Structure, function and regulation of the tricarboxylate transport protein from rat liver mitochondria

Insights into the malfunctioning of the mitochondrial citrate carrier: Implications for cell pathology

The mitochondrial citrate carrier (CIC) is a member of the mitochondrial carrier family and is responsible for the transit of tricarboxylates and dicarboxylates across the inner membrane. By modulating the flux of these molecules, it represents the molecular link between catabolic and anabolic reactions that take place in distinct cellular sub-compartments. Therefore, this transport protein represents an important element of investigation both in physiology and in pathology. In this review we critically analyze the involvement of the mitochondrial CIC in several human pathologies, which can be divided into two subgroups, one characterized by a decrease and the other by an increase in the flux of citrate across the inner mitochondrial membrane. In particular, a decrease in the activity of the mitochondrial CIC is responsible for several congenital diseases of different severity, which are also characterized by the increase in urinary levels of L-2- and D-2-hydroxyglutaric acids. On the other hand, an increase in the activity of the mitochondrial CIC is involved, in various ways, in the onset of inflammation, autoimmune diseases, and cancer. Then, understanding the role of CIC and the mechanisms driving the flux of metabolic intermediates between mitochondria and cytosol would potentially allow for manipulation and control of metabolism in pathological conditions.

Molecular Mechanism of Citrate Efflux by the Mitochondrial Citrate Transporter CT in Filamentous Fungus Mucor circinelloides WJ11

The mitochondrial citrate transporter (MCT) plays an important role in citrate efflux from the mitochondria in eukaryotes, and hence provides a direct correlation between carbohydrate metabolism and lipid synthesis. Our previous studies on transporters confirmed the presence of two MCTs (TCT and CT) in oleaginous Mucor circinelloides WJ11 associated with high lipid accumulation. However, the molecular mechanism of citrate efflux from the mitochondria by MCT in M. circinelloides is still unclear. To study the citrate transport mechanism of CT, the citrate transporter gene was expressed in Escherichia coli, and its product was purified. The citrate transport activity of the protein was studied in CT reconstituted liposomes. Our results showed high efficiency of CT for [14C] citrate/citrate exchange with K m 0.01 mM at 25°C. Besides citrate, other molecules such as oxaloacetate, malate, fumarate, succinate aconitate, oxoadipate, isocitrate, and glutamate also promote citrate transport. In addition, the ct overexpression and knockout plasmids were constructed and transferred into M. circinelloides WJ11, and the mitochondria were isolated, and the transport activity was studied. Our findings showed that in the presence of 10 mM malate, the mitochondria of ct-overexpressing transformant showed 51% increase in the efflux rate of [14C] citrate, whereas the mitochondria of the ct-knockout transformant showed 18% decrease in citrate efflux compared to the mitochondria of wild-type WJ11. This study provided the first mechanistic evidence of citrate efflux from the mitochondria by citrate transporter in oleaginous filamentous fungus M. circinelloides, which is associated with high lipid accumulation.

Allatostatin-C reversibly blocks the transport of citrate out of the mitochondria and inhibits juvenile hormone synthesis in mosquitoes

Aedes aegypti allatostatin-C (AeaAST-C or PISCF-AST) is a strong and fast reversible inhibitor of juvenile hormone III (JH III) synthesis by the corpora allata (CA) of mosquitoes; however, its mechanism of action remains poorly understood. AeaAST-C showed no inhibitory activity in the presence of any of the intermediate precursors of JH III indicating that the AeaAST-C target is located before the entry of acetyl-CoA in the pathway. Stimulation experiments using different sources of carbon (glucose, pyruvate, acetate and citrate) suggest that AST-C acts after pyruvate is transformed to citrate in the mitochondria. In vitro inhibition of the citrate mitochondrial carrier (CIC) mimicked the effect of AeaAST-C, and was overridden by addition of citrate or acetate. Our results provide compelling evidence that AeaAST-C inhibits JH III synthesis by blocking the CIC carrier that transports citrate from the mitochondria to the cytosol, obstructing the production of cytoplasmic acetyl-CoA that sustains JH III synthesis in the CA of mosquitoes.

