Analysis of membrane topology of the human reduced folate carrier protein by hemagglutinin epitope insertion and scanning glycosylation insertion mutagenesis

Partial proteolysis improves the identification of the extracellular segments of transmembrane proteins by surface biotinylation

Transmembrane proteins (TMP) play a crucial role in several physiological processes. Despite their importance and diversity, only a few TMP structures have been determined by high-resolution protein structure characterization methods so far. Due to the low number of determined TMP structures, the parallel development of various bioinformatics and experimental methods was necessary for their topological characterization. The combination of these methods is a powerful approach in the determination of TMP topology as in the Constrained Consensus TOPology prediction. To support the prediction, we previously developed a high-throughput topology characterization method based on primary amino group-labelling that is still limited in identifying all TMPs and their extracellular segments on the surface of a particular cell type. In order to generate more topology information, a new step, a partial proteolysis of the cell surface has been introduced to our method. This step results in new primary amino groups in the proteins that can be biotinylated with a membrane-impermeable agent while the cells still remain intact. Pre-digestion also promotes the emergence of modified peptides that are more suitable for MS/MS analysis. The modified sites can be utilized as extracellular constraints in topology predictions and may contribute to the refined topology of these proteins.

Covalently modified carboxyl side chains on cell surface leads to a novel method toward topology analysis of transmembrane proteins

The research on transmembrane proteins (TMPs) is quite widespread due to their biological importance. Unfortunately, only a little amount of structural data is available of TMPs. Since technical difficulties arise during their high-resolution structure determination, bioinformatics and other experimental approaches are widely used to characterize their low-resolution structure, namely topology. Experimental and computational methods alone are still limited to determine TMP topology, but their combination becomes significant for the production of reliable structural data. By applying amino acid specific membrane-impermeable labelling agents, it is possible to identify the accessible surface of TMPs. Depending on the residue-specific modifications, new extracellular topology data is gathered, allowing the identification of more extracellular segments for TMPs. A new method has been developed for the experimental analysis of TMPs: covalent modification of the carboxyl groups on the accessible cell surface, followed by the isolation and digestion of these proteins. The labelled peptide fragments and their exact modification sites are identified by nanoLC-MS/MS. The determined peptides are mapped to the primary sequences of TMPs and the labelled sites are utilised as extracellular constraints in topology predictions that contribute to the refined low-resolution structure data of these proteins.

Identification of the Receptor Used by the Ecotropic Mouse GLN Endogenous Retrovirus

Approximately 10% of the mouse genome is composed of endogenous retroviruses belonging to different families. In contrast to the situation in the human genome, several of these families correspond to recent, still-infectious elements capable of encoding complete viral particles. The mouse GLN endogenous retrovirus is one of these active families. We previously identified one fully functional provirus from the sequenced genome of the C57BL/6 mouse strain. The GLN envelope protein gives the infectious viral particles an ecotropic host range, and we had demonstrated that the receptor was neither CAT1 nor SMIT1, the two previously identified receptors for mouse ecotropic retroviral envelope proteins. In this study, we have identified SLC19A1, the reduced folate carrier, as the cellular protein used as a receptor by the GLN retrovirus. The ecotropic tropism exhibited by this envelope is due to the presence or absence of an N-linked glycosylation site in the first extracellular loop as well as the specific amino acid sequence of the extracellular domains of the receptor. Like all the other retroviral envelope proteins from the gammaretrovirus genus whose receptors have been identified, the GLN envelope protein uses a member of the solute carrier superfamily as a receptor.IMPORTANCE Endogenous retroviruses are genomic traces of past infections present in all vertebrates. Most of these elements degenerate over time and become nonfunctional, but the mouse genome still contains several families with full infection abilities. The GLN retrovirus is one of them, and its members encode particles that are able to infect only mouse cells. Here, we identified the cellular protein used as a receptor by GLN for cell entry. It is SLC19A1, the reduced folate carrier. We show that GLN infection is limited to mouse cells due to both a mutation in the mouse gene preventing the glycosylation of SLC19A1 and also other residues conserved within the rat but not in the hamster and human proteins. Like all other gammaretroviruses whose receptors have been identified, GLN uses a member of the solute carrier superfamily for cell entry, highlighting the role of these proteins for retroviral infection in mammals.

Identification of Extracellular Segments by Mass Spectrometry Improves Topology Prediction of Transmembrane Proteins

Transmembrane proteins play crucial role in signaling, ion transport, nutrient uptake, as well as in maintaining the dynamic equilibrium between the internal and external environment of cells. Despite their important biological functions and abundance, less than 2% of all determined structures are transmembrane proteins. Given the persisting technical difficulties associated with high resolution structure determination of transmembrane proteins, additional methods, including computational and experimental techniques remain vital in promoting our understanding of their topologies, 3D structures, functions and interactions. Here we report a method for the high-throughput determination of extracellular segments of transmembrane proteins based on the identification of surface labeled and biotin captured peptide fragments by LC/MS/MS. We show that reliable identification of extracellular protein segments increases the accuracy and reliability of existing topology prediction algorithms. Using the experimental topology data as constraints, our improved prediction tool provides accurate and reliable topology models for hundreds of human transmembrane proteins.

Functional and mechanistic roles of the human proton-coupled folate transporter transmembrane domain 6-7 linker

The proton-coupled folate transporter (PCFT; SLC46A1) is a folate-proton symporter expressed in solid tumors and is used for tumor-targeted delivery of cytotoxic antifolates. Topology modeling suggests that the PCFT secondary structure includes 12 transmembrane domains (TMDs) with TMDs 6 and 7 linked by an intracellular loop (positions 236-265) including His247, implicated as functionally important. Single-cysteine (Cys) mutants were inserted from positions 241 to 251 in Cys-less PCFT and mutant proteins were expressed in PCFT-null (R1-11) HeLa cells; none were reactive with 2-aminoethyl methanethiosulfonate biotin, suggesting that the TMD6-7 loop is intracellular. Twenty-nine single alanine mutants spanning the entire TMD6-7 loop were expressed in R1-11 cells; activity was generally preserved, with the exception of the 247, 250, and 251 mutants, partly due to decreased surface expression. Coexpression of PCFT TMD1-6 and TMD7-12 half-molecules in R1-11 cells partially restored transport activity, although removal of residues 252-265 from TMD7-12 abolished transport. Chimeric proteins, including a nonhomologous sequence from a thiamine transporter (ThTr1) inserted into the PCFT TMD6-7 loop (positions 236-250 or 251-265), were active, although replacement of the entire loop with the ThTr1 sequence resulted in substantial loss of activity. Amino acid replacements (Ala, Arg, His, Gln, and Glu) or deletions at position 247 in wild-type and PCFT-ThTr1 chimeras resulted in differential effects on transport. Collectively, our findings suggest that the PCFT TMD6-7 connecting loop confers protein stability and may serve a unique functional role that depends on secondary structure rather than particular sequence elements.

