Identification of the intracellular gate for a member of the equilibrative nucleoside transporter (ENT) family

Toward a Molecular Basis of Cellular Nucleoside Transport in Humans

Nucleosides play central roles in all facets of life, from metabolism to cellular signaling. Because of their physiochemical properties, nucleosides are lipid bilayer impermeable and thus rely on dedicated transport systems to cross biological membranes. In humans, two unrelated protein families mediate nucleoside membrane transport: the concentrative and equilibrative nucleoside transporter families. The objective of this review is to provide a broad outlook on the current status of nucleoside transport research. We will discuss the role played by nucleoside transporters in human health and disease, with emphasis placed on recent structural advancements that have revealed detailed molecular principles of these important cellular transport systems and exploitable pharmacological features.

Equilibrative Nucleoside Transporter 2: Properties and Physiological Roles

Equilibrative nucleoside transporter 2 (ENT2) is a bidirectional transporter embedded in the biological membrane and is ubiquitously found in most tissue and cell types. ENT2 mediates the uptake of purine and pyrimidine nucleosides and nucleobase besides transporting a variety of nucleoside-derived drugs, mostly in anticancer therapy. Since high expression of ENT2 has been correlated with advanced stages of different types of cancers, consequently, this has gained significant interest in the role of ENT2 as a potential therapeutic target. Furthermore, ENT2 plays critical roles in signaling pathway and cell cycle progression. Therefore, elucidating the physiological roles of ENT2 and its properties may contribute to a better understanding of ENT2 roles beyond their transportation mechanism. This review is aimed at highlighting the main roles of ENT2 and at providing a brief update on the recent research.

IMPROvER: the Integral Membrane Protein Stability Selector

Identifying stabilising variants of membrane protein targets is often required for structure determination. Our new computational pipeline, the Integral Membrane Protein Stability Selector (IMPROvER) provides a rational approach to variant selection by employing three independent approaches: deep-sequence, model-based and data-driven. In silico tests using known stability data, and in vitro tests using three membrane protein targets with 7, 11 and 16 transmembrane helices provided measures of success. In vitro, individual approaches alone all identified stabilising variants at a rate better than expected by random selection. Low numbers of overlapping predictions between approaches meant a greater success rate was achieved (fourfold better than random) when approaches were combined and selections restricted to the highest ranked sites. The mix of information IMPROvER uses can be extracted for any helical membrane protein. We have developed the first general-purpose tool for selecting stabilising variants of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upalpha$$\end{document}α-helical membrane proteins, increasing efficiency and reducing workload. IMPROvER can be accessed at http://improver.ddns.net/IMPROvER/.

Expansion of the Major Facilitator Superfamily (MFS) to include novel transporters as well as transmembrane-acting enzymes

The Major Facilitator Superfamily (MFS) is currently the largest characterized superfamily of transmembrane secondary transport proteins. Its diverse members are found in essentially all organisms in the biosphere and function by uniport, symport, and/or antiport mechanisms. In 1993 we first named and described the MFS which then consisted of 5 previously known families that had not been known to be related, and by 2012 we had identified a total of 74 families, classified phylogenetically within the MFS, all of which included only transport proteins. This superfamily has since expanded to 89 families, all included under TC# 2.A.1, and a few transporter families outside of TC# 2.A.1 were identified as members of the MFS. In this study, we assign nine previously unclassified protein families in the Transporter Classification Database (TCDB; http://www.tcdb.org) to the MFS based on multiple criteria and bioinformatic methodologies. In addition, we find integral membrane domains distantly related to partial or full-length MFS permeases in Lysyl tRNA Synthases (TC# 9.B.111), Lysylphosphatidyl Glycerol Synthases (TC# 4.H.1), and cytochrome b₅₆₁ transmembrane electron carriers (TC# 5.B.2). Sequence alignments, overlap of hydropathy plots, compatibility of repeat units, similarity of complexity profiles of transmembrane segments, shared protein domains and 3D structural similarities between transport proteins were analyzed to assist in inferring homology. The MFS now includes 105 families.