Dietary fat and hepatic lipogenesis: mitochondrial citrate carrier as a sensor of metabolic changes

Citrate carrier (CIC) is an integral protein of the inner mitochondrial membrane that has a fundamental role in hepatic intermediary metabolism. Its primary function is to catalyze the transport of citrate from mitochondria, where this molecule is formed, to cytosol, where this molecule is used for fatty acid (FA) and cholesterol synthesis. Therefore, mitochondrial CIC acts upstream of cytosolic lipogenic reactions, and its regulation is particularly important in view of the modulation of hepatic lipogenesis. Although a great deal of data are currently available on the dietary modulation of cytosolic lipogenic enzymes, little is known about the nutritional regulation of CIC transport activity. In this review, we describe the differential effects of distinct FAs present in the diet on the activity of mitochondrial CIC. In particular, polyunsaturated FAs were powerful modulators of the activity of mitochondrial CIC by influencing its expression through transcriptional and posttranscriptional mechanisms. On the contrary, saturated and monounsaturated FAs did not influence mitochondrial CIC activity. Moreover, variations in CIC activity were connected to similar alterations in the metabolic pathways to which the transported citrate is channeled. Therefore, CIC may be considered as a sensor for changes occurring inside the hepatocyte and may represent an important target for the regulation of hepatic lipogenesis. The crucial role of this protein is reinforced by the recent discovery of its involvement in other cellular processes, such as glucose-stimulated insulin secretion, inflammation, tumorigenesis, genome stability, and sperm metabolism.

The effects of bioactive compounds from plant foods on mitochondrial function: a focus on apoptotic mechanisms

Mitochondria are essential organelles for cellular integrity and functionality maintenance and their imparement is implicated in the development of a wide range of diseases, including metabolic, cardiovascular, degenerative and hyperproliferative pathologies. The identification of different compounds able to interact with mitochondria for therapeutic purposes is currently becoming of primary importance. Indeed, it is well known that foods, particularly those of vegetable origin, present several constituents with beneficial effects on health. This review summarizes and updates the most recent findings concerning the mechanisms through which different dietary compounds from plant foods affect mitochondria functionality in healthy and pathological in vitro and in vivo models, paying particular attention to the pathways involved in mitochondrial biogenesis and apoptosis.

Metabolism of activated T lymphocytes

Activated T cells undergo metabolic reprogramming which promotes glycolytic flux and lactate production as well as elevated production of lipids, proteins, nucleic acids and other carbohydrates (i.e. induction of biomass) even in the presence of oxygen. Activated T cells show induced expression of, among other things, Glucose Transporter 1 and several glycolytic enzymes, including ADP-Dependent Glucokinase and the low affinity isoform Pyruvate Kinase-M2 (which promote glycolytic flux), as well Glutamine Transporters and Glycerol-3-phosphate Dehydrogenase 2 which make available glutamate and glycerol-3-phosphate as mitochondrial energy sources. Intracellular leucine concentrations critically regulate mammalian target of rapamycin (mTOR) signaling to promote Th1, Th2, and Th17 CD4(+) T effector cell differentiation. In contrast, T regulatory (Treg) cells are generated when AMP-Activating Protein Kinase signaling is activated and mTOR activation is suppressed. Unlike effector CD4(+) and CD8(+) T cells, Tregs and memory T cells oxidize fatty acids for fuel. Effector and memory T cells perform different functions and thus show distinct metabolic profiles which are exquisitely controlled by cellular signaling. Upon activation, T cells express the insulin and leptin receptors on their surface and become sensitive to insulin signaling and nutrient availability and show changes in differentiation. Thus, metabolism and nutrient availability influence T cell activation and function.

Mitochondrial roles and cytoprotection in chronic liver injury

The liver is one of the richest organs in terms of number and density of mitochondria. Most chronic liver diseases are associated with the accumulation of damaged mitochondria. Hepatic mitochondria have unique features compared to other organs' mitochondria, since they are the hub that integrates hepatic metabolism of carbohydrates, lipids and proteins. Mitochondria are also essential in hepatocyte survival as mediator of apoptosis and necrosis. Hepatocytes have developed different mechanisms to keep mitochondrial integrity or to prevent the effects of mitochondrial lesions, in particular regulating organelle biogenesis and degradation. In this paper, we will focus on the role of mitochondria in liver physiology, such as hepatic metabolism, reactive oxygen species homeostasis and cell survival. We will also focus on chronic liver pathologies, especially those linked to alcohol, virus, drugs or metabolic syndrome and we will discuss how mitochondria could provide a promising therapeutic target in these contexts.