Biology of the major facilitative folate transporters SLC19A1 and SLC46A1

This chapter focuses on the biology of the major facilitative membrane folate transporters, the reduced folate carrier (RFC), and the proton-coupled folate transporter (PCFT). Folates are essential vitamins, and folate deficiency contributes to a variety of heath disorders. RFC is ubiquitously expressed and is the major folate transporter in mammalian cells and tissues. PCFT mediates intestinal absorption of dietary folates. Clinically relevant antifolates such as methotrexate (MTX) are transported by RFC, and the loss of RFC transport is an important mechanism of MTX resistance. PCFT is abundantly expressed in human tumors and is active under pH conditions associated with the tumor microenvironment. Pemetrexed (PMX) is an excellent substrate for PCFT as well as for RFC. Novel tumor-targeted antifolates related to PMX with selective membrane transport by PCFT over RFC are being developed. The molecular picture of RFC and PCFT continues to evolve relating to membrane topology, N-glycosylation, energetics, and identification of structurally and functionally important domains and amino acids. The molecular bases for MTX resistance associated with loss of RFC function, and for the rare autosomal recessive condition, hereditary folate malabsorption (HFM), attributable to mutant PCFT, have been established. From structural homologies to the bacterial transporters GlpT and LacY, homology models were developed for RFC and PCFT, enabling new mechanistic insights and experimentally testable hypotheses. RFC and PCFT exist as homo-oligomers, and evidence suggests that homo-oligomerization of RFC and PCFT monomeric proteins may be important for intracellular trafficking and/or transport function. Better understanding of the structure and function of RFC and PCFT should facilitate the rational development of new therapeutic strategies for cancer as well as for HFM.

Identification and characterization of a 66-68-kDa protein as a methotrexate-binding protein in murine leukemia L1210 cells

We previously observed an unidentified, tyrosine-phosphorylated, membrane-associated, 66-68-kDa protein which was present in the L1210 murine leukemia cells but not present, at least in the tyrosine-phosphorylated form, in cisplatin-methotrexate (CDDP-MTX) cross-resistant L1210/DDP cells. We purified and characterized this 66-68-kDa protein by affinity chromatography purification using its two identified properties, tyrosine phosphorylation and MTX-binding, and yielded a single band of 66-68 kDa. The purified protein was subjected to trypsin digestion and the isolated peptide fragments were sequenced and yielded two partial peptide sequences: VEIIANDQ and VTNAVVTVPAYFNDSQRQA. The two peptide sequences were used to search for the mouse genome at the national center for biotechnology information (NCBI) database for Open Reading Frame Sequence (ORFs) containing these peptides using the TBLASTN function. A single gene was identified containing both sequences, the HSPa8 gene, which codes for the heat shock family protein, HSC70. We further demonstrated that HSC70 is a MTX-binding protein using a binding assay with MTX-agarose beads followed by Western blotting. The HSC70 also existed in various cancer cell lines and showed binding to MTX. Additionally, the HSC70 protein, cloned from the L1210 murine leukemia cells, was expressed and purified from E. coli cells using a polyhistidine-tag purification system and it also showed the binding properties with MTX. DnaK, the HSC70 homologue in E. coli, also binds to MTX. By using the purified truncated HSC70 domains, we identified the adenosine triphosphatase (ATPase) domain of HSC70 that can bind to MTX. Thus, we have tentatively characterized a new, novel property of HSC70 as a MTX-binding protein.

Mechanisms of membrane transport of folates into cells and across epithelia

Until recently, the transport of folates into cells and across epithelia has been interpreted primarily within the context of two transporters with high affinity and specificity for folates, the reduced folate carrier and the folate receptors. However, there were discrepancies between the properties of these transporters and characteristics of folate transport in many tissues, most notably the intestinal absorption of folates, in terms of pH dependency and substrate specificity. With the recent cloning of the proton-coupled folate transporter (PCFT) and the demonstration that this transporter is mutated in hereditary folate malabsorption, an autosomal recessive disorder, the molecular basis for this low-pH transport activity is now understood. This review focuses on the properties of PCFT and briefly addresses the two other folate-specific transporters along with other facilitative and ATP-binding cassette (ABC) transporters with folate transport activities. The role of these transporters in the vectorial transport of folates across epithelia is considered.

Membrane topological analysis of the proton-coupled folate transporter (PCFT-SLC46A1) by the substituted cysteine accessibility method

The proton-coupled folate transporter (PCFT) mediates intestinal folate absorption. Loss-of-function mutations in this gene are the molecular basis for the autosomal recessive disorder, hereditary folate malabsorption. In this study, the substituted cysteine accessibility method was utilized to localize extra- or intracellular loops connecting predicted PCFT transmembrane domains. Cysteine-less PCFT was generated by replacement of all seven cysteine residues with serine and was shown to be functional, following which cysteine residues were introduced into predicted loops. HeLa cells, transiently transfected with these PCFT mutants, were then labeled with an impermeant, cysteine-specific biotinylation reagent (MTSEA-biotin) with or without permeabilization of cells. The biotinylated proteins were precipitated by streptavidin beads and assessed by Western blotting analysis. The biotinylation of PCFT was further confirmed by blocking cysteine residues with impermeant 2-sulfonatoethyl methanethiosulfonate. Two extracellular cysteine residues (66, 298) present in WT-PCFT were not biotinylated; however, in the absence of either one, biotinylation occurred. Likewise, biotinylation occurred after treatment with beta-mercaptoethanol. Taken together, these analyses establish a PCFT secondary structure of 12 transmembrane domains with the N- and C- termini directed to the cytoplasm. The data indicate further that there is a disulfide bridge, which is not required for function, between the native C66 and C298 residues in the first and fourth transmembrane domains, respectively.

Alternative transcripts of rat slc19a1: Cloning, genomic organisation, tissue specific promoters and alternative splicing

Recently, the rat genome project revealed the genomic sequence of slc19a1, coding for the methotrexate carrier-1, identical to the reduced folate carrier-1 of humans, on rat chromosome 20. At the same time, we have cloned and analysed the complete or partial cDNAs of now at least six different transcripts from rat liver and kidneys. Alignment with the genomic sequence revealed seven exons. The first two non-coding exons, exon I and Ia were used alternatively in kidneys and liver, respectively, suggesting usage of alternative promoters. Three minor mRNA forms resulted from absent splicing of intron III, a shortened exon III (exon IIIa), and a shortened exon IV (exon IVa). The minor transcripts were predicted to result in translation products with 7 or 6 instead of 12 transmembrane domains (TMDs) and a peptide mass of 38, 39 and 40 kDa instead of 58 kDa.

