# Patent

## Title
DNA encoding a testicular carnitine transporter

## Abstract

It is intended to identify and provide a novel carnitine transporter gene participating in carnitine transport in the testis and epididymis and carnitine transporter which is a protein encoded by the gene. A protein comprising the amino acid sequence represented by SEQ ID NO: 2 or an amino acid sequence derived therefrom by deletion, substitution or addition of one to several amino acids and being capable of transporting carnitine or its analog; and a gene encoding this protein.

## Background
This application is a Divisional Application of U.S. Ser. No. 10/479,013, filed on Nov. 25, 2003, now issued as U.S. Pat. No. 7,244,556, which was a U.S. National Phase Application of International Application No. PCT/JP02/05095, filed on May 27, 2002, which in turn claimed the prior benefit of Japanese Patent Application No. 2001-161212, filed on May 29, 2001.


TECHNICAL FIELD 
The present invention relates to a gene participating in sodium dependent transport of carnitine and its analog in the testis and the epididymis and a polypeptide encoded by the gene.
BACKGROUND ART 
Carnitine has an important function in beta-oxidation of fatty acids in every organ of a living body. Cells in every organ produce ATP (adenosine triphosphate), which is the energy source for the cells, in mitochondria owing to the beta-oxidation of fatty acids. An acyl group of the fatty acids is transported in the mitochondria through a carnitine action to be transformed into acyl Co-A, and then the acyl Co-A is beta-oxidized to generate ATP. Many carnitines are diet-induced, and, though a part thereof is biosynthesized in the liver and the brain, almost all the cells need to uptake carnitines from outside; therefore, it has been assumed that a carnitine-specific transporter exists in the cell membrane according to past physiological studies.
Studies have been conducted on the carnitine uptake by various cells by the use of experiment systems employing the removed organ perfusion method or the isolated cell membrane vesicular system. However, it has been difficult to analyze the system of carnitine transport through cell membrane in detail by the conventional methods, and there has been a demand for isolation and analysis of the transporter itself.
The rat CT1 (Sekine, T et al., Biochem. Biophis. Res. Commun., Vol. 251, pp. 586-591, 1998) has been isolated as the carnitine transporter. The hOCTN2 (Tamai, L et al., .1 Biol. Chem., Vol. 273, pp. 20378-20382, 1998) has been reported to be a human homologous gene of the CT1. The carnitine transporters are widely distributed in organs of the whole body to contribute to the carnitine uptake by various cells. They exist also in the kidney to carry out a function of reabsorption of carnitine from the renal tubules.
According to the past studies, it has been clarified that a carnitine concentration in the testis and the epididymis is the highest in the living body and reached the 2,000 times of the serum level depending on the specie of the carnitine. Since the duct of epididymis is an organ governing nutrition and maturity of sperms, the relationship between carnitine and sperm function has attracted attention.
It has been clarified that the sperms produced in the testis are premature and low in motility and lacks in fertilizability at that stage. The sperms leaving the testis move to the epididymis and subjected to various modifications therein and matured. Further, flagellar movement is necessary for the sperm fertility, which requires enormous energy. It is reasonable to consider that a large amount of carnitine is required for the production of ATP which is the energy source for the flagellar movement.
Actually, a study revealing a positive relationship between the carnitine concentration in the human semen and the number and motility of sperms has been reported. Also, in the clinical medicine, it has been reported that carnitine administration had resulted in an increase in fertilization probability of idiopathic infertility patients.
Thus, the carnitine transport specific to the testis and the epididymis has been considered to have an important function in the maturity and the fertilizability of sperms and has been studied by the experiment systems employing the removed organ perfusion method or the isolated cell membrane vesicular system. Though the existence of an active carnitine transport system has been clarified from the studies, its molecular substance has been indefinite.
Organic anion transporters OAT1 (organic anion transporter 1) (Sekine, T et al., J. Biol. Chem., Vol. 272, pp. 18526-18529, 1997), OAT2 (Sekine, T. FEBS letter, Vol. 429, pp. 179-182, 1998), OAT3 (Kusuhara, H et al., J. Biol. Chem., Vol. 274, pp. 13675-13680, 1999), and OAT4 (Cha, S. H. et al., J. Biol. Chem., Vol. 275, pp. 4507-4512, 2000), which have the central role in drug transport in the kidney, liver, brain, and placenta, have been isolated and reported by some of the inventors of the present invention. These transporters belonging to an OAT family are transporters that can transport many different organic anions in chemical structure and also transport of various anionic drugs.
Recently, an OCT (organic cation transporter) family which recognizes organic cations also has recently been identified. Since carnitine is an organic ion having both of a positive charge and a negative charge, it is assumed that the carnitine transporter is positioned between the OAT family and the OCT family from the point of molecular evolution.
In view of these facts, the present inventors have presumed that the carnitine transporter specific to the testis and the epididymis belongs to the organic ion transporter family.
DISCLOSURE OF THE INVENTION 
An object of the present invention is to identify and provide a novel carnitine transporter gene participating in carnitine transport in the testis and the epididymis and a carnitine transporter which is a polypeptide encoded by the gene.



BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows (A) a result and (B) a time dependence of an experiment on carnitine uptake by oocytes into which a cRNA of human CT2 gene was injected.
FIG. 2 shows (A) a result of a Michaelis-Menten kinetics experiment on carnitine uptake by oocytes into which the cRNA of human CT2 gene was injected and (B) a result of D-carnitine inhibition effect in L-carnitine transport by the CT2.
FIG. 3 shows (A) a result of influences of added salts and (B) a result of influences of pH of an experiment on carnitine uptake by oocytes into which the cRNA of human CT2 gene was injected.
FIG. 4 shows a result of influence of additions of carnitine analogs to a system of an experiment on carnitine uptake by oocytes into which the cRNA of human CT2 gene was injected.
FIG. 5 is a photo substituted for drawing, showing a result of analyses of CT2 gene mRNA expressions in the human organs by the northern blotting method.
FIGS. 6A-6F show the results of immunohistological analyses of CT2 protein expression in the testis and the epididymis, an experiment described in Example 1 (8): FIGS. 6A-6C are photos of testis and FIGS. 6D-6F are photos of epididymis; FIGS. 6A and 6D show testis and epididymis, respectively, which were incubated with polyclonal anti-CT2 antibody and then reacted with anti-rabbit IgG antibody labeled with peroxidase and stained by diaminobenzidine; FIGS. 6B and 6E are amplified pictures of FIGS. 6A and 6D, respectively. FIGS. 6C and 6F show testis and epididymis, respectively, which were incubated with polyclonal anti-CT2 antibody and then reacted with anti-rabbit IgG antibody labeled with peroxidase and stained by diaminobenzidine.
FIG. 7 shows a result of analyzing a structure of a human genome of CT2 gene.