Purine and pyrimidine transporters of pathogenic protozoa - conduits for therapeutic agents

Purines and pyrimidines are essential nutrients for any cell. Most organisms are able to synthesize their own purines and pyrimidines, but this ability was lost in protozoans that adapted to parasitism, leading to a great diversification in transporter activities in these organisms, especially for the acquisition of amino acids and nucleosides from their hosts throughout their life cycles. Many of these transporters have been shown to have sufficiently different substrate affinities from mammalian transporters, making them good carriers for therapeutic agents. In this review, we summarize the knowledge obtained on purine and pyrimidine activities identified in protozoan parasites to date and discuss their importance for the survival of these parasites and as drug carriers, as well as the perspectives of developments in the field.

Accessibility of substituted cysteines in TM2 and TM10 transmembrane segments in the Plasmodium falciparum equilibrative nucleoside transporter PfENT1

Infection with Plasmodium species parasites causes malaria. Plasmodium parasites are purine auxotrophic. They import purines via an equilibrative nucleoside transporter (ENT). In P. falciparum, the most virulent species, the equilibrative nucleoside transporter 1 (PfENT1) represents the primary purine uptake pathway. This transporter is a potential target for the development of antimalarial drugs. In the absence of a high-resolution structure for either PfENT1 or a homologous ENT, we used the substituted cysteine accessibility method (SCAM) to investigate the membrane-spanning domain structure of PfENT1 to identify potential inhibitor-binding sites. We previously used SCAM to identify water-accessible residues that line the permeation pathway in transmembrane segment 11 (TM11). TM2 and TM10 lie adjacent to TM11 in an ab initio model of a homologous Leishmania donovani nucleoside transporter. To identify TM2 and TM10 residues in PfENT1 that are at least transiently on the water-accessible transporter surface, we assayed the reactivity of single cysteine-substitution mutants with three methanethiosulfonate (MTS) derivatives. Cysteines substituted for 12 of 14 TM2 segment residues reacted with MTS-ethyl-ammonium-biotin (MTSEA-biotin). At eight positions, MTSEA-biotin inhibited transport, and at four positions substrate transport was potentiated. On an α helical wheel projection of TM2, the four positions where potentiation occurred were located in a cluster on one side of the helix. In contrast, although MTSEA-biotin inhibited 9 of 10 TM10 cysteine-substituted mutants, the reactive residues did not form a pattern consistent with either an α helix or β sheet. These results may help identify the binding site(s) of PfENT1 inhibitors.

Role of cysteine 416 in N-ethylmaleimide sensitivity of human equilibrative nucleoside transporter 1 (hENT1)

Human equilibrative nucleoside transporter 1 (hENT1), the first identified member of the ENT family of integral membrane proteins, is the primary mechanism for cellular uptake of physiologic nucleosides and many antineoplastic and antiviral nucleoside drugs. hENT1, which is potently inhibited by nitrobenzylthioinosine (NBMPR), possesses 11 transmembrane helical domains with an intracellular N-terminus and an extracellular C-terminus. As a protein with 10 endogenous cysteine residues, it is sensitive to inhibition by the membrane permeable sulfhydryl-reactive reagent N-ethylmaleimide (NEM) but is unaffected by the membrane impermeable sulfhydryl-reactive reagent p-chloromercuriphenyl sulfonate. To identify the residue(s) involved in NEM inhibition, we created a cysteine-less version of hENT1 (hENT1C-), with all 10 endogenous cysteine residues mutated to serine, and showed that it displays wild-type uridine transport and NBMPR-binding characteristics when produced in the Xenopus oocyte heterologous expression system, indicating that endogenous cysteine residues are not essential for hENT1 function. We then tested NEM sensitivity of recombinant wild-type hENT1, hENT1 mutants C1S to C10S (single cysteine residues replaced by serine), hENT1C- (all cysteine residues replaced by serine), and hENT1C- mutants S1C to S10C (single serine residues converted back to cysteine). Mutants C9S (C416S/hENT1) and S9C (S416C/hENT1C-) were insensitive and sensitive, respectively, to inhibition by NEM, identifying Cys⁴¹⁶ as the endofacial cysteine residue in hENT1 responsible for NEM inhibition. Kinetic experiments suggested that NEM modification of Cys⁴¹⁶, which is located at the inner extremity of TM10, results in the inhibition of hENT1 uridine transport and NBMPR binding by constraining the protein in its inward-facing conformation.

