Abstract:
Four novel transporter genes were successfully cloned by screening novel transporter genes based on the human OATP transporter gene sequence. These transporters are useful in the development of drugs by taking advantage of the activity of transporting biological substances and various drugs. It was also found that these transporter genes have single nucleotide polymorphisms (SNP). Gene diagnosis based on the polymorphisms (such as SNP) in these transporter genes enables one to judge, for example, the efficacy of a drug therapy.

Description:
This application is a continuation-in-part of International Application PCT/JP00/06416, filed Sep. 20, 2000, which claims priority to Japanese Patent Application No. 11/267835, filed Sep. 21, 1999. 
    
    
     TECHNICAL FIELD 
     The present invention relates to transporter families, proteins involved in the transport of substances from the outside to the inside of cells and vice versa. 
     BACKGROUND 
     Recently, the involvement of various transporters localized on the plasma membrane in the uptake system for nutrients and endogenous substances into cells and their transport mechanisms have been clarified (Tsuji et al., Pharm. Res. 13:963–977, 1996). These transporters recognize the structures of substances to be transported to selectively transport specific substances across biological membranes. Transporters that recognize structures of a relatively wide range may possibly recognize foreign substances, such as drugs, by mistake, and actively take in them into cells. It is believed that drugs permeate through the plasma membrane fundamentally by simple diffusion, depending on their physicochemical properties such as molecular size, fat-solubility, and hydrogen-binding capacity. Particularly, according to the pH partition hypothesis, in the case of ionic drugs, only molecules in the non-dissociated form can permeate through the plasma membrane. However, it has become evident that a number of drugs penetrate through the cell membrane by a specific mechanism other than simple diffusion, that is, an active transport mediated by transporters, particularly in organs that require efficient exchange of intracellular and extracellular substances, including small intestine, uriniferous tubule, placenta, epithelial cells of choroid plexus, hepatocytes, and blood-brain barrier (Tamai et al., Pharmacia 31:493–497, 1995; Saito et al., Igaku no Ayumi 179:393–397, 1996; Tamai, I., Yakubutsu Dotai (Pharmacokinetics) 11:642–650, 1996). For example, it is known that although oral β-lactam antibiotics of the non-esterified type are amphoteric or negatively charged in physiological pHs and sparingly soluble in fat, they are readily absorbed through the intestine. A transport study using the isolated membrane-vesicles system demonstrated that an H + -driven peptide transporter localized on the brush-border membrane is involved in the absorption process of these drugs (Okano et al., J. Biol. Chem. 261:14130–14134, 1986). Although the specificity of a peptide transport system in terms of the peptide size is so strict as to recognize di- or tri-peptides but not tetrapeptides or larger peptides, it has a rather broad substrate specificity so as to recognize peptides comprising non-natural amino acids. The peptide transporter mistakenly mediates transport of β-lactam antibiotics due to its broad substrate specificity. This property has been unexpectedly utilized in the clinical field (Tsuji, A., American Chemical Society (eds. Taylor, M. D., Amidon, G. L.), Washington, D.C., 101–134, 1995). Furthermore, it has been reported that a transporter is possibly also involved in permeation of substances with a high fat-solubility such as fatty acids through the plasma membrane (Schaffer et al., Cell 79:427–436, 1994). 
     Since various transporters are presumed to be distributed in organs and cells based on the physiological roles of the organs and cells, their distribution and functions may be specific to organs. Therefore, transporters are expected to be used to impart an organ specificity to pharmacokinetics. In other words, an organ-specific drug delivery system (DDS) can be constructed utilizing transporters. If drug absorption that relies solely on simple diffusion is improved by elevating its fat-solubility, the effect of the drug obtained in the initial transport in the liver can be enhanced and the drug can non-specifically translocate into any organ. In addition, it would also be possible to increase the drug absorption independently of its fat-solubility by designing the drug based on the substrate specificity of transporters (Hayashi et al., Drug Delivery System 11:205–213, 1996). For this purpose, it is necessary to identify various transporters at the molecular level and analyze their properties in detail. However, molecular level identification is greatly behind studies on membrane physiology because the transporters are difficult to handle biochemically and require complicated processes in their functional assays. 
     Recently, cDNAs of several transporters have been cloned by the expression cloning method using  Xenopus  oocytes, a foreign gene expression system, and the structural homology among them has been revealed (Fei et al., Nature 368:563–566, 1994). For example, Koepsell et al. cloned an organic cation transporter, OCT1, which is presumed to be localized on a basement membrane, using the expression cloning method in 1994 (Grundemann et al., Nature 372:549–552, 1994). Subsequently, OCT2 was identified by homology cloning based on the sequence of OCT1 (Okuda et al., Biochem. Biophys. Res. Commun. 224:500–507, 1996). OCT1 and OCT2 show homology as high as 67% to each other (Grundemann et al., J. Biol. Chem. 272:10408–10413, 1997). Both of them are intensely express in the kindey, but differ in the organ distribution; OCT1 is also expressed in the liver, colon, and small intestine, while OCT2 expression is specific to the kidney. 
     In addition, another transporter, the human OATP transporter (hereinafter, referred to as “OATP-A”; Gastroenterology 109(4):1274–1282, 1995), has been reported. This transporter is a protein capable of transporting various endogenous and foreign substances in a sodium ion-independent manner. Known substances transported by OATP-A include bromosulfophthalein, bile acids, steroid hormones, etc. Since PGT, a transporter capable of transporting prostaglandins, also shows significant homology to OATP-A, genes encoding these transporters are thought to form a gene family (the OATP family). 
     Only a few reports are available on identifications of transporters at the molecular level, including above reports, and it is believed that many unidentified transporters exist that can be clinically useful. 
     SUMMARY 
     It is an object of this invention to provide novel transporter genes belonging to the OATP family, proteins encoded by these genes, and use of the genes and proteins. 
     In order to find genes encoding novel transporters, the present inventors performed a tBLASTn search of the human EST (Expressed Sequence Tag) database (found on the World Wide Web at ncbi.nlm.nih.gov/blast/blast.cgi) in NCBI (National Center for Biotechnology Information in U.S.A., found on the World Wide Web at ncbi.nlm.nih.gov/index.html) with the protein sequence of human OATP transporter (Gastroenterology 109(4):1274–1282, 1995) as a query. As a result, several ESTs which may encode amino acid sequences having significant homologies with the human OATP-A protein were found. Next, further searches of the database with these EST sequences as the query revealed that all of the ESTs are derived from genes of unknown functions, with the exception of those which were clearly judged to be derived from the known human OATP-A gene and human prostaglandin transporter gene (J. Clin. Invest. 98(5):1142–1149, 1996) (hereinafter, abbreviated as PGT). This finding indicated that these ESTs are derived from transporter genes which have not been identified so far. Therefore, cloning of the full-length cDNA was performed by screening cDNA libraries with these EST sequences, using PCR and plaque hybridization methods, which resulted in the successful cloning of four genes encoding four novel transporter-like proteins. Since all of the proteins encoded by these genes have significant homologies with the human OATP-A protein, the genes have been designated as OATP-B, C, D and E, respectively. Thus, the present inventors discovered that all of the ESTs found by the tBLASTn search are those derived from human OATP-A, B, C, D and E as well as human PGT gene. 
     As described above, it has been known that the human OATP-A is a transporter protein capable of transporting a variety of endogenous and foreign substances, including bromosulfophthalein, bile acids, steroid hormones, etc., in a sodium ion-independent manner, and that the PGT protein capable of transporting prostaglandins shows a significant homology with the OATP-A protein. These transporter genes have been thought to form a family of genes (the OATP family) potentially involved in the removal of substances unnecessary for living bodies and regulation of concentrations of a variety of substances in vivo (J. Biol. Chem. 273(35): 22395–401, 1998). 
     It is presumed that novel members of the OATP family found in the present invention also share similar functions to OATP-A and PGT, in the regulation of in vivo concentrations of substances essential or unnecessary to living bodies. Furthermore, the present inventors also demonstrated a capability of the OATP-C protein to transport drugs, such as β-lactam antibiotics. According to these facts, it was presumed that drugs which are originally foreign substances to living bodies may possibly be taken up into or excreted from cells in a manner mediated by the OATP family proteins. Therefore, it may be possible to control pharmacodynamics and speedily design or screen drugs with a higher absorbability by utilizing the transport specificity, such as substrate specificity, and distribution pattern in vivo of the OATP-family proteins. In particular, the present inventors discovered, by analysis using RT-PCR method, that OATP-E is highly expressed in a variety of solid cancer cells but seldom in hemocytes. Thus, anticancer agents may be obtained by constructing a screening system using the OATP-E gene to screen compounds which are specifically transported into cells mediated by the OATP-E protein. Such agents are expected to have reduced cytotoxicity to hemocytes. 
     In addition, if the OATP family proteins are involved in the control of pharmacodynamics of drugs in vivo, the pharmacodynamics are expected to be modified by genetic polymorphism of the proteins. It has been already known that the genetic polymorphism such as the single nucleotide polymorphism (SNP) causes differences in gene expression level and in the amino acid sequences encoded among individuals (Nat. Genet. 22(3):231–8, 1999; Nat. Genet. 22(3):239–47, 1999). It is expected that the genetic polymorphism in OATP family genes causes differences in transport specificities, such as transport activity and substrate specificity, of OATP proteins among individuals, leading to individual differences in in vivo pharmacodynamics of drugs and such which are controlled by OATP family proteins. In fact, it is presumed that such differences among individuals may lead to differences in effectiveness and reactivity of particular drugs. Thus, it is postulated that examination of the polymorphism, such as SNP, in OATP family genes in detail, particularly the accumulation of information on relationship between genotype and phenotype (reactivity to drugs) of these genes, can enable the prediction of the reactivity of individuals to drugs by performing genetic diagnosis of genotype of these genes in a particular individual. 
     In fact, the present inventors found SNPs with the following three types of amino acid mutations in normal individuals during the cloning process of OATP family genes:
     polymorphism in the 486th codon of OATP-B gene (tct:Ser or ttt:Phe),   polymorphism in the 130th codon of OATP-C gene (aat:Asn or gat:Asp), and   polymorphism in the 174th codon of OATP-C gene (gtg:Val or gcg:Ala).   