BEST MODE FOR CARRYING OUT THE INVENTION 
The present invention relates to (1) a protein having an ability to transport carnitine or its analog, comprising an amino acid sequence represented by SEQ ID NO: 2 or an amino acid sequence derived from the said amino acid sequence by deletion, substitution, or addition of one to several amino acids.
Also, the present invention relates to (2) a gene encoding the above-described protein.
Further, the invention relates to (3) a gene comprising a DNA having a base sequence represented by SEQ ID NO. 1 or a DNA encoding a protein which hybridizes with the said DNA under stringent conditions and has an ability to transport carnitine and its analog in a sodium dependent manner.
Still further, the invention relates to (4) an expression vector having the above-described gene or a gene encoding a protein of the said gene.
Also, the invention relates to (5) a host cell transformed by the above-described expression vector.
Further, the invention relates to (6) a nucleotide including a partial sequence of consecutive 14 or more bases of the base sequence represented by SEQ ID NO. 1 or a complementary sequence thereof.
Still further, the invention relates to (7) a probe comprising the above-described nucleotide and detecting the gene encoding a protein having an ability to transport carnitine and its analog in a sodium dependent manner.
Also, the invention relates to (8) a method of modulating an expression of the gene encoding a protein having an ability to transport carnitine and its analog in a sodium dependent manner by using the above-described nucleotide.
Further, the invention relates to (9) an antibody to the protein defined in (1).
Still further, the invention relates to (10) a method of detecting a function of subjected materials as a substrate to the ability of the protein of transporting carnitine and its analog in a sodium dependent manner, by using the protein defined in (1).
Also, the invention relates to (11) a method of adjusting kinetics of carnitine and its analog transported by the protein defined in (1) in the testis and the epididymis, by modulating the ability of the said protein of transporting carnitine and its analog, by using the protein, its specific antibody, its promoter, or its inhibitor.
Further, the invention relates to (12) a method of adjusting an influence of carnitine and its analog transported by the protein defined in (1) on spermatids, by modulating the ability of the said protein of transporting carnitine and its analog, by using the said protein, its specific antibody, its promoter, or its inhibitor.
Still further, the invention relates to (13) a method of adjusting an influence of carnitine and its analog transported by the protein defined in (1) on a fertilizability of spermatids, by modulating the ability of the said protein of transporting carnitine and its analog, by using the protein, its specific antibody, its promoter, or its inhibitor.
Also, the invention relates to (14) a method of detecting and adjusting an influence of carnitine and its analog transported by the protein defined in (1) on a fertilizability of spermatids or fertilizability of an individual, by making the protein express in an excessive amount in a specific cell or modulating the ability of the said protein existing in the cell of transporting carnitine and its analog, by using the said protein, its specific antibody, its promoter, or its inhibitor.
Namely, the inventors have isolated the four organic anion transporters OAT1, OAT2, OAT3, and OAT4 as described above. Amino acid sequences of these organic anion transporters are about 40% homologous with each other. The inventors have investigated EST (expressed sequence tag) database based on these sequences to identify a novel cDNA fragment which is homologous with OAT1, OAT2, OAT3 and OAT4. Using the cDNA fragment, the inventors have identified a novel clone (CT2) which had not been reported according to the human testis cDNA library to accomplish the present invention.
The carnitine transporter CT2 of the present invention comprises an ability of transporting L-carnitine and its analog from one cell to another through a cell membrane, and a narrow substrate selectivity comparing other drug transporters belonging to the OAT family or the OCT family.
The protein of the present invention includes a protein having the amino acid sequence represented by SEQ ID NO: 2 and a protein having the amino acid sequence derived from the said amino acid sequence by deletion, substitution, or addition of one to several amino acids. An amount of the deletion, the substitution, or the addition of the amino acid(s) may be to such an extent that organic anion transport activity is not lost, and the number of the amino acids may generally be from 1 to about 110, preferably from 1 to about 55. Amino acid sequence of each of these proteins has a homology to the amino acid sequence represented by SEQ ID NO. 1 of generally 75%, preferably 90%.