Functional Characterization of the Saccharomyces cerevisiae Equilibrative Nucleoside Transporter 1 (ScENT1)

Equilibrative nucleoside transporters (ENTs) are polytopic membrane transporters responsible for the translocation of nucleosides, nucleobases—to a lesser extent—and nucleoside analog therapeutics across cellular membranes. ENTs function in a diffusion controlled bidirectional manner and are thought to utilize an alternating access transport mechanism. However, a detailed understanding of ENT function at the molecular level has remained elusive. ScENT1 (formerly known as Function Unknown Now 26 or FUN26) is the only known ENT ortholog endogenously expressed in S. cerevisiae, and a proteoliposome assay system was used to study homogenously overexpressed and purified ScENT1 (wildtype relative to L390A and F249I mutants). L390 and F249 are highly conserved residues and were found to alter transporter function. L390A produced a reduction of mean transport activity while F249I increased mean substrate translocation relative to wildtype protein. However, both mutations resulted in transport of UTP—a novel gain of function for any ENT. These residues were then mapped onto an ab initio model of FUN26 which suggests they function in substrate translocation (L390) or cytoplasmic gating (F249). Furthermore, wildtype, L390A, and F249I were found to be sensitive to the presence of alcohols. Ethanol attenuated ScENT1-mediated transport of uridine by ~50%. These findings further demonstrate functional similarities between ScENT1 and human ENT isoforms and support identification of FUN26 as ScENT1, the first ENT isoform in S. cerevisiae.

Purinergic Regulation of Neutrophil Function

Purinergic signaling, which utilizes nucleotides (particularly ATP) and adenosine as transmitter molecules, plays an essential role in immune system. In the extracellular compartment, ATP predominantly functions as a pro-inflammatory molecule through activation of P2 receptors, whereas adenosine mostly functions as an anti-inflammatory molecule through activation of P1 receptors. Neutrophils are the most abundant immune cells in circulation and have emerged as an important component in orchestrating a complex series of events during inflammation. However, because of the destructive nature of neutrophil-derived inflammatory agents, neutrophil activation is fine-tuned, and purinergic signaling is intimately involved in this process. Indeed, shifting the balance between P2 and P1 signaling is critical for neutrophils to appropriately exert their immunologic activity. Here, we review the role of purinergic signaling in regulating neutrophil function, and discuss the potential of targeting purinergic signaling for the treatment of neutrophil-associated infectious and inflammatory diseases.

Substituted-cysteine accessibility and cross-linking identify an exofacial cleft in the 7th and 8th helices of the proton-coupled folate transporter (SLC46A1)

