Molecular identification and functional characterization of a novel glutamate transporter in yeast and plant mitochondria.

The importance of glutamate is well established in human as well as in plants and fungi where mitochondria play a key role in its metabolism[1,2]. Glutamate is transported inside and/or out from mitochondria by proteins localized in the inner mitochondrial membrane. The genome of S. cerevisiae encodes 35 members of the mitochondrial carrier family (MCF) and that of A. thaliana 58 MCF members. Here two members of this family, Ymc2p from S. cerevisiae and BOU from Arabidopsis, have been characterized. These proteins were overproduced in bacteria and reconstituted into liposomes. Their transport properties and kinetic parameters demonstrate that Ymc2p and BOU transport glutamate. Transport catalyzed by both carriers was saturable, inhibited by mercuric chloride and dependent on the proton gradient across the proteoliposomal membrane. The growth phenotype of S. cerevisiae cells lacking the genes ymc2 and agc1, which encodes the only other S. cerevisiae carrier capable to transport also glutamate, was fully complemented by expressing Ymc2p, Agc1p or BOU. Furthermore, mitochondria isolated from wild-type, ymc2? and agc1? strains, but not from the double mutant ymc2?agc1? strain, swell in isosmotic ammonium glutamate showing that glutamate is transported together with a H+. It is proposed that the primary function of these proteins is to import glutamate into the mitochondrial matrix for energy production, C1 metabolism and protein synthesis.