REVIEW ARTICLE published: 09 January 2012 doi: 10.3389/fpls.2011.00110 Phylogenetic analysis of the thylakoid ATP/ADP carrier reveals new insights into its function restricted to green plants Cornelia Spetea1*, Bernard E. Pfeil 1 and Benoît Schoefs 2 1 Department of Plant and Environmental Sciences, University of Gothenburg, Gothenburg, Sweden 2 Mer, Molécules, Santé, Faculté des Sciences et Techniques, Université du Maine à Le Mans, Le Mans, France Edited by: ATP is the common energy currency of cellular metabolism in all living organisms. Most Heven Sze, University of Maryland, of them synthesize ATP in the cytosol or on the mitochondrial inner membrane, whereas USA land plants, algae, and cyanobacteria also produce it on the thylakoid membrane during Reviewed by: Daniel Hofius, University of the light-dependent reactions of photosynthesis. From the site of synthesis, ATP is trans- Copenhagen, Denmark ported to the site of utilization via intracellular membrane transporters. One major type Uener Kolukisaoglu, University of of ATP transporters is represented by the mitochondrial ADP/ATP carrier family. Here we Tuebingen, Germany review a recently characterized member, namely the thylakoid ATP/ADP carrier from Ara- *Correspondence: bidopsis thaliana (AtTAAC). Thus far, no orthologs of this carrier have been characterized Cornelia Spetea, Department of Plant and Environmental Sciences, in other organisms, although similar sequences can be recognized in many sequenced University of Gothenburg, 40530 genomes. Protein Sequence database searches and phylogenetic analyses indicate the Gothenburg, Sweden. absence of TAAC in cyanobacteria and its appearance early in the evolution of photosyn- e-mail: cornelia.spetea.wiklund@ thetic eukaryotes. The TAAC clade is composed of carriers found in land plants and some dpes.gu.se green algae, but no proteins from other photosynthetic taxa, such as red algae, brown algae, and diatoms.This implies thatTAAC-like sequences arose only once before the divergence of green algae and land plants. Based on these findings, it is proposed thatTAAC may have evolved in response to the need of a new activity in higher photosynthetic eukaryotes.This activity may provide the energy to drive reactions during biogenesis and turnover of pho- tosynthetic complexes, which are heterogeneously distributed in a thylakoid membrane system composed of appressed and non-appressed regions. Keywords: green alga, chloroplast, plant, photosynthesis, ADP/ATP carrier, thylakoid,TAAC phylogeny INTRODUCTION host cytosol and the cyanobiont (Facchinelli and Weber, 2011). Oxygenic photosynthesis is a biophysicochemical process that con- There are so far no evolutionary studies dedicated to solute trans- verts carbon dioxide into organic compounds using sunlight as a porters from the outer envelope or thylakoid membranes, but source of energy. It occurs in the chloroplasts of land plants and it is believed that they originate from proteins in the ancestral algae, and also in the cytoplasm of cyanobacteria, and uses water cyanobacterial outer and thylakoid membranes, respectively. as a source of electrons, releasing oxygen as a waste product (for Mitochondria and chloroplasts are the two organelles able to a recent review, see Hohmann-Marriott and Blankenship, 2011). synthesize ATP, which is the universal energy currency of cellu- The chloroplast in algae and plants has evolved from a cyanobac- lar metabolism in all living organisms. The difference between terial ancestor via endosymbiosis with a primitive eukaryotic host. these organelles in this respect is that in all eukaryotes mito- It is a highly compartmentalized organelle, with three mem- chondria produce ATP via oxidative phosphorylation on the inner brane systems (outer envelope, inner envelope, and thylakoid) and membrane, to be used during cell metabolism. Chloroplasts and three soluble spaces (intermembrane space, stroma, and thylakoid also cyanobacteria use sunlight as a source of energy to pro- lumen). A wide variety of solute and metabolite transporters reside duce ATP (photophosphorylation) on the thylakoid membrane, within the different types of chloroplast membranes and mediate which is consumed during CO2 fixation in the stroma. In addi- communication between the cytosol, stroma, and lumen. Several tion, ATP is used for energy-dependent reactions on the envelope excellent reviews on the identification and functional characteri- and thylakoid membrane or inside the thylakoid lumen (Spetea zation of these transporters have recently become available (Spetea and Thuswaldner, 2008). and Schoefs, 2010; Breuers et al., 2011; Flügge et al., 2011; Weber ATP is the largest and most highly charged solute transported and Linka, 2011). across organellar membranes. Two