     It is presumed that additional polymorphisms, besides those described herein, exist in the OATP gene family to be related to phenotypes. 
     In addition, in view of such critical roles of the OATP family proteins in vivo, it is presumed that disorders exist which are caused by deficiencies in transport functions due to gene mutations of these proteins. In fact, it has been reported that the gene mutation in the OCTN2 transporter, which is one of the organic cation transporters, causes systemic carnitine deficiency (SCD) (Nat. Genet. 21(1):91–4, 1999), proving the actual existence of genetic disorders caused by the mutation of transporter genes. Genetic diagnosis based on direct examination of causative transporter genes is clinically very important for such genetic disorders caused by the mutations in the transporter genes. 
     Genetic diagnosis to detect polymorphisms and mutations in genes of the OATP family is made possible by the structure of the OATP family genes revealed in this invention. Specifically, genetic diagnosis can be made using genes of the OATP family themselves or synthetic oligonucleotides prepared from the nucleotide sequences thereof as the primers for PCR. In addition, it has recently become possible to more conveniently detect structures or expression levels of genes by techniques referred to as DNA chip or DNA microarray techniques (Nat. Genet. 21:1–60, 1999; Science 283(5398):83–7, 1999). Such methods can be also carried out using genes of the OATP family themselves or synthetic oligonucleotides prepared from nucleotide sequences thereof. 
     Accordingly, this invention relates to novel transporters OATP-B, C, D, and E, genes encoding these transporters, and uses thereof. More specifically, the present invention provides:
         (1) a DNA encoding a protein having a transporter activity selected from the group of:
           (a) a DNA encoding a protein comprising the amino acid sequence of SEQ ID NOs:2, 4, 6, or 8;   (b) a DNA comprising a coding region of the nucleotide sequence of SEQ ID NOs:1, 3, 5, or 7;   (c) a DNA encoding a protein comprising the amino acid sequence of SEQ ID NOs:2, 4, 6, or 8, wherein one or more amino acids have been substituted, deleted, inserted, and/or added; and   (d) a DNA that hybridizes with the DNA consisting of the nucleotide sequence of SEQ ID NOs:1, 3, 5, or 7;   
           (2) a DNA encoding a partial peptide of a protein comprising the amino acid sequence of SEQ ID NOs:2, 4, 6, or 8;   (3) a vector into which the DNA of (1) or (2) is inserted;   (4) a transformed cell harboring the DNA of (1) or (2), or the vector of (3);   (5) a protein or a peptide encoded by the DNA of (1) or (2);   (6) a method for producing the protein or peptide of (5), comprising the steps of: culturing the transformed cell of (4), and recovering the expressed protein from said transformed cell or the culture supernatant thereof;   (7) an antibody binding to the protein of (5);   (8) a polynucleotide comprising at least 15 nucleotides that is complementary to the DNA consisting of the nucleotide sequence of SEQ ID NOs:1, 3, 5, or 7, or the complementary strand thereof;   (9) a method of screening for a compound that is transported from the outside to the inside of a cell through the intermediary of the protein of (5), comprising the steps of:
           (a) providing a cell that expresses the protein of (5) on the cell membrane;   (b) contacting a labeled compound with said cell;   (c) detecting whether or not the labeled compound has been taken up into the cell; and   (d) selecting the compound that is taken up into the cell;   
           (10) a method of screening for a test compound that promotes or suppresses the transporter activity of the protein of (5), comprising the steps of:
           (a) providing a cell that expresses the protein of (5) on the cell membrane;   (b) contacting a test compound and a labeled organic compound to be transported through the intermediary of the protein of (5) with said cell;   (c) measuring the amount of the labeled organic compound that has been taken up into said cell; and   (d) selecting the test compound that increases or decreases the amount of the labeled organic compound taken up into said cell as compared with that observed in the absence of the test compound (control).   
               

     The nucleotide sequences of cDNAs of novel transporters isolated by the present inventors, “OATP-B”, “OATP-C”, “OATP-D”, and “OATP-E”, are set forth in SEQ ID NOs:1, 3, 5, and 7, respectively, and the amino acid sequences of the proteins encoded by said cDNAs are set forth in SEQ ID NOs:2, 4, 6, and 8, respectively. All of these proteins have structural similarity with the human OATP-A transporter, and all are thought to form a family (the “OATP” family). 
     The transporters of this invention are presumed to regulate the in vivo concentrations of substances, which are either essential or unnecessary for living bodies. It is also presumed that a variety of drugs are incorporated into cells or are excreted from cells by the OATP family proteins. Therefore, it may be possible to control the pharmacodynamics of drugs, and speedily design or screen drugs with an improved absorbability using the proteins of the OATP family. 
     The transporter proteins of this invention include mutants of the above-described human transporters, “OATP-B”, “OATP-C”, “OATP-D”, and “OATP-E” protein. Herein, “mutants” are referred to as proteins that have amino acid sequences which have been mutated by substitution, deletion, addition, or insertion of amino acids from the natural “OATP-B”, “OATP-C”, “OATP-D”, or “OATP-E” proteins of SEQ ID NOs:2, 4, 6, or 8, and that still retain the transporter activity. Mutations of amino acids in proteins may occur artificially or naturally. 
     The phrase “having the transporter activity” as used herein means that a protein has the activity to transport an organic compound across membranes. Examples of organic compounds include estradiol-17β-glucuronide, estron-3-sulfate, benzyl penicillin, prostaglandin E2, and so on, but are not limited thereto. 
     In addition, the phrase “activity to transport” as used herein includes not only the activity to transport an organic compound from the outside to inside of cells, but also that from the inside to outside of cells. The transporter proteins of this invention include those having both of these activities and those with either one of the activities. The activity of a protein to transport an organic compound can be measured, for example, by adding a labeled organic compound to cells to detect the uptake or excretion thereof, for example, by the method described in Examples. 
     During the cloning process of genes of the OATP family, the present inventors found SNPs with following three types of amino acid mutations in normal individuals:
     polymorphism in the 486th codon of OATP-B gene (tct:Ser or ttt:Phe),   polymorphism in the 130th codon of OATP-C gene (aat:Asn or gat:Asp), and   polymorphism in the 174th codon of OATP-C gene (gtg:Val or gcg:Ala).   

