Mitochondrial Metal Homeostasis: A look into iron and copper mobilization within mitochondria

Cellular iron and copper homeostasis is interdependent with mitochondrial iron and copper homeostasis. Mitochondria must import iron to form iron-sulfur clusters and heme, while it must import copper for usage and storage. These cofactors are incorporated into mitochondrial proteins that support essential functions, including cellular respiration and maintaining redox homeostasis. In turn, mitochondria also provide heme to the cell and enables the biogenesis of cytosolic iron-sulfur cluster containing proteins, while also providing copper when needed. Due to both metals (and their modified species) reactivity, iron and copper are stored and trafficked within the mitochondria carefully. Although these cofactors are crucial for mitochondrial homeostasis, mechanistic details are lacking regarding the meticulous process of iron and copper transport. With the goal in mind to determine the mitochondrial factors necessary for proper heme biosynthesis and transport within the mitochondrion, our lab investigated the physical and genetic interactions of the terminal heme biosynthetic enzyme ferrochelatase, Hem15. These analyses revealed a dynamic association between Hem15 and the core component of the mitochondrial contact site and cristae organizing system (MICOS) Mic60. The loss of Mic60 negatively impacts Hem15 activity, affects the size of the Hem15 high mass complex, and results in the accumulation of reactive and toxic tetrapyrrole precursors. Furthermore, using fluorescent heme sensors, our lab determined that heme is trafficked to the nucleus approximately 25% faster than to the cytosol and mitochondrial matrix. We, also, discovered that mitochondrial heme efflux to the nucleus is regulated by the initial heme biosynthetic enzyme and GTPases that control mitochondrial dynamics and endoplasmic reticulum contact sites. Lastly, we probed the copper cofactor site assembly within cytochrome c oxidase. To this end, we determined that human cytochrome c oxidase copper chaperones form macromolecular assemblies and cooperate with several twin-CX9C proteins to control heme a biosynthesis and coordinate copper transfer to the CuA and CuB sites. Taken together, these results provide insight for the meticulous transport of iron-containing cofactors and its precursors and copper within the mitochondria