The gene of the present invention includes a gene comprising a DNA having a base sequence represented by SEQ ID NO. 1 and a gene comprising a DNA encoding a protein which hybridizes with the DNA having the base sequence represented by SEQ ID NO. 1 under stringent conditions and has an ability to transport carnitine and its analog in a sodium dependent manner.
The hybridization under stringent conditions of the present invention may be conducted in a hybridization solution of 5×SSC (standard saline citrate) or a hybridization solution having a salt concentration equivalent to 5×SSC under a temperature condition of from 37° C. to 42° C. for about 12 hours and then preliminarily washed with a solution of 5×SSC, or a solution having a salt concentration equivalent to 5×SSC, followed by washing with a solution of 1×SSC or a solution having a salt concentration equivalent to 1×SSC. The higher stringency can be achieved by conducting the washing with a solution of 0.1×SSC or a solution having a salt concentration equivalent to 0.1×SSC.
The carnitine transporter gene and the protein of the present invention can be isolated and obtained by screening using an organ, a tissue, or a cultured cell of a suitable mammal as a gene source. The mammal includes non-human animals; such as a dog, a cow, a horse, a goat, a sheep, a monkey, a pig, a rabbit, a rat, and a mouse; and a human.
The screening and isolation of the gene may preferably be conducted by a homology screening, a PCR screening or the like.
The base sequence of the obtained cDNA may be determined by an ordinary method to analyze a translation region, whereby determining an amino acid sequence of a protein, CT2, encoded by the base sequence.
It is possible to verify that the obtained cDNA is the cDNA of a carnitine transporter, namely, the gene product encoded in the cDNA is a carnitine transporter, by the following manner, for example.
It is possible to confirm the ability to transport (uptake) carnitine into a cell by expressing a cRNA prepared from the obtained CT2cDNA by introducing the cRNA into an oocyte and measuring a substrate uptake into the cell employing an ordinary uptake experiment (Sekine, T et al., Biochem. Biophis. Res. Commun., Vol. 251, pp. 586-591, 1998) wherein carnitine is used as a substrate.
Also, it is possible to investigate a transport characteristic and a substrate specificity of the CT2 by applying a similar uptake experiment to the expressed cell.
Further, it is possible to investigate characteristics of the CT2 such as a time dependent transport, a sodium dependence, a substrate selectivity, and a pH dependence by applying a similar uptake experiment to the expressed cell.
By screening a suitable cDNA library or a genomic DNA library prepared from a different gene source using the obtained cDNA of the CT2 gene, it is possible to isolate a homologous gene, a chromogene, or the like, derived from a different tissue or a different organism.
Also, by using a synthetic primer designed based on disclosed information of the base sequence of the gene of the present invention (represented by SEQ ID NO. 1 or a part thereof), it is possible to isolate the gene from the cDNA library according to an ordinary PCR method.
DNA libraries such as the cDNA library and the genomic DNA library can be prepared by the method. For example, described in Molecular Cloning; written by Sambrook, J, Fritsh, E. F. and Maniatis, T; published by Cold Spring Harbor Laboratory Press in 1989. Alternatively, a commercially available library may be used when it is purchasable.
In order to obtain a human genomic structure of the CT2 gene, the genomic DNA library is screened using the cDNA of the obtained CT2 gene to analyze the clone obtained by the screening. Alternatively, it may be searched using a homology search program on the basis of publicly disclosed information of human genome analysis results.
The carnitine transporter (CT2) of the present invention can be produced by a gene recombinant technology using, for example, the cDNA encoding the carnitine transporter. For instance, the DNA (such as cDNA) encoding the carnitine transporter is incorporated into a suitable expression vector, such as a plasmid vector, a phage vector, and a virus vector to introduce the obtained recombinant DNA into a suitable host cell. An expression system (host vector system) for producing a polypeptide includes expression systems of bacteria, yeast, an insect cell, and a mammal cell.
Among the above expression systems, it is desirable to use the insect cell and the mammal cell for the purpose of obtaining a functional protein.
