Identification by site-directed mutagenesis of a hydrophobic binding site of the mitochondrial carnitine/acylcarnitine carrier involved in the interaction with acyl groups

Insights into Transient Dimerisation of Carnitine/Acylcarnitine Carrier (SLC25A20) from Sarkosyl/PAGE, Cross-Linking Reagents, and Comparative Modelling Analysis

The carnitine/acylcarnitine carrier (CAC) is a crucial protein for cellular energy metabolism, facilitating the exchange of acylcarnitines and free carnitine across the mitochondrial membrane, thereby enabling fatty acid β-oxidation and oxidative phosphorylation (OXPHOS). Although CAC has not been crystallised, structural insights are derived from the mitochondrial ADP/ATP carrier (AAC) structures in both cytosolic and matrix conformations. These structures underpin a single binding centre-gated pore mechanism, a common feature among mitochondrial carrier (MC) family members. The functional implications of this mechanism are well-supported, yet the structural organization of the CAC, particularly the formation of dimeric or oligomeric assemblies, remains contentious. Recent investigations employing biochemical techniques on purified and reconstituted CAC, alongside molecular modelling based on crystallographic AAC dimeric structures, suggest that CAC can indeed form dimers. Importantly, this dimerization does not alter the transport mechanism, a phenomenon observed in various other membrane transporters across different protein families. This observation aligns with the ping-pong kinetic model, where the dimeric form potentially facilitates efficient substrate translocation without necessitating mechanistic alterations. The presented findings thus contribute to a deeper understanding of CAC's functional dynamics and its structural parallels with other MC family members.

In Silico Analysis of the Structural Dynamics and Substrate Recognition Determinants of the Human Mitochondrial Carnitine/Acylcarnitine SLC25A20 Transporter

The Carnitine-Acylcarnitine Carrier is a member of the mitochondrial Solute Carrier Family 25 (SLC25), known as SLC25A20, involved in the electroneutral exchange of acylcarnitine and carnitine across the inner mitochondrial membrane. It acts as a master regulator of fatty acids β-oxidation and is known to be involved in neonatal pathologies and cancer. The transport mechanism, also known as "alternating access", involves a conformational transition in which the binding site is accessible from one side of the membrane or the other. In this study, through a combination of state-of-the-art modelling techniques, molecular dynamics, and molecular docking, the structural dynamics of SLC25A20 and the early substrates recognition step have been analyzed. The results obtained demonstrated a significant asymmetry in the conformational changes leading to the transition from the c- to the m-state, confirming previous observations on other homologous transporters. Moreover, analysis of the MD simulations' trajectories of the apo-protein in the two conformational states allowed for a better understanding of the role of SLC25A20 Asp231His and Ala281Val pathogenic mutations, which are at the basis of Carnitine-Acylcarnitine Translocase Deficiency. Finally, molecular docking coupled to molecular dynamics simulations lend support to the multi-step substrates recognition and translocation mechanism already hypothesized for the ADP/ATP carrier.

Proline/Glycine residues of the PG-levels guide conformational changes along the transport cycle in the mitochondrial carnitine/acylcarnitine carrier (SLC25A20)

Mitochondrial carnitine/acylcarnitine carrier (CAC) is a member of the mitochondrial carrier (MC) family and imports acylcarnitine into the mitochondrial matrix in exchange for carnitine, playing a pivotal role in carnitine shuttle, crucial for fatty acid oxidation. The crystallized structure of CAC has not been solved yet, however, the availability of several in vitro/in silico studies, also based on the crystallized structures of the ADP/ATP carrier in the cytosolic-conformation and in the matrix-conformation, has made possible to confirm the hypothesis of the single-binding centered-gated pore mechanism for all the members of the MC family. In addition, our recent bioinformatics analyses allowed quantifying in silico the importance of protein residues of MC substrate binding region, of those involved in the formation of the matrix and cytosolic gates, and of those belonging to the Pro/Gly (PG) levels, proposed to be crucial for the tilting/kinking/bending of the six MC transmembrane helices, funneling the substrate translocation pathway. Here we present a combined in silico/in vitro analysis employed for investigating the role played by a group of 6 proline residues and 6 glycine residues, highly conserved in CAC, belonging to MC PG-levels. Residues of the PG-levels surround the similarly located MC common substrate binding region, and were proposed to lead conformational changes and substrate translocation, following substrate binding. For our analysis, we employed 3D molecular modeling approaches, alanine scanning site-directed mutagenesis and in vitro transport assays. Our analysis reveals that P130 (H3), G268 (H6) and G220 (H5), mutated in alanine, affect severely CAC transport activity (mutant catalytic efficiency lower than 5 % compared to the wild type CAC), most likely due to their major role in triggering CAC conformational changes, following carnitine binding. Notably, P30A (H1) and G121A (H3) CAC mutants, increase the carnitine uptake up to 217 % and 112 %, respectively, compared to the wild type CAC.