The conserved carboxy-terminal region of the ammonia channel AmtB plays a critical role in channel function

The ammonium transport (Amt) proteins are a highly conserved family of integral membrane proteins found in eubacteria, archaea, fungi and plants. Genetic, biochemical and bioinformatic analyses suggest that they have a common tertiary structure comprising eleven trans-membrane helices with an N-out, C-in topology. The cytoplasmic C-terminus is variable in length but includes a core region of some 22 residues with considerable sequence conservation. Previous studies have indicated that this C-terminus is not absolutely required for Amt activity but that mutations that alter C-terminal residues can have very marked effects. Using the Escherichia coli AmtB protein as a model system for Amt proteins, we have carried out an extensive site-directed mutagenesis study to investigate the possible role of this region of the protein. Our data indicate that nearly all mutations fall into two phenotypic classes that are best explained in terms of two distinct effects of the C-terminal region on AmtB activity. Residues within the C-terminus play a significant role in normal AmtB function and the C-terminal region might also mediate co-operativity between the three subunits of AmtB.

Membrane transporters and folate homeostasis: intestinal absorption and transport into systemic compartments and tissues

Members of the family of B9 vitamins are commonly known as folates. They are derived entirely from dietary sources and are key one-carbon donors required for de novo nucleotide and methionine synthesis. These highly hydrophilic molecules use several genetically distinct and functionally diverse transport systems to enter cells: the reduced folate carrier, the proton-coupled folate transporter and the folate receptors. Each plays a unique role in mediating folate transport across epithelia and into systemic tissues. The mechanism of intestinal folate absorption was recently uncovered, revealing the genetic basis for the autosomal recessive disorder hereditary folate malabsorption, which results from loss-of-function mutations in the proton-coupled folate transporter gene. It is therefore now possible to piece together how these folate transporters contribute, both individually and collectively, to folate homeostasis in humans. This review focuses on the physiological roles of the major folate transporters, with a brief consideration of their impact on the pharmacological activities of antifolates.

N-linked glycosylation and its impact on the electrophoretic mobility and function of the human proton-coupled folate transporter (HsPCFT)

The human proton-coupled folate transporter (HsPCFT, SLC46A1) mediates intestinal absorption of folates and transport of folates into the liver, brain and other tissues. On Western blot, HsPCFT migrates as a broad band (~55 kDa), higher than predicted (~50 kDa) in cell lines. Western blot analysis required that membrane preparations not be incubated in the loading buffer above 50 degrees C to avoid aggregation of the protein. Treatment of membrane fractions from HsPCFT-transfected HeLa cells with peptidyl N-glycanase F, or cells with tunicamycin, resulted in conversion to a ~35 kDa species. Substitution of asparagine residues of two canonical glycosylation sites to glutamine, individually, yielded a ~47 kDa protein; substitution of both sites gave a smaller (~35 kDa) protein. Single mutants retained full transport activity; the double mutant retained a majority of activity. Transport function and molecular size were unchanged when the double mutant was hemagglutinin (HA) tagged at either the NH(2) or COOH terminus and probed with an anti-HA antibody excluding degradation of the deglycosylated protein. Wild-type or deglycosylated HsPCFT HA, tagged at amino or carboxyl termini, could only be visualized on the plasma membrane when HeLa cells were first permeabilized, consistent with the intracellular location of these domains.

Structure and function of the reduced folate carrier a paradigm of a major facilitator superfamily mammalian nutrient transporter

Folates are essential for life and folate deficiency contributes to a host of health problems including cardiovascular disease, fetal abnormalities, neurological disorders, and cancer. Antifolates, represented by methotrexate, continue to occupy a unique niche among the modern day pharmacopoeia for cancer along with other pathological conditions. This article focuses on the biology of the membrane transport system termed the "reduced folate carrier" or RFC with a particular emphasis on RFC structure and function. The ubiquitously expressed RFC is the major transporter for folates in mammalian cells and tissues. Loss of RFC expression or function portends potentially profound physiological or developmental consequences. For chemotherapeutic antifolates used for cancer, loss of RFC expression or synthesis of mutant RFC protein with impaired function results in antifolate resistance due to incomplete inhibition of cellular enzyme targets and low levels of substrate for polyglutamate synthesis. The functional properties for RFC were first documented nearly 40 years ago in murine leukemia cells. Since 1994, when RFC was first cloned, tremendous advances in the molecular biology of RFC and biochemical approaches for studying the structure of polytopic membrane proteins have led to an increasingly detailed picture of the molecular structure of the carrier, including its membrane topology, its N-glycosylation, identification of functionally and structurally important domains and amino acids, and helix packing associations. Although no crystal structure for RFC is yet available, biochemical and molecular studies, combined with homology modeling, based on homologous bacterial major facilitator superfamily transporters such as LacY, now permit the development of experimentally testable hypotheses designed to establish RFC structure and mechanism.

Human reduced folate carrier: translation of basic biology to cancer etiology and therapy

This review attempts to provide a comprehensive overview of the biology of the physiologically and pharmacologically important transport system termed the "reduced folate carrier" (RFC). The ubiquitously expressed RFC has unequivocally established itself as the major transport system in mammalian cells and tissues for a group of compounds including folate cofactors and classical antifolate therapeutics. Loss of RFC expression or function may have potentially profound pathophysiologic consequences including cancer. For chemotherapeutic antifolates used for cancer such as methotrexate or pemetrexed, synthesis of mutant RFCs or loss of RFC transcripts and proteins results in antifolate resistance due to incomplete inhibition of cellular enzyme targets and insufficient substrate for polyglutamate synthesis. Since RFC was first cloned in 1994, tremendous advances have been made in understanding the complex transcriptional and posttranscriptional regulation of RFC, in identifying structurally and functionally important domains and amino acids in the RFC molecule as a prelude to establishing the mechanism of transport, and in characterizing the molecular defects in RFC associated with loss of transport in antifolate resistant cell line models. Many of the insights gained from laboratory models of RFC portend opportunities for modulating carrier expression in drug resistant tumors, and for designing a new generation of agents with improved transport by RFC or substantially enhanced transport by other folate transporters over RFC. Many of the advances in the basic biology of RFC in cell line models are now being directly applied to human cancers in the clinical setting, most notably pediatric acute lymphoblastic leukemia and osteogenic sarcoma.

Membrane topology structure of human high-affinity, sodium-dependent dicarboxylate transporter

High-affinity, sodium-dependent dicarboxylate transporter (NaDC3) is responsible for transport of Krebs cycle intermediates and may involve in regulation of aging and life span. Hydropathy analysis predicts that NaDC3 contains 11 or 12 hydrophobic transmembrane (TM) domains. However, the actual membrane topological structure of NaDC3 remains unknown. In this study, confocal immunofluorescence microscopy and membrane biotinylation of epitope-tagged N and C termini of NaDC3 provide evidence of an extracellular C terminus and an intracellular N terminus, indicating an odd number of transmembrane regions. The position of hydrophilic loops within NaDC3 was identified with antibodies against the loops domains combined with cysteine accessibility methods. A confocal image of membrane localization and transport activity assay of the cysteine insertion mutants show behavior similar to that of wild-type NaDC3 in transfected HEK293 cells, suggesting that these mutants retain a native protein configuration. We find that NaDC3 contains 11 transmembrane helices. The loops 1, 3, 5, 7, and 9 face the extracellular side, and loops 2, 4, 6, and 10 face the cytoplasmic side. A re-entrant loop-like structure between TM8 and TM9 may protrude into the membrane. Our results support the topography of 11 transmembrane domains with an extracellular C terminus and an intracellular N terminus of NaDC3, and for the first time provide experimental evidence for a novel topological model for NaDC3.

