Mitochondrial metabolite carrier proteins: purification, reconstitution, and transport studies

Identification and purification of the reconstitutively active glutamine carrier from rat kidney mitochondria

The glutamine carrier from rat kidney mitochondria, solubilized in dodecyl octaoxyethylene ether (C12E8) and partly purified on hydroxyapatite, was identified and completely purified by Celite chromatography. On SDS/PAGE, the purified glutamine carrier consisted of a single protein band with an apparent molecular mass of 41.5 kDa. When reconstituted into liposomes, the glutamine carrier catalysed both the unidirectional flux of glutamine and the glutamine/glutamine countertransport, which were completely inhibitable by a mixture of pyridoxal 5'-phosphate and N-ethylmaleimide. The carrier protein was purified 474-fold with a recovery of 58% and a protein yield of 0.12% with respect to the mitochondrial extract. The glutamine carrier-mediated transport is quite specific for l-glutamine. l-Asparagine is the only other amino acid that is efficiently transported by the reconstituted carrier protein. d-Glutamine, l-glutamate and l-aspartate are very poor substrates. The transport activity was inhibited by several thiol-group and amino-group reagents.

Identification of the yeast ACR1 gene product as a succinate-fumarate transporter essential for growth on ethanol or acetate

The protein encoded by the ACR1 gene in Saccharomyces cerevisiae belongs to a family of 35 related membrane proteins that are encoded in the fungal genome. Some of them are known to transport various substrates and products across the inner membranes of mitochondria, but the functions of 28 members of the family are unknown. The yeast ACR1 gene was introduced into Escherichia coli on an expression plasmid. The protein was over-produced as inclusion bodies, which were purified and solubilised in the presence of sarkosyl. The solubilised protein was reconstituted into liposomes and shown to transport fumarate and succinate. Its physiological role in S. cerevisiae is probably to transport cytoplasmic succinate, derived from isocitrate by the action of isocitrate lyase in the cytosol, into the mitochondrial matrix in exchange for fumarate. This exchange activity and the subsequent conversion of fumarate to oxaloacetate in the cytosol would be essential for the growth of S. cerevisiae on ethanol or acetate as the sole carbon source.

The purified and reconstituted ornithine/citrulline carrier from rat liver mitochondria: electrical nature and coupling of the exchange reaction with H+ translocation

The mechanism and the electrical nature of ornithine/citrulline exchange has been investigated in proteoliposomes reconstituted with the ornithine/citrulline carrier purified from rat liver mitochondria. The stoichiometry of the exchanging substrates was close to 1:1. The exchange was not affected by inducing electrogenic flux of K+ with valinomycin. In contrast, the pH gradient generated by the K+/H+ exchanger nigericin in the presence of an outwardly directed K+ gradient stimulated the ornithineout/citrullinein exchange, but not the ornithine/ornithine homoexchange. Experiments in which either the internal or the external pH was varied, while keeping constant the pH in the other compartment, indicated that maximal exchange rates are found at pH 6 in the compartment containing citrulline and at pH 8 in the compartment containing ornithine. Changes in fluorescence of the pH indicator pyranine, included inside the proteoliposomes, showed that the exchanges ornithineout/citrullinein and citrullineout/ornithinein are accompanied by translocation of H+ in the same direction as citrulline. It is concluded that the mitochondrial ornithine/citrulline carrier catalyses an electroneutral exchange of ornithine+ for citrulline plus an H+. A reasonable model is one in which ornithine binds to a deprotonated carrier and citrulline to a protonated carrier and both substrate-carrier complexes are neutral. The physiological implications of this transport process are discussed.