     The transporter of this invention includes proteins having the above-described mutations, namely: a protein comprising the amino acid sequence of SEQ ID NO:2, wherein the amino acid at position 486 is substituted with Phe; a protein comprising the amino acid sequence of SEQ ID NO:4, wherein the amino acid at position 130 is substituted with Asp; and a protein comprising the amino acid sequence of SEQ ID NO:4, wherein the amino acid at position 174 is substituted with Ala. 
     Polymorphisms other than those described above are presumed to exist for “OATP-B”, “OATP-C”, “OATP-D”, and “OATP-E”, and such polymorphisms of “OATP-B”, “OATP-C”, “OATP-D”, and “OATP-E” are also included in this invention. These polymorphisms are presumed to affect the expression level or activity of transporters and are probably related to their phenotypes. Genetic diagnosis, wherein polymorphism and mutation of the OATP family genes are detected, is made possible by further elucidating the relationship between the genes of OATP family and their phenotypes. 
     Examples of methods for artificially altering amino acids well known to those skilled in the art include the site-specific mutagenesis system by PCR (GIBCO-BRL, Gaithersburg, Md.); site-specific mutagenesis using oligonucleotides (Kramer et al., Methods in Enzymol. 154:350–367, 1987); the Kunkel&#39;s method (Methods Enzymol. 85:2763–2766, 1988); etc. There is no particular limitation in the number and site of amino acid mutations so long as the mutant proteins retain the transporter activity of this invention. The preferred number of amino acids to be substituted is typically 10 amino acid residues or less, preferably 6 or less, and more preferably 3 or less. 
     As for the amino acid residue to be mutated, it is preferable that the amino acid be mutated into a different amino acid that allows the properties of the amino acid side-chain to be conserved. Examples of properties of amino acid side chains include hydrophobic amino acids (A, I, L, M, F, P, W, Y, V), hydrophilic amino acids (R, D, N, C, E, Q, G, H, K, S, T), and amino acids comprising the following side chains: an aliphatic side-chain (G, A, V, L, I, P); a hydroxyl group containing side-chain (S, T, Y); a sulfur atom containing side-chain (C, M); a carboxylic acid and amide containing side-chain (D, N, E, Q); a base containing side-chain (R, K, H); and an aromatic containing side-chain (H, F, Y, W) (The parenthetic letters indicate the one-letter codes of amino acids). 
     It is well known that a protein having deletion, addition, and/or substitution of one or more amino acid residues in its protein sequence can retain the biological activity of the original protein (Mark, D. F. et al., Proc. Natl. Acad. Sci. USA 81:5662–5666, 1984; Zoller et al., Nucleic Acids Res. 10:6487–6500, 1982; Wang et al., Science 224:1431–1433; Dalbadie-McFarland et al., Proc. Natl. Acad. Sci. USA 79:6409–6413, 1982). 
     An example of a protein to which plural amino acid residues are added to the amino acid sequence of human “OATP-B”, “OATB-C”, “OATP-D”, or “OATP-E” protein (SEQ ID NOs:2, 4, 6, or 8) is a fusion protein comprising human “OATP-B”, “OATP-C”, “OATP-D”, or “OATP-E” protein. Fusion proteins are fusions of the human “OATP-B”, “OATP-C”, “OATP-D”, or “OATP-E” protein and other peptides or proteins, and are included in the present invention. Fusion proteins can be made by techniques well known to a person skilled in the art, such as by linking the DNA encoding the human “OATP-B”, “OATP-C”, “OATP-D”, or “OATP-E” protein of the invention with DNA encoding the other peptides or proteins, so that the frames match, inserting this fusion DNA into an expression vector, and expressing it in a host. There is no restriction as to the peptides or proteins fused to the protein of the present invention. 
     However, examples of known peptides that can be used as peptides to be fused to the protein of the present invention include: FLAG (Hopp et al., Biotechnology 6:1204–1210, 1988), 6× His consisting of six His (histidine) residues, 10× His, Influenza agglutinin (HA), human c-myc fragment, VSV-GP fragment, p18HIV fragment, T7-tag, HSV-tag, E-tag, SV40T antigen fragment, 1ck tag, α-tubulin fragment, B-tag, Protein C fragment, and such. Additional examples of proteins that can be fused to a protein of the present invention include GST (glutathione-S-transferase), Influenza agglutinin (HA), immunoglobulin constant region, β-galactosidase, MBP (maltose-binding protein), and such. 
     Fusion proteins can be prepared by fusing commercially available DNA encoding these peptides or proteins with the DNA encoding a protein of the present invention and expressing the fused DNA prepared. 
     The term “substantially pure” as used herein in reference to a given polypeptide means that the polypeptide is substantially free from other biological macromolecules. For example, the substantially pure polypeptide is at least 75%, 80%, 85%, 95%, or 99% pure by dry weight. Purity can be measured by any appropriate standard method known in the art, for example, by column chromatography, polyacrylamide gel electrophoresis, or HPLC analysis. 
     Accordingly, the invention includes a polypeptide having a sequence shown as SEQ ID NO:2, 4, 6 or 8. The invention also includes a polypeptide, or fragment thereof, that differs from the corresponding sequence shown as SEQ ID NO:2, 4, 6 or 8. The differences are, preferably, differences or changes at a non-essential residue or a conservative substitution. In one embodiment, the polypeptide includes an amino acid sequence at least about 60% identical to a sequence shown as SEQ ID NO:2, 4, 6 or 8, or a fragment thereof. Preferably, the polypeptide is at least 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99% or more identical to SEQ ID NO: 2, 4, 6 or 8 and has at least one transporter activity described herein. Preferred polypeptide fragments of the invention are at least 10%, preferably at least 20%, 30%, 40%, 50%, 60%, 70%, or more, of the length of the sequence shown as SEQ ID NO:2, 4, 6 or 8 and have at least one transporter activity described herein. Or alternatively, the fragment can be merely an immunogenic fragment. 
     In addition, the transporter proteins of the present invention include proteins having a high structural homology with the above-described human “OATP-B”, “OATP-C”, “OATP-D”, or “OATP-E” proteins, and retaining the transporter activity. Such proteins include proteins, for example, derived from non-human mammals, which correspond to the human “OATP-B”, “OATP-C”, “OATP-D”, or “OATP-E” proteins. A protein having a high structural homology with the human “OATP-B”, “OATP-C”, “OATP-D”, or “OATP-E” proteins can be isolated by, for example, the hybridization technique (Sambrook et al., Molecular Cloning 2nd ed., 9.47–9.58, Cold Spring Harbor Lab. press, 1989). Specifically, based on the DNA sequences (SEQ ID NOs:1, 3, 5, or 7) encoding the above-described human “OATP-B”, “OATP-C”, “OATP-D”, or “OATP-E” proteins or portions thereof, a DNA derived from non-human mammals having a high homology with the aforementioned DNAs can be isolated using the affinity among DNAs, and the desired protein can be prepared from the isolated DNA. Non-human mammals to be used for isolating DNAs include monkeys, mice, rats, rabbits, cattle, pigs, dogs, cats, and so on, but the invention is not limited to them. 
     One example of hybridization conditions (stringent conditions) for isolating such DNAs is as follows. That is, after the pre-hybridization at 55° C. for 30 min or more in the “ExpressHyb Hybridization Solution” (CLONTECH), a labeled probe is added, and hybridization is performed by heating the reaction mixture at 37° C. to 55° C. for 1 h or more. Then, the reaction product is successively washed in 2×SSC and 0.1% SDS three times at room temperature for 20 min, and then in 1×SSC and 0.1% SDS once at 37° C. for 20 min. 
     More preferable conditions (more stringent conditions) are as follows: After the pre-hybridization at 60° C. for 30 min or more in the “ExpressHyb Hybridization Solution” (CLONTECH), a labeled probe is added, and hybridization is performed by heating the reaction mixture at 60° C. for 1 h or more. Then, the reaction product is successively washed in 2×SSC and 0.1% SDS three times at room temperature for 20 min, and then in 1×SSC and 0.1% SDS twice at 50° C. for 20 min. 
     Still more preferable conditions (still more stringent conditions) are as follows: After pre-hybridization at 68° C. for 30 min or more in the “ExpressHyb Hybridization Solution” (CLONTECH), a labeled probe is added, and hybridization is performed by heating the reaction mixture at 68° C. for 1 h or more. Then, the reaction product is successively washed in 2×SSC and 0.1% SDS three times at room temperature for 20 min, and then in 0.1×SSC and 0.1% SDS twice at 50° C. for 20 min. However, several factors, such as temperature or salt concentration, can influence the stringency of hybridization and one skilled in the art can suitably select the factors to accomplish a similar stringency. 
     It is also possible for those skilled in the art to similarly isolate a gene having a high homology with the human “OATP-B”, “OATP-C”, “OATP-D”, or “OATP-E” genes, and obtain desired proteins from these genes using techniques other than the hybridization technique, for example, the polymerase chain reaction. 
     Such proteins isolated by hybridization and polymerase chain reaction techniques are thought to have a high homology with the human “OATP-B”, “OATP-C”, “OATP-D”, or “OATP-E” proteins. “High homology” means at least 80% or more, preferably 90% or more, and more preferably 95% or more of homology at the amino acid level. The homology of a protein can be determined by following the algorithm in “Wilbur, W. J. and Lipman, D. J., Proc. Natl. Acad. Sci. USA (1983) 80, 726–730”. 
     A protein of the present invention may have variations in amino acid sequence, molecular weight, isoelectric point, the presence or absence of sugar chains, or form, depending on the after-mentioned cell or host used to produce it or the purification method utilized. Nevertheless, so long as the obtained protein has the transporter activity, it is within the scope of the present invention. 
     A protein of the present invention can be prepared as a recombinant protein or a natural protein by methods well known to those skilled in the art. A recombinant protein can be prepared by inserting a DNA (for example, the DNA comprising the nucleotide sequence of SEQ ID NOs:1, 3, 5, or 7) encoding a protein of the present invention into an appropriate expression vector, introducing the vector into appropriate host cells, collecting the recombinant, obtaining the extract, and purifying by subjecting the extract to chromatography such as ion exchange, reverse, gel filtration, or affinity chromatography in which an antibody against a protein of the present invention is fixed on column or by combining more than one of these columns. 
     Also, when a protein of the present invention is expressed within host cells (for example, animal cells and  E. coli ) as a fusion protein with glutathione-S-transferase protein or as a recombinant protein supplemented with multiple histidines, the expressed recombinant protein can be purified using a glutathione column or nickel column. 
     After purifying the fusion protein, it is also possible to exclude regions other than the objective protein by cutting with thrombin or factor-Xa as required. 
     A natural protein can be isolated by methods well known to those skilled in the art, for example, by contacting the extract of tissues or cells expressing a protein of the present invention to an affinity column in which an antibody binding to a protein of the present invention (described below) is bound so as to isolate and purify the protein from the extract. An antibody can be a polyclonal or a monoclonal antibody. 
     The present invention also includes a partial peptide of a protein of the present invention. A partial peptide comprising the amino acid sequence specific to the protein of the present invention comprises at least 7 amino acids, preferably 8 amino acids or more, and more preferably 9 amino acids or more. The partial peptide can be used, for example, to prepare an antibody against a protein of the present invention, to screen for a compound binding to a protein of the present invention, and to screen for accelerators or inhibitors of a protein of the present invention. Partial peptides of the proteins of the present invention include those peptides comprising the functional domains of original proteins, for example, comprising the amino acid sequence of SEQ ID NOs:2, 4, 6, or 8. Other examples of partial peptides are those comprising one or more hydrophobic and hydrophilic regions predicted from the hydrophobicity plot analysis. These partial peptides may contain either a partial or entire area of one hydrophobic or hydrophilic region. 
     A partial peptide of the invention can be produced by genetic engineering, known methods of peptide synthesis, or by digesting a protein of the invention with an appropriate peptidase. For peptide synthesis, for example, solid phase synthesis or liquid phase synthesis may be used. 
     A DNA encoding a protein of the present invention can be used to produce a protein of the present invention in vivo or in vitro as described above. In addition, it may, for example, find application to gene therapy for diseases attributed to genetic abnormality in a gene encoding a protein of the present invention and diseases treatable with a protein of the present invention. Any form of the DNA of the present invention can be used, so long as it encodes a protein of the present invention. Specifically, cDNA synthesized from the mRNA, genomic DNA, or chemically synthesized DNA can be used. The present invention includes a DNA comprising a given nucleotide sequence based on degeneracy of genetic codons, so long as it encodes a protein of the present invention. 
     As used herein, an “isolated nucleic acid” is a nucleic acid, the structure of which is not identical to that of any naturally occurring nucleic acid or to that of any fragment of a naturally occurring genomic nucleic acid spanning more than three genes. The term therefore covers, for example, (a) a DNA which has the sequence of part of a naturally occurring genomic DNA molecule but is not flanked by both of the coding sequences that flank that part of the molecule in the genome of the organism in which it naturally occurs; (b) a nucleic acid incorporated into a vector or into the genomic DNA of a prokaryote or eukaryote in a manner such that the resulting molecule is not identical to any naturally occurring vector or genomic DNA; (c) a separate molecule such as a cDNA, a genomic fragment, a fragment produced by polymerase chain reaction (PCR), or a restriction fragment; and (d) a recombinant nucleotide sequence that is part of a hybrid gene, i.e., a gene encoding a fusion protein. Specifically excluded from this definition are nucleic acids present in random, uncharacterized mixtures of different DNA molecules, transfected cells, or cell clones, e.g., as these occur in a DNA library such as a cDNA or genomic DNA library. 
     Accordingly, in one aspect, the invention provides an isolated or purified nucleic acid molecule that encodes a polypeptide described herein or a fragment thereof. Preferably, the isolated nucleic acid molecule includes a nucleotide sequence that is at least 60% identical to the nucleotide sequence shown in SEQ ID NO:1, 3, 5 or 7. More preferably, the isolated nucleic acid molecule is at least 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or more, identical to the nucleotide sequence shown in SEQ ID NO:1, 3, 5 or 7. In the case of an isolated nucleic acid molecule which is longer than or equivalent in length to the reference sequence, e.g., SEQ ID NO:1, 3, 5 or 7, the comparison is made with the full length of the reference sequence. Where the isolated nucleic acid molecule is shorter that the reference sequence, e.g., shorter than SEQ ID NO:1, 3, 5 or 7, the comparison is made to a segment of the reference sequence of the same length (excluding any loop required by the homology calculation). 
     As used herein, “% identity” of two amino acid sequences, or of two nucleic acid sequences, is determined using the algorithm of Karlin and Altschul (Proc. Natl. Acad. Sci. USA 87:2264–2268, 1990), modified as in Karlin and Altschul, Proc. Natl. Acad. Sci. USA 90:5873–5877, 1993). Such an algorithm is incorporated into the NBLAST and XBLAST programs of Altschul et al. (J. Mol. Biol. 215:403–410, 1990). BLAST nucleotide searches are performed with the NBLAST program, score=100, wordlength=12. BLAST protein searches are performed with the XBLAST program, score=50, wordlength=3. To obtain gapped alignment for comparison purposes GappedBLAST is utilized as described in Altschul et al., (Nucleic Acids Res. 25:3389–3402, 1997). When utilizing BLAST and GappedBLAST programs the default parameters of the respective programs (e.g., XBLAST and NBLAST) are used to obtain nucleotide sequences homologous to a nucleic acid molecule of the invention. 
     The DNA of the present invention can be prepared by methods known to those skilled in the art. For example, a DNA of the present invention can be prepared by preparing a cDNA library from cells which express the protein of the present invention, and conducting hybridization using a partial sequence of a DNA of the present invention (e.g., SEQ ID NOs:1, 3, 5, or 7) as a probe. A cDNA library can be prepared, for example, by the method described in Sambrook et al., Molecular Cloning, Cold Spring Harbor Laboratory Press, 1989), or by using commercially available cDNA libraries. A cDNA library can be also prepared by preparing RNA from cells expressing a protein of the present invention, synthesizing cDNA based on the RNA using reverse transcriptase, synthesizing an oligo DNA based on the sequence of the DNA of the present invention (for example, SEQ ID NOs:1, 3, 5, or 7), conducting PCR by using these as primers, and amplifying cDNA encoding the protein of the present invention. 
     In addition, by sequencing the nucleotides of the obtained cDNA, a translation region encoded by it can be determined, and an amino acid sequence of a protein of the present invention can be obtained. Moreover, by screening the genomic DNA library using the obtained cDNA as a probe, genomic DNA can be isolated. 
     More specifically, mRNAs may first be isolated from a cell, tissue, or organ in which a protein of the invention is expressed. Known methods can be used to isolate mRNAs: for instance, total RNA may be prepared by guanidine ultracentrifugation (Chirgwin et al., Biochemistry 18:5294–5299, 1979) or by the AGPC method (Chomczynski et al., Anal. Biochem. 162:156–159, 1987), and mRNA may then be purified from total RNA using an mRNA Purification Kit (Pharmacia) and such. Alternatively, mRNA may be directly purified by QuickPrep mRNA Purification Kit (Pharmacia). 
     The obtained mRNA is used to synthesize cDNA using reverse transcriptase. cDNA may be synthesized using a kit such as the AMV Reverse Transcriptase First-strand cDNA Synthesis Kit (Seikagaku Kogyo). Alternatively, cDNA may be synthesized and amplified following the 5′-RACE method (Frohman et al., Proc. Natl. Acad. Sci. USA 85:8998–9002, 1988; Belyavsky et al., Nucleic Acids Res. 17:2919–2932, 1989) which uses a primer and such, described herein, the 5′-Ampli FINDER RACE Kit (Clontech), and polymerase chain reaction (PCR). 
     A desired DNA fragment is prepared from the PCR products and ligated with a vector DNA. The recombinant vectors are used to transform  E. coli  and such, and a desired recombinant vector is prepared from a selected colony. The nucleotide sequence of the desired DNA can be verified by conventional methods, such as dideoxynucleotide chain termination. 
     A DNA of the invention may be designed to have a nucleotide sequence that is expressed more efficiently by taking into account the frequency of codon usage in the host to be used for expression (Grantham et al., Nucleic Acids Res. 9:43–74, 1981). The DNA of the present invention may be altered by a commercially available kit or a conventional method. For instance, the DNA may be altered by digestion with restriction enzymes, insertion of synthetic oligonucleotides or appropriate DNA fragments, addition of a linker, or insertion of the initiation codon (ATG) and/or a stop codon (TAA, TGA, or TAG). 
     Specifically, DNAs of this invention include DNAs comprising bases A179 through G2305 of the nucleotide sequence of SEQ ID NO:1, bases A100 through T2172 of the nucleotide sequence of SEQ ID NO:3, bases A1 through A2130 of the nucleotide sequence of SEQ ID NO:5, and bases A92 through C2257 of the nucleotide sequence of SEQ ID NO:7. 
     Moreover, DNAs of this invention includes DNAs comprising bases A179 through G2305 of the nucleotide sequence of SEQ ID NO:1, wherein the C1635 has been substituted with T; bases A100 through T2172 of the nucleotide sequence of SEQ ID NO:3, wherein the A487 has been substituted with G; bases A100 through T2172 of the nucleotide sequence of SEQ ID NO:3, wherein the T620 has been substituted with C. 
     Furthermore, the present invention provides DNAs that are capable of hybridizing with a DNA having a nucleotide sequence of SEQ ID NOs:1, 3, 5, or 7, and encoding a protein having the transporter activity. Suitable hybridization conditions include the above conditions. The hybridizing DNA is preferably a natural DNA, for example, cDNA or chromosomal DNA. 