For example, when expressing the polypeptide in the mammal, the DNA encoding the carnitine transporter is inserted downstream of a suitable promoter (such as SV40 promoter, LTR promoter, and elongation 1 a promoter) in a suitable expression vector (such as a plasmid vector, a retrovirus-based vector, a papillomavirus vector, a vacciniavirus vector, and a SV40-based vector) to construct the expression vector. Then, a suitable animal cell is transformed by the obtained expression vector, and the transformant is cultured in a proper medium to produce the desired polypeptide. The host mammal cells includes a monkey COS-7 cell, a Chinese hamster CHO cell, a human Hela cell or a cell strain of a primary culture cell derived from the kidney tissue, a pig kidney-derived LLC-PK1 cell, an opossum kidney-derived OK cell, mouse-derived proximal convoluted tubules S1, S2, and S3 cells, and the like.
Examples of the cDNA encoding the carnitine transporter CT2 includes, but not limited to, the cDNA having the base sequence represented by SEQ ID NO. 1, and, also, a DNA corresponding to the amino acid sequence and encoding the polypeptide may be used as the cDNA. In this case, it is known that one amino acid is encoded by 1 to 6 kinds of codons, and the codon(s) to be used may be selected arbitrarily, but, it is desirable to design a sequence which enables a higher level of expression in view of frequency of using codon of the host used for the expression. The DNA having the designed base sequence is obtainable by a DNA chemical synthesis, a combination with a fragment of the above-described cDNA, a partial alteration of the base sequence, or the like. Artificial partial alteration and mutation introduction of the base sequence can be carried out by the site specific mutagenesis disclosed in Mark D. F et al., Proc. Natl. Acad. Sci. USA, Vol. 18, pp. 5662-5666, 1984, using a primer constituted of synthetic oligonucleotides encoding the desired alteration.
The nucleotide (oligonucleotide or polynucleotide) which hybridizes with the carnitine transporter gene of the present invention under stringent conditions can be used as the probe for detecting the carnitine transporter gene and also as an antisense oligonucleotide, a ribozyme, or a decoy for modulating the carnitine transporter expression. The nucleotide includes a nucleotide comprising a partial sequence of consecutive 14 or more bases of the base sequence represented by SEQ ID NO. 1 or a complementary sequence thereof, and, the longer sequence such as that including 20 or more bases and 30 or more bases may be used as the partial sequence in order to enhance specificity of the hybridization.
Further, by using the carnitine transporter of the present invention or its immunologically equivalent polypeptide, it is possible to obtain an antibody thereof, and the antibody can be used for carnitine transporter detection and purification. The antibody can be produced by using the carnitine transporter of the present invention, a synthetic peptide having a fragment or a partial sequence of the carnitine transporter, as an antigen. A polyclonal antibody can be produced by an ordinary method of inoculating the antibody to a host animal (such as a rat and a rabbit) and recovering an antiserum, and a monoclonal antibody can be produced by an ordinary hybridoma method or the like.
By using the protein of the present invention, it is possible to detect function of subjected materials as a substrate to the ability of the said protein transporting carnitine and its analog in a sodium dependent manner.
Further, by modulating the ability of the protein of the present invention of transporting carnitine and its analog using the protein, its specific antibody, its promoter (for example hormones such as an androgen), or its inhibitor (such as an antisense nucleotide), it is possible to adjust kinetics of carnitine and its analog transported by the said protein in the testis and the epididymis.
Furthermore, by modulating the ability of the protein of the present invention of transporting carnitine and its analog using the protein, its specific antibody, its promoter (for example hormones such as an androgen), or its inhibitor (such as an antisense nucleotide), it is possible to adjust an influence on fertilizability of the spermatids of carnitine and its analog transported by the said protein.
Still further, by modulating the ability of the protein of the present invention of transporting carnitine and its analog using the protein, its specific antibody, its promoter (for example hormones such as an androgen), or its inhibitor (such as an antisense nucleotide), it is possible to adjust an influences on fertilizability of the spermatids or an individual of carnitine and its analog transported by the said protein.