The proton-coupled folate transporter (PCFT-SLC46A1) is required for folate transport across the apical membrane of the small intestine and across the choroid plexus. This study focuses on the structure/function of the 7th transmembrane domain (TMD), and its relationship to the 8th TMD as assessed by the substituted cysteine accessibility method (SCAM) and dicysteine cross-linking. Nine exofacial residues (I278C; H281C-L288C) of 23 residues in the 7th TMD were accessible to 2-((biotinoyl)amino)ethyl methanethiosulfonate (MTSEA-biotin). Pemetrexed, a high-affinity substrate for PCFT, decreased or abolished biotinylation of seven of these residues consistent with their location in or near the folate binding pocket. Homology models of PCFT based on Glut5 fructose transporter structures in both inward- and outward- open conformations were constructed and predicted that two pairs of residues (T289-I304C and Q285-Q311C) from the 7th and 8th TMDs should be in sufficiently close proximity to form a disulfide bond when substituted with cysteines. The single Cys-substituted mutants were accessible to MTSEA-biotin and functional with and without pretreatment with dithiotreitol. However, the double mutants were either not accessible at all, or accessibility was markedly reduced and function markedly impaired. This occurred spontaneously without inclusion of an oxidizing agent. Dithiotreitol restored accessibility and function consistent with disulfide bond disruption. The data establish the proximity of exofacial regions of the 7th and 8th TMDs and their role in defining the aqueous translocation pathway and suggest that these helices may be a component of an exofacial cleft through which substrates enter the protein binding pocket in its outward-open conformation.

Functional characterization of human equilibrative nucleoside transporter 1

Equilibrative nucleoside transporters (ENTs), which facilitate cross-membrane transport of nucleosides and nucleoside-derived drugs, play an important role in the salvage pathways of nucleotide synthesis, cancer chemotherapy, and treatment for virus infections. Functional characterization of ENTs at the molecular level remains technically challenging and hence scant. In this study, we report successful purification and biochemical characterization of human equilibrative nucleoside transporter 1 (hENT1) in vitro. The HEK293F-derived, recombinant hENT1 is homogenous and functionally active in proteoliposome-based counter flow assays. hENT1 transports the substrate adenosine with a Km of 215 ± 34 µmol/L and a Vmax of 578 ± 23.4 nmol mg-1 min-1. Adenosine uptake by hENT1 is competitively inhibited by nitrobenzylmercaptopurine ribonucleoside (NBMPR), nucleosides, deoxynucleosides, and nucleoside-derived anti-cancer and anti-viral drugs. Binding of hENT1 to adenosine, deoxyadenosine, and adenine by isothermal titration calorimetry is in general agreement with results of the competitive inhibition assays. These results validate hENT1 as a bona fide target for potential drug target and serve as a useful basis for future biophysical and structural studies.

Impact of posttranslational modifications of engineered cysteines on the substituted cysteine accessibility method: evidence for glutathionylation

The substituted cysteine accessibility method (SCAM) is widely used to study the structure and function of channels, receptors and transporters. In its usual application, a cysteine residue is introduced into a protein which lacks native cysteines following which the accessibility of the residue to the aqueous compartment is assessed. Implicit, and generally assumed, is that if the cysteine-substituted residue is not available to react with sulfhydryl reagents it is not exposed to the extracellular compartment or within the aqueous translocation pathway. We demonstrate here, in a Hela-derived cell line, that some cysteine-substituted residues of the proton-coupled folate transporter (PCFT, SLC46A1) that are inaccessible to 2-((biotinoyl)amino)ethyl methanethiosulfonate are glutathionylated by biotinylated glutathione ethyl ester in the absence of an oxidizing agent. Intramolecular disulfide formation involving cysteine-substituted residues was also identified in some instances. These posttranslational modifications limit the accessibility of the cysteine residues to sulfhydryl-reactive reagents and can have a profound impact on the interpretation of SCAM but may not alter function. When a posttranslationally modified residue is used as a reference extracellular control, the high level of exposure required for detection on Western blot results in erroneous detection of otherwise inaccessible intracellular cysteine-substituted residues. The data indicate that in the application of SCAM, when a cysteine-substituted residue does not appear to be accessible to sulfhydryl-reactive reagents, the possibility of a posttranslational modification should be excluded. The data explain the discrepancies in the assessment, and confirm the localization, of the first intracellular loop of PCFT.