     The present invention also relates to a vector into which a DNA of the present invention is inserted. A vector of the present invention is useful for keeping the DNA of the present invention in a host cell, or to express a protein of the present invention. 
     When  E. coli  is the host cell and the vector is amplified and produced in large amounts in  E. coli  (e.g., JM109, DH5α, HB101, or XL1Blue) and such, the vector should have “ori” to be amplified in  E. coli  and a marker gene for selecting transformed  E. coil  (for example, a drug-resistance gene selected by a drug (e.g., ampicillin, tetracycline, kanamycin, or chloramphenicol)). For example, M13-series vectors, pUC-series vectors, BR322, pBluescript, pCR-Script, and so on can be used. Additionally, pGEM-T, pDIRECT, and pT7 can also be used for subcloning and extracting cDNA as well as the vectors described above. When a vector is used to produce a protein of the present invention, an expression vector is especially useful. For example, an expression vector to be expressed in  E. coli  should have the above characteristics so as to be amplified in  E. coli . When  E. coli , such as JM109, DH5α, HB101, or XL1 Blue, are used as the host cell, the vector should have a promoter as well as the above characters such as the vector is copied in the host; for example, the lacZ promoter (Ward et al., Nature 341:544–546, 1989; FASEB J. 6:2422–2427, 1992), the araB promoter (Better et al., Science 240:1041–1043, 1988), and the T7 promoter and such can efficiently express the desired gene in  E. coli . As such a vector, in addition to the above vectors, pGFX-5X-1 (Pharmacia), “QIAexpress system” (Qiagen), pEGFP, and pET (in this case, the host is preferably BL21 which expresses T7 RNA polymerase) can be used. 
     A vector also may contain a signal sequence for polypeptide secretion. As a signal sequence for protein secretion, pelB signal sequence (Lei et al., J. Bacteriol. 169:4379, 1987) can be used in the case of producing protein into the periplasm of  E. coli . For introducing a vector into host cells, for example, the calcium chloride method, and the electroporation method can be used. 
     In addition to  E. coli , for example, expression vectors derived from mammals (for example, pcDNA3 (Invitrogen), pEGF-BOS (Nucleic Acids. Res. 18(17):p5322, 1990), pEF, and pCDM8); expression vectors derived from insect cells (for example, “Bac-to-BAC baculovirus expression system” (GIBCO BRL), pBacPAK8); expression vectors derived from plants (for example, pMH1, pMH2), expression vectors derived from animal viruses (for example, pHSV, pMV, pAdexLcw); expression vectors derived from retroviruses (for example, pZIpneo); expression vector derived from yeast (for example, “Pichia Expression Kit” (Invitrogen), pNV11, SP-Q01); and expression vectors derived from  Bacillus subtilis  (for example, pPL608, pKTH50) can be used to produce a protein of the present invention. 
     In order to express the vector in animal cells, such as CHO, COS, or NIH3T3 cells, the vector should have a promoter necessary for expression in such cells, for example, the SV40 promoter (Mulligan et al., Nature 277:108, 1979), the MMLV-LTR promoter, the EF1 α promotor (Mizushima et al., Nucleic Acids Res. 18:5322, 1990), or the CMV promoter, and such, and preferably should also include a marker gene for selecting transformants (for example, a drug resistance gene selected by a drug (e.g., neomycin, G418)). Examples of the vectors with these characteristics include pMAM, pDR2, pBK-RSV, pBK-CMV, pOPRSV, pOp13, and so on. 
     In addition, in order to stably express a gene and amplify the copy number of the gene in cells, an exemplary method can use the following steps: introducing a vector comprising the complementary DHFR gene (for example pCHO I) into CHO cells in which the nucleic acid synthesizing pathway is deleted, and amplifying by methotrexate (MTX); in the case of transient expression of a gene, an exemplary method can use the steps of transforming with a vector (e.g., pcD) comprising replication origin of SV40 using COS cells comprising the gene expressing SV40 T antigen on chromosomes. The origin used for replication may also be those of polyomavirus, adenovirus, bovine papilloma virus (BPV), and the like. In addition, the expression vector may include a selection marker gene for amplification of the gene copies in host cells. Examples of such markers include, but are not limited to, the aminoglycoside transferase (APH) gene, the thymidine kinase (TK) gene, the  E. coli  xanthine-guanine phosphoribosyl transferase (Ecogpt) gene, and the dihydrofolate reductase (dhfr) gene. 
     On the other hand, a DNA of the present invention can be expressed in vivo in animals, for example, by inserting a DNA of the present invention into an appropriate vector and introducing it into the living body by a method such as the retrovirus method, the liposome method, the cationic liposome method, or the adenovirus method. By using these, gene therapy against diseases attributed to the mutation of a transporter gene of the present invention can be effected. As a vector to be used, for example, adenovirus vector (for example pAdexlcw), and retrovirus vector (for example, pZIPneo) can be used; however, the present invention is not restricted thereto. Common gene manipulation, for example, insertion of the DNA of the present invention to a vector, can be performed according to conventional methods (Molecular Cloning, 5.61–5.63). Administration into a living body can be either an ex vivo method, or in vivo method. 
     The present invention also relates to a transformed cell into which a DNA or a vector of the present invention has been introduced. The host cell into which the vector of the invention is introduced is not particularly limited. For example,  E. coli  or various animal cells can be used. The transformed cells of the present invention can be used as, for example, a production system for producing or expressing a protein of the present invention. The present invention provides methods of producing a protein of the invention both in vitro or in vivo. For in vitro production, eukaryotic cells or prokaryotic cells can be used as host cells. 
     Eukaryotic cells useful as host cells may be animal, plant, or fungi cells. As animal cells, mammalian cells, such as CHO (J. Exp. Med. 108:945, 1995), COS, 3T3, myeloma, baby hamster kidney (BHK), HeLa, and Vero cells; or amphibian cells, such as Xenopus oocytes (Valle et al., Nature 291:340–358, 1981); or insect cells, such as Sf9, Sf21, or Tn5 cells, can be used. CHO cells lacking the DHFR gene (dhfr-CHO) (Proc. Natl. Acad. Sci. USA 77:4216–4220, 1980) or CHO K-1 (Proc. Natl. Acad. Sci. USA 60:1275, 1968) may also be used. In animal cells, CHO cells are particularly preferable for mass expression. A vector can be introduced into host cells by, for example, the calcium phosphate method, the DEAE dextran method, the cationic liposome DOTAP (Boehringer Mannheim), the electroporation method, the lipofection method, and such. A recombinant protein derived from the obtained transformants can be purified by standard methods, for example, the method described in “The Qiaexpressionist handbook, Qiagen, Hilden, Germany”. 
     As plant cells, plant cells originating from  Nicotiana tabacum  are known as protein-production system, and may be used as callus cultures. As fungi cells, yeast cells, such as  Saccharomyces,  including  Saccharomyces cerevisiae , or filamentous fungi, such as  Aspergillus,  including  Aspergillus niger , are known and may be used herein. 
     Prokaryotic cells suitable for use in the production system of the present invention include, but are not limited to, bacterial cells. A known example of these bacterial cells is  E. coli , such as JM109, DH5α, HB101. Regarding others,  Bacillus subtilis  is known in the art. 
     These cells are transformed by a desired DNA, and the resulting transformants are cultured in vitro to obtain the protein. Transformants can be cultured using known methods. Culture medium for animal cells, for example, DMEM, MEM, RPMI1640, or IMDM, may be used with or without serum supplement, such as fetal calf serum (FCS). The pH of the culture medium is preferably between about pH 6 to 8. Such cells are typically cultured at about 30 to 40° C. for about 15 to 200 hr, and the culture medium may be replaced, aerated, and/or stirred if necessary. 
     On the other hand, animal and plant hosts may be used for in vivo production. For example, a desired DNA can be introduced into an animal or plant host. Encoded proteins are produced in vivo, and then recovered. These animal and plant hosts are included in the host cells of the present invention. 
     Animals to be used for the production system described above include, but are not limited to, mammals and insects. Mammals such as goat, porcine, sheep, mouse, and bovine may be used (Vicki Glaser, SPECTRUM Biotechnology Applications, 1993). Alternatively, the mammals may be transgenic animals. 
     For instance, a desired DNA may be prepared as a fusion gene with a gene encoding a protein specifically produced into milk, such as goat β casein. DNA fragments, comprising the fusion gene having the desired DNA, are injected into goat embryos, which are then introduced back to female goats. Proteins are recovered from milk produced by the transgenic goats (i.e., those born from the goats that had received the modified embryos) or from their offspring. To increase the amount of milk containing the proteins produced by transgenic goats, appropriate hormones may be administered to them (Ebert et al., Bio/Technology 12:699–702, 1994). 
     Alternatively, insects, such as the silkworm, may be used. Baculovirus inserted a DNA encoding a desired protein can be used to infect silkworms, and the desired protein is recovered from their body fluid (Susumu et al., Nature 315:592–594, 1985). 
     As plants, for example, tobacco can be used. In use of tobacco, a DNA encoding a desired protein may be inserted into a plant expression vector, such as pMON530, which is then introduced into bacteria, such as  Agrobacterium tumefaciens . Then, the bacteria is used to infect a tobacco plant, such as  Nicotiana tabacum , and a desired polypeptide is recovered from the plant&#39;s leaves (Julian et al., Eur. J. Immunol. 24:131–138, 1994). 
     A protein of the present invention obtained as above may be isolated from the interior or the exterior (e.g., the culture medium) of transformed cells, and purified as a substantially pure homogeneous protein. The method for protein isolation and purification is not limited to any specific method; in fact, any standard method may be used. For instance, column chromatography, filter, ultrafiltration, salting-out, solvent precipitation, solvent extraction, distillation, immunoprecipitation, SDS-polyacrylamide gel electrophoresis, isoelectric point electrophoresis, dialysis, recrystallization, and such may be appropriately selected and combined to isolate and purify the protein. 
     For chromatography, for example, affinity chromatography, ion-exchange chromatography, hydrophobic chromatography, gel filtration, reverse-phase chromatography, adsorption chromatography, and such may be used (Strategies for Protein Purification and Characterization: A Laboratory Course Manual. Ed. Daniel R. Marshak et al., Cold Spring Harbor Laboratory Press, 1996). These chromatographies maybe performed by liquid-phase chromatography, such as HPLC and FPLC. Thus, the present invention provides for highly purified proteins produced by the above methods. 
     A protein of the present invention may be optionally modified or partially deleted by treating it with an appropriate protein modification enzyme before or after purification. Useful protein modification enzymes include, but are not limited to, trypsin, chymotrypsin, lysylendopeptidase, protein kinase, and glucosidase. 
     The present invention also provides antibodies that bind to the proteins of the invention. An antibody of the invention can be used in any form, such as monoclonal or polyclonal antibodies, and includes antiserum obtained by immunizing a rabbit with a protein of the invention, all classes of polyclonal and monoclonal antibodies, human antibodies, and humanized antibodies produced by genetic recombination. 
     A protein of the invention used as an antigen to obtain an antibody may be derived from any animal species, but preferably from a mammal such as a human, mouse, or rat, or more preferably from a human. A human-derived protein may be obtained from the nucleotide or amino acid sequences disclosed herein. 
     In the present invention, a protein to be used as an immunization antigen may be a complete protein or a partial peptide of the protein. A partial peptide may be, for example, an amino (N)-terminal or carboxy (C)-terminal fragment of the protein. Herein, “an antibody” is defined as a molecule that specifically reacts with either the full-length protein or a fragment of the protein. 
     A gene encoding a protein of the invention or its fragment may be inserted into a known expression vector, which may then be used to transform a host cell as described herein. The desired protein or its fragment may be recovered from the exterior or the interior of the transformed host cells, by any standard method, and may be used as an antigen. Alternatively, cells expressing the protein or their lysates, or a chemically synthesized protein of the present invention may be used as an antigen. 
     Any mammalian animal may be immunized with the antigen, but preferably the compatibility with parental cells used for cell fusion is taken into account. In general, animals of the Rodentia order, Lagomorphs, or Primates may be used. 
     Animals of the order Rodentia include, for example, mouse, rat, and hamster. Lagomorphs include, for example, rabbit. Primates include, for example, a monkey of catarrhine (old world monkey) such as  Macaca fascicularis , rhesus monkey, sacred baboon, or chimpanzee. 
     Methods for immunizing animals against antigens are known in the art. Intraperitoneal injection or subcutaneous injection of antigens is used as a standard method for immunization of mammals. More specifically, antigens may be diluted and suspended in an appropriate amount with phosphate buffered saline (PBS), physiological saline, etc. If desired, the antigen suspension may be mixed with an appropriate amount of a standard adjuvant, such as Freund&#39;s complete adjuvant, made into emulsion, and then administered to mammalian animals. Preferably, it is followed by several administrations of antigen mixed with an appropriately amount of Freund&#39;s incomplete adjuvant every 4 to 21 days. An appropriate carrier may also be used for immunization. After immunization as above, serum is examined for increase of the amount of desired antibodies by a standard method. 
     Polyclonal antibodies against the proteins of the present invention may be prepared by collecting blood from the immunized mammal examined for the increase of desired antibodies in the serum, and by separating serum from the blood by any conventional method. Polyclonal antibodies may be used as serum containing the polyclonal antibodies, or if necessary, a fraction containing the polyclonal antibodies may be isolated from the serum. Immunoglobulin G or M can be prepared by obtaining a fraction which recognizes only the protein of the present invention using an affinity column coupled with the protein of the present invention and further purifying this fraction by using protein A or protein G column. 
     To prepare monoclonal antibodies, immune cells are collected from the mammal immunized against the antigen and checked for increased level of desired antibodies in the serum as described above, and are subjected to cell fusion. The immune cells used for cell fusion are preferably obtained from spleen. As the other parental cells to be fused with the above immunocyte, for example, preferably myeloma cells of mammalians, more preferably myeloma cells which acquired the property for selecting fused cells by drugs, can be used. 
     The above immunocyte and myeloma cells can be fused by known methods, for example, the method by Milstein et al. (Galfre et al., Methods Enzymol. 73:3–46, 1981). 
     Resulting hybridomas obtained from cell fusion may be selected by cultivating them in a standard selection medium, such as HAT medium (culture medium containing hypoxanthine, aminopterin, and thymidine). The cell culture is typically continued in the HAT medium for several days to several weeks, the sufficient time to allow all the other cells, except desired hybridoma (non-fused cells), to die. Then, by the standard limiting dilution method, a hybridoma cell producing the desired antibody is screened and cloned. 
     In addition to the above method, in which a non human animal is immunized against an antigen for preparing hybridoma, human lymphocytes, such as that infected by EB virus, may be immunized with a protein, protein-expressing cells, or their lysates in vitro. Then, the immunized lymphocytes are fused with human-derived myeloma cells capable of indefinitely dividing, such as U266, to yield a hybridoma producing a desired human antibody having binding activity to the protein(Unexamined Published Japanese Patent Application (JP-A) No. Sho 63-17688). 
     Next, the monoclonal antibody obtained by transplanting the obtained hybridomas into the abdominal cavity of a mouse and by extracting ascites can be purified by, for example, ammonium sulfate precipitation, protein A or protein G column, DEAE ion exchange chromatography, or an affinity column to which the protein of the present invention is coupled. An antibody of the present invention can be used not only for purification and detection of the protein of the present invention, but also as a candidate agonist or antagonist of a protein of the present invention. In addition, the antibody can be utilized in antibody treatment for diseases associated with a protein of the present invention. When the obtained antibody is used for the administration to the human body (antibody treatment), a human antibody or a humanized antibody is preferable for reducing immunogenicity. 
     For example, transgenic animals having a repertory of human antibody genes may be immunized against a protein, protein-expressing cells, or their lysates as an antigen. Antibody-producing cells are collected from the animals, and fused with myeloma cells to obtain hybridoma, from which human antibodies against the protein can be prepared (see WO92-03918, WO93-2227, WO94-02602, WO94-25585, WO96-33735, and WO96-34096). 
     Alternatively, an immune cell, such as an immunized lymphocyte, producing antibodies may be immortalized by an oncogene and used for preparing monoclonal antibodies. 
     Monoclonal antibodies thus obtained can be also recombinantly prepared using genetic engineering techniques (see, for example, Borrebaeck et al., THERAPEUTIC MONOCLONAL ANTIBODIES, published in the United Kingdom by MACMILLAN PUBLISHERS LTD, 1990). A DNA encoding an antibody may be cloned from an immune cell, such as a hybridoma or an immunized lymphocyte producing the antibody, inserted into an appropriate vector, and introduced into host cells to prepare a recombinant antibody. The present invention also provides recombinant antibodies prepared as described above. 
     Furthermore, an antibody of the present invention may be a fragment of an antibody or modified antibody, so long as it binds to one or more of the proteins of the invention. For instance, the antibody fragment may be Fab, F(ab′) 2 , Fv, or single chain Fv (scFv), in which Fv fragments from H and L chains are ligated by an appropriate linker (Huston et al., Proc. Natl. Acad. Sci. USA 85:5879–5883, 1988). More specifically, an antibody fragment may be generated by treating an antibody with an enzyme such as papain or pepsin. Alternatively, a gene encoding the antibody fragment may be constructed, inserted into an expression vector, and expressed in an appropriate host cell (see, for example, Co et al., J. Immunol. 152:2968–2976, 1994; Better et al., Methods Enzymol. 178:476–496, 1989; Pluckthun et al., Methods Enzymol. 178:497–515, 1989; Lamoyi, Methods Enzymol. 121:652–663, 1986; Rousseaux et al., Methods Enzymol. 121:663–669, 1986; Bird et al., Trends Biotechnol. 9:132–137, 1991). 
     An antibody may be modified by conjugation with a variety of molecules, such as polyethylene glycol (PEG). The term “an antibody” as used herein includes such modified antibodies. The modified antibody can be obtained by chemically modifying an antibody. These modification methods are conventional in this field. 
     Alternatively, an antibody of the present invention may be obtained as a chimeric antibody by using known technique, between a variable region derived from nonhuman antibody and the constant region derived from human antibody, or as a humanized antibody, comprising the complementarity determining region (CDR) derived from nonhuman antibody, the frame work region (FR) derived from human antibody, and the constant region. 
     Obtained antibodies may be purified into homogeneity. An antibody used in the present invention can be separated and purified by the method used for separating and purifying usual proteins. For example, the separation and purification of the protein can be performed by the appropriately selected and combined use of column chromatography such as affinity chromatography, filter, ultrafiltration, salting-out, dialysis, SDS polyacrylamide gel electrophoresis, isoelectric focusing, and others (Antibodies: A Laboratory Manual. Ed Harlow and David Lane, Cold Spring Harbor Laboratory, 1988), but the methods are not limited thereto. The level of the obtained antibody can be determined by measurement of absorbance, enzyme-linked immunosorbent assay (ELISA), and such. 
