Mitochondrial Carriers for Aspartate, Glutamate and Other Amino Acids: A Review

Abstract

Members of the mitochondrial carrier (MC) protein family transport various molecules across the mitochondrial inner membrane to interlink steps of metabolic pathways and biochemical processes that take place in different compartments; i.e., are localized partly inside and outside the mitochondrial matrix. MC substrates consist of metabolites, inorganic anions (such as phosphate and sulfate), nucleotides, cofactors and amino acids. These compounds have been identified by in vitro transport assays based on the uptake of radioactively labeled substrates into liposomes reconstituted with recombinant purified MCs. By using this approach, 18 human, plant and yeast MCs for amino acids have been characterized and shown to transport aspartate, glutamate, ornithine, arginine, lysine, histidine, citrulline and glycine with varying substrate specificities, kinetics, influences of the pH gradient, and capacities for the antiport and uniport mode of transport. Aside from providing amino acids for mitochondrial translation, the transport reactions catalyzed by these MCs are crucial in energy, nitrogen, nucleotide and amino acid metabolism. In this review we dissect the transport properties, phylogeny, regulation and expression levels in different tissues of MCs for amino acids, and summarize the main structural aspects known until now about MCs. The effects of their disease-causing mutations and manipulation of their expression levels in cells are also considered as clues for understanding their physiological functions.

Keywords: amino acids; arginine; aspartate; glutamate; glycine; lysine; membrane transport; mitochondrial carrier; mitochondrial transporter; ornithine.

Figure 1

Structures of AAC. The carboxytractyloside-inhibited bovine AAC (PDB ID 1OKC) (A,B) and a bongkrekic acid-inhibited AAC of Thermothelomyces thermophila (PDB ID 6GCI) (C,D) are viewed laterally in the plane of the inner mitochondrial membrane (A,C), and from the c- (B) and m-sides (D). The first and second parts of the signature motif sequence are colored in cyan and magenta, respectively. Contact points I–III are shown as red spheres.

Figure 2

Phylogenetic tree of biochemically characterized mitochondrial carries that transport amino acids. The tree was constructed using PhyML v3.1 from a multiple-sequence alignment with ClustalW in Seaview4 and drawn in FigTree v1.4.2. Bootstrap values for 1000 replicates are reported on each node. The names of mitochondrial carriers (MCs) from humans, Arabidopsis thaliana and Saccharomyces cerevisiae are shown in black, brown and blue, respectively. The clusters are color coded as in Table 1. The transported amino acid substrates for each carrier group are indicated with those that vary between one carrier and another in parentheses: histidine is not transported by ORC1 or Ort1p; citrulline is not transported by Ort1p, BAC1 or SLC25A29. AtUCP1 and AtUCP2 also transport malate, oxaloacetate, 2-oxoglutarate, oxalate, succinate, phosphate, sulfate and thiosulfate. Human UCP2 also transports malate, oxaloacetate, phosphate and sulfate.

Figure 3

The metabolic roles of MCs transporting amino acids. The schemes do not show all the metabolic pathways in which the individual carriers are involved. The carriers for amino acids are in color, whereas the oxoglutarate carrier is not colored. Substrates are abbreviated in black/gray: 2-OA, 2-oxoacid; 2-OG, 2-oxoglutarate; OAA, oxaloacetate. Enzymes are abbreviated in blue: ARG-I, arginase I; ASL, argininosuccinase; ASS, argininosuccinate synthase; ATs, amino transferases; cGOT and mGOT, cytoplasmic and mitochondrial glutamate oxaloacetate transferase; cFUM, cytoplasmic fumarase; cMDH and mMDH, cytoplasmic and mitochondrial malate dehydrogenase; CPS-I, carbamoyl phosphate synthetase I; GDH, glutamate dehydrogenase; mtNOS, mitochondrial nitric oxide synthetase; OTC, ornithine transcarbamoylase.