Membrane proteins constitute about one third of all proteins encoded in the human genome . The largest family of membrane-bound proteins consists of over 800 G protein-coupled receptors [2, 3] while the second largest is the solute carrier (SLC) family including 384 human genes . The SLC genes encode proteins related to passive transporters, ion-coupled transporters and exchangers. SLCs were functionally grouped into forty-three subfamilies . Since then additional families have been added including five new subfamilies, SLC44 - SLC48, according to the Hugo Gene Nomenclature Committee .
A systematic phylogenetic analysis of the entire repertoire of SLC genes reveals that 15 of the SLC subfamilies, along with synaptic vesicle 2 (SV2) proteins, can be clustered into 5 major groups, named α-, β-, γ- and δ-groups . The α-group is largest with SV2 proteins and seven SLC subfamilies (SLC2, 16, 17, 18, 22, 37 and 46). The main common features of the members of the α-group are the presence of 12 putative transmembrane (TM) regions, N- and C-termini at the cytosolic side, a large extracellular loop between TM1 and TM2 (except SLC17) and a large third intracellular loop.
The SLC17 family belonging to the α-group is known as the type I phosphate/vesicular glutamate transporter family . The SLC17 family consists of nine genes that have previously been functionally divided into four subgroups: (i) type I phosphate transporters, SLC17A1-4, (ii) vesicular excitatory amino acid transporter, SLC17A5 (previously known as sialin) (iii) vesicular glutamate transporters (VGLUT), SLC17A6-17A8, and (iv) vesicular nucleotide transporter (VNUT), SLC17A9 [7, 8].
The type I phosphate transporters are known to cotransport sodium (Na) and phosphate (Pi), with a capacity to also transport organic anions. Their ionic coupling properties have not been determined and the identity of their endogenous substrates remains unresolved. Moreover, data on the tissue distribution of the type I transporters seem rather limited. The SLC17A1 expression has been studied by northern blot and it has been identified in the kidney, liver and, at very low levels, in the brain [9, 10]. The SLC17A2 has a different expression pattern, with relatively high levels in the heart and skeletal muscle and lower levels in the kidney, liver, lung, placenta, pancreas and brain [11, 12]. SLC17A3 is limited to the liver and kidney , whereas SLC17A4 is expressed in the intestine, colon, liver, and pancreas .
The second group of SLC17 proteins consists of the vesicular excitatory amino acid transporter (VEAT/SLC17A5), previously known as sialin. It was first identified as a lysosomal sialic acid transporter implicated in the Salla disease and infantile sialic acid storage disorder . However, a recent study showed that SLC17A5 serves as a vesicular protein transporting aspartate and glutamate and, hence, the name vesicular excitatory amino acid transporter (VAT) was suggested . Slc17a5 shows ubiquitous expression ; in the brain it is predominantly expressed in the hippocampus, striatum and cerebral cortex [16, 17].
The third group contains vesicular glutamate transporters involved in loading glutamate into synaptic vesicles of glutamatergic cells. In the SLC nomenclature the three identified VGLUT transporters, VGLUT1 through 3 are classified as Sl17A7, A6 and A8, respectively. Slc17a7 and Slc17a6 are predominantly expressed in glutamatergic neurons of the central nervous system (CNS). Interestingly, these genes have a complementary expression pattern with limited overlaps [18, 19]. Unlike Slc17a7 and Slc17a6, Slc17A8 is expressed in CNS neurons not classically considered as glutamatergic [20, 21]. The peripheral expression of Slc17a6 and Slc17a7 is limited to the pancreatic islets whereas Slc17a8 is expressed in the liver and, at lower levels, in the kidney.
The fourth type of the SLC17 genes is represented by a single gene in humans and it is named SLC17A9. Recently it has been identified as the first vesicular nucleotide transporter. The vesicular storage of this protein is confirmed by studies on PC12 cells. Functional characterization of SLC17A9 in liposomes containing the purified protein revealed that it transports nucleotides, such as ATP and ADP . This is likely a major breakthrough in understanding neuronal signaling because ATP co-transmission is crucial in many neurons. The purinergic receptors that respond to ATP, UTP and adenosine serve as important drug targets and this transporter is likely to draw large attention for pharmaceutical development. Northern blot analysis showed wide expression of SLC17A9 in various organs, but predominantly in the brain, adrenals and the thyroid gland . However, the detailed expression profile of SLC17A9 in the brain is unknown.
Vesicular glutamate transporters are fundamental components in glutamate signaling and are evolutionarily old. A vesicular glutamate transporter has been characterized in the tunicate Ciona intestinalis  as well as in Drosophila melanogaster  and Caenorhabditis elegans . Also the presence of SLC17A9 in many animal species has been suggested . However, despite the obvious biological importance of the SLC17 family, the evolutionary history of these genes has not been systematically mapped.
In the current project, we thoroughly mined the entire SLC17 family and explained the evolutionary events that shaped different branches of this family. We also performed comprehensive expression analysis of the nine SLC17 genes using quantitative real-time PCR (RT-PCR) in the rat. We then looked in more detail at newly discovered Slc17a9, the only known vesicular nucleotide transporter, and performed a comprehensive expression profiling of the Slc17a9 in the mouse brain by in situ hybridization.