Other Mitochondrial Proteins
ANT is a reversible transporter of ATP and ADP. During OXPHOS the protein exchanges ATP out for ADP in, but when it becomes important to maintain a membrane potential, at times when OXPHOS is not working, ANT can work in reverse. Mutant ANT has been identified in cardiomyopathy, and autoimmune reactions to this protein can cause heart related diseases.
AIF is a multi-functional protein with a vital oxidoreductase function, an anti-oxidant activity, an assembly function in Complex I formation, and an apoptogenic function. AIF has a major role in the caspase-independent apoptosis process.
Cyclophilin D is the binding site for cyclosporins and this protein may be the gate for PTP.
Frataxin is a nuclear-encoded mitochondrial protein. In humans the gene is localized on chromosome 9 and is highly conserved during evolution. The gene is expressed in every cell, although in varying levels in different tissues and during development. The specific function of frataxin is still unknown, but it has been shown to play a role in iron metabolism.
Mitofilin is a transmembrane protein of the inner mitochondrial membrane and is associated with a large multimeric protein complex of about 1200 kDa. Mitofilin has critical functions in mitochondrial morphology and mitochondrial fusion and fission, specifically in the formation of tubular cristae and cristae junctions. Abnormal mitochondrial morphology has been implicated in many human diseases such as myopathy, cardiomyopathy, rhabdomyosarcoma and Whartin's tumor.
TFP is part of the mitochondrial fatty acid beta-oxidation pathway. It is located in the inner mitochondrial membrane and it catalyzes three out of the four steps in the beta-oxidation cycle with a specific affinity for long chain substrates. TFP is hetero-octamer composed of four α and four β-subunits.
Nicotinamide nucleotide transhydrogenase (NNT) is a mitochondrial inner membrane protein catalyzing the interconversion of NADH and NADPH within the matrix space of the organelle. It couples this reaction to proton translocation across the inner membrane. In the presence of an electrochemical proton gradient the production of NADPH is strongly favored. NADPH is required in the mitochondrial matrix to keep the ratio GSH:GSSG high via the NADPH dependent glutathione reductase. Thus NNT has an important role to play in defences against oxidative stress produced by the organelle. A novel, unexpected role of NNT is in insulin secretion. The absence of NNT prevents glucose from closing the K+ATP channel and from opening the Ca++ channel in pancreatic beta cells. These defective beta cells also show a profound increase inROS.
The channel-forming protein porin (also called the voltage dependent anion channel, VDAC) is involved in both aerobic metabolism and apoptotic processes. Porin is localized in the outer membrane of mitochondria where it forms the primary pathway of metabolites across the membrane. Porin can also serve as a binding site for enzymes or a regulatory molecule.
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