Molecular basis of antifolate resistance

Folates play a key role in one-carbon metabolism essential for the biosynthesis of purines, thymidylate and hence DNA replication. The antifolate methotrexate has been rationally-designed nearly 60 years ago to potently block the folate-dependent enzyme dihydrofolate reductase (DHFR) thereby achieving temporary remissions in childhood acute leukemia. Recently, the novel antifolates raltitrexed and pemetrexed that target thymidylate synthase (TS) and glycineamide ribonucleotide transformylase (GARTF) were introduced for the treatment of colorectal cancer and malignant pleural mesothelioma. (Anti)folates are divalent anions which predominantly use the reduced folate carrier (RFC) for their cellular uptake. (Anti)folates are retained intracellularly via polyglutamylation catalyzed by folylpoly-gamma-glutamate synthetase (FPGS). As the intracellular concentration of antifolates is critical for their pharmacologic activity, polyglutamylation is a key determinant of antifolate cytotoxicity. However, anticancer drug resistance phenomena pose major obstacles towards curative cancer chemotherapy. Pre-clinical and clinical studies have identified a plethora of mechanisms of antifolate-resistance; these are frequently associated with qualitative and/or quantitative alterations in influx and/or efflux transporters of (anti)folates as well as in folate-dependent enzymes. These include inactivating mutations and/or down-regulation of the RFC and various alterations in the target enzymes DHFR, TS and FPGS. Furthermore, it has been recently shown that members of the ATP-binding cassette (ABC) superfamily including multidrug resistance proteins (MRP/ABCC) and breast cancer resistance protein (BCRP/ABCG2) are low affinity, high capacity ATP-driven (anti)folate efflux transporters. This transport activity is in addition to their established facility to extrude multiple cytotoxic agents. Hence, by actively extruding antifolates, overexpressed MRPs and/or BCRP confer antifolate resistance. Moreover, down-regulation of MRPs and/or BCRP results in decreased folate efflux thereby leading to expansion of the intracellular folate pool and antifolate resistance. This chapter reviews and discusses the panoply of molecular modalities of antifolate-resistance in pre-clinical tumor cell systems in vitro and in vivo as well as in cancer patients. Currently emerging novel strategies for the overcoming of antifolate-resistance are presented. Finally, experimental evidence is provided that the identification and characterization of the molecular mechanisms of antifolate-resistance may prove instrumental in the future development of rationally-based novel antifolates and strategies that could conceivably overcome drug-resistance phenomena.

The role of multidrug resistance efflux transporters in antifolate resistance and folate homeostasis

Members of the ATP-binding cassette (ABC) transporters including P-glycoprotein (Pgp/ABCB1), multidrug resistance proteins (MRPs/ABCC) as well as breast cancer resistance protein (BCRP/ABCG2) function as ATP-dependent drug efflux transporters, which form a unique defense network against multiple chemotherapeutic drugs and cellular toxins. Among antitumor agents is the important group of folic acid antimetabolites known as antifolates. Antifolates such as methotrexate (MTX), pemetrexed and raltitrexed exert their cytotoxic activity via potent inhibition of folate-dependent enzymes essential for purine and pyrimidine nucleotide biosynthesis and thereby block DNA replication. Overexpression of MRPs and BCRP confers resistance upon malignant cells to various hydrophilic and lipophilic antifolates. Apart from their central role in mediating resistance to antifolates and other anticancer drugs, MRPs and BCRP have been recently shown to transport naturally occurring reduced folates. This was inferred from various complementary systems as follows: (a) Cell-free systems including ATP-dependent uptake of radiolabeled folate/MTX into purified inside-out membrane vesicles from stable transfectants and/or cells overexpressing these transporters, (b) Decreased accumulation of radiolabeled folate/MTX in cultured tumor cells overexpressing these transporters, as well as (c) In vivo rodent models such as Eisi hyperbillirubinemic rats (EHBR) that hereditarily lack MRP2 in their canalicular membrane and thereby display a bile that is highly deficient in various reduced folate cofactors and MTX, when compared with wild type Sprague-Dawley (SD) rats. In all cases, these folate/antifolate transporters functioned as high capacity, low affinity ATP-driven exporters. While the mechanism of cellular retention of (anti)folates is mediated via (anti)folylpolyglutamylation, certain efflux transporters including MRP5 (ABCC5) and BCRP were shown to transport both mono-, di- as well as triglutamate derivatives of MTX and folic acid. Furthermore, overexpression of MRPs and BCRP has been shown to result in decreased cellular folate pools, whereas loss of ABC transporter expression brought about a significant expansion in the intracellular reduced folate pool. The latter finding has important implications to antifolate-based chemotherapy as an augmented cellular folate pool results in a significant level of resistance to certain antifolates. Hence, the aims of the present review are: (a) To summarize and discuss the cumulative evidence supporting a functional role for various multidrug resistance efflux transporters of the ABC superfamily which mediate resistance to hydrophilic and lipophilic antifolates, (b) To describe and evaluate the recent data suggesting a role for these efflux transporters in regulation of cellular folate homeostasis under folate replete and deplete conditions. Furthermore, novel developments and future perspectives regarding the identification of novel antifolate target proteins and mechanisms of action, as well as rationally designed emerging drug combinations containing antifolates along with receptor tyrosine kinase inhibitors are being discussed.

Transmembrane domains 4, 5, 7, 8, and 10 of the human reduced folate carrier are important structural or functional components of the transmembrane channel for folate substrates

The human reduced folate carrier (hRFC) facilitates membrane transport of folates and antifolates. hRFC is characterized by 12 transmembrane domains (TMDs). To identify residues or domains involved in folate binding, we used substituted cysteine (Cys) accessibility methods (SCAM) with sodium (2-sulfonatoethyl)methanethiosulfonate (MTSES). We previously showed that residues in TMD11 of hRFC were involved in substrate binding, whereas those in TMD12 were not (Hou, Z., Stapels, S. E., Haska, C. L., and Matherly, L. H. (2005) J. Biol. Chem. 280, 36206-36213). In this study, 232 Cys-substituted mutants spanning TMDs 1-10 and conserved stretches within the TMD6-7 (residues 204-217) and TMD10-11 connecting loop domains were transiently expressed in hRFC-null HeLa cells. All Cys-substituted mutants showed moderate to high levels of expression on Western blots, and only nine mutants including R133C, I134C, A135C, Y136C, S138C, G163C, Y281C, R373C, and S313C were inactive for methotrexate transport. MTSES did not inhibit transport by any of the mutants in TMDs 1, 3, 6, and 9 or for positions 204-217. Whereas most of the mutants in TMDs 2, 4, 5, 7, 8, and 10, and in the TMD10-11 connecting loop were insensitive to MTSES, this reagent inhibited methotrexate transport (25-75%) by 26 mutants in these TMDs. For 13 of these (Y126C, S137C, V160C, S168C, W274C, S278C, V284C, V288C, A311C, T314C, Y376C, Q377C, and V380C), inhibition was prevented by leucovorin, another hRFC substrate. Combined with our previous findings, these results implicate amino acids in TMDs 4, 5, 7, 8, 10, and 11, but not in TMDs 1, 2, 3, 6, 9, or 12, as important structural or functional components of the putative hydrophilic cavity for binding of anionic folate substrates.

