Permease protein subunits of the MC family (the human SLC25 family) possess six transmembrane α-helical spanners. The proteins are of fairly uniform size of about 300 residues. They arose by tandem intragenic triplication events in which a genetic element encoding two spanners gave rise to one encoding six spanners. This event may have occurred less than 1.5 billion years ago when mitochondria first developed their specialized endosymbiotic functions within eukaryotic cells. Members of the family are found exclusively in eukaryotic organelles although they are nuclearly encoded. Most are found in mitochondria, but some are found in peroxisomes of animals, in hydrogenosomes of anaerobic fungi, and in amyloplasts of plants. Structurally characterized members of the MC family are dimers. Many of them preferentially catalyze the exchange of one solute for another (antiport). Fifteen paralogues of the MC family are encoded within the genome of Saccharomyces cerevisiae.

The high resolution 3-D structure of a single subunit of one MC family member, the bovine ATP/ADP antiporter (TC #2.A.29.1.1), has been solved by x-ray crystallography to 2.2 Å resolution (Pebay-Peyroula et al., 2003). The carrier was crystalized in complexation with the inhibitor, carboxyatractyloside. The six TMSs of each subunit (with the N- and C-termini normally facing the cytoplasmic side of the membrane and the three hairpin loops of the repeat sequences facing the matrix) form a compact barrel domain which shows a deep cone-shaped depression at the surface facing the intermembrane space. At its base was found the signature sequence of these nucleotide carriers (R R R M M M). The cavity has a maximal diameter of 20 Å and a depth of 30 Å. The fold of the three repeat elements is very similar. Each odd-numbered helix exhibits a sharp kink, due to a conserved prolyl residue located in the conserved P X(D/E) X X (K/R) motif, characteristic of all mitochondrial carriers. The even-numbered helices pass straight through the membrane without a kink. The structure reveals large hydrophilic surfaces in the interior of the conical pit, due to the weak hydrophobicities of these proteins. A positive electrostatic surface potential on the matrix side and at the bottom of the pit provides the force for anionic substrate binding. Two lipid molecules, both cardiolipin molecules, are tightly bound to the carrier.

The transport substrates of MC family members probably bind to the bottom of the cavity, and translocation results in a transient transition from a 'pit' to a 'channel' conformation. However, each subunit in the dimeric carrier is believed to assume different conformations, one exhibiting the ADP site, facing the cytoplasm (out), and the other, exhibiting the ATP site, facing the matrix. Transport is believed to take place once ADP binds to one monomer from the outside and ATP simultaneously binds to the second monomer from the inside. A 'half of sites' mechanism is therefore proposed. In the absence of substrate, the carrier may be multiconformational. Kinetic analyses have suggested a sequential mechanism.

The inhibitor, carboxyatractyloside, probably binds where ADP binds, in the pit on the outer surface, thus blocking the transport cycle. Another inhibitor, bongkrekic acid, is believed to stabilize a second conformation, with the pit facing the matrix. In this conformation, the inhibitor may bind to the ATP-binding site.

One of the MC family members, the uncoupling protein, UCP1 (TC# 2.A.29.3.1), functions to dissipate the proton motive force, thereby generating heat. This protein has been shown to be capable of transporting fatty acids, long chain alkylsulfonates and chloride. It is believed to allow transport of protons down their electrochemical gradient in a cyclic, fatty acid-dependent process by first exporting fatty acyl anions and then allow the free diffusion of the protonated fatty acid across the bilayer into the mitochondrion. UNC1 is therfore probably an anion translocator that may not require that transport occurs by an antiport mechanism. The fatty acid behaves as a cycling protonophore (Garlid et al., 2000). UNC1 uses coenzyme Q (ubiquinone) as a cofactor (Echtay et al., 2000). Like many other MC family members, uncoupling proteins are found in the mitochondria of plants as well as animals. Various compounds such as the reactive aldehyde (produced under oxidative stress conditions), 4-hydroxy-2-nonenal, as well as trans-retinal and other 2-alkenals activate uncoupling via UCP1-3 (TC #2.A.29.3.1) as well as the ATP/ADP antiporter (TC #2.A.29.1.1) (Echtay et al., 2003).

The generalized transport reaction for carriers of the MC family is:

S1 (out) + S2 (in) S1 (in) + S2 (out).