Citrate carrier promoter is target of peroxisome proliferator-activated receptor alpha and gamma in hepatocytes and adipocytes

Citrate carrier (CiC), a mitochondrial inner membrane protein, is an essential component of the shuttle system which transports acetyl-CoA from mitochondria to the cytosol where lipogenesis occurs. CiC is regulated by SREBP-1, a transcription factor that controls the expression of several lipogenic genes. CiC is also implicated in cholesterol synthesis, glycolysis and gluconeogenesis, suggesting that besides SREBP-1 other transcription factors could modulate the expression of its gene. Here, we provide evidences demonstrating that CiC expression is regulated by peroxisome proliferator-activated receptor (PPAR) alpha and gamma in hepatocytes and adipocytes, respectively. CiC expression increased in rat BRL-3A hepatocytes treated with WY-14,643, agonist of PPARα, and in murine 3T3-L1 adipocytes treated with rosiglitazone, agonist of PPARγ. The overexpression of PPARα/RXRα and PPARγ/RXRα heterodimer enhanced CiC promoter activity in BRL-3A and 3T3-L1, respectively. Luciferase reporter gene and gel mobility shift assays indicated that a functional peroxisome proliferator-activated receptor response element (PPRE), identified in the CiC promoter, conferred responsiveness to activation by PPARs. The binding of PPRE of CiC promoter by PPARα and PPARγin vivo was confirmed by ChIP assay in BRL-3A and 3T3-L1 cells, respectively.

Streptozotocin-induced diabetes affects in rat liver citrate carrier gene expression by transcriptional and posttranscriptional mechanisms

Citrate carrier (CiC), also known as tricarboxylate carrier, is an integral protein of the mitochondrial inner membrane. It is an essential component of the shuttle system by which mitochondrial acetyl-CoA, primer for both fatty acid and cholesterol synthesis, is transported into the cytosol, where lipogenesis occurs. Here, we report the effect of streptozotocin-induced diabetes on the activity and expression of CiC in rat liver mitochondria. A significant reduction of CiC activity and a parallel decline in the abundance of CiC mRNA were found in liver from diabetic rats. Diabetes did not influence CiC mRNA stability, whereas nuclear run-on assay revealed that the transcriptional rate of CiC mRNA decreased, when compared to control, in the nuclei from diabetic rats. The ratio of mature to precursor CiC RNA decreased in diabetic animals, indicating that the splicing of CiC RNA was also affected. The 3'-end processing rate of CiC mRNA was not altered in diabetes. These results suggest that diabetes affects CiC expression at both transcriptional and posttranscriptional levels. In addition, by in vitro transfection experiments in rat hepatocytes cultured in the absence of insulin, a reduction of CiC promoter activity was observed, and this was ascribed to a decreased expression of sterol regulatory element-binding protein-1 transcriptional factor. Furthermore, the binding of sterol regulatory element-binding protein-1 to the CiC promoter was reduced in STZ-diabetic rats with respect to control ones, and it was restored to the control values after insulin treatment.

Reduced activity and expression of mitochondrial citrate carrier in streptozotocin-induced diabetic rats