     Examples of columns used for affinity chromatography include protein A column and protein G column. Examples of columns using protein A column include Hyper D, POROS, Sepharose F. F. (Pharmacia), etc. 
     In addition to affinity chromatography, chromatography includes, for example, ion-exchange chromatography, hydrophobic chromatography, gel filtration, reverse-phase chromatography, adsorption chromatography, and such (Strategies for Protein Purification and Characterization: A Laboratory Course Manual. Ed Daniel R. Marshak et al., Cold Spring Harbor Laboratory Press, 1996). The chromatographic procedures can be carried out by liquid-phase chromatography such as HPLC, FPLC, and so on. 
     For example, measurement of absorbance, enzyme-linked immunosorbent assay (ELISA), enzyme immunoassay (EIA), radioimmunoassay (RIA), and/or immunofluorescence may be used to measure the antigen binding activity of the antibody of the invention. In ELISA, an antibody of the present invention is immobilized on a plate, protein of the invention is applied to the plate, and then a sample containing a desired antibody, such as culture supernatant of antibody-producing cells or purified antibodies, is applied. Then, a secondary antibody that recognizes the primary antibody and is labeled with an enzyme, such as alkaline phosphatase, is applied, and the plate is incubated. Next, after washing, an enzyme substrate, such as p-nitrophenyl phosphate, is added to the plate, and the absorbance is measured to evaluate the antigen binding activity of the sample. A fragment of the protein, such as a C-terminal or N-terminal fragment, may be used as a protein. BIAcore (Pharmacia) may be used to evaluate the activity of the antibody according to the present invention. 
     The above methods allow for the detection or measurement of a protein of the invention, by exposing an antibody of the invention to a sample assumed to contain a protein of the invention, and detecting or measuring the immune complex formed by the antibody and the protein. Because the method of detection or measurement of proteins according to the invention can specifically detect or measure proteins, the method may be useful in a variety of experiments in which the protein is used. 
     The present invention also provides a polynucleotide comprising at least 15 nucleotides which is complementary to a DNA encoding a human “OATP-B”, “OATP-C”, “OATP-D”, or “OATP-E” protein (SEQ ID NOs:1, 3, 5, or 7), or complementary strand thereof. 
     Herein, the term “complementary strand” is defined as one strand of a double strand DNA composed of A:T and G:C base pair to the other strand. Also, “complementary” is defined as not only those completely matching within a continuous region of at least 15 nucleotides, but also those having a homology of at least 70%, preferably at least 80%, more preferably 90%, and still more preferably 95% or more within that region. The homology may be determined using the algorithm described herein. 
     Such nucleotide includes, probes, primers, nucleotides and nucleotide derivatives (for example, antisense oligonucleotides, ribozymes, and so on) for inhibiting the expression of a protein of the present invention, which are used to detect or amplify DNA encoding a protein of the invention. Moreover, such DNA can be utilized in preparation of DNA chip. 
     When used as primers, such nucleic acids are complementary at the 3′-end, and restriction enzyme recognition sequences or tags can be added to the 5′-end. 
     The antisense oligonucleotides of the present invention include nucleotides that hybridize with any site within the nucleotide sequence any one of SEQ ID NOs:1, 3, 5, and 7. An antisense oligonucleotide is preferably against at least 15 continuous nucleotides in the nucleotide sequence any one of SEQ ID NO:1, 3, 5, or 7. The above-mentioned antisense oligonucleotide, which contains an initiation codon in the above-mentioned at least 15 continuous nucleotides, is even more preferred. 
     Derivatives or modified products of antisense oligonucleotides can be used as antisense oligonucleotides. Examples of such modified products are lower alkyl phosphonate modifications, such as methyl-phosphonate-type or ethyl-phosphonate-type; phosphothioate modifications; and phosphoamidate modifications. 
     The term “antisense oligonucleotides” as used herein means, not only those in which the entire nucleotides corresponding to those constituting a specified region of a DNA or mRNA are complementary, but also those having a mismatch of one or more nucleotides, so long as DNA or mRNA and an oligonucleotide can specifically hybridize with the nucleotide sequence of SEQ ID NO:1, 3, 5, or 7. 
     An antisense oligonucleotide derivative of the present invention has inhibitory effect on the function of a protein of the present invention wherein the derivative inhibits the expression of the protein of the invention by acting upon cells producing the protein of the invention and by binding to the DNA or mRNA encoding the protein to inhibit its transcription or translation or to promote the degradation of the mRNA. 
     An antisense oligonucleotide derivative of the present invention can be made into an external preparation, such as a liniment or a poultice, by mixing with a suitable base material which is inactive against the derivatives. 
     Also, as necessary, the derivatives can be formulated into tablets, powders, granules, capsules, liposome capsules, injections, solutions, nose-drops, and freeze-drying agents, and such by adding excipients, isotonic agents, solubilizing agents, stabilizers, preservative substance, pain-killers, and such. These can be prepared by following usual methods. 
     An antisense oligonucleotide derivative of the present invention is given to the patient by directly applying onto the ailing site or by injecting into a blood vessel so that it will reach the site of ailment. An antisense-mounting medium can also be used to increase durability and membrane-permeability. Examples are, liposome, poly-L-lysine, lipid, cholesterol, lipofectin, or derivatives of these. 
     The dosage of the antisense oligonucleotide derivative of the present invention can be adjusted suitably according to the patient&#39;s condition and used in desired amounts. For example, a dose range of 0.1 to 100 mg/kg, preferably 0.1 to 50 mg/kg can be administered. 
     An antisense oligonucleotide of the invention inhibits the expression of a protein of the invention and is thereby useful for suppressing the biological activity of the protein. Also, expression-inhibitors comprising the antisense oligonucleotide of the invention are useful in that they can inhibit the biological activity of a protein of the invention. 
     The transporter proteins of this invention can be used to control internal absorption and dynamics of drugs. Based on the results of detailed analysis of the substrate specificity of transporter proteins of this invention, drugs can be designed so as to be transported by these transporters and internal absorbability of the drugs mediated by these transporter proteins can be improved. Conventional modifications to enhance fat-solubility are no longer necessary for drugs so designed, which enables the speedy and efficient development of water-soluble drugs that are easy to handle. The drugs thus developed are thought to be absorbed principally depending on the internal distribution pattern of transporter proteins of this invention, and an organ-specific delivery of the drugs thus becomes possible. In particular, if the transporter proteins of this invention are distributed in the target organ of the drug, an ideal drug delivery system (DDS) can be developed. If a drug is to be absorbed mediated by not the transporter proteins of this invention but other transporters, the drug can be designed so as to be specific to other transporter proteins by designing it considering the substrate specificity of the transporter proteins of this invention. For example, it has been revealed that the OATP-E gene is expressed in high frequency in a variety of solid cancer cells but seldom in hemocytes. Therefore, anticancer agents can be obtained by constructing a screening system using the OATP-E gene to screen compounds which are specifically transported into cells mediated by the OATP-E protein; these agents are expected to be anticancer agents with reduced cytotoxicity to hemocytes. 
     Screening of compounds which are transported into cells from the outside of the cells mediated by a protein of the present invention can be carried out, for example, as follows. First, cells are provided which express a protein of this invention on their cell membranes. More specifically, for example, a vector to express a protein of this invention may be constructed and transferred into appropriate cells. Then, a labeled compound to be tested is brought into contact with said cells. For example, low molecular weight compounds can be used as compounds to be tested. There is no particular limitation on the label used for labeling the compounds to be tested, so long as it can be readily detected; for example, radiolabels, fluorescence labels, and such can be used. Then, labeled test compounds which are taken up into said cells are detected. The detection can be performed by measuring radioactivity using a liquid scintillation counter, and such in case of radiolabeled compounds, and fluorescence using a fluorometer, etc., in case of fluorescence labeled compounds. In addition, even when using non-labeled compounds, the amount of the compounds which has been transported into the cell mediated by a protein of the present invention can be measured by the bioassay relied biological activities (e.g., cytotoxicity, cell proliferation stimulating activity), and such of said compounds as the indicator. Then, based on the results of the above-described detection, the compound taken up into cells is selected. Specifically, in this screening, the detection system for the transport activity as described in Example 3 can be used. Compounds thus isolated can be used to create the above-described drugs. 
     Another possible application of this invention is to develop a drug that targets the transporter proteins of this invention. The transporters play important roles in the absorption mechanism of nutrients and drugs, or the excretion mechanism of drugs and internal metabolites. Thus, damage or abnormal elevation of the transporter&#39;s functions may cause some disorders. It is considered to be efficacious against such disorders to administer a drug containing a compound that inhibits or enhances functions of the transporter proteins of this invention, or regulates the expression level of the transporter gene of this invention and the amount of the transporter proteins. 
     Screening of compounds which promote or inhibit the transporter activity of the proteins of this invention can be carried out, for example, as follows. First, cells are provided which express one or more of the proteins of this invention on their cell membranes. Then, a test compound and a labeled organic compound which is transported through the intermediary of the proteins of this invention are brought into contact with said cells. Examples of organic compounds to be used include estradiol-17β- glucuronide, estron-3-sulfate, benzyl penicillin, prostaglandin E2, and such, but the present invention is not limited to them. Then, the amount of labeled organic compound which has been taken up into said cells is measured. Then, a compound is selected which increases or decreases the amount of the labeled organic compound taken up into said cell compared with that in case of a similar measurement conducted in the absence of a test compound (control). Specifically, in this screening, the detection system for the transport activity as described in Example 3 can be used. As a result, when the amount of labeled organic compound to be taken up into said cells is increased by the contact with a test compound, said compound is judged to promote the activity of a protein of this invention to transport the organic compound. On the other hand, when the amount of labeled organic compound to be taken up into said cells is decreased by the contact with a compound to be tested, said compound is judged to inhibit the activity of a protein of this invention to transport the organic compound. 
     Compounds thus obtained by the screening method of this invention may be applied to drug therapy using a protein of this invention and treatment mediated by the control of substance transport by the protein. The structure of compounds obtained by the screening method of this invention may be partially modified by addition, deletion and/or substitution. 
     When a compound binding to a protein of the invention, a protein of the invention, and a partial peptide thereof are used as a pharmaceutical for humans and other mammals, such as mice, rats, guinea pigs, rabbits, chicken, cats, dogs, sheep, pigs, bovines, monkeys, baboons, chimpanzees, the protein or the isolated compound can be administered not only directly, but also in a dosage form using known pharmaceutical preparation methods. For example, according to needs, the drugs can be taken orally, as sugarcoated tablets, capsules, elixirs, and microcapsules, or non-orally, in the form of injections of sterile solutions or suspensions with water or any other pharmaceutically acceptable liquid. For example, the compounds can be mixed with pharmacologically acceptable carriers or medium, specifically, sterilized water, physiological saline, plant-oil, emulsifiers, suspending agent, surface-active agent, stabilizers, flavoring agents, excipients, vehicles, preservatives, binders, and such into a unit dose form required for generally accepted drug implementation. The amount of active ingredient in these preparations makes a suitable dosage within the indicated range acquirable. 
     Examples of additives which can be mixed to tablets and capsules include: binders, such as gelatin, corn starch, tragacanth gum, and gum acacia; excipients, such as crystalline cellulose; swelling agents, such as corn starch, gelatin, and alginic acid; lubricants, such as magnesium stearate; sweeteners, such as sucrose, lactose, or saccharin; and flavoring agents, such as peppermint,  Gaultheria adenothrix  oil, and cherry. When the unit dosage form is a capsule, a liquid carrier, such as oil, can also be included in the above ingredients. Sterile composites for injections can be formulated following normal drug implementations using vehicles such as distilled water used for injections. 
     Physiological saline, glucose, and other isotonic liquids, including adjuvants, such as D-sorbitol, D-mannose, D-mannitol, and sodium chloride, can be used as aqueous solutions for injections. These can be used in conjunction with suitable solubilizers, such as alcohol, specifically ethanol, polyalcohols such as propylene glycol and polyethylene glycol, and non-ionic surfactants, such as Polysorbate 80 (TM) and HCO-50. 
     Sesame oil or soy-bean oil can be used as an oleaginous liquid and may be used in conjunction with benzyl benzoate or benzyl alcohol as solubilizers; alternatively, they may be formulated with a buffer such as phosphate buffer and sodium acetate buffer, a pain-killer such as procaine hydrochloride, a stabilizer such as benzyl alcohol and phenol, and an anti-oxidant. The prepared injection may then be filled into a suitable ampule. 
     Methods well known to those skilled in the art may be used to administer the pharmaceutical compounds of the present invention to patients. Examples of suitable administration methods include intraarterial, intravenous, subcutaneous injections and also as intranasal, transbronchial, intramuscular, percutaneous, or oral administrations. The dosage varies according to the body-weight and age of a patient and the administration method, but one skilled in the art can suitably select them. If the compound can be encoded by a DNA, the DNA can be inserted into a vector for gene therapy and perform the therapy. The dosage and method of administration vary according to the body-weight, age, and symptoms of a patient, but one skilled in the art can select them suitably. 
     The DNAs of this invention can be applied to gene therapy for disorders caused by aberrations in the activity and expression level of the proteins of this invention. In the case of using DNA in gene therapy, a DNA of this invention is inserted to an adenovirus vector (e.g., pAdexLcw), a retrovirus vector (e.g., pZIPneo), and such for administration into the living body. The transformed vector may be administered into the living body by the ex vivo method or in vivo method. Gene therapy can also be performed by administering a synthetic antisense DNA to the living body either directly or after being inserted into above-described vectors. The DNAs of this invention can be applied also to the diagnosis for disorders caused by aberrant activities and expression levels of the proteins of this invention. 
     All publications and patents cited herein are incorporated by reference in their entirety. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a photograph showing the results of examination by the RT-PCR method for the expression level of respective OATP family genes in human fetal and adult tissues: 1. fetal brain, 2. fetal heart, 3. fetal kidney, 4. fetal liver, 5. fetal lung, 6. fetal skeletal muscle, 7. fetal spleen, 8. fetal thymus, 9. adult pancreas, 10. adult kidney, 11. adult skeletal muscle, 12. adult liver, 13. adult lung, 14. adult placenta, 15. adult brain, 16. adult heart, 17. adult peripheral blood leukocytes, 18. adult large intestine, 19. adult small intestine, 20. adult ovary, 21. adult testis, 22. adult prostate, 23. adult thymus, 24. adult spleen, 25. adult bone marrow, 26. adult lymph node, and 27. adult tonsil. 
         FIG. 2  is a photograph showing the results of examination by the RT-PCR method for the expression level of respective OATP family genes in human cancer cells: 1. mammary cancer cells (GI-101), 2. lung cancer cells (LX-1), 3. large intestinal adenoma cells (CX-1), 4. lung cancer cells (GI-117), 5. prostatic adenoma cells, 6. large intestinal adenoma cells (GI-112), 7. ovary cancer cells (GI-102), and 8. pancreatic adenoma cells (GI-103). 
         FIG. 3  is a bar graph showing the results of transport experiments for various labeled compounds. More particularly, the transport activities for various labeled compounds of HEK293 cells transfected with OATP-C (OATP-C) or with the vector alone (mock) are shown. 
         FIG. 4  is a graph showing the concentration-dependency of the transport activity for PCG of HEK293 cells expressing the OATP-C protein. 
         FIG. 5  is a bar graph showing the effect of various β-lactam antibiotics on the transport activity for PCG in HEK293 cells wherein the OATP-C protein is expressed. The transport activity in the control (with no inhibitor) is taken as 100%. 
         FIG. 6  is a bar graph showing the sodium ion and chloride ion dependencies of the transport activity for estradiol-17β-glucuronide. “OATP-C” represents the transport activity for estradiol-17β-glucuronide of HEK293 cells transfected with OATP-C, and “Mock” represents the transport activity for estradiol-17β-glucuronide of HEK293 cells transfected with the vector alone. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is described below in more detail with reference to examples, but is not construed as being limited thereto. Molecular biological experimental techniques in general have been performed principally according to methods described in usual experimental textbooks, such as “Sambrook, J., Fritsch, E. F., Maniatis, T., Molecular Cloning, Cold Spring Harbor Lab. press (1989)”. 
     EXAMPLE 1 
     Cloning of cDNAs Comprising the Entire Open Reading Frame (ORF) of OATP-B, C, D, or E Gene 
     OATP-B 
     The OABE-1 primer (5′ gat aag ctt ctg tgt ggc cca aga aga act gac 3′ /SEQ ID NO:9) and OABE-6 primer (5′ gat aag ctt tac tgc tgt ggc tgc tac tct tgg 3′ /SEQ ID NO:10) were prepared based on the nucleotide sequences of W19504 and AI052501, ESTs possibly encoding the amino acid sequence having significant homology with the human OATP-A protein. Using these primers, PCR was performed against human adult brain polyA +  RNA-derived cDNA as a template to amplify the OATP-B cDNA comprising the entire ORF. The OATP-B cDNA thus amplified by PCR was cleaved at the Hind III site added to the primer, and incorporated into the Hind III site of the pcDNA3 vector (Invitrogen), an expression vector for mammalian cells. By sequencing a plurality of clones, a clone (pcDNA3/OATP-B) with no PCR error was selected to be used in expression experiments. 
     OATP-C 
     Respective primers were prepared from the nucleotide sequences of the following ESTs possibly encoding amino acid sequences having a significant homology with the human OATP-A:
     EST H62893:2893-4 primer (5′ aag ctt ccg tca ata aaa cca aca 3′ /SEQ ID NO:11), and 2893-1 primer (5′ ctt ctc ttg ttg gtt tta ttg acg 3′ /SEQ ID NO:12);   EST R29414; 9414-2 primer (5′ tgt aag tta ttc cat tgt ttc cac 3′ /SEQ ID NO:13); and   EST T73863; 3863-1 primer (5′ ttg gtg ctt tta ctt atg tct tca 3′ /SEQ ID NO:14).   

