2.A.54 The Mitochondrial Tricarboxylate Carrier (MTC) Family

The MTC family consists of a limited number of homologues, all from eukaryotes. A single member of the family has been functionally characterized and sequenced. This is the tricarboxylate carrier from rat liver mitochondria. It is 357 amino acyl residues in length with 5 or 6 putative transmembrane α-helical spanners (TMSs). It does not exhibit internal repeats or show homology to proteins of the mitochondrial carrier family (TC #2.A.29). Homologues are found in Caenorhabditis elegans, Saccharomyces cerevisiae and Leishmania major. They are of 285-293 amino acyl residues with one exception, and are reported to possess 3-6 putative TMSs.

The rat liver mitochondrial tricarboxylate carrier has been reported to transport citrate, cis-aconitate, threo-D-isocitrate, D- and L-tartrate, malate, succinate and phosphoenolpyruvate. Trans-aconitate, α-ketoglutarate and malonate are not substrates. It presumably functions by a proton symport mechanism for the uptake of the variety of anionic substrates listed above.

The generalized transport reaction catalyzed by the MTC protein of rat liver mitochondria is therefore probably:

anionic substrate (out) + nH+ (out) → anionic substrate (in) + nH+ (in).