Citrate carrier (CiC), an integral protein of the mitochondrial inner membrane, plays an important role in hepatic intermediary metabolism, supplying the cytosol with acetyl-coenzyme A for fatty acid and cholesterol synthesis. Here, the effect of streptozotocin-induced diabetes on CiC activity and expression in rat liver was investigated. The rate of citrate transport was reduced by about 35% in mitochondria from diabetic vs. control rats. Kinetic studies in mitochondria from diabetic rats showed a reduction in maximum velocity and almost unchanged Michaelis-Menten constant of the CiC protein. Mitochondrial phospholipid amount was not significantly affected, whereas an increase in the cholesterol content and in the cholesterol/phospholipid ratio was observed. To thoroughly investigate the mechanism responsible for the reduced CiC activity in the diabetic state, molecular studies were performed. Ribonuclease protection assays and Western blotting analysis indicated that both hepatic CiC mRNA accumulation and protein level decreased similarly to the CiC activity. The reduced mRNA level and the lower content of the mitochondrial CiC protein, might account for the decline of CiC activity in diabetic animals. To discriminate between the role played by hyperglycemia from that of hypoinsulinemia in the reduction of CiC activity and expression, studies were conducted administrating phlorizin or insulin to streptozotocin-diabetic rats. Our data indicated that both insulin and glucose affect CiC activity and expression in diabetic rats, although they act at different regulatory steps.

Long-term prediction of fish growth under varying ambient temperature using a multiscale dynamic model

Background

Feed composition has a large impact on the growth of animals, particularly marine fish. We have developed a quantitative dynamic model that can predict the growth and body composition of marine fish for a given feed composition over a timespan of several months. The model takes into consideration the effects of environmental factors, particularly temperature, on growth, and it incorporates detailed kinetics describing the main metabolic processes (protein, lipid, and central metabolism) known to play major roles in growth and body composition.

Results

For validation, we compared our model's predictions with the results of several experimental studies. We showed that the model gives reliable predictions of growth, nutrient utilization (including amino acid retention), and body composition over a timespan of several months, longer than most of the previously developed predictive models.

Conclusion

We demonstrate that, despite the difficulties involved, multiscale models in biology can yield reasonable and useful results. The model predictions are reliable over several timescales and in the presence of strong temperature fluctuations, which are crucial factors for modeling marine organism growth. The model provides important improvements over existing models.

Functional analysis of rat liver citrate carrier promoter: differential responsiveness to polyunsaturated fatty acids

CiC (citrate carrier), a mitochondrial membrane protein, plays an important metabolic role by transporting acetyl-CoA into the cytosol for fatty acid and cholesterol synthesis. Several studies showed that CiC activity and expression is regulated by dietary fatty acids. In the present study we report data on the structural and functional characterization of the 5'-flanking region of the rat Cic gene. By transient transfection assays in H4IIE rat hepatoma cells, a PUFA (polyunsaturated fatty acids) response region has been identified within the CiC promoter. A cluster of putative binding sites for several transcription factors, composed of a NF-Y (nuclear factor-Y) site, an E-box-like site, a SRE1 (sterol regulatory element 1)-like site and four Sp1 (stimulatory protein 1) sites, was localized in the promoter region. Luciferase reporter gene and gel mobility shift assays indicated that a functional E-box-like, essential to the basal CiC promoter activity, confers responsiveness to activation by SREBP (SRE-binding protein)-1c. This study provides evidence for SREBP-1c as a principal target for PUFA regulation of CiC transcription. In H4IIE cells, overexpression of nSREBP (nuclear SREBP)-1c over-rides arachidonic acid (C(20:4, n-6)) suppression, but does not prevent the repression by docosahexaenoic acid (C(22:6, n-3)). ChIP (chromatin immunoprecipitation) assays in H4IIE cells showed that docosahexaenoic acid affects the binding of NF-Y, Sp1 and SREBP-1 to the PUFA response region of CiC promoter, whereas arachidonic acid alters only the binding of SREBP-1. Our data show that PUFA inhibition of hepatic Cic gene transcription is mediated not only by the nuclear level of SREBP-1c, but also might involve a reduction in Sp1 and NF-Y DNA binding, suggesting differential mechanisms in the Cic gene regulation by different PUFA.