     Using these primers, the human OATP-C divided into three fragments was cloned. 
     The 5′-end fragments were cloned by the 5′ RACE (Rapid Amplification cDNA Ends) method. More specifically, PCR was performed against the human fetal liver-derived Marathon-Ready™ cDNA (CLONTECH) as a template using a combination of the API primer, a linker primer attached to the kit, and the 2893-4 primer to amplify the 5′-end fragment of the human OATP-C cDNA of about 400 bp. This cDNA fragment was incorporated into the pT7Blue-T vector (Novagen) by the TA cloning method, and a plurality of subclones thus obtained were sequenced to determine the 5′-end sequence of the human OATP-C cDNA. The 3′-end sequence was similarly cloned by the 3′ RACE method. Specifically, PCR was performed with the human fetal liver-derived Marathon-Ready™ cDNA (CLONTECH) as a template using a combination of the AP1 primer, a linker primer attached to the kit, and the 3863-1 primer to amplify the 3′-end fragment of the human OATP-C cDNA of about 1.5 kbp. This cDNA fragment was incorporated into the pT7Blue-T vector by the TA cloning method, and a plurality of subclones thus obtained were sequenced to determine the 3′-end sequence of the human OATP-C cDNA. In addition, the intermediate fragment between the 5′-end sequence and the 3′-end sequence was amplified by PCR with cDNA derived from the human adult liver as a template using a combination of the 2893-1 and 9414-2 primers. The fragment of about 1.2 kbp thus obtained was purified by the gel filtration method and directly sequenced to determine the nucleotide sequence. By combining the obtained sequences, the cDNA sequence comprising the entire ORF of human OATP-C was determined. 
     The expression plasmid was constructed as follows. The human OATP-C was divided into two fragments, and they were amplified by PCR with the human adult liver-derived cDNA as a template using a combination of the following primers: 
     
