Adenine nucleotide transporters in organelles: novel genes and functions

Abstract

In eukaryotes, cellular energy in the form of ATP is produced in the cytosol via glycolysis or in the mitochondria via oxidative phosphorylation and, in photosynthetic organisms, in the chloroplast via photophosphorylation. Transport of adenine nucleotides among cell compartments is essential and is performed mainly by members of the mitochondrial carrier family, among which the ADP/ATP carriers are the best known. This work reviews the carriers that transport adenine nucleotides into the organelles of eukaryotic cells together with their possible functions. We focus on novel mechanisms of adenine nucleotide transport, including mitochondrial carriers found in organelles such as peroxisomes, plastids, or endoplasmic reticulum and also mitochondrial carriers found in the mitochondrial remnants of many eukaryotic parasites of interest. The extensive repertoire of adenine nucleotide carriers highlights an amazing variety of new possible functions of adenine nucleotide transport across eukaryotic organelles.

Fig. 1a–d

Structure and transport mechanism of the mitochondrial carriers. a Mitochondrial carriers contain three repetitions of 100 amino acids (separated by dashed lines), and each consists of two α-helixes connected by a segment that faces the mitochondrial matrix and makes a short helix parallel to the membrane plane. The substrate binding site of the carrier is located at the midpoint of the membrane (black arrow). The location of the matrix and cytoplasmic salt-bridge networks is indicated by red arrows. Conserved prolines are indicated by orange circles and conserved glycines are indicated by green circles. TM1–TM6 indicates the transmembrane helixes, and h12, h34, and h56 indicate the short helixes. b Putative substrate binding site viewed from the intermembrane space: the substrate binds to three binding points, I, II, and III, located in helixes 2, 4, and 6, respectively. In adenine nucleotide carriers, the adenine ring binds to contact point II, a hydrophobic pocket formed by G-[IVLM], whereas the phosphate groups bind to contact point I and contact point III, which are normally [RK]. c, d Cytoplasmic network of salt bridges (located in even-numbered helixes close to the intermembrane space) and matrix network of salt bridges (located in odd-numbered helixes close to the mitochondrial matrix). The positively and negatively charged residues of the salt bridges are shown in blue and red, respectively. The matrix network blocks the access to the matrix when the carrier is in the c conformation, whereas the cytoplasmic network blocks the access to the intermembrane space when the carrier is in the m conformation. Binding of the substrate to the carrier disrupts the salt bridges and triggers its conversion to the other state

Fig. 2

Phylogenetic tree of MCF members that transport nucleotides. Nucleotide carriers are subdivided into dinucleotide carriers (D; NAD⁺ carriers, pyrimidine nucleotide carriers, FAD carriers, and peroxisomal adenine nucleotide carriers) and mononucleotide carriers (M; thiamine pyrophosphate carriers, ATP-Mg/Pi carriers, SLC25A43-like carriers, ADP/ATP carriers, Ypr011c-like carriers, and coenzyme A carriers). Carriers characterized by direct transport measurements are marked with a black asterisk. Carriers whose function has been inferred by genetic studies or complementation assays are marked with a red asterisk. Adenine nucleotide carriers from amitochondriate organisms mentioned in the text are in red. Protein sequence multiple alignment was performed by Clustal W software. The phylogenetic unrooted tree was obtained by neighbor-joining based on the amino acid pairwise distance with the Poisson-correction method using the Mega4.0 software

Fig. 3

Function of the mitochondrial ADP/ATP carrier. Schematic representation of the oxidative phosphorylation process. The NADH generated by lipid oxidation (β Ox), in the Krebs cycle (TCA) and other reactions, and succinate (Succ) generated in the TCA are oxidated by the respiratory chain complexes (complexes I, II, III, and IV in green, cytochrome C in blue), and protons are pumped to the intermembrane space. The proton electrochemical gradient (∆μH⁺) is used by the H⁺-ATP synthase (complex V) to generate ATP from ADP and phosphate (Pi). Pi is imported into the mitochondrial matrix in symport with a proton through its own transporter (PiC, SLC25A3). The ATP generated is transported to the cytosol in exchange for ADP through the ADP/ATP carrier (AAC). The AAC may be inhibited by CAT and BKA, which act on opposite sides of the membrane