Alternative transcripts of rat slc19a1: defective sorting and inefficient expression of alternative splicing products--an addendum

Recently, we have reported about the existence and cloning of alternative transcripts of the solute carrier 19a1 in rat liver and kidney, and have explained their origin from tissue specific promoters and by alternative splicing [DNA Seq. 2005, 16(1), 1-6]. The variant open reading frames (ORFs) of these transcripts were now expressed as fusion proteins with an N-terminal GFP. Transfection of HPCT-1E3 hepatocytoma cells as well as the MDCK kidney epithelial cell line with the GFP-fusion of the major splicing form containing 12 transmembrane domains (TMD) resulted in a strong fluorescence at the plasma membrane, which was stable over at least 3 days. Expression of GFP alone gave a comparable overall fluorescence intensity, although, staining was distributed evenly throughout the cells. Fusions of GFP with the shorter ORFs of all alternative slc19a1 transcripts containing 6-7 predicted TMD were expressed less efficiently and not sorted to the plasma membrane. Instead, these proteins accumulated in intracellular granules, and were, apparently, degraded. Hence, it is unlikely that these minor splicing forms are directly involved in solute transport across the plasma membrane.

Infection-dependent nuclear localization of US17, a member of the US12 family of human cytomegalovirus-encoded seven-transmembrane proteins

The human cytomegalovirus (HCMV) US12 gene family is a group of predicted seven-transmembrane, G-protein-coupled receptor-related proteins, about which little is known. Specific rabbit polyclonal antibodies detected US17 and US18 beginning 54 and 36 h after infection, respectively, with expression of both proteins dependent on viral DNA synthesis. While US14 and US18 are expressed exclusively in the cytoplasm, we unexpectedly found abundant expression of US17 in both the cytoplasm and nucleoplasm. N- and C-terminally tagged versions of US17 were readily detected in the cytoplasm of transfected mammalian cells, but not in nuclei, suggesting that nuclear localization involves other viral proteins or an infection-triggered cellular process. There was no specific colocalization between US17 and other nuclear expressed HCMV-encoded proteins (IE-2, DNA polymerase processivity factor, and pp28/UL99). To determine whether the observed nuclear localization might be the product of a process by which a soluble C-terminal segment of the full-length protein is expressed, we constructed a recombinant virus that incorporates a synthetic epitope at its N terminus, which in conjunction with the antipeptide antibody that targets its predicted cytoplasmic C-terminal segment, enables simultaneous independent detection of both termini. In cells infected with the recombinant, the US17 N and C termini had limited colocalization, with the N-terminal segment not detected in nuclei, supporting the segmentation hypothesis. Consistent with this, a fragment with an apparent molecular size of 10 kDa was detected by immunoblotting. We have identified the first viral example of a seven-transmembrane protein that is either segmented or expressed in nuclei. Further study will be required to learn the mechanism by which this occurs and the function of the nuclear localizing segment. This likely represents yet another mechanism by which a virus has hijacked or modified cellular regulatory pathways for its benefit.

Membrane topology of mouse stearoyl-CoA desaturase 1

Stearoyl-CoA desaturase (SCD) is an integral membrane protein anchored in the endoplasmic reticulum. It catalyzes the biosynthesis of monounsaturated fatty acids that are required for the synthesis of triglycerides, cholesteryl esters, and phospholipids. Four mouse isoforms of SCD (SCD1-4) and two human isoforms have been characterized. In the current study, we characterize the topology of the mouse SCD1 isoform. Hydropathy analysis of the 355-amino acid mouse SCD1 protein predicts that the protein contains four transmembrane domains (TMDs) and three loops connecting the membrane-spanning domains. To define the topology of the protein, recombinant SCD1 constructs containing epitope tags were transiently expressed in HeLa cells and analyzed by indirect immunofluorescence and cysteine derivatization. Our data provide evidence that the N and C termini of SCD1 are oriented toward the cytosol with four transmembrane domains separated by two very short hydrophilic loops in the ER lumen and one large hydrophilic loop in the cytosol. In addition, based on the previous observation that SCD is a thiol enzyme, we sought to investigate whether the cysteine residues were essential for enzyme activity through mutagenesis studies, and our data suggest that the cysteines in SCD are not catalytically essential.

Localization of a substrate binding domain of the human reduced folate carrier to transmembrane domain 11 by radioaffinity labeling and cysteine-substituted accessibility methods

The human reduced folate carrier (hRFC) mediates the membrane transport of reduced folates and classical anti-folates into mammalian cells. RFC is characterized by 12 transmembrane domains (TMDs), internally oriented N and C termini, and a large central linker connecting TMDs 1-6 and 7-12. By co-expression and N-hydroxysuccinimide methotrexate (Mtx) radioaffinity labeling of hRFC TMD 1-6 and TMD 7-12 half-molecules, combined with endoproteinase GluC digestion, a substrate binding domain was previously localized to within TMDs 8-12 (Witt, T. L., Stapels, S. E., and Matherly, L. H. (2004) J. Biol. Chem. 279, 46755-46763). In this report, this region was further refined to TMDs 11-12 by digestion with 2-nitro-5-thiocyanatobenzoic acid. A transportcompetent cysteine-less hRFC was used as a template to prepare single cysteine-replacement mutant constructs in which each residue from Glu-394 to Asp-420 of TMD 11 and Tyr-435 to His-457 of TMD 12 was replaced individually by a cysteine. The mutant constructs were transfected into hRFC-null HeLa cells. Most of the 50 single cysteine-substituted constructs were expressed at high levels on Western blots. With the exception of G401C hRFC, all mutants were active for Mtx transport. Treatment with sodium (2-sulfonatoethyl) methanethiosulfonate (MTSES) had no effect on hRFC activity for all of the cysteine mutants within TMD 12 and for the majority of the cysteine mutants within TMD 11. However, MTSES inhibited Mtx uptake by the T404C, A407C, T408C, T412C, F416C, I417C, V418C, and S419C mutants by 25-65%. Losses of activity by MTSES treatment for T404C, A407C, T412C, and I417C hRFCs were appreciably reversed in the presence of excess leucovorin, a hRFC substrate. Our results strongly suggest that residues within TMD 11 are likely critical structural and/or functional components of the putative hRFC transmembrane channel for anionic folate and anti-folate substrates.

