cells Review Regulation of COX Assembly and Function by Twin CX9C Proteins—Implications for Human Disease Stephanie Gladyck 1, Siddhesh Aras 1,2, Maik Hüttemann 1 and Lawrence I. Grossman 1,2,* 1 Center for Molecular Medicine and Genetics, Wayne State University School of Medicine, Detroit, MI 48201, USA;
[email protected] (S.G.);
[email protected] (S.A.);
[email protected] (M.H.) 2 Perinatology Research Branch, Division of Obstetrics and Maternal-Fetal Medicine, Division of Intramural Research, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, U.S. Department of Health and Human Services, Bethesda, Maryland and Detroit, MI 48201, USA * Correspondence:
[email protected] Abstract: Oxidative phosphorylation is a tightly regulated process in mammals that takes place in and across the inner mitochondrial membrane and consists of the electron transport chain and ATP synthase. Complex IV, or cytochrome c oxidase (COX), is the terminal enzyme of the electron transport chain, responsible for accepting electrons from cytochrome c, pumping protons to contribute to the gradient utilized by ATP synthase to produce ATP, and reducing oxygen to water. As such, COX is tightly regulated through numerous mechanisms including protein–protein interactions. The twin CX9C family of proteins has recently been shown to be involved in COX regulation by assisting with complex assembly, biogenesis, and activity. The twin CX9C motif allows for the import of these proteins into the intermembrane space of the mitochondria using the redox import machinery of Mia40/CHCHD4. Studies have shown that knockdown of the proteins discussed in this review results in decreased or completely deficient aerobic respiration in experimental models ranging from yeast to human cells, as the proteins are conserved across species.