structurally and phylogeneti- Regarding their evolution, the majority of the inner envelope cally different types of ATP transporters are represented in chloro- metabolite transporters have been shown to have a host origin,dri- plasts, namely ATP/ADP antiporters (AAA, TC #2.1.12, according ven by the requirement to establish communication between the to Saier et al., 2009) and mitochondrial ADP/ATP carriers (AAC, www.frontiersin.org January 2012 | Volume 2 | Article 110 | 1 Spetea et al. Evolution and function of TAAC TC #2.A.29.1.1). There are two ATP/ADP antiporters (AATPs) to a lesser degree in the connecting loops (Thuswaldner et al., in the inner envelope membrane of photosynthetic and het- 2007). The selectivity filter for adenine nucleotides, represented erotrophic plastids, supplying cytosolic ATP to the stroma (for by residues K-130, R-186, Y-282, and K-369, is fully conserved, recent reviews, see Haferkamp et al., 2011; Traba et al., 2011). The indicating adenine nucleotides as the most likely substrates for the transport is electroneutral since the counter-ions for ATP4− are transport activity of TAAC. 3− − ADP together with H2PO4 (Trentmann et al., 2008). AATPs Arabidopsis TAAC was characterized in E. coli as an ATP possess 12 putative transmembrane helices, and share a common importer in exchange for cytosolic ADP (Thuswaldner et al.,2007), origin with the ATP/ADP antiporters found in the parasite bacte- and its activity was found sensitive to bongkrekic acid (Thuswald- ria Rickettsia prowazekii and Chlamydia psittaci (Haferkamp et al., ner, S., and Spetea, C., unpublished data). Pi is not a substrate 2011). for transport by TAAC (Thuswaldner et al., 2007), implying the The mitochondrial ADP/ATP carriers are the first and most requirement for a separate thylakoid Pi transporter. Indeed, such studied members of the mitochondrial carrier (MC) family and a protein has been identified in Arabidopsis, and functionally char- are present only in eukaryotic cells (Haferkamp et al., 2011; Traba acterized in yeast and E. coli (Guo et al., 2008; Pavón et al., 2008). et al., 2011). Like the members of the AAA family, AACs are When assessed in thylakoid membranes, TAAC transports stromal exchangers, but the transport is electrogenic (ATP4−/ADP3−) and ATP into the thylakoid lumen in exchange for ADP. The direc- proceeds in the opposite direction, since they transport matrix tion of TAAC-mediated transport determined in both E. coli and ATP through the intermembrane space out into the cytosol. Yet thylakoids is opposite to the direction of transport by mitochon- another functional difference from AAA is that AACs are sensitive drial AACs (Thuswaldner et al., 2007). Therefore, to distinguish to specific inhibitors, such as bongkrekic acid and carboxyatracty- it from the mitochondrial ADP/ATP carrier, the thylakoid protein loside (Klingenberg, 2008). The 3D structure of the bovine AAC has been named ATP/ADP carrier. Through adenine nucleotide has been resolved at 2.2 Å resolution in a conformation stabi- exchange, TAAC was proposed to supply ATP for nucleotide- lized with carboxyatractyloside (Pebay-Peyroula et al., 2003). The dependent reactions in the thylakoid lumen (Spetea et al., 2004). structure revealed six transmembrane helices and a selectivity filter An extensive review on the structure,function,and evolution of the for adenine nucleotides, whose sequence could be used to predict MC family has become recently available and provides insights into other AACs and even other MCs (Nury et al., 2010). From a total their roles in plants (Palmieri et al., 2011). This review focuses on of 58 MC in Arabidopsis thaliana, three classical mitochondrial the evolutionary origin of the TAAC subfamily of the mitochon- AACs have been characterized, with at least four more paralogous drial AACs, which aids in elucidating its function in the thylakoid sequences awaiting validation (Palmieri et al., 2011). In addition to membrane. the mitochondrion, AAC members have also been found in per- oxisomes, endoplasmic reticulum, amyloplasts, and chloroplasts WHEN AND WHERE IN THE TREE OF LIFE DID TAAC (Haferkamp et al., 2011; Traba et al., 2011). ORIGINATE? Using western blotting and activity inhibition with an antibody We assembled a set of protein sequences with which to place TAAC against the bovine AAC, the activity of an AAC was reported in the in an evolutionary context. We extracted protein sequences from spinach thylakoid membrane (Spetea et al., 2004). BLAST searches the curated gene families at ARAMEMNON (Schwacke et al., with the bovine AAC against the Arabidopsis protein database 2003)1 and from search results to specific clades and genomes combined with prediction of chloroplast transit peptides revealed at NCBI2 and PHYTOZOME