The Novel Membrane-Bound Proteins MFSD1 and MFSD3 are Putative SLC Transporters Affected by Altered Nutrient Intake

Membrane-bound solute carriers (SLCs) are essential as they maintain several physiological functions, such as nutrient uptake, ion transport and waste removal. The SLC family comprise about 400 transporters, and we have identified two new putative family members, major facilitator superfamily domain containing 1 (MFSD1) and 3 (MFSD3). They cluster phylogenetically with SLCs of MFS type, and both proteins are conserved in chordates, while MFSD1 is also found in fruit fly. Based on homology modelling, we predict 12 transmembrane regions, a common feature for MFS transporters. The genes are expressed in abundance in mice, with specific protein staining along the plasma membrane in neurons. Depriving mouse embryonic primary cortex cells of amino acids resulted in upregulation of Mfsd1, whereas Mfsd3 is unaltered. Furthermore, in vivo, Mfsd1 and Mfsd3 are downregulated in anterior brain sections in mice subjected to starvation, while upregulated specifically in brainstem. Mfsd3 is also attenuated in cerebellum after starvation. In mice raised on high-fat diet, Mfsd1 was specifically downregulated in brainstem and hypothalamus, while Mfsd3 was reduced consistently throughout the brain.

Novel nuclear hENT2 isoforms regulate cell cycle progression via controlling nucleoside transport and nuclear reservoir

Nucleosides participate in many cellular processes and are the fundamental building blocks of nucleic acids. Nucleoside transporters translocate nucleosides across plasma membranes although the mechanism by which nucleos(t)ides are translocated into the nucleus during DNA replication is unknown. Here, we identify two novel functional splice variants of equilibrative nucleoside transporter 2 (ENT2), which are present at the nuclear envelope. Under proliferative conditions, these splice variants are up-regulated and recruit wild-type ENT2 to the nuclear envelope to translocate nucleosides into the nucleus for incorporation into DNA during replication. Reduced presence of hENT2 splice variants resulted in a dramatic decrease in cell proliferation and dysregulation of cell cycle due to a lower incorporation of nucleotides into DNA. Our findings support a novel model of nucleoside compartmentalisation at the nuclear envelope and translocation into the nucleus through hENT2 and its variants, which are essential for effective DNA synthesis and cell proliferation.

Equilibrative nucleoside transporters-A review

Equilibrative nucleoside transporters (ENTs) are polytopic integral membrane proteins that mediate the transport of nucleosides, nucleobases, and therapeutic analogs. The best-characterized ENTs are the human transporters hENT1 and hENT2. However, non-mammalian eukaryotic ENTs have also been studied (e.g., yeast, parasitic protozoa). ENTs are major pharmaceutical targets responsible for modulating the efficacy of more than 30 approved drugs. However, the molecular mechanisms and chemical determinants of ENT-mediated substrate recognition, binding, inhibition, and transport are poorly understood. This review highlights findings on the characterization of ENTs by surveying studies on genetics, permeant and inhibitor interactions, mutagenesis, and structural models of ENT function.

Characterization of a Novel Endoplasmic Reticulum Protein Involved in Tubercidin Resistance in Leishmania major

BACKGROUND

Tubercidin (TUB) is a toxic adenosine analog with potential antiparasitic activity against Leishmania, with mechanism of action and resistance that are not completely understood. For understanding the mechanisms of action and identifying the potential metabolic pathways affected by this drug, we employed in this study an overexpression/selection approach using TUB for the identification of potential targets, as well as, drug resistance genes in L. major. Although, TUB is toxic to the mammalian host, these findings can provide evidences for a rational drug design based on purine pathway against leishmaniasis.