       
         
               
               
               
               
             
           
               
                 5′-end 
                   
                   
                   
               
               
                   
               
               
                     OAHC17 primer 
                 (5′ gat ggt acc aaa ctg agc atc aac aac aaa aac 3′ 
                 /SEQ ID NO:15), and 
               
               
                   
               
               
                     OAHC18 primer 
                 (5′ gat ggt acc cat cga gaa tca gta gga gtt atc 3′ 
                 /SEQ ID NO:16). 
               
               
                   
               
               
                 3′-end 
               
               
                   
               
               
                     OAHC21 primer 
                 (5′ gat ggt acc tac cct ggg atc tct gtt ttc taa 3′ 
                 /SEQ ID NO:17), and 
               
               
                   
               
               
                     OAHC22 primer 
                 (5′ gat ggt acc gtt tgg aaa cac aga agc aga agt 3′ 
                 /SEQ ID NO:18). 
               
             
          
         
       
     
     These fragments were subcloned into the pT7Blue-T vector, respectively, to select clones with no PCR error. After both clones were linked at the Bgl II sites existing in the overlapping regions, the product was cleaved at the Kpn I sites existing at both ends, and incorporated into the Kpn I site of the pcDNA3 vector to obtain the expression plasmid, pcDNA3/OATP-C. 
     OATP-D 
     0224-3 primer (5′ cgc cct cgt ggt ttt tga tgt agc 3′ /SEQ ID NO:19) was prepared from EST AA280224, an EST possibly encoding the amino acid sequence having significant homology with the human OATP-A protein. Furthermore, it was found that a partial sequence of the PAC clone (pDJ430i19) derived from q26.1 region of human chromosome 15 could also encode the amino acid sequence having significant homology with the human OATP-A protein. PAC151-2 primer (5′ gcg gtg cct tac tct tct tct ctt 3′ /SEQ ID NO:20) was prepared from this sequence. PCR was performed using these primers on the human adult brain-derived cDNA as a template to amplify a cDNA fragment of about 1.1 kbp. Using this cDNA fragment as a probe, the human adult kidney-derived 5′-STRETCH PLUS cDNA library (CLONTECH) was screened by the plaque hybridization method. PCR was performed with a phage suspension of obtained positive clones as a template using a combination of the above-described primers, or OATP-D gene specific primer prepared from the above-elucidated sequence and GT10 S1 primer (5′ ctt ttg agc aag ttc agc ct 3′ /SEQ ID NO:21) or GT10A1 primer (5′ aga ggt ggc tta tga gta ttt ctt 3′ /SEQ ID NO:22) prepared from the sequence of λgt-10 vector, and the fragments thus amplified were directly sequenced to determine the nucleotide sequence. Furthermore, the region covered by the phage clone was extended to determine the entire ORF sequence by the screening using the DNA fragment comprising the newly obtained region as a probe. 
     OATP-E 
     7130-1 primer (5′ tgt aca agg tgc tgg gcg tcc tct 3′ /SEQ ID NO:23) and 7130-4 primer (5′ cga tcg ggt ata aaa cac att cta 3′ /SEQ ID NO:24) were prepared from EST A1347130, an EST potentially encoding the amino acid sequence having significant homology with the human OATP-A protein. PCR was performed using these primers with the human adult lung-derived cDNA as a template to amplify a cDNA fragment of about 400 bp. The human adult kidney-derived 5′-STRETCH PLUS cDNA library (CLONTECH) was screened using this cDNA as a probe by the plaque hybridization method. PCR was performed with a phage suspension of obtained positive clones as a template using a combination of the above-described primers or the OATP-E gene specific primer prepared from the above-elucidated sequence and the GT10-S1 primer or GT10-A1 primer prepared from the λkgt-10 vector sequence, and amplified fragments thus obtained were directly sequenced to determine the nucleotide sequence. Furthermore, the region covered by the phage clone was extended to determine the entire ORF sequence by the screening using the DNA fragment comprising the newly obtained region as a probe. 
     The expression plasmid was constructed as follows. The human OATP-E was divided into two fragments, 5′-end and 3′-end fragments, which were amplified by PCR using combinations of the following primers. The 5′-end fragment was amplified using human adult lung-derived cDNA as a template, and the 3′-end fragment was amplified using human fetal lung-derived cDNA as a template, respectively. 
     
       
         
               
               
               
               
             
           
               
                 5′-end: 
                   
                   
                   
               
               
                   
               
               
                     OAE17 primer 
                 (5′ gat aag ctt tgc gtg gct gaa gcc tcg aag tca 3′ 
                 /SEQ ID NO:25), and 
               
               
                   
               
               
                     OAE18 primer 
                 (5′ gat gga tcc act ggt gca ttt ccg ccg ctc tca 3′ 
                 /SEQ ID NO:26). 
               
               
                   
               
               
                 3′-end: 
               
               
                   
               
               
                     OAE21 primer 
                 (5′ gat aag ctt tct tca ccg ccg ttc cca tcc ttg 3′ 
                 /SEQ ID NO:27), and 
               
               
                   
               
               
                     OAE22 primer 
                 (5′ gat gga tcc act gtt ctg tca tca gga aat gct 3′ 
                 /SEQ ID NO:28). 
               