Hypothyroidism Reduces Tricarboxylate Carrier Activity and Expression in Rat Liver Mitochondria by Reducing Nuclear Transcription Rate and Splicing Efficiency

The tricarboxylate carrier (TCC), also known as citrate carrier, is an integral protein of the mitochondrial inner membrane. It is an essential component of the shuttle system by which mitochondrial acetyl-CoA, primer for both fatty acid and cholesterol synthesis, is transported into the cytosol, where lipogenesis occurs. The effect of hypothyroidism on the activity and expression of the hepatic mitochondrial TCC was investigated in this study. TCC activity was significantly decreased in hypothyroid rats as compared with euthyroid animals. This hormone deficiency effect was due to a reduction in the amount of carrier protein, which resulted from a proportionate decrease of the specific mRNA. Hypothyroidism did not influence TCC mRNA stability. On the other hand, nuclear run-on assay revealed that the transcriptional rate of TCC mRNA decreased by approximately 40% in the nuclei from hypothyroid versus euthyroid rats. In addition, the ribonuclease protection assay showed that, in the nuclei of hypothyroid rats, the ratio of mature to precursor RNA decreased, indicating that the splicing of TCC RNA is affected. Furthermore, we found that the ratio of polyadenylated/unpolyadenylated TCC RNA as well as the length of the TCC RNA poly(A) tail were similar in both euthyroid and hypothyroid rats. Thus, the rate of formation of the TCC 3'-end is not altered in hypothyroidism. These results suggest that hypothyroidism affects TCC expression at both the transcriptional and post-transcriptional levels.

Essential role of citrate export from mitochondria at early differentiation stage of 3T3-L1 cells for their effective differentiation into fat cells, as revealed by studies using specific inhibitors of mitochondrial di- and tricarboxylate carriers

1,2,3-Benzenetricarboxylate (BTA) and n-butylmalonate (BM), specific inhibitors of the mitochondrial tricarboxylate and dicarboxylate carrier, respectively, have been used to study the contribution of citrate export from mitochondria to the accumulation of fat in 3T3-L1 cells. Continuous treatment of the cells with BTA or BM for 5 days after the induction of differentiation caused a significant reduction in fat accumulation in the cells in an inhibitor concentration-dependent manner. These inhibitory effects of BTA and BM were not due to their side effects on DNA replication, since similar inhibition of fat accumulation was not observed with ordinary inhibitors of DNA replication. A similar reduction in fat accumulation was also observed when the cells were treated with BTA or BM for only 2 days just after induction of differentiation. However, interestingly, treatment of the cells with an inhibitor starting 2 days after the induction did not result in reduced fat accumulation. Furthermore, Northern analysis clearly indicated that transcript levels of peroxisome proliferator-activated receptor gamma (PPARgamma) and adipose-type fatty acid binding protein (A-FABP) were well correlated with the levels of fat accumulation. These results clearly indicate the essential role of citrate export from the mitochondrial matrix to the cytosol at the early differentiation stage of 3T3-L1 cells for their effective differentiation into fat cells.

Different dietary fatty acids have dissimilar effects on activity and gene expression of mitochondrial tricarboxylate carrier in rat liver

The tricarboxylate carrier (TCC), an integral protein of the mitochondrial inner membrane, transports mitochondrial acetyl-CoA into the cytosol, where lipogenesis occurs. We investigated in rat liver mitochondria the effect of diets enriched with saturated fatty acids (beef tallow, BT), monounsaturated fatty acids (olive oil, OO) or n-3 polyunsaturated fatty acids (fish oil, FO), respectively, on the activity and expression of TCC. TCC activity decreased, in parallel with TCC mRNA abundance, only upon FO-feeding. The TCC transcription rate, mRNA turnover and RNA processing indicated that FO administration regulates TCC gene at transcriptional and post-transcriptional steps, whereas BT- and OO-feeding do not seem to affect either TCC activity or gene expression.

n-6 PUFAs downregulate expression of the tricarboxylate carrier in rat liver by transcriptional and posttranscriptional mechanisms

The tricarboxylate (citrate) carrier (TCC), a protein of the mitochondrial inner membrane, is an obligatory component of the shuttle system by which mitochondrial acetyl-CoA is transported into the cytosol, where lipogenesis occurs. The aim of this study was to investigate the molecular basis for the regulation of TCC gene expression by a high-fat, n-6 PUFA-enriched diet. Rats received for up to 4 weeks a diet enriched with 15% safflower oil (SO), which is high in linoleic acid (70.4%). We found a gradual decrease of TCC activity and a parallel decline in the abundance of TCC mRNA, the maximum effect occurring after 4 weeks of treatment. At this time, the estimated half-life of TCC mRNA was the same in the hepatocytes from rats on both diets, whereas the transcriptional rate of TCC mRNA, tested by nuclear run-on assay, was reduced by approximately 38% in the rats on the SO-enriched diet. The RNase protection assay showed that the ratio of mature to precursor RNA, measured in the nuclei, decreased with the change to the n-6 PUFA diet. These results suggest that administration of n-6 PUFAs to rats leads to changes not only in the transcriptional rate of the TCC gene but also in the processing of the nuclear precursor for TCC RNA.