METHODOLOGY/PRINCIPAL FINDINGS

After transfection of a cosmid genomic library into L. major Friedlin (LmjF) parasites and application of the overexpression/selection method, we identified two cosmids (cosTUB1 and cosTU2) containing two different loci capable of conferring significant levels of TUB resistance. In the cosTUB1 contained a gene encoding NUPM1-like protein, which has been previously described as associated with TUB resistance in L. amazonensis. In the cosTUB2 we identified and characterized a gene encoding a 63 kDa protein that we denoted as tubercidin-resistance protein (TRP). Functional analysis revealed that the transfectants were less susceptible to TUB than LmjF parasites or those transfected with the control vector. In addition, the trp mRNA and protein levels in cosTUB2 transfectants were higher than LmjF. TRP immunolocalization revealed that it was co-localized to the endoplasmic reticulum (ER), a cellular compartment with many functions. In silico predictions indicated that TRP contains only a hypothetical transmembrane domain. Thus, it is likely that TRP is a lumen protein involved in multidrug efflux transport that may be involved in the purine metabolic pathway.

CONCLUSIONS/SIGNIFICANCE

This study demonstrated for the first time that TRP is associated with TUB resistance in Leishmania. The next challenge is to determine how TRP mediates TUB resistance and whether purine metabolism is affected by this protein in the parasite. Finally, these findings may be helpful for the development of alternative anti-leishmanial drugs that target purine pathway.

A Novel and Fast Purification Method for Nucleoside Transporters

Nucleoside transporters (NTs) play critical biological roles in humans, and to understand the molecular mechanism of nucleoside transport requires high-resolution structural information. However, the main bottleneck for structural analysis of NTs is the production of pure, stable, and high quality native protein for crystallization trials. Here we report a novel membrane protein expression and purification strategy, including construction of a high-yield membrane protein expression vector, and a new and fast purification protocol for NTs. The advantages of this strategy are the improved time efficiency, leading to high quality, active, stable membrane proteins, and the efficient use of reagents and consumables. Our strategy might serve as a useful point of reference for investigating NTs and other membrane proteins by clarifying the technical points of vector construction and improvements of membrane protein expression and purification.

Novel regulation of equlibrative nucleoside transporter 1 (ENT1) by receptor-stimulated Ca2+-dependent calmodulin binding

Equilibrative nucleoside transporters (ENTs) facilitate the flux of nucleosides, such as adenosine, and nucleoside analog (NA) drugs across cell membranes. A correlation between adenosine flux and calcium-dependent signaling has been previously reported; however, the mechanistic basis of these observations is not known. Here we report the identification of the calcium signaling transducer calmodulin (CaM) as an ENT1-interacting protein, via a conserved classic 1-5-10 motif in ENT1. Calcium-dependent human ENT1-CaM protein interactions were confirmed in human cell lines (HEK293, RT4, U-87 MG) using biochemical assays (HEK293) and the functional assays (HEK293, RT4), which confirmed modified nucleoside uptake that occurred in the presence of pharmacological manipulations of calcium levels and CaM function. Nucleoside and NA drug uptake was significantly decreased (∼12% and ∼39%, respectively) by chelating calcium (EGTA, 50 μM; BAPTA-AM, 25 μM), whereas increasing intracellular calcium (thapsigargin, 1.5 μM) led to increased nucleoside uptake (∼26%). Activation of N-methyl-d-aspartate (NMDA) receptors (in U-87 MG) by glutamate (1 mM) and glycine (100 μM) significantly increased nucleoside uptake (∼38%) except in the presence of the NMDA receptor antagonist, MK-801 (50 μM), or CaM antagonist, W7 (50 μM). These data support the existence of a previously unidentified novel receptor-dependent regulatory mechanism, whereby intracellular calcium modulates nucleoside and NA drug uptake via CaM-dependent interaction of ENT1. These findings suggest that ENT1 is regulated via receptor-dependent calcium-linked pathways resulting in an alteration of purine flux, which may modulate purinergic signaling and influence NA drug efficacy.