Mitochondrial transport and metabolism of the vitamin B-derived cofactors thiamine pyrophosphate, coenzyme A, FAD and NAD+ , and related diseases: A review

Multiple mitochondrial matrix enzymes playing key roles in metabolism require cofactors for their action. Due to the high impermeability of the mitochondrial inner membrane, these cofactors need to be synthesized within the mitochondria or be imported, themselves or one of their precursors, into the organelles. Transporters belonging to the protein family of mitochondrial carriers have been identified to transport the coenzymes: thiamine pyrophosphate, coenzyme A, FAD and NAD+ , which are all structurally similar to nucleotides and derived from different B-vitamins. These mitochondrial cofactors bind more or less tightly to their enzymes and, after having been involved in a specific reaction step, are regenerated, spontaneously or by other enzymes, to return to their active form, ready for the next catalysis round. Disease-causing mutations in the mitochondrial cofactor carrier genes compromise not only the transport reaction but also the activity of all mitochondrial enzymes using that particular cofactor and the metabolic pathways in which the cofactor-dependent enzymes are involved. The mitochondrial transport, metabolism and diseases of the cofactors thiamine pyrophosphate, coenzyme A, FAD and NAD+ are the focus of this review.

Mitochondrial transport and metabolism of the major methyl donor and versatile cofactor S-adenosylmethionine, and related diseases: A review†

S-adenosyl-L-methionine (SAM) is a coenzyme and the most commonly used methyl-group donor for the modification of metabolites, DNA, RNA and proteins. SAM biosynthesis and SAM regeneration from the methylation reaction product S-adenosyl-L-homocysteine (SAH) take place in the cytoplasm. Therefore, the intramitochondrial SAM-dependent methyltransferases require the import of SAM and export of SAH for recycling. Orthologous mitochondrial transporters belonging to the mitochondrial carrier family have been identified to catalyze this antiport transport step: Sam5p in yeast, SLC25A26 (SAMC) in humans, and SAMC1-2 in plants. In mitochondria SAM is used by a vast number of enzymes implicated in the following processes: the regulation of replication, transcription, translation, and enzymatic activities; the maturation and assembly of mitochondrial tRNAs, ribosomes and protein complexes; and the biosynthesis of cofactors, such as ubiquinone, lipoate, and molybdopterin. Mutations in SLC25A26 and mitochondrial SAM-dependent enzymes have been found to cause human diseases, which emphasizes the physiological importance of these proteins.

NMR Characterization of Long-Chain Fatty Acylcarnitine Binding to the Mitochondrial Carnitine/Acylcarnitine Carrier

The mitochondrial carnitine/acylcarnitine carrier (CAC) transports short-, medium- and long-carbon chain acylcarnitines across the mitochondrial inner membrane in exchange for carnitine. How CAC recognizes the substrates with various fatty acyl groups, especially long-chain fatty acyl groups, remains unclear. Here, using nuclear magnetic resonance (NMR) technology, we have shown that the CAC protein reconstituted into a micelle system exhibits a typical six transmembrane structure of the mitochondrial carrier family. The chemical shift perturbation patterns of different fatty acylcarnitines suggested that the segment A76–G81 in CAC specifically responds to the long-chain fatty acylcarnitine. Molecular dynamics (MD) simulations of palmitoyl-L-carnitine inside the CAC channel showed the respective interaction and motion of the long-chain acylcarnitine in CAC at the cytosol-open state and matrix-open state. Our data provided a molecular-based understanding of CAC structure and transport mechanism.

The Mitochondrial Carnitine Acyl-carnitine Carrier (SLC25A20): Molecular Mechanisms of Transport, Role in Redox Sensing and Interaction with Drugs

The SLC25A20 transporter, also known as carnitine acyl-carnitine carrier (CAC), catalyzes the transport of short, medium and long carbon chain acyl-carnitines across the mitochondrial inner membrane in exchange for carnitine. The 30-year story of the protein responsible for this function started with its purification from rat liver mitochondria. Even though its 3D structure is not yet available, CAC is one of the most deeply characterized transport proteins of the inner mitochondrial membrane. Other than functional, kinetic and mechanistic data, post-translational modifications regulating the transport activity of CAC have been revealed. CAC interactions with drugs or xenobiotics relevant to human health and toxicology and the response of the carrier function to dietary compounds have been discovered. Exploiting combined approaches of site-directed mutagenesis with chemical targeting and bioinformatics, a large set of data on structure/function relationships have been obtained, giving novel information on the molecular mechanism of the transport catalyzed by this protein.