Transmembrane protein topology mapping by the substituted cysteine accessibility method (SCAM(TM)): application to lipid-specific membrane protein topogenesis

We provide an overview of lipid-dependent polytopic membrane protein topogenesis, with particular emphasis on Escherichia coli strains genetically altered in their lipid composition and strategies for experimentally determining the transmembrane organization of proteins. A variety of reagents and experimental strategies are described including the use of lipid mutants and thiol-specific chemical reagents to study lipid-dependent and host-specific membrane protein topogenesis by substituted cysteine site-directed chemical labeling. Employing strains in which lipid composition can be controlled temporally during membrane protein synthesis and assembly provides a means to observe dynamic changes in protein topology as a function of membrane lipid composition.

Placental folate transport and binding are not impaired in pregnancies complicated by fetal growth restriction

Maternal folate deficiency is associated with fetal growth restriction, however, transfer of folate across placentae of pregnancies complicated by fetal growth restriction has never been investigated. We studied whether maternal to fetal 5-methyltetrahydrofolate (5MTF) transport in the ex vivo dually perfused isolated cotyledon, binding of [(3)H] folate (PteGlu) to the syncytial microvillous membrane, and protein expression of folate receptor alpha (FR-alpha) and reduced folate carrier (RFC) in these placentae are disturbed. Placental clearance of 5MTF from the maternal perfusate appeared to be non-saturable over a range of 50 to 500 nm, independent of albumin and flow-independent. No statistically significant differences between placentae complicated with fetal growth restriction and uncomplicated pregnancies were observed. Binding characteristics of [(3)H-]PteGlu to microvillous membranes of fetal growth restriction versus control placentae were similar: B(max)of 3.9+/-2.0 (mean+/-s.d.) versus 4.0+/-1.6 pmol/mg protein and a K(d)of 0.037+/-0.010 versus 0.040+/-0.018 nm. Expression of FR-alpha and RFC were not different in placentae of both groups studied. In conclusion, fetal growth restriction appears not to be associated with impaired maternal to fetal placental folate transport, placental receptor binding, or expression of FR-alpha and RFC.

A prominent low-pH methotrexate transport activity in human solid tumors: contribution to the preservation of methotrexate pharmacologic activity in HeLa cells lacking the reduced folate carrier

Whereas the major folate transporter, the reduced folate carrier (RFC), has a physiological pH optimum, transport activities for folates and antifolates have been detected with low pH optima. Because the interstitial pH in solid tumors is generally acidic, the mechanisms by which antifolates are transported at low pH could be an important determinant of drug activity under these conditions. The current study quantitated the low pH methotrexate (MTX) transport activity in human solid tumor cell lines from the National Cancer Institute tumor panel and other sources. MTX influx at pH 5.5 was equal to, or greater than, influx at pH 7.4 in 29 of 32 cell lines. To assess the role of RFC in transport at low pH in one of these cell lines, a HeLa clonal line (R5) was selected for MTX resistance due to a genomic deletion of the carrier gene. MTX influx was depressed by 70% in R5 versus wild-type HeLa cells at pH 7.4. At pH 6.5, influx in these two lines was similar; as the pH was decreased to 5.5 influx increased in both cell lines. Similarly, whereas net MTX uptake over 1 h was markedly decreased in R5 cells at pH 7.4, net uptake in HeLa and R5 cells was comparable at pH 6.5. Also, as compared with MCF7 breast cancer cells, MTX uptake was markedly decreased at pH 7.4, but only minimally at pH 6.5, in the MDA-MB-231 human breast cancer cell line that lacks RFC expression. When grown with folic acid (2 micro M) at pH 7.4, the IC(50) for MTX was 14-fold higher in R5 as compared with wild-type HeLa cells; the difference was only 4-fold when cells when grown at pH 6.9; the IC(50)s were identical at this pH when the medium folate was 25 nM 5-formyltetrahydrofolate. These data demonstrate that transport activity at low pH is prevalent in human solid tumors, is RFC-independent in R5 cells and MDA-MB-231 breast cancer cells, and can preserve MTX activity in the absence of RFC at an acidic pH relevant to solid tumors in vivo.

Restoration of transport activity by co-expression of human reduced folate carrier half-molecules in transport-impaired K562 cells: localization of a substrate binding domain to transmembrane domains 7-12

Reduced folates such as 5-methyl tetrahydrofolate and classical antifolates such as methotrexate are actively transported into mammalian cells by the reduced folate carrier (RFC). RFC is characterized by 12 stretches of mostly hydrophobic, alpha-helix-promoting amino acids, internally oriented N and C termini, and a large central linker connecting transmembrane domains (TMDs) 1-6 and 7-12. Previous studies showed that deletion of the majority of the central loop domain between TMDs 6 and 7 abolished transport, but this segment could be replaced with mostly non-homologous sequence from the SLC19A2 thiamine transporter to restore transport function. In this report, we expressed RFC from separate TMD1-6 and TMD7-12 RFC half-molecule constructs, each with a unique epitope tag, in RFC-null K562 cells to restore transport activity. Restored transport exhibited characteristic transport kinetics for methotrexate, a capacity for trans-stimulation by pretreatment with leucovorin, and inhibition by N-hydroxysuccinimide methotrexate, a documented affinity inhibitor of RFC. The TMD1-6 half-molecule migrated on SDS gels as a 38-58 kDa glycosylated species and was converted to 27 kDa by N-glycosidase F or tunicamycin treatments. The 40 kDa TMD7-12 half-molecule was unaffected by these treatments. Using transfected cells expressing both TMDs 1-6 and TMDs 7-12 as separate polypeptides, the TMD7-12 half-molecule was covalently radiolabeled with N-hydroxysuccinimide [(3)H]methotrexate. No radioactivity was incorporated into the TMD1-6 half-molecule. Digestion with endoproteinase GluC decreased the size of the radiolabeled 40 kDa TMD7-12 polypeptide to approximately 20 kDa. Our results demonstrate that a functional RFC can be reconstituted with RFC half-molecules and localize a critical substrate binding domain to within TMDs 7-12.

Analysis of the membrane topology for transmembrane domains 7-12 of the human reduced folate carrier by scanning cysteine accessibility methods

The hRFC (human reduced folate carrier) is the major membrane transporter for both reduced folates and antifolates in human tissues and tumours. The primary amino acid sequence of hRFC predicts a membrane topology involving 12 TMDs (transmembrane domains) with cytosolic oriented N- and C-termini, and a large internal loop connecting TMDs 6 and 7. Previous studies using haemagglutinin epitope insertion and scanning glycosylation mutagenesis methods verified portions of the predicted topology model, including TMDs 1-8 and the N- and C-termini of hRFC. However, the topology structure for TMDs 9-12 remains controversial. To further determine the membrane topology of the hRFC protein, single cysteine residues were introduced into the predicted extracellular or cytoplasmic loops of a fully functional cysteine-less hRFC expressed in transport impaired MtxRIIOua(R)2-4 Chinese hamster ovary cells. The membrane orientations of the substituted cysteines were determined by treatments with the thiol reagents 3-(N-maleimidylpropionyl)-biocytin (biotin maleimide) and 4-acetamido-4'maleimidylstilbene-2,2'-disulphonic acid (stilbenedisulphonate maleimide; SM) or N-ethylmaleimide, combined with the cell-permeabilizing reagent SLO (streptolysin O). We found that cysteine residues placed in the predicted extracellular loops between TMDs 7 and 8 (position 301), 9 and 10 (360), and 11 and 12 (429) could be biotinylated with 200 microM biotin maleimide, and labelling could be blocked with SM. However, biotinylation of cysteines placed in the predicted intracellular loops between TMDs 8 and 9 (position 332) and TMDs 10 and 11 (position 388) was only detected after cell permeabilization with SLO and was abolished by pre-treatment with N -ethylmaleimide. These results strongly support a 12-TMD topology structure for the hRFC protein.