Identification of the yeast ARG-11 gene as a mitochondrial ornithine carrier involved in arginine biosynthesis

The ARG-11 gene in Saccharomyces cerevisiae encodes a protein with the characteristic features of a family of 35 related membrane proteins that are encoded in the fungal genome. Some of them are known to transport various substrates and products across the inner membranes of mitochondria, but the functions of 29 members of the family are unknown. The yeast ARG-11 protein has been over-produced as inclusion bodies in Escherichia coli. It has been solubilized in the presence of sarkosyl, re-constituted into liposomes and shown to transport ornithine in exchange for protons. Its main physiological role is probably to take ornithine synthesized from glutamate in the mitochondrial matrix to the cytosol where it is converted to arginine.

The mitochondrial carnitine carrier protein: cDNA cloning, primary structure and comparison with other mitochondrial transport proteins

GENBANK/o acid sequence of the rat carnitine carrier protein, a component of the inner membranes of mitochondria, has been deduced from the sequences of overlapping cDNA clones. These clones were generated in polymerase chain reactions with primers and probes based on amino acid sequence information, obtained from the direct sequencing of internal peptides of the purified carnitine carrier protein from rat. The protein sequence of the carrier, including the initiator methionine, has a length of 301 amino acids. The mature protein has a modified alpha-amino group, although the nature of this modification and the precise position of the N-terminal residue have not been ascertained. Analysis of the carnitine carrier sequence shows that the protein contains a 3-fold repeated sequence about 100 amino acids in length. Dot plot comparisons and sequence alignment demonstrate that these repeated domains are related to each other and also to the repeats of similar length that are present in the other mitochondrial carrier proteins sequenced so far. The hydropathy analysis of the carnitine carrier supports the view that the domains are folded into similar structural motifs, consisting of two transmembrane alpha-helices joined by an extensive extramembranous hydrophilic region. Southern blotting experiments suggest that both the human and the rat genomes contain single genes for the carnitine carrier. These studies provide the primary structure of the mitochondrial carnitine carrier protein and allow us to identify this metabolically important transporter as a member of the mitochondrial carrier family, and the sixth of the members whose biochemical function has already been identified.

Identification by bacterial expression and functional reconstitution of the yeast genomic sequence encoding the mitochondrial dicarboxylate carrier protein

The inner membranes of mitochondria contain a family of transport proteins of related sequence and structure. The DNA sequence of the genome of Saccharomyces cerevisiae encodes at least 35 members of this family. Three of them can be recognised as known isoforms of the ADP-ATP translocase and two others as the phosphate and citrate carriers. The transport functions of the remainder cannot be identified with certainty. One of them, encoded on yeast chromosome xii, shows a fairly close sequence relationship to the known sequence of the bovine mitochondrial oxoglutarate-malate carrier. The yeast protein has been obtained by over-expression in Escherichia coli, reconstituted into phospholipid vesicles and shown to have transport properties characteristic of the mitochondrial carrier for dicarboxylate ions, such as malate, and also phosphate, previously biochemically characterised, but not sequenced, from both mammalian and yeast mitochondria. This is the first example of the biochemical identification of an unknown membrane protein encoded in the yeast genome since the completion of the genomic sequence.

The mitochondrial oxoglutarate carrier protein contains a disulfide bridge between intramembranous cysteines 221 and 224

The oxoglutarate carrier (OGC) purified from bovine heart mitochondria was treated, both in its active and in its SDS-denatured state, with the fluorescent N-(1-pyrenyl)maleimide and other SH reagents before and after reduction with dithioerythritol or beta-mercaptoethanol. The number of SH groups per OGC polypeptide chain was found to be about 1 for the oxidized carrier and 3 for the reduced carrier. The bovine oxoglutarate carrier contains three cysteines: Cys-184, Cys-221 and Cys-224. Sequencing of BrCN cleavage products of oxoglutarate carrier showed that N-(1-pyrenyl)maleimide binds to only Cys-184 of the oxidized protein and also to Cys-221 and Cys-224 after reduction of the protein. These results show the presence of a disulfide bridge between the latter two cysteines of the purified carrier. The oxidized and the reduced forms of the oxoglutarate carrier exhibited different Vmax but virtually the same K(m) values for oxoglutarate.