of cell cycle progression showed that in excess Cu, yeast displayed a cell cycle defect while Cu deficient conditions did not affect cell cycle progression. Strikingly, changing the primary carbon source to galactose led to a cell cycle defect in the presence of a Cu specific chelator. Overall, we suggest that the presence of mitochondrial Sod1 is essential for cell cycle progression. The protein machinery responsible for Cu homeostasis has been well characterized, however the transport of Cu to cellular compartments and regulatory mechanisms involved in Cu import and export are not fully understood. We now demonstrate a novel phenotype that provides insight into regulation of Cu within the mitochondria. Additionally, we find that mitochondrial Cu distribution is affected by carbon source, and is associated with changes in cell cycle progression. These findings further our understanding of Cu homeostasis in yeast, and will likely have implications on Cu-related disorders, such as Menkes disease.