             
          
         
       
     
     These fragments were subcloned into the Hind III/BamH I site of the pcDNA3 vector, respectively, to select clones with no PCR error. Both clones were linked at the BstP I sites existing in the overlapping regions to obtain the expression plasmid, pcDNA3/OATP-E. 
     PCR 
     PCR was fundamentally performed under the following basic conditions with appropriate modifications if necessary. 
     &lt;Composition of reaction solution&gt;
         template DNA,   10×ExTaq buffer (TaKaRa) 5 μl,   2.5 mM dNTPs (TaKaRa) 4 μl,   ExTaq (TaKaRa) 0.5 μl,   TaqStart™ Antibody (CLONTECH) 0.5 μl,   sense primer 10 to 20 pmol, and   antisense primer 10 to 20 pmol   /total volume 50 μl.       

     &lt;Reaction conditions&gt; 
     PCR in general:
         94° C., 2 min→(94° C., 30 s→55 to 62° C., 30 s→72° C., 2 to 3 min)×25 to 40 cycles→72° C., 10 min.       

     RACE method:
         94° C., 2 min→(94° C., 30 s→68° C., 4 min)×5 cycles→(94° C., 30 s→62° C., 30 s→72° C., 2 min)×30 cycles→72° C., 10 min.
 
Synthesis of cDNA
       

     cDNAs used as templates for PCR were prepared using the SUPERSCRIPT™ II RNase H −  reverse transcriptase (GIBCO BRL) according to the usual method recommended by the supplier. Specifically, 10 μg of the total RNA or 2 μg of poly A +  RNA and about 1 μg of the oligo dT primer (GIBCO BRL) or about 0.5 μg of the random hexamer primer (GIBCO BRL) were mixed, heated at 70° C. for 10 min, and then cooled on ice. The first strand buffer (GIBCO BRL), DDT (final concentration of 10 mM), dNTPs (final concentration of 0.5 mM; GIBCO BRL), and 400 to 800 U of SUPERSCRIPT™ II RNase H −  reverse transcriptase were added to this mixture, and the resulting mixture was warmed at 42° C. for 1 h to synthesize cDNA. The mixture was then heated at 70° C. for 15 min, and a portion thereof was used as the template. 
     Hybridization 
     DNA fragments amplified by PCR, or purified from gels after agarose electrophoresis, and such were labeled with [α− 32 P]dCTP using the Ready-to Go DNA labeling beads (Pharmacia) by the random primer method to be used as the primer. The hybridization was performed using the ExpressHyb Hybridization Solution (CLONTECH) by heating at 68° C. for 2 h or more according to the method recommended by the supplier. After the hybridization, the filter was washed twice in a solution of 2×SSC and 0.1% SDS solution at room temperature for 20 min, and then, twice in a solution of 0.1×SSC and 0.1% SDS at 50° C. for 20 min. 
     EXAMPLE 2 
     Analysis by RT-PCR Method 
     Using the following primers specific for respective genes, the tissue distribution of the expression of each gene was analyzed by the RT-PCR method. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 OATP-A 
                   
                   
                   
               
               
                   
                   OAA-1 primer 
                 (5′ aag aag agg tca aga agg aaa aat 3′ 
                 /SEQ ID NO:29), and 
               
               
                   
                   
               
               
                   
                   OAA-2 primer 
                 (5′ gga gca tca agg aac agt cag gtc 3′ 
                 /SEQ ID NO:30). 
               
               
                   
                   
               
               
                   
                 OATP-B 
               
               
                   
                   
               
               
                   
                   4742-1 primer 
                 (5′ cgt gcg gcc aag tgt gtt cca taa 3′ 
                 /SEQ ID NO:31), and 
               
               
                   
                   
               
               
                   
                   4742-2 primer 
                 (5′ gaa gga gta gcc cca tag cca atc 3′ 
                 /SEQ ID NO:32). 
               
               
                   
                   
               
               
                   
                 OATP-C 
               
               
                   
                   
               
               
                   
                   9414-1 primer 
                 (5′ tgt cat tgt cct ttt acc tat tat 3′ 
                 /SEQ ID NO:33), and 
               
               
                   
                   
               
               
                   
                   9414-2 primer 
                 (above-described, 5′ tgt aag tta ttc cat tgt ttc cac 3′ 
                 /SEQ ID NO:13). 
               
               
                   
                   
               
               
                   
                 OATP-D 
               
               
                   
                   
               
               
                   
                   0224-2 primer 
                 (5′ ctc aaa tcc ttc gcc ttc atc ctg 3′ 
                 /SEQ ID NO:34), and 
               
               
                   
                   
               
               
                   
                   0224-4 primer 
                 (5′ agg gtc aga gta gag gca aag aac 3′ 
                 /SEQ ID NO:35). 
               
               
                   
                   
               
               
                   
                 OATP-E 
               
               
                   
                   
               
               
                   
                   7130-2 primer 
                 (5′ cac ggc ggg cac tca gca ttt cct 3′ 
                 /SEQ ID NO:36), and 
               
               
                   
                   
               
               
                   
                   7130-4 primer 
                 (above-described, 5′ cga tcg ggt ata aaa cac att cta 3′ 
                 /SEQ ID NO:24). 
               
               
                   
                   
               
               
                   
                 G3PDH 
               
               
                   
                   
               
               
                   
                   Upstream primer 
                 (5′ TGAAGGTCGGAGTCAACGGATTTGGT 3′ 
                 /SEQ ID NO:37), and 
               
               
                   
                   
               
               
                   
                   Downstream primer 
                 (5′ CATGTGGGCCATGAGGTCCACCAC 3′ 
                 /SEQ ID NO:38). 
               
             
          
         
       
     
     PCR was performed with appropriate amounts of cDNAs derived from various organs and cells contained in the Multiple Tissue cDNA (MTC™) Panel (CLONTECH) as template using the above-described primers. PCR-amplified products were analyzed by agarose electrophoresis ( FIGS. 1 and 2 ). OATP-A showed an expression pattern comparatively restricted in brain and liver. Expression of OATP-C was discovered to be restricted in liver in both fetal and adult tissues examined. Although OATP-B, OATP-D, and OATP-E were found to be expressed in a relatively wide range of tissues, it was revealed that OATP-B and OATP-E, among them, were expressed extremely low in the peripheral blood leukocytes, thymus, and spleen. These findings strongly indicate that expressions of OATP-B and OATP-E are low in hemocytes. On the other hand, the examination of OATPs in cancer cells have revealed that both OATP-D and OATP-E are expressed in a high frequency in these cells ( FIG. 2 ). From these results, the potential production of an anticancer agent which is specifically incorporated into cells by the OATP-E protein is expected, such an agent being an anticancer agent with an attenuated side effects on hematopoietic cells (e.g., bone marrow suppression, and such). 
     EXAMPLE 3 
     Transport Experiment 
     The human fetal kidney-derived cell strain, HEK 293 cells, were transfected with the plasmid pcDNA3/OATP-C or the pcDNA3 vector containing no insert as a control (mock) by the calcium phosphate method. Specifically, the plasmid DNA (10 μg), a Hepes buffer solution (137 mM NaCl 5 mM KCl, 0.7 mM Na 2 HPO 4 , 6 mM dextrose, and 21 mM Hepes pH 7.1) (1 ml), and 2 M CaCl 2  (62.5 μl) were mixed and the resultant mixture was allowed to stand at room temperature for 30 min or more to form calcium phosphate coprecipitates. After the cells were plated on 10-cm diameter plates at a concentration of 1.5×10 6  cells per plate, and the cells were cultured for 24 h. The above-described calcium phosphate coprecipitates were added thereto, and the cells were further cultured for 24 h. Then, plates were washed with phosphate buffered saline (PBS), and after the addition of fresh culture medium, the cells were further cultured for 24 h. 
     Transport experiment was performed using the cells transfected with the plasmid DNA according to the following procedures. The cells were detached from the plates using a rubber policeman, suspended in a transport buffer (containing 125 mM NaCl, 4.8 mM KCl, 5.6 mM (+)-glucose, 1.2 mM CaCl 2 , 1.2 mM KH 2 PO 4,  1.2 mM MgSO 4 , and 25 mM Hepes pH 7.4), and pre-incubated for 20 min. Each of various labeled substrates ([ 3 H]methotrexate, [ 3 H]digoxin, [ 3 H]ouabain, [ 3 H]prostaglandin E2, [ 3 H]estradiol-17β-glucuronide, [ 3 H]estron-3-sulfate, [ 14 C]PCG&lt;benzylpenicillin&gt;, and so on) was then added in an appropriate amount to the above-described cell suspension, and the resulting mixture was incubated at 37° C. for a predetermined period of time. Incubated cells were overlaid on a silicon layer formed by laying a mixture of silicon oil and liquid paraffin (specific gravity=1.022) on a 3 M KCl layer, and separated by centrifugation. Radioactivity of cells was measured to determine the into-the-cell transport activity. In this case, 1×10 6  cells were used as one point of cells. 
     Herein, the culture of HEK 293 cells was performed using Dulbecco&#39;s MEM containing 10% FCS (fetal calf serum) as the culture medium in an atmosphere of 5% carbon dioxide at 37° C. 
     From measuring the transport activity in HEK293 cells wherein the OATP-C proteins had been expressed, transport was obviously observed with estradiol-17β-glucuronide, estron-3-sulfate, and PCG. A weak transport activity was also observed with methotrexate, ouabain, and prostaglandin E2 ( FIG. 3 ). 
     Further, to obtain the Km (Michaelis constant) value of the PCG transport mediated by the OATP-C protein, the uptake of [ 14 C]PCG added at various concentrations was measured ( FIG. 4 ). From the Lineweaver-Burk reciprocal plot of the net PCG uptake obtained in the cells wherein the OATP-C proteins are expressed, a Km value of 983±289 μM PCG was obtained with a maximal velocity Vmax of 5.45±0.63 (nmol/mg/15 min). 
     Furthermore, the effects of the addition of various β-lactam antibiotics on the PCG transport mediated by the OATP-C protein were examined ( FIG. 5 ). Remarkable inhibitory activity was observed with cefazolin, cefoperazone, cefpiramide, and nafcillin when effects of various β-lactam antibiotics were examined by adding them at a concentration of 1 mM, respectively, to the transport activity of 4 μM [ 14 C]PCG. A weak inhibitory activity was also seen with cefaloridine and cefalexin. These results strongly indicate that these β-lactam antibiotics can be also transported by the OATP family proteins similarly as PCG belonging to the same β-lactam antibiotics. 
     Further, the requirement for sodium and chloride ions in the transport of estradiol-17β-glucuronide mediated by the OATP-C protein was examined ( FIG. 6 ). No alteration was observed in the transport of estradiol-17β-glucuronide either in case where the sodium ion was replaced with N-methylglucamine, or where the chloride ion was substituted with gluconate. According to these results, it has been revealed that the transport mediated by the OATP-C protein is independent to sodium ion. 
     INDUSTRIAL APPLICABILITY 
     The present invention provides novel transporter proteins and genes encoding these proteins. These proteins and genes are useful for developing drugs with a new design that can be transported by transporter proteins of this invention, and pharmaceuticals for the treatment of disorders caused by expressional and functional aberrations of these transporter proteins. Furthermore, these proteins and genes can be applied to genetic diagnosis and gene therapy. For example, by the SNP diagnosis and such of transporter genes of this invention, it is possible to design a tailor-made treatment plan, taking individual differences in efficacy of drugs into consideration.