Identification of an Extracellular Gate for the Proton-coupled Folate Transporter (PCFT-SLC46A1) by Cysteine Cross-linking

The proton-coupled folate transporter (PCFT, SLC46A1) is required for intestinal folate absorption and folate homeostasis in humans. A homology model of PCFT, based upon theEscherichia coliglycerol 3-phosphate transporter structure, predicted that PCFT transmembrane domains (TMDs) 1, 2, 7, and 11 form an extracellular gate in the inward-open conformation. To assess this model, five residues (Gln(45)-TMD1, Asn(90)-TMD2, Leu(290)-TMD7, Ser(407)-TMD11 and Asn(411)-TMD11) in the predicted gate were substituted with Cys to generate single and nine double mutants. Transport function of the mutants was assayed in transient transfectants by measurement of [(3)H]substrate influx as was accessibility of the Cys residues to biotinylation. Pairs of Cys residues were assessed for spontaneous formation of a disulfide bond, induction of a disulfide bond by oxidization with dichloro(1,10-phenanthroline)copper (II) (CuPh), or the formation of a Cd(2+)complex. The data were consistent with the formation of a spontaneous disulfide bond between the N90C/S407C pair and a CuPh- and Cd(2+)-induced disulfide bond and complex, respectively, for the Q45C/L290C and L290C/N411C pairs. The decrease in activity induced by cross-linkage of the Cys residue pairs was due to a decrease in the influxVmaxconsistent with restriction in the mobility of the transporter. The presence of folate substrate decreased the CuPh-induced inhibition of transport. Hence, the data support the glycerol 3-phosphate transporter-based homology model of PCFT and the presence of an extracellular gate formed by TMDs 1, 2, 7, and 11.

The Role of Flexible Loops in Folding, Trafficking and Activity of Equilibrative Nucleoside Transporters

Equilibrative nucleoside transporters (ENTs) are integral membrane proteins, which reside in plasma membranes of all eukaryotic cells and mediate thermodynamically downhill transport of nucleosides. This process is essential for nucleoside recycling, and also plays a key role in terminating adenosine-mediated cellular signaling. Furthermore, ENTs mediate the uptake of many drugs, including anticancer and antiviral nucleoside analogues. The structure and mechanism, by which ENTs catalyze trans-membrane transport of their substrates, remain unknown. To identify the core of the transporter needed for stability, activity, and for its correct trafficking to the plasma membrane, we have expressed human ENT deletion mutants in Xenopus laevis oocytes and determined their localization, transport properties and susceptibility to inhibition. We found that the carboxyl terminal trans-membrane segments are essential for correct protein folding and trafficking. In contrast, the soluble extracellular and intracellular loops appear to be dispensable, and must be involved in the fine-tuning of transport regulation.

Visualizing nucleic acid metabolism using non-natural nucleosides and nucleotide analogs

Nucleosides and their corresponding mono-, di-, and triphosphates play important roles in maintaining cellular homeostasis. In addition, perturbations in this homeostasis can result in dysfunctional cellular processes that cause pathological conditions such as cancer and autoimmune diseases. This review article discusses contemporary research areas applying nucleoside analogs to probe the mechanistic details underlying the complexities of nucleoside metabolism at the molecular and cellular levels. The first area describes classic and contemporary approaches used to quantify the activity of nucleoside transporters, an important class of membrane proteins that mediate the influx and efflux of nucleosides and nucleobases. A focal point of this section is describing how biophotonic nucleosides are replacing conventional assays employing radiolabeled substrates to study the mechanism of these proteins. The second section describes approaches to understand the utilization of nucleoside triphosphates by cellular DNA polymerases during DNA synthesis. Emphasis here is placed on describing how novel nucleoside analogs such as 5-ethynyl-2'-deoxyuridine are being used to quantify DNA synthesis during normal replication as well as during the replication of damaged DNA. In both sections, seminal research articles relevant to these areas are described to highlight how these novel probes are improving our understanding of these biological processes. This article is part of a Special Issue entitled: Physiological Enzymology and Protein Functions.

