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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. This article highlights and discusses the importance of COX regulation by twin CX9C proteins in the mitochondria via COX assembly and control of its activity through protein–protein interactions, which is further modulated by cell signaling pathways. Interestingly, select members of the CX9C protein family, including MNRR1 and CHCHD10, show a novel feature in that they not only localize to the mitochondria but also to the nucleus, where they mediate oxygen- and stress-induced transcriptional regulation, opening a new view of mitochondrial-nuclear crosstalk and its involvement in human disease.

Predicting protein submitochondrial locations has been studied for about ten years. A dozen of methods were developed in this regard. Although a mitochondrion has four submitochondrial compartments, all existing studies considered only three of them. The mitochondrial intermembrane space proteins were always excluded in these studies. However, there are over 50 mitochondrial intermembrane space proteins in the recent release of UniProt database. We think it is time to incorporate these proteins in predicting protein submitochondrial locations. We proposed the functional domain enrichment score, which can be used as an enhancement to our positional-specific physicochemical properties method. We constructed a high-quality working dataset from the UniProt database. This dataset contains proteins from all four submitochondrial locations. Proteins with multiple submitochondrial locations are also included. Our method achieved over 70% prediction accuracy for proteins with single location on this dataset. On the M3-317 benchmarking dataset, our method achieved comparable prediction performance to other state-of-the-art methods. Our results indicate that the intermembrane space proteins can be incorporated in predicting protein submitochondrial locations. By evaluating our method with the proteins that have multiple submitochondrial locations, we conclude that our method is capable of predicting multiple submitochondrial locations. This is the first report of ab initio methods that can identify intermembrane space proteins. This is also the first attempt to incorporate proteins with multiple submitochondrial locations. The benchmarking dataset can be obtained by emails to the corresponding author. [ABSTRACT FROM AUTHOR]

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. This article highlights and discusses the importance of COX regulation by twin CX9C proteins in the mitochondria via COX assembly and control of its activity through protein–protein interactions, which is further modulated by cell signaling pathways. Interestingly, select members of the CX9C protein family, including MNRR1 and CHCHD10, show a novel feature in that they not only localize to the mitochondria but also to the nucleus, where they mediate oxygen- and stress-induced transcriptional regulation, opening a new view of mitochondrial-nuclear crosstalk and its involvement in human disease. [ABSTRACT FROM AUTHOR]