Differential effects of coconut oil- and fish oil-enriched diets on tricarboxylate carrier in rat liver mitochondria

The mitochondrial tricarboxylate carrier (TCC) plays an important role in lipogenesis being TCC-responsible for the efflux from the mitochondria to the cytosol of acetyl-CoA, the primer for fatty acid synthesis. In this study, we investigated the effects of two high-fat diets with different fatty acid composition on the hepatic TCC activity. Rats were fed for 3 weeks on a basal diet supplemented with 15% of either coconut oil (CO), abundant in medium-chain saturated fatty acids, or fish oil (FO), rich in n-3 polyunsaturated fatty acids. Mitochondrial fatty acid composition was differently influenced by the dietary treatments, while no appreciable change in phospholipid composition and cholesterol level was observed. Compared with CO, the TCC activity was markedly decreased in liver mitochondria from FO-fed rats; kinetic analysis of the carrier revealed a decrease of the Vmax, with no change of the Km. No difference in the Arrhenius plot between the two groups was observed. Interestingly, the carrier protein level and the corresponding mRNA abundance decreased following FO treatment. These data indicate that FO administration markedly decreased the TCC activity as compared with CO. This effect is most likely due to a reduced gene expression of the carrier protein.

Citrate carrier activity and cardiolipin level in eel (Anguilla anguilla) liver mitochondria

The activity of the tricarboxylate (citrate) carrier has been assayed in intact liver mitochondria from yellow eel (Anguilla anguilla) and compared to that from rat. The eel-citrate carrier specific activity was approximately 1.7-fold higher than that assayed in rat-liver mitochondria. The content of the main mitochondrial phospholipids, phosphatidylethanolamine and phosphatidylcholine, did not show a significant difference between the two species, while in eel a higher cardiolipin level was observed. Fatty acid composition of eel-liver mitochondrial phospholipids was characterised by a large amount of unsaturated fatty acids, dominated by octadecaenoic acid (C(18:1) (n-9)) and docosahexaenoic acid (C(22:6) (n-3)). The cardiolipin fatty acid pattern of eel-liver mitochondria showed, with respect to the rat, a higher C(20:5) (n-3) and C(22:6) (n-3) content and a lower amount of C(18:2) (n-6) and C(20:4) (n-6). A noticeable activity of lipogenic enzymes was also detected in eel liver cytosol. The results of this study suggest that the remarkable activity of the citrate carrier in eel-liver mitochondria can most likely be ascribed to a considerable cardiolipin level. A covariance of citrate carrier and lipogenic enzyme activities was observed.

Molecular cloning of Aralar, a new member of the mitochondrial carrier superfamily that binds calcium and is present in human muscle and brain

We have identified a new calcium-dependent subfamily of mitochondrial carrier proteins with members in Saccharomyces cerevisiae, Caenorhabditis elegans, and various mammalian species. The members of this subfamily have a bipartite structure: a carboxyl-terminal half with the characteristic features of the mitochondrial solute carrier superfamily and an amino-terminal extension harboring various EF-hand domains. A member of this subfamily (that we have termed Aralar) was cloned from a human heart cDNA library. The corresponding cDNA comprises an open reading frame of 2037 base pairs encoding a polypeptide of 678 amino acids. The carboxyl-terminal half of Aralar (amino acids 321-678) has high similarity with the oxoglutarate, citrate, and adenine nucleotide carriers (28-29% identity), whereas the amino-terminal half (amino acids 1-320) contains three canonical EF-hands. Aralar amino-terminal half was shown to bind calcium by 45Ca2+ overlay and calcium-dependent mobility shift assays. The subcellular localization of the protein in COS cells transfected with Aralar was exclusively mitochondrial. Antibodies against Aralar amino-terminal fusion protein recognized a 70-kDa protein in brain mitochondrial fractions. Northern blot analysis showed that the protein was expressed in heart, brain, and skeletal muscle. The domain structure, mitochondrial localization, and presence in excitable tissues suggests a possible function of Aralar as calcium-dependent mitochondrial solute carrier.

