Transport proteins (carriers) of mitochondria.

Mitochondria are subcellular structures essential to the aerobic eukaryotic cell. Their role extends much beyond their basic reactions of oxidative phosphorylation. It encompasses the steps critical for cellular metabolic pathways, for apoptosis, and for other processes such as antiviral signaling. This short review is limited to transport proteins (carriers) that catalyze the transport of metabolites across the inner mitochondrial membrane and thus link metabolic pathway reactions in the cytosol and the mitochondrial matrix. Such transport must minimally affect the electrochemical proton gradient essential for oxidative phosphorylation (chemiosmotic mechanism of oxidative phosphorylation). Many of these transport proteins belong to a family of membrane proteins, and the major part of this review will consider their structures and functions. First studies of these transporters were carried out with intact mitochondria and with inhibitors that appeared transporter-specific. Such an inhibitor was then utilized in the first purification of one of these transporter proteins. Its substrate-specificity was then established after functionally active incorporation into liposomes. Questions about copurification of other transporters and thus a definitive identification of transported substrate with the purified protein were resolved definitively only after heterologous expression in bacteria, most generally as inclusion bodies, and followed by reconstitution in liposomes. Site-specific mutations permitted the identification of amino acids essential to their transport function. These mutagenesis studies then also helped interpret human diseases with mutations in these transport proteins. The high-resolution structure of a member of this transporter protein family dramatically advanced these studies. It raised new questions because this structure complexed with a high-affinity inhibitor showed a monomeric protein, while purification and inhibitor stoichiometry studies suggest a functional homodimeric transport protein. Remaining key questions need to address: the homodimeric nature of the transporters, details of their transport mechanism, and the functional identification of many members of this family whose existence has only been suggested from genomic data.