Resistance to antifolates

The antifolates were the first class of antimetabolites to enter the clinics more than 50 years ago. Over the following decades, a full understanding of their mechanisms of action and chemotherapeutic potential evolved along with the mechanisms by which cells develop resistance to these drugs. These principals served as a basis for the subsequent exploration and understanding of the mechanisms of resistance to a variety of diverse antineoplastics with different cellular targets. This section describes the bases for intrinsic and acquired antifolate resistance within the context of the current understanding of the mechanisms of actions and cytotoxic determinants of these agents. This encompasses impaired drug transport into cells, augmented drug export, impaired activation of antifolates through polyglutamylation, augmented hydrolysis of antifolate polyglutamates, increased expression and mutation of target enzymes, and the augmentation of cellular tetrahydrofolate-cofactor pools in cells. This chapter also describes how these insights are being utilized to develop gene therapy approaches to protect normal bone marrow progenitor cells as a strategy to improve the efficacy of bone marrow transplantation. Finally, clinical studies are reviewed that correlate the cellular pharmacology of methotrexate with the clinical outcome in children with neoplastic diseases treated with this antifolate.

The region between transmembrane domains 1 and 2 of the reduced folate carrier forms part of the substrate-binding pocket

A functional cysteine-less form of the hamster reduced folate carrier protein was generated by alanine replacement of the 14 cysteine residues. The predicted 12-transmembrane topology was examined by replacing selected amino acids, predicted to be exposed to the extracellular or cytosolic environments, with cysteines. The location of these cysteines was defined by their accessibility to biotin maleimide in the presence or absence of specific blocking agents. Amino acids predicted to be exposed to the extracellular environment (S46C, S179C, L300C, Y355C, and K430C) could be labeled with biotin maleimide; this modification could be blocked by prior treatment with nonpermeable reagents. Amino acids predicted to be within the cytosol (S152C, Cys224, and L475C) could be labeled only after streptolysin O permeabilization. In addition, the cysteine-less reduced folate carrier was exploited to evaluate a potential substrate-binding domain as suggested by previous studies. Nineteen cysteine replacements were generated between residues 39 and 75, a region located between the first and second transmembrane segments. From the biotinylation of these sites and the ability of various reagents to block this labeling, it appears that L41C, E45C, S46C, T49C, I66C, and L70C are exposed to the extracellular environment, whereas Q54C, Q61C, and T63C are slightly less accessible. Cysteines 39, 42, 44, 47, 51, and 73 were inefficiently biotinylated, suggesting that these sites are located in the membrane or within a tightly folded domain of the protein. Furthermore, biotinylation of cysteines 41, 46, 49, 70, and 71 could be prevented by prior treatment with either methotrexate or folinic acid, indicating that these sites form part of a substrate-binding pocket.

Characterization of a cysteine-less human reduced folate carrier: localization of a substrate-binding domain by cysteine-scanning mutagenesis and cysteine accessibility methods

The human reduced folate carrier (hRFC) mediates the transport of reduced folates and classical anti-folates into mammalian cells. Whereas the functionally important domains in hRFC are poorly characterized, previous studies with anti-folate-resistant cells suggest critical roles for transmembrane domain (TMD) 1 and residues (Gly44, Glu45, Ser46 and Ile48) in or flanking this region. An hRFC mutant devoid of cysteine residues was prepared by deleting the C-terminal 56 amino acids, including four cysteine residues, and mutagenizing the remaining cysteine residues to serine residues. A fully functional cysteine-less hRFC protein was expressed in transport-impaired MtxRIIOuaR2-4 Chinese-hamster ovary cells. To explore the role of residues in or flanking TMD1 in transport, all 24 amino acids from Trp25 to Ile48 of hRFC were mutated individually to cysteine residues, and the mutant hRFCs were transfected into MtxRIIOuaR2-4 cells. All of the 24 cysteine mutants were expressed and, with the exception of R42C (Arg42-->Cys), were capable of mediating methotrexate uptake above the low level in MtxRIIOuaR2-4 cells. We found that by treating the transfected cells with the small, water-soluble, thiol-reactive anionic reagent, sodium (2-sulphonatoethyl) methanethiosulphonate, methotrexate transport by several of the cysteine-substituted hRFC mutants was significantly inhibited, including Q40C, G44C, E45C and I48C. Sodium (2-sulphonatoethyl) methanethiosulphonate transport inhibition of the Q40C, G44C and I48C mutants was protected by leucovorin [(6R, S)-5-formyltetrahydrofolate], indicating that these residues lie at or near a substrate-binding site. Using surface-labelling reagents [N-biotinylaminoethyl methanethiosulphonate and 3-(N-maleimidylpropionyl)biocytin, combined with 4-acetamido-4'-maleimidylstilbene-2,2'-disulphonic acid] with cysteine mutants from positions 37-48, the extracellular TMD1 boundary was found to lie between residues 39 and 40, and amino acids 44-46 and 48 were localized to the TMD1 exofacial loop. Collectively, our results imply that amino acids 40, 44, 48 and, possibly, 42 serve important roles in hRFC transport, albeit not as structural components of the putative transmembrane channel for folate substrates.

Membrane transport of folates

The chapter reviews the current understanding of the transport mechanisms for folates in mammalian cells--their molecular identities and organization, tissue expression, regulation, structures, and their kinetic and thermodynamic properties. This encompasses a variety of diverse processes. Best characterized is the reduced folate carrier, a member of the SLC19 family of facilitative carriers. But other facilitative organic anion carriers (SLC21), largely expressed in epithelial tissues, transport folates as well. In addition to these bi-directional carrier systems are the membrane-localized folate receptors alpha and beta, that mediate folate uptake unidirectionally into cells via an endocytotic process. There are also several transporters, typified by the family of multidrug resistance-associated proteins, that unidirectionally export folates from cells. There are transport activities for folates, that function optimally at low pH, related in part to the reduced folate carrier, with at least one activity that is independent of this carrier. The reduced folate carrier-associated low-pH route mediates intestinal folate transport. This review considers how these different transport processes contribute to the generation of transmembrane folate gradients and to vectorial flows of folates across epithelia. The role of folate transporters in mouse development, as assessed by homologous deletion of folate receptors and the reduced folate carrier, is described. Much of the focus is on antifolate cancer chemotherapeutic agents that are often model surrogates for natural folates in transport studies. In particular, antifolate transport mediated by the reduced folate carrier is a major determinant of the activity of, and resistance to, these agents. Finally, many of the key in vitro findings on the properties of antifolate transporters are now beginning to be extended to patient specimens, thus setting the stage for understanding response to these drugs in the clinical setting at the molecular level.