High level expression and characterization of the mitochondrial citrate transport protein from the yeast Saccharomyces cerevisiae

The gene encoding the mitochondrial citrate transport protein (CTP) in the yeast Saccharomyces cerevisiae has been identified, and its protein product has been overexpressed in Escherichia coli. The expressed CTP accumulates in inclusion bodies and can be solubilized with sarkosyl. Approximately 25 mg of solubilized CTP at a purity of 75% is obtained per liter of E. coli culture. The function of the solubilized CTP has been reconstituted in a liposomal system where both its kinetic parameters (i.e. Km = 0.36 mM and Vmax = 2.5 mumol/min/mg protein) and its substrate specificity have been determined. Notably, the yeast CTP displays a stricter specificity for tricarboxylates than do CTPs from higher eukaryotic organisms. Dot matrix analysis of the yeast CTP sequence indicates the presence of three homologous sequence domains (each approximately 100 residues in length), which are also related to domains in other CTPs. Thus, the yeast CTP displays the tripartite structure characteristic of other mitochondrial transporters. Alignment of the yeast CTP sequence with CTPs from other sources defines a consensus sequence that displays 89 positions of amino acid identity, as well as the more generalized mitochondrial transporter-associated sequence motif. Based on hydropathy analysis, the yeast CTP contains six putative membrane-spanning alpha-helices. Finally, Southern blot analysis indicates that the yeast genome contains a single gene encoding the mitochondrial CTP. Our data indicate that, based on both its structural and functional properties, the expressed yeast CTP can be assigned membership in the mitochondrial carrier family. The identification of the yeast CTP gene, and the expression and purification of large quantities of its protein product, pave the way for investigations into the roles of specific amino acids in the CTP translocation mechanism, as well as for the initiation of crystallization trials.

Mitochondrial carrier proteins

Ten mitochondrial carriers have been purified from animal mitochondria. They are small proteins with a molecular mass ranging from 28 to 34 kDa on SDS-PAGE. So far, five of these proteins have been sequenced. Their polypeptide chain consists of three tandemly related sequences of about 100 amino acids. The repeats of the different proteins are related and probably fold into two transmembrane alpha-helices linked by an extra-membrane loop. The features of this family are also present in several proteins of unknown function characterized by DNA sequencing. Isoforms of some carriers have been found. All mitochondrial carriers investigated in proteoliposomes function according to a simultaneous (sequential) mechanism of transport. The only exception is the carnitine carrier that proceeds via a ping-pong mechanism. Three mitochondrial carriers have been expressed in yeast and two overexpressed in E. coli and refolded in active form.

Phosphate transport in mitochondria: past accomplishments, present problems, and future challenges

The requirement of inorganic phosphate (Pi) for oxidative phosphorylation in eukaryotic cells is fulfilled through specific Pi transport systems. The mitochondrial proton/phosphate symporter (Pic) is a membrane-embedded protein which translocates Pi from the cytosol into the mitochondrial matrix. Pic is responsible for the very rapid transport of most of the Pi used in ATP synthesis. During the past five years there have been advances on several fronts. Genomic and cDNA clones for yeast, bovine, rat, and human Pic have been isolated and sequenced. Functional expression of yeast Pic in yeast strains deficient in Pi transport and expression in Escherichia coli of a chimera protein involving Pic and ATP synthase alpha subunit have been accomplished. Pic, in contrast to other members of the family of transporters involved in energy metabolism, was demonstrated to have a presequence, which optimizes the import of the precursor protein into mitochondria. Six transmembrane segments appear to be a structural feature shared between Pic and other mitochondrial anion carriers, and recent-site directed mutagenesis studies implicate structure-functional relationships to bacteriorhodopsin. These recent advances on Pic will be assessed in light of a more global interpretation of transport mechanism across the inner mitochondrial membrane.