Identification of the mitochondrial glutamate transporter. Bacterial expression, reconstitution, functional characterization, and tissue distribution of two human isoforms

Identification of a mitochondrial transporter for basic amino acids in Arabidopsis thaliana by functional reconstitution into liposomes and complementation in yeast

We describe the identification and functional characterization of two Arabidopsis mitochondrial basic amino acid carriers (BAC), AtmBAC1 and AtmBAC2, which are related to the yeast ornithine (Orn) carrier Ort1p, also known as Arg11p. The arg11 mutant requires arginine (Arg) supplementation because it fails to export sufficient ornithine from the mitochondrion to the cytosol where it is converted to arginine. AtmBAC1 and, to a lesser extent, AtmBAC2 partially replaced the function of Ort1p in yeast arg11. The more efficient putative carrier, AtmBAC1, was expressed in E. coli, purified, and reconstituted into phospholipid vesicles, where it transported the basic l-amino acids arginine, lysine, ornithine and histidine (in order of decreasing affinity). AtmBAC1 recognized l-histidine whereas both yeast Ort1p and the mammalian ortholog ORNT1p do not. Also different from ORNT1p, AtmBAC1 did not transport citrulline. AtmBAC1 appeared to be more stereospecific than the yeast and mammalian ornithine carriers, exhibiting greater preference for the l-forms of arginine, lysine and ornithine. By RT-PCR, both AtmBAC1 and AtmBAC2 transcripts were detected in stems, leaves, flowers, siliques, and seedlings. Expression of AtmBAC1 in seedlings is consistent with its involvement in Arg breakdown in early seedling development, i.e. delivery of Arg to mitochondrial arginase. The Km (0.19 mm) for Arg uptake by AtmBAC1 was close to the value we previously determined for the saturable component of Arg uptake into intact mitochondria from soybean seedling cotyledons.

Expression pattern of a nuclear encoded mitochondrial arginine-ornithine translocator gene from Arabidopsis

BACKGROUND

Arginine and citrulline serve as nitrogen storage forms, but are also involved in biosynthetic and catabolic pathways. Metabolism of arginine, citrulline and ornithine is distributed between mitochondria and cytosol. For the shuttle of intermediates between cytosol and mitochondria transporters present on the inner mitochondrial membrane are required. Yeast contains a mitochondrial translocator for ornithine and arginine, Ort1p/Arg11p. Ort1p/Arg11p is a member of the mitochondrial carrier family (MCF) essential for ornithine export from mitochondria. The yeast arg11 mutant, which is deficient in Ort1p/Arg11p grows poorly on media lacking arginine.

RESULTS

High-level expression of a nuclear encoded Arabidopsis thaliana homolog (AtmBAC2) of Ort1p/Arg11p was able to suppress the growth deficiency of arg11. RT-PCR analysis demonstrated expression of AtmBAC2 in all tissues with highest levels in flowers. Promoter-GUS fusions showed preferential expression in flowers, i.e. pollen, in the vasculature of siliques and in aborted seeds. Variable expression was observed in leaf vasculature. Induction of the promoter was not observed during the first two weeks in seedlings grown on media containing NH4NO3, arginine or ornithine as sole nitrogen sources.

CONCLUSION

AtmBAC2 was isolated as a mitochondrial transporter for arginine in Arabidopsis. The absence of expression in developing seeds and in cotyledons of seedlings indicates that other transporters are responsible for storage and mobilization of arginine in seeds.

Infantile citrullinemia caused by citrin deficiency with increased dibasic amino acids

In an infant who suffered from prolonged icterus and hepatocellular dysfunction we detected an increase of citrulline and dibasic amino acids in plasma and urine. The amino acid levels along with all the abnormal liver tests normalized upon replacing breast-milk by formula feeding; there was no relapse after human milk was tentatively reintroduced. A novel mutation, a approximately 9.5-kb genomic duplication, was identified in the citrin gene (SLC25A13) resulting in the insertion of exon 15. No mutation was detected in the CAT2A specific exon of the SLC7A2 gene which encodes for the liver transporter of cationic amino acids. This is the first report of infantile citrin deficiency in non-Asian patients.

Identification and reconstitution of the yeast mitochondrial transporter for thiamine pyrophosphate

The genome of Saccharomyces cerevisiae contains 35 members of a family of transport proteins that, with a single exception, are found in the inner membranes of mitochondria. The transport functions of the 15 biochemically identified mitochondrial carriers are concerned with shuttling substrates, biosynthetic intermediates and cofactors across the inner membrane. Here the identification of the mitochondrial carrier for the essential cofactor thiamine pyrophosphate (ThPP) is described. The protein has been overexpressed in bacteria, reconstituted into phospholipid vesicles and identified by its transport properties. In confirmation of its identity, cells lacking the gene for this carrier had reduced levels of ThPP in their mitochondria, and decreased activity of acetolactate synthase, a ThPP-requiring enzyme found in the organellar matrix. They also required thiamine for growth on fermentative carbon sources.

Identification of a novel transporter for dicarboxylates and tricarboxylates in plant mitochondria. Bacterial expression, reconstitution, functional characterization, and tissue distribution

A cDNA from Arabidopsis thaliana and four related cDNAs from Nicotiana tabacum that we have isolated encode hitherto unidentified members of the mitochondrial carrier family. These proteins have been overexpressed in bacteria and reconstituted into phospholipid vesicles. Their transport properties demonstrate that they are orthologs/isoforms of a novel mitochondrial carrier capable of transporting both dicarboxylates (such as malate, oxaloacetate, oxoglutarate, and maleate) and tricarboxylates (such as citrate, isocitrate, cis-aconitate, and trans-aconitate). The newly identified dicarboxylate-tricarboxylate carrier accepts only the single protonated form of citrate (H-citrate2-) and the unprotonated form of malate (malate2-) and catalyzes obligatory, electroneutral exchanges. Oxoglutarate, citrate, and malate are mutually competitive inhibitors, showing K(i) close to the respective K(m). The carrier is expressed in all plant tissues examined and is largely spread in the plant kingdom. Furthermore, nitrate supply to nitrogen-starved tobacco plants leads to an increase in its mRNA in roots and leaves. The dicarboxylate-tricarboxylate carrier may play a role in important plant metabolic functions requiring organic acid flux to or from the mitochondria, such as nitrogen assimilation, export of reducing equivalents from the mitochondria, and fatty acid elongation.