Arginine transport in mitochondria of Neurospora crassa

The role of mitochondrial transport in energy metabolism

Since mitochondria are closed spaces in the cell, metabolite traffic across the mitochondrial membrane is needed to accomplish energy metabolism. The mitochondrial carriers play this function by uniport, symport and antiport processes. We give here a survey of about 50 transport processes catalysed by more than 30 carriers with a survey of the methods used to investigate metabolite transport in isolated mammalian mitochondria. The role of mitochondria in metabolic pathways including ammoniogenesis, amino acid metabolism, mitochondrial shuttles etc. is also reported in more detail, mainly in the light of the existence of new transport processes.

Isolation and characterization of strains defective in vacuolar ornithine permease in Neurospora crassa

Vacuolar uptake of ornithine and lysine was characterized in Neurospora crassa using a cupric ion permeabilization system. Michaelis constants were measured as 1.4 mM for lysine and 11.0 mM for ornithine, and maximal velocities were determined. Vacuolar lysine uptake was shown to be inhibited competitively by L-arginine and histidine while ornithine uptake was inhibited by a variety of amino acids. Strains defective in the vacuolar ornithine permease were isolated using a filtration enrichment method. Two isolates--RSC-39 and RSC-63--had a reduced ability to accumulate ornithine. Vacuolar uptake of amino acids was measured using cupric ion-permeabilized mycelia; both strains had reduced ornithine uptake while lysine uptake and arginine uptake were normal. For both isolates, both the Michaelis constant and the maximal velocity for ornithine uptake were reduced compared to those of wild type. These results suggest that both strains are defective in the gene which encodes the vacuolar ornithine permease.

Characterization of vacuolar arginine uptake and amino acid efflux in Neurospora crassa using cupric ion to permeabilize the plasma membrane

Treatment of Neurospora crassa mycelia with cupric ion has been shown to permeabilize the plasma and mitochondrial membranes. Permeabilized mycelia were shown to take up arginine into the vacuoles. Uptake was ATP-independent and appeared to be driven by an existing K+-gradient. The kinetic characteristics of the observed uptake were similar to those observed using vacuolar membrane vesicles: the Km for arginine uptake was found to be 4.2-4.5 mM. Permeabilized mycelia were used to study the regulation of arginine uptake into vacuoles. The results suggest that uptake is relatively indifferent to the contents of the vacuoles and is not affected by growth of mycelia in amino acid-supplemented medium. Efflux of arginine, lysine, and ornithine from vacuoles was also measured using mycelia permeabilized with cupric ion. Arginine release was shown to be specifically enhanced by cytosolic ornithine and/or increases in the vacuolar pool of arginine or ornithine. Lysine efflux was shown be indifferent to the presence of other amino acids. These observations emphasize the importance of vacuolar compartmentation in controlling arginine and ornithine metabolism and suggest that vacuolar compartmentation may play an important role in nitrogen homeostasis of filamentous fungi.

The mitochondrial carrier family of transport proteins: structural, functional, and evolutionary relationships

Energy transduction in mitochondria requires the transport of many specific metabolites across the inner membrane of this eukaryotic organelle. We have screened the protein sequence database for proteins homologous to the mitochondrial ATP/ADP exchange carrier, and the homologous proteins found were similarly screened to ensure that all currently sequenced members of the mitochondrial carrier family (MCF) had been identified. Thirty-seven proteins were identified, 28 of which were less than 90% identical to any other sequenced member of the MCF, and the latter proteins fell into 10 clusters or subfamilies as follows: (1) ATP/ADP exchangers of mammals, plants, algae, yeast, and fungi (11 members); (2) a bovine oxoglutarate/malate exchanger (one member); (3) mammalian uncoupling carriers (five members); (4) yeast and mammalian phosphate carriers (three members); (5) MRS proteins that suppress mitochondrial splicing defects in Saccharomyces cerevisiae (two members); (6) a putative peroxysomal carrier of Candida boidinii; (7) a putative solute carrier from the protozoan, Oxytricha fallax; (8) a putative solute carrier from S. cerevisiae; (9) a putative solute carrier from Zea mays, and (10) two putative solute carriers from the mammalian thyroid gland. The specificities of proteins in clusters 5 to 10 are not known. A multiple alignment and an evolutionary tree of the 28 selected members of the MCF were constructed, thus defining the conserved residues and the phylogenetic relationships of the proteins. Hydropathy plots of the homologous regions were determined and averaged, and the average hydropathy plots were evaluated for sequence similarity. These analyses revealed that the six transmembrane spanners exhibited varying degrees of sequence conservation and hydrophilicity. These spanners, and immediately adjacent hydrophilic loop regions, were more highly conserved than other regions of these proteins. All members of the MCF appear to consist of a tripartite structure with each of the three repeated segments being about 100 residues in length. Each repeat contains two transmembrane spanners, the first being more hydrophobic with conserved glycyl and prolyl residues, the second, preceded by a highly conserved glycyl residue, being more hydrophilic with largely conserved hydrophilic residues in certain positions. Five of the six spanners are followed by the largely conserved sequence (D/E)-Hy (K/R)[- = any residue; Hy = a hydrophobic residue]. Based on both intracluster and intercluster statistical comparisons, repeats 1, 2, and 3 are homologous, but repeats 1 are more similar to each other than they are to repeats 2 or 3 or repeats 2 or 3 are to each other.(ABSTRACT TRUNCATED AT 400 WORDS)