Purine import into malaria parasites as a target for antimalarial drug development

Infection with Plasmodium species parasites causes malaria. Plasmodium parasites are purine auxotrophs. In all life cycle stages, they require purines for RNA and DNA synthesis and other cellular metabolic processes. Purines are imported from the host erythrocyte by equilibrative nucleoside transporters (ENTs). They are processed via purine salvage pathway enzymes to form the required purine nucleotides. The Plasmodium falciparum genome encodes four putative ENTs (PfENT1-4). Genetic, biochemical, and physiologic evidence suggest that PfENT1 is the primary purine transporter supplying the purine salvage pathway. Protein mass spectrometry shows that PfENT1 is expressed in all parasite stages. PfENT1 knockout parasites are not viable in culture at purine concentrations found in human blood (<10 μM). Thus, PfENT1 is a potential target for novel antimalarial drugs, but no PfENT1 inhibitors have been identified to test the hypothesis. Identifying inhibitors of PfENT1 is an essential step to validate PfENT1 as a potential antimalarial drug target.

Understanding transporter specificity and the discrete appearance of channel-like gating domains in transporters

Transporters are ubiquitous proteins mediating the translocation of solutes across cell membranes, a biological process involved in nutrition, signaling, neurotransmission, cell communication and drug uptake or efflux. Similarly to enzymes, most transporters have a single substrate binding-site and thus their activity follows Michaelis-Menten kinetics. Substrate binding elicits a series of structural changes, which produce a transporter conformer open toward the side opposite to the one from where the substrate was originally bound. This mechanism, involving alternate outward- and inward-facing transporter conformers, has gained significant support from structural, genetic, biochemical and biophysical approaches. Most transporters are specific for a given substrate or a group of substrates with similar chemical structure, but substrate specificity and/or affinity can vary dramatically, even among members of a transporter family that show high overall amino acid sequence and structural similarity. The current view is that transporter substrate affinity or specificity is determined by a small number of interactions a given solute can make within a specific binding site. However, genetic, biochemical and in silico modeling studies with the purine transporter UapA of the filamentous ascomycete Aspergillus nidulans have challenged this dogma. This review highlights results leading to a novel concept, stating that substrate specificity, but also transport kinetics and transporter turnover, are determined by subtle intramolecular interactions between a major substrate binding site and independent outward- or cytoplasmically-facing gating domains, analogous to those present in channels. This concept is supported by recent structural evidence from several, phylogenetically and functionally distinct transporter families. The significance of this concept is discussed in relationship to the role and potential exploitation of transporters in drug action.

FUN26 (function unknown now 26) protein from saccharomyces cerevisiae is a broad selectivity, high affinity, nucleoside and nucleobase transporter

Equilibrative nucleoside transporters (ENTs) are polytopic integral membrane proteins that transport nucleosides and, to a lesser extent, nucleobases across cell membranes. ENTs modulate efficacy for a range of human therapeutics and function in a diffusion-controlled bidirectional manner. A detailed understanding of ENT function at the molecular level has remained elusive. FUN26 (function unknown now 26) is a putative ENT homolog from S. cerevisiae that is expressed in vacuole membranes. In the present system, proteoliposome studies of purified FUN26 demonstrate robust nucleoside and nucleobase uptake into the luminal volume for a broad range of substrates. This transport activity is sensitive to nucleoside modifications in the C(2')- and C(5')-positions on the ribose sugar and is not stimulated by a membrane pH differential. [(3)H]Adenine nucleobase transport efficiency is increased ∼4-fold relative to nucleosides tested with no observed [(3)H]adenosine or [(3)H]UTP transport. FUN26 mutational studies identified residues that disrupt (G463A or G216A) or modulate (F249I or L390A) transporter function. These results demonstrate that FUN26 has a unique substrate transport profile relative to known ENT family members and that a purified ENT can be reconstituted in proteoliposomes for functional characterization in a defined system.