Fig. 4

Function of the mitochondrial ATP-Mg/Pi carrier. The ATP-Mg/Pi carrier (SCaMC) exchanges ATP-Mg/Pi for Pi (as HPO₄ ²⁻) between the cytosol and the mitochondria. The electron transport chain generates a ΔpH, which in turn generates a Pi gradient through the Pi carrier (PiC). This gradient allows the entry of ATP-Mg into the mitochondria, even against concentration gradient, whenever the carrier is activated by a Ca²⁺ signal in the intermembrane space. The ATP-Mg/Pi carrier contributes to the regulation of the net content of matrix adenine nucleotides (ATP + ADP + AMP). The ADP/ATP carrier (AAC) is unable to do so, as it always exchanges one nucleotide for another. Changes in the matrix adenine nucleotide pool regulate the reactions shown in the figure (Adapted from [29, 30])

Fig. 5

Function of the mitochondrial coenzyme A carrier. The CoA carrier exchanges cytosolic CoA for mitochondrial ADP. CoA is used: 1 in the β-oxidation of fatty acids (β Ox); 2 in the synthesis of N-acetylglutamate, which is an activator of the urea cycle; 3 in the catabolism of branched amino acids (leucine, isoleucine, and valine), generating succinyl-CoA, which goes into the Krebs cycle; 4 in the Krebs cycle (TCA) at the level of pyruvate dehydrogenase (to generate acetyl-CoA) and α-ketoglutarate dehydrogenase (to generate succinyl-CoA, which may also be used for heme synthesis); 5 to provide the mitochondrial acyl carrier protein, a key component of the mitochondrial type II fatty acid synthase (TII FAS, involved in the generation of octanoate, a precursor of the key mitochondrial cofactor lipoate), with its prosthetic group, phosphopantetheine, with production of adenosine 3′,5′-diphosphate, which is exported to the cytosol by the CoA carrier instead of ADP

Fig. 6

Function of the Arabidopsis mitochondrial AMP/ATP carrier. In plants that emerge from dormancy or in plants under anoxic stress, AMP is the predominant nucleotide in the cytosol. Cytosolic AMP is exchanged for catalytic amounts of mitochondrial ATP through the AMP/ATP carrier (1). The ATP that appears in the intermembrane space reacts with another AMP (2) and generates two molecules of ADP (adenylate kinase reaction). The newly formed ADP is exchanged with mitochondrial ATP through the ADP/ATP carrier (AAC), and this allows oxidative phosphorylation and respiration to start (3). In order for the AMP/ATP carrier reaction to proceed, AMP must somehow leave the mitochondrial matrix (4)

Fig. 7a, b

Function of hydrogenosomal and mitosomal adenine nucleotide transporters. a Hydrogenosomes generate H₂ and ATP by substrate-level phosphorylation. The ATP is used for protein import, chaperone function, or synthesis of Fe-S clusters, and is also exported to the cytosol in exchange for ADP by an adenine nucleotide transporter (ANT). b Mitosomes do not produce ATP, and thus it must be imported from the cytosol to support mitosomal reactions by an adenine nucleotide transporter (ANT)

Fig. 8

Plastidic adenine nucleotide transporters. In plastids there are several adenine nucleotide transporters. The plastid envelope adenine nucleotide carriers (AATP) exchange cytosolic ATP for mitochondrial ADP and Pi, and this ATP is used for night reactions, including synthesis of lipids and starch synthesis. The thylakoid adenine nucleotide carriers (TAAC) exchange stromal ATP (generated by photophosphorylation in the plastid ATP synthase) for thylakoidal ADP, and this ATP is used for thylakoid reactions, such as protein phosphorylation, folding, import, and degradation, turnover and repair of the photosystem II complex. The ubiquitous BT1 (BT1-2) transporter exports adenine nucleotides, which are exclusively synthesized de novo in plastids, to the cytosol. The related BT1 (BT1-1) transporter, which is only present in monocotyledonous plants, imports cytosolic ADP-glucose, required for starch synthesis, in exchange for plastidic ADP

Fig. 9

Function of the peroxisomal adenine nucleotide transporter. The peroxisomal adenine nucleotide transporter (pANT) exchanges cytosolic ATP and an as yet undetermined number of protons for peroxisomal AMP. ATP is used in the activation of fatty acids for peroxisomal β oxidation (β Ox), and the protons generate a peroxisomal ∆pH