Diseases Caused by Mutations in Mitochondrial Carrier Genes SLC25: A Review

In the 1980s, after the mitochondrial DNA (mtDNA) had been sequenced, several diseases resulting from mtDNA mutations emerged. Later, numerous disorders caused by mutations in the nuclear genes encoding mitochondrial proteins were found. A group of these diseases are due to defects of mitochondrial carriers, a family of proteins named solute carrier family 25 (SLC25), that transport a variety of solutes such as the reagents of ATP synthase (ATP, ADP, and phosphate), tricarboxylic acid cycle intermediates, cofactors, amino acids, and carnitine esters of fatty acids. The disease-causing mutations disclosed in mitochondrial carriers range from point mutations, which are often localized in the substrate translocation pore of the carrier, to large deletions and insertions. The biochemical consequences of deficient transport are the compartmentalized accumulation of the substrates and dysfunctional mitochondrial and cellular metabolism, which frequently develop into various forms of myopathy, encephalopathy, or neuropathy. Examples of diseases, due to mitochondrial carrier mutations are: combined D-2- and L-2-hydroxyglutaric aciduria, carnitine-acylcarnitine carrier deficiency, hyperornithinemia-hyperammonemia-homocitrillinuria (HHH) syndrome, early infantile epileptic encephalopathy type 3, Amish microcephaly, aspartate/glutamate isoform 1 deficiency, congenital sideroblastic anemia, Fontaine progeroid syndrome, and citrullinemia type II. Here, we review all the mitochondrial carrier-related diseases known until now, focusing on the connections between the molecular basis, altered metabolism, and phenotypes of these inherited disorders.

Characterization of Exosomal SLC22A5 (OCTN2) carnitine transporter

Exosomes are extracellular vesicles involved in cell-to-cell communication. Previous large scale proteomics revealed that they contain SLC proteins. However, no data on the function of exosomal SLCs is available, so far. An SLC localized in exosomes was here characterized for the first time: the carnitine transporter OCTN2 (SLC22A5). The protein was detected by Western Blot analysis in HEK293 exosomes. To investigate the functional properties of the exosomal OCTN2, the proteins extracted from vesicles were reconstituted into proteolipsomes and the transport function was measured as uptake of ³H-carnitine. Transport was stimulated by sodium and was dependent on pH. ³H-carnitine uptake was inhibited by Acetyl-carnitine, but not by Asn, Gln and Arg thus excluding interference by ATB^0,+, an amino acid transporter which also recognizes carnitine. Cardiolipin failed to stimulate transport, excluding the activity of the mitochondrial Carnitine/acylcarnitine transporter. Increased level of exosomal OCTN2 was induced by treatment of HEK293 with the pro-inflammatory cytokine INFγ. All data concurred to demonstrate that OCTN2 present in exosomes is fully functional and is in its native conformation. Functional OCTN2 was detected also in human urinary exosomes, thus suggesting the OCTN2 exosomal protein as a candidate biomarker for inflammation related pathologies.

The molecular features of uncoupling protein 1 support a conventional mitochondrial carrier-like mechanism

Uncoupling protein 1 (UCP1) is an integral membrane protein found in the mitochondrial inner membrane of brown adipose tissue, and facilitates the process of non-shivering thermogenesis in mammals. Its activation by fatty acids, which overcomes its inhibition by purine nucleotides, leads to an increase in the proton conductance of the inner mitochondrial membrane, short-circuiting the mitochondrion to produce heat rather than ATP. Despite 40 years of intense research, the underlying molecular mechanism of UCP1 is still under debate. The protein belongs to the mitochondrial carrier family of transporters, which have recently been shown to utilise a domain-based alternating-access mechanism, cycling between a cytoplasmic and matrix state to transport metabolites across the inner membrane. Here, we review the protein properties of UCP1 and compare them to those of mitochondrial carriers. UCP1 has the same structural fold as other mitochondrial carriers and, in contrast to past claims, is a monomer, binding one purine nucleotide and three cardiolipin molecules tightly. The protein has a single substrate binding site, which is similar to those of the dicarboxylate and oxoglutarate carriers, but also contains a proton binding site and several hydrophobic residues. As found in other mitochondrial carriers, UCP1 has two conserved salt bridge networks on either side of the central cavity, which regulate access to the substrate binding site in an alternating way. The conserved domain structures and mobile inter-domain interfaces are consistent with an alternating access mechanism too. In conclusion, UCP1 has retained all of the key features of mitochondrial carriers, indicating that it operates by a conventional carrier-like mechanism.

Glutamine transport. From energy supply to sensing and beyond

Glutamine is the most abundant amino acid in plasma and is actively involved in many biosynthetic and regulatory processes. It can be synthesized endogenously but becomes "conditionally essential" in physiological or pathological conditions of high proliferation rate. To accomplish its functions glutamine has to be absorbed and distributed in the whole body. This job is efficiently carried out by a network of membrane transporters that differ in transport mechanisms and energetics, belonging to families SLC1, 6, 7, 38, and possibly, 25. Some of the transporters are involved in glutamine traffic across different membranes for metabolic purposes; others are involved in specific signaling functions through mTOR. Structure/function relationships and regulatory aspects of glutamine transporters are still at infancy. In the while, insights in involvement of these transporters in cell redox control, cancer metabolism and drug interactions are arising, stimulating basic research to uncover molecular mechanisms of transport and regulation. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

Mitochondrial carnitine/acylcarnitine transporter, a novel target of mercury toxicity

The effect of Hg(2+) and CH3Hg(+) on the mitochondrial carnitine/acylcarnitine transporter (CACT) has been studied on the recombinant protein and on the CACT extracted from HeLa cells or Zebrafish and reconstituted in proteoliposomes. Transport was abolished upon treatment of the recombinant CACT in proteoliposomes by Hg(2+) or CH3Hg(+). Inhibition was reversed by the SH reducing agent 1,4-dithioerythritol, GSH, and N-acetylcysteine. IC50 for Hg(2+) and CH3Hg(+) of 90 nM and 137 nM, respectively, were measured by dose-response analyses. Inhibition was abolished in the C-less CACT mutant. Strong reduction of inhibition by both reagents was observed in the C136A and some reduction in the C155A mutants. Inhibition similar to that of the WT was observed in the C23V/C58V/C89S/C155V/C283S mutant, containing only C136. Optimal inhibition by Hg(2+)was found in the four replacement mutants C23V/C58V/C89S/C283S containing both C136 and C155 indicating cross-reaction of Hg(2+) with the two Cys residues. Inhibition kinetic analysis showed mixed inhibition by Hg(2+) or competitive inhibition by CH3Hg(+). HeLa cells or Zebrafish were treated with the more potent inhibitor. Ten micromolar HgCl2 caused clear impairment of viability of HeLa cells. The transport assay in proteoliposomes with CACT extracted from treated cells showed that the transporter was inactivated and that DTE rescued the activity. Nearly identical results were observed with Zebrafish upon extraction of the CACT from the liver of the treated animals that, indeed, showed accumulation of the mercurial compound.

Antiporters of the mitochondrial carrier family

The eukaryotic transport protein family SLC25 consists of mitochondrial carriers (MCs) that are recognized on the sequence level by a threefold repeated and conserved signature motif. The majority of MCs characterized so far catalyzes strict exchanges of substrates across the mitochondrial inner membrane. The substrates are nucleotides, metabolic intermediates, and cofactors that are required in cytoplasmic and matrix metabolism. This review summarizes and discusses the current knowledge of the antiport mechanism(s) of MCs that has been deduced from determining transport characteristics and by analyzing structural, sequence, and mutagenesis data. The mode of transport varies among different MCs with respect to how the substrate translocation depends on the electrical and pH gradients across the mitochondrial inner membrane, for example, the ADP/ATP carrier is electrogenic (electrophoretic), the GTP/GDP carrier is dependent on the pH gradient, the aspartate/glutamate carrier is dependent on both, and the oxoglutarate/malate carrier is independent of them. The structure of the bovine ADP/ATP carrier consists of a six-transmembrane α-helix bundle with a pseudo-threefold symmetry and a closed matrix gate. By using this structure as a template in homology modeling, residues engaged in substrate binding and the formation of a cytoplasmic gate in MCs have been proposed. The functional importance of the residues of the binding site, the matrix, and the cytoplasmic gates is supported by transport activities of different MCs with single point mutations. Cumulative evidence has been used to postulate a general transport mechanism for MCs.

Single-nucleotide evolution quantifies the importance of each site along the structure of mitochondrial carriers

Mitochondrial carriers are membrane-embedded proteins consisting of a tripartite structure, a three-fold pseudo-symmetry, related sequences, and similar folding whose main function is to catalyze the transport of various metabolites, nucleotides, and coenzymes across the inner mitochondrial membrane. In this study, the evolutionary rate in vertebrates was screened at each of the approximately 50,000 nucleotides corresponding to the amino acids of the 53 human mitochondrial carriers. Using this information as a starting point, a scoring system was developed to quantify the evolutionary pressure acting on each site of the common mitochondrial carrier structure and estimate its functional or structural relevance. The degree of evolutionary selection varied greatly among all sites, but it was highly similar among the three symmetric positions in the tripartite structure, known as symmetry-related sites or triplets, suggesting that each triplet constitutes an evolutionary unit. Based on evolutionary selection, 111 structural sites (37 triplets) were found to be important. These sites play a key role in structure/function of mitochondrial carriers and are involved in either conformational changes (sites of the gates, proline-glycine levels, and aromatic belts) or in binding and specificity of the transported substrates (sites of the substrate-binding area in between the two gates). Furthermore, the evolutionary pressure analysis revealed that the matrix short helix sites underwent different degrees of selection with high inter-paralog variability. Evidence is presented that these sites form a new sequence motif in a subset of mitochondrial carriers, including the ADP/ATP translocator, and play a regulatory function by interacting with ligands and/or proteins of the mitochondrial matrix.

Molecular mechanism of inhibition of the mitochondrial carnitine/acylcarnitine transporter by omeprazole revealed by proteoliposome assay, mutagenesis and bioinformatics

The effect of omeprazole on the mitochondrial carnitine/acylcarnitine transporter has been studied in proteoliposomes. Externally added omeprazole inhibited the carnitine/carnitine antiport catalysed by the transporter. The inhibition was partially reversed by DTE indicating that it was caused by the covalent reaction of omeprazole with Cys residue(s). Inhibition of the C-less mutant transporter indicated also the occurrence of an alternative non-covalent mechanism. The IC₅₀ of the inhibition of the WT and the C-less CACT by omeprazole were 5.4 µM and 29 µM, respectively. Inhibition kinetics showed non competitive inhibition of the WT and competitive inhibition of the C-less. The presence of carnitine or acylcarnitines during the incubation of the proteoliposomes with omeprazole increased the inhibition. Using site-directed Cys mutants it was demonstrated that C283 and C136 were essential for covalent inhibition. Molecular docking of omeprazole with CACT indicated the formation of both covalent interactions with C136 and C283 and non-covalent interactions in agreement with the experimental data.

Localization of mitochondrial carnitine/acylcarnitine translocase in sensory neurons from rat dorsal root ganglia

The carnitine/acylcarnitine transporter is a transport system whose function is essential for the mitochondrial β-oxidation of fatty acids. Here, the presence of carnitine/acylcarnitine carrier (CACT) in nervous tissue and its sub-cellular localization in dorsal root ganglia (DRG) neurons have been investigated. Western blot analysis using a polyclonal anti-CACT antibody produced in our laboratory revealed the presence of CACT in all the nervous tissue extracts analyzed. Confocal microscopy experiments performed on fixed and permeabilized DRG neurons co-stained with the anti-CACT antibody and the mitochondrial marker MitoTracker Red clearly showed a mitochondrial localization for the carnitine/acylcarnitine transporter. The transport activity of CACT from DRG extracts reconstituted into liposomes was about 50 % in respect to liver extracts. The experimental data here reported represent the first direct evidence of the expression of the carnitine/acylcarnitine transporter in sensory neurons, thus supporting the existence of the β-oxidation pathway in these cells.

Oleoyl-L-carnitine inhibits glycine transport by GlyT2

BACKGROUND AND PURPOSE

Concentrations of extracellular glycine in the CNS are regulated by two Na(+)/Cl(-) -dependent glycine transporters, GlyT1 and GlyT2. Selective inhibitors of GlyT1 have been developed for the treatment of schizophrenia, whilst selective inhibitors of GlyT2 are analgesic in animal models of pain. We have assessed a series of endogenous lipids as inhibitors of GlyT1 and GlyT2.

EXPERIMENTAL APPROACH

Human GlyT1 and GlyT2 were expressed in Xenopus laevis oocytes, and the inhibitory actions of a series of acylcarnitines on glycine transport were measured using electrophysiological techniques.

KEY RESULTS

Oleoyl-L-carnitine inhibited glycine transport by GlyT2, with an IC(50) of 340 nM, which is 15-fold more potent than the previously identified lipid inhibitor N-arachidonyl-glycine. Oleoyl-L-carnitine had a slow onset of inhibition and a slow washout. Using a series of chimeric GlyT1/2 transporters and point mutant transporters, we have identified an isoleucine residue in extracellular loop 4 of GlyT2 that conferred differences in sensitivity to oleoyl-L-carnitine between GlyT2 and GlyT1.

CONCLUSIONS AND IMPLICATIONS

Oleoyl-L-carnitine is a potent non-competitive inhibitor of GlyT2. Previously identified GlyT2 inhibitors show potential as analgesics and the identification of oleoyl-L-carnitine as a novel GlyT2 inhibitor may lead to new ways of treating pain.

Transcriptional Regulation of the Mitochondrial Citrate and Carnitine/Acylcarnitine Transporters: Two Genes Involved in Fatty Acid Biosynthesis and β-oxidation

Transcriptional regulation of genes involved in fatty acid metabolism is considered the major long-term regulatory mechanism controlling lipid homeostasis. By means of this mechanism, transcription factors, nutrients, hormones and epigenetics control not only fatty acid metabolism, but also many metabolic pathways and cellular functions at the molecular level. The regulation of the expression of many genes at the level of their transcription has already been analyzed. This review focuses on the transcriptional control of two genes involved in fatty acid biosynthesis and oxidation: the citrate carrier (CIC) and the carnitine/ acylcarnitine/carrier (CAC), which are members of the mitochondrial carrier gene family, SLC25. The contribution of tissue-specific and less tissue-specific transcription factors in activating or repressing CIC and CAC gene expression is discussed. The interaction with drugs of some transcription factors, such as PPAR and FOXA1, and how this interaction can be an attractive therapeutic approach, has also been evaluated. Moreover, the mechanism by which the expression of the CIC and CAC genes is modulated by coordinated responses to hormonal and nutritional changes and to epigenetics is highlighted.

The substrate specificity of mitochondrial carriers: mutagenesis revisited

Mitochondrial carriers transport inorganic ions, nucleotides, amino acids, keto acids and cofactors across the mitochondrial inner membrane. Structurally they consist of three domains, each containing two transmembrane α-helices linked by a short α-helix and loop. The substrate binds to three major contact points in the central cavity. The class of substrate (e.g., adenine nucleotides) is determined by contact point II on transmembrane α-helix H4 and the type of substrate within the class (e.g., ADP, coenzyme A) by contact point I in H2, whereas contact point III on H6 is most usually a positively charged residue, irrespective of the type or class. Two salt bridge networks, consisting of conserved and symmetric residues, are located on the matrix and cytoplasmic side of the cavity. These residues are part of the gates that are involved in opening and closing of the carrier during the transport cycle, exposing the central substrate binding site to either side of the membrane in an alternating way. Here we revisit the plethora of mutagenesis data that have been collected over the last two decades to see if the residues in the proposed binding site and salt bridge networks are indeed important for function. The analysis shows that the major contact points of the substrate binding site are indeed crucial for function and in defining the specificity. The matrix salt bridge network is more critical for function than the cytoplasmic salt bridge network in agreement with its central position, but neither is likely to be involved in substrate recognition directly.

The peroxisomal NAD+ carrier of Arabidopsis thaliana transports coenzyme A and its derivatives

The peroxisomal protein PXN encoded by the Arabidopsis gene At2g39970 has very recently been found to transport NAD+, NADH, AMP and ADP. In this work we have reinvestigated the substrate specificity and the transport properties of PXN by using a wide range of potential substrates. Heterologous expression in bacteria followed by purification, reconstitution in liposomes, and uptake and efflux experiments revealed that PNX transports coenzyme A (CoA), dephospho-CoA, acetyl-CoA and adenosine 3', 5'-phosphate (PAP), besides NAD+, NADH, AMP and ADP. PXN catalyzed fast counter-exchange of substrates and much slower uniport and was strongly inhibited by pyridoxal 5'-phosphate, bathophenanthroline and tannic acid. Transport was saturable with a submillimolar affinity for NAD+, CoA and other substrates. The physiological role of PXN is probably to provide the peroxisomes with the essential coenzymes NAD+ and CoA.