Residues 45 and 404 in the murine reduced folate carrier may interact to alter carrier binding and mobility

The reduced folate carrier (RFC), a facilitative transporter, plays a major role in the delivery of reduced folates and antifolates into cells. Previous studies indicated that mutations of E45K in the first transmembrane domain (TMD), and K404L in the 11th TMD, produce selective and opposite alterations in binding of natural folate substrates to murine RFC. The former mutation is frequently associated with antifolate resistance. The current study was designed to determine whether there might be an interaction between these sites by comparing the transport properties of RFC-null cell lines stably transfected with K404E, E45K, or E45K/K404E carriers. These studies demonstrated that: (1) All mutant carriers were inserted into the plasma membrane. (2) In the K404E mutant, the influx K(t)'s for 5-formyltetrahydrofolate and 5-methyltetrahydrofolate were markedly increased, and to a much smaller extent folic acid, as compared to L1210 cells. However, with introduction of a second E45K mutation the influx K(t) for these folates reverted to those of the E45K cells which retained wild-type binding for 5-methyltetrahydrofolate and enhanced binding of 5-formyltetrahydrofolate and folic acid. (3) The influx V(max) of the E45K mutant was markedly reduced. Introduction of the second K404E mutation doubled this parameter and the ratio of V(max) to K(t) for 5-formytetrahydrofolate was restored to approximately 50% that of the wild-type carrier consistent with a substantial increase in function. (4) Chloride inhibits wild-type RFC but the E45K mutant requires chloride for activity. The K404E mutant is also suppressed by chloride but introduction of the K404E mutation decreased the chloride-dependence of E45K. The results suggest that there is an interaction between the E45 and K404 residues in the first and 11th TMDs, respectively, but that the E45 residue appears to be the more dominant determinant of binding and anion sensitivity.

SLC19: the folate/thiamine transporter family

The SLC19 gene family of solute carriers is a family of three transporter proteins with significant structural similarity, transporting, however, substrates with different structure and ionic charge. The three members of this gene family are expressed ubiquitously and mediate the transport of two important water-soluble vitamins, folate and thiamine. The concentrative transport of substrates mediated by the members of this gene family is energized by transcellular H(+)/OH(-) gradient. SLC19A1 is expressed at highest levels in absorptive cells where it is located in a polarized manner either in the apical or basal membrane, depending on the cell type. It mediates the transport of reduced folate and its analogs, such as methotrexate, which are anionic at physiological pH. SLC19A2 is expressed ubiquitously and mediates the transport of thiamine, a cation at physiological pH. SLC19A3 is also widely expressed and is capable of transporting thiamine. This review summarizes the current knowledge on the structural, functional, molecular and physiological aspects of the SLC19 gene family.

Restoration of high-level transport activity by human reduced folate carrier/ThTr1 thiamine transporter chimaeras: role of the transmembrane domain 6/7 linker region in reduced folate carrier function

The reduced folate carrier (RFC; SLC19A1) is closely related to the thiamine transporter, SLC19A2 (ThTr1). Hydropathy models for these homologous transporters predict up to 12 transmembrane domains (TMDs), with internally oriented N- and C-termini and a large central loop between TMDs 6 and 7. The homologies are localized mostly in the TMDs. However, there is little similarity in their N- and C-terminal domains and the central peptide linkers connecting putative TMDs 1-6 and TMDs 7-12. To explore the functional role of the 61-amino acid central linker in the human RFC (hRFC), we introduced deletions of 49 and 60 amino acids into this region, differing by the presence of a stretch of 11 highly conserved amino acids between the human and rodent RFCs (positions 204-214). An additional hRFC construct was prepared in which only the 11 conserved amino acids were deleted. The resulting hRFC(D215-R263 Delta), hRFC(K204-R263 Delta) and hRFC(K204-R214 Delta) proteins were transfected into transport-impaired K562 cells. The deletion constructs were all expressed in plasma membranes; however, they were completely inactive for methotrexate and (6 S )5-formyl tetrahydrofolate transport. Insertion of non-homologous 73- and 84-amino acid fragments from the structurally analogous ThTr1 linker region into position 204 of hRFC(K204-R263 Delta) restored low levels of transport (16-21% of the wild type). Insertion of the ThTr1 linkers into hRFC(D215-R263 Delta) at position 215 restored 60-80% of wild-type levels of transport. Collectively, our results suggest that the role of the hRFC linker peptide is to provide the proper spatial orientation between the two halves of the hRFC protein for optimal function, and that this is largely independent of amino acid sequence. Our results also demonstrate a critical transport role for the stretch of 11 conserved amino acids starting at position 204 of hRFC.

Identification of lysine-411 in the human reduced folate carrier as an important determinant of substrate selectivity and carrier function by systematic site-directed mutagenesis

Site-directed mutagenesis was used to characterize the functional role of lysine-411, a conserved amino acid located in putative transmembrane domain (TMD) 11 of the human reduced folate carrier (hRFC). Lysine-411 was mutagenized to arginine, glutamate, and leucine, and the mutant constructs (K411R-, K411E-, and K411L-hRFC, respectively) were transfected into hRFC-deficient K562 cells. The mutant hRFC constructs were all expressed at high levels and restored 22-36% of the methotrexate (MTX) transport level in wild-type (K43-6) hRFC transfectants. Although 5-formyl tetrahydrofolate (5-CHO-H(4)PteGlu) uptake levels for both the K411E- and K411L-hRFCs were also impaired (approximately 33% and 28%, respectively), a complete restoration of the wild-type level was observed for K411R-hRFC. While loss of MTX transport activity for the K411R-hRFC transfectant was associated with an incomplete restoration of MTX sensitivity compared to K43-6 cells, these cells were similarly sensitive to Tomudex. The K411R-hRFC transfectants showed an approximately threefold decreased growth requirement for 5-CHO-H(4)PteGlu compared to K43-6 cells. The 5-CHO-H(4)PteGlu transport stimulation observed for the wild-type carrier in chloride-free buffer was also observed for K411R-hRFC, however, this response was decreased for the K411E- and K411L-hRFCs. The preservation of low levels of transport for the K411E- and K411L-hRFCs suggest that the amino acid at position 411 does not directly participate in the binding of anionic hRFC substrates. However, a functionally important role for a basic amino acid at position 411 was, nonetheless, implied by the increased MTX transport for wild-type hRFC over the K411 mutant hRFCs, and the highly selective uptake of 5-CHO-H(4)PteGlu over MTX for K411R-hRFC.