In addition, by making the protein of the present invention express in an excessive amount in a particular cell, or by modulating the ability of the said protein already existing in a cell of transporting carnitine and its analog using the protein, its specific antibody, its promoter (for example hormones such as an androgen), or its inhibitor (such as an antisense nucleotide), it is possible to detect or adjust an influence on the fertilizability of spermatids or an individual by carnitine and its analog transported by the said protein.
EXAMPLES
The present invention will hereinafter be described in more detail by examples, but it is not limited to these examples at all.
Note that operations in the following examples are performed in accordance with the method described in Molecular Cloning: written by Sambrook J, Fritsh, E. F, and Maniatis, T; published by Cold Spring Harbor Laboratory Press in 1989 or, when using a commercially available kit, in accordance with instructions thereof unless otherwise noted.
Example 1
Isolation and Analysis of Carnitine Transporter (CT2) cDNA Specific to Testis and Epididymis
Based on base sequence information of OAT1, OAT2, OAT3, OAT4, and CT1, which were isolated by some of the inventors, publicly disclosed EST database was searched. As a result of this search, a novel cDNA fragment AA778598 which is homologous with OAT 1, OAT2, OAT3, OAT4, and CT1 was obtained.
By using a probe labeled AA778598 with 32P, a human testis cDNA library prepared in advance was screened. Hybridization was conducted in a hybridization solution at 50° C. all day and night, and then a filter film was washed with 0.1×SSC (standard saline citrate)/0.1% SDS (sodium dodecyl sulfate) at 50° C. Used as the hybridization solution was a buffer solution of pH 6.5 containing 50% formamide, 5>SSC, 3× Denhard solution, 0.2% SDS, 10% dextran sulfate, 0.2 mg/ml denatured sermon sperm DNA, 2.5 mM sodium pyrophosphate, 25 mM MES, and 0.01% Antifoam B (foam inhibitor; a product of Sigma-Aldrich, Co.). A clone isolated in λZipLox was subcloned to a plasmid vector pZL1 by an in vitro excision method. As a result, a novel cDNA (CT2cDNA) having carnitine transport activity was obtained.
Determination of a base sequence of the cDNA (CT2cDNA) obtained by the above operation was performed by an automatic sequencer (product of Applied Biosystems) using a specific primer (see SEQ ID NO.1).
Next, a cRNA (RNA complementary to cDNA) was prepared in vitro from a plasmid containing the CT2cDNA by using a T7RNA polymerase (see Sekine, T., et al., J. Biol. Chem., Vol. 272, pp. 18526-18529, 1997).
The thus-obtained cRNA was injected into oocytes of a platanna in accordance with the already reported method (Sekine, T. et al., J. Biol. Chem., Vol. 272, pp. 18526-18529, 1997), and an uptake experiment by radioactive labeled L-carnitine was conducted on the oocytes. As a result, it was proved that the oocytes which expressed the CT2 exhibit 3H-L-carnitine uptake as shown in FIG. 1 A. In contrast, uptake of 14C-PAH (paraamino hippuric acid) which is a representative organic anion and uptake of 14C-TEA (tetraethyl ammonium) which is an organic cation were not observed.
Hereinafter, results of a transport function analysis, a study on substrate selectivity, an analysis of gene expression, an immunological study on the protein encoded by the gene in the testis and the epididymis, and a structure analysis of the CT2 gene in a human genome, all of which were conducted using the CT2 of the present invention, will be described in order.
(1) An experiment on time dependence of the carnitine transport by CT2 was conducted.
As a result of the experiment, the oocytes which expressed the CT2 exhibited time dependence of 3H-L-carnitine uptake as shown in FIG. 1B. From this result, it was proved that the CT2 not only combines with carnitine but also serves as a transporter transporting carnitine in cells.
(2) A Michaelis-Menten kinetics experiment on the carnitine transport by CT2 was conducted.
A concentration dependence of the carnitine transport by CT2 was studied through an investigation of changes in amounts of uptake of various concentration carnitines by CT2. The uptake experiment of radioactive labeled carnitines was performed by using oocytes into which the CT2cRNA was injected and in accordance with the above-described method. As a result (FIG. 2A), a Km value of the L-carnitine uptake was about 20.3 μM.
(3) An influence to be exerted by D-Carnitine, which is an isomer of L-carnitine, on the L-carnitine transport by CT2 was studied.
As shown in FIG. 2B, D-carnitine inhibited the L-carnitine transport by CT2 in a concentration dependent manner. A Ki value was about 30.1 μM.
(4) A sodium dependence of the CT2 in the carnitine transport was studied.
When extracellular sodium is substituted with lithium and N-methyl-D-glucamine, the carnitine transport by CT2 was decreased to reveal that the CT2 is an extracellular sodium dependent carnitine transporter (FIG. 3A). However, the uptake was not perfectly decreased when the extracellular sodium was substituted with lithium and N-methyl-D-glucamine, thereby it was revealed that CT2 is dependent on a partial extracellular sodium.
(5) A pH dependence of the CT2 in the carnitine transport was studied.
As shown in FIG. 3B, when an extracellular pH was inclined to acidity, the carnitine transport by CT2 was decreased.
(6) In order to study further on the substrate selectivity of CT2, in the 3H-carnitine uptake experiment system by oocytes into which CT2cRNA was injected, various carnitine analogs were added to the system and its influences were investigated (inhibition experiment).
The 3H-carnitine uptake experiment was performed in accordance with the above-described method using oocytes into which CT2cRNA was injected. 50 nM 3H-carnitine uptake was measured in the presence or absence of 5 μM or 50 μM of various compounds (unlabelled). As a result, various carnitine analogs (D-carnitine, acetyl L-carnitine, acetyl DL-carnitine, octanoyl L-carnitine, betaine and the like) significantly inhibited the 3H-carnitine transport by CT2 (FIG. 4). On the other hand, anionic substances and cationic substances such as paraamino hippuric acid and tetraethylammnonium did not show inhibition effect (FIG. 4). From the above results, it was revealed that the CT2 is a specific transporter of carnitine and carnitine analogs.
(7) Expressions of CT2 gene in human tissues (northern blotting) were analyzed.
The full length of the CT2cDNA was labeled with 32P-dCTP, and using it as a probe, hybridization of a filter (product of Clontech) blotted RNAs extracted from various human tissues was conducted. The hybridization was conducted overnight in a hybridization solution in which the full length of the CT2cDNA was contained, and then the filter was washed with 0.1×SSC containing 0.1% SDS at 65° C. As a result of the northern blotting (FIG. 5), a strong band was detected only in the testis.
(8) An immunological study on a protein encoded by the CT2 gene in the testis and the epididymis was conducted.
A polypeptide comprising 14 amino acid residues of a carboxyl terminal of the CT2 protein was synthesized, and it was immunized a rabbit to obtain an antiserum. A paraffin section of the human testis was subjected to a deparaffinization and then treated with 3% hydrogen peroxide solution to inactivate endogenous peroxidase. The section was incubated with an affinity purified polyclonal anti-CT2 antibody (primary antibody) for 24 hours, and then reacted with anti-rabbit IgG antibody labeled with peroxidase. Finally, by adding diaminobenzidine, which is a substrate for peroxidase, sites in which brown precipitation was obtained were observed with an optical microscope. As shown in FIG. 6, significant stains were observed in a cytoplasm of a Sertoli cell of the testis and a luminal side of an epithelial cell of the epididymis (an arrow in FIG. 6 indicates the CT2 expression site). Accordingly, the CT2 is a transporter which provides carnitine in Sertoli cells of the testis and transports carnitine from blood to the lumens of the epididymis.
(9) A structure of the CT2 gene in a human genome was analyzed.
Publicly disclosed information of human genome analysis results was searched by using a homology search program to find an exon-intron structure of the CT2 gene. As shown in FIG. 7, the CT2 gene is constituted of 10 exons, and a starting codon exists in the second exon.
INDUSTRIAL APPLICABILITY 
The carnitine transporter specific to the testis and the epididymis selectively transporting carnitine and a gene thereof of the present invention enable an in vitro study on transport of carnitine and its analog at an expression site of the said transporter as well as a prediction of kinetics of the compounds in the genital organ based on the study. Carnitine is the essential factor for producing energy required for mobility and fertilizability of the sperms, and it is considered that the invention of the said transporter will contribute to clarification of the cause of the idiopathic male infertility in future. Further, by clarifying a control factor modulating the expression of the said transporter, the invention can be applied to developments of a method of modulating the ability of transporting carnitine and a method of modulating the sperm fertilizability.

## Claims

1. An isolated DNA molecule encoding a protein having an ability to transport carnitine or its analog, wherein said protein comprises the amino acid sequence represented by SEQ ID NO:2.

2. An isolated DNA molecule having the nucleotide sequence represented by SEQ ID NO: 1.

3. The DNA molecule according to claim 2, wherein the DNA molecule is derived from a human.

4. The DNA molecule according to claim 2, wherein the DNA molecule is derived from an organ, a tissue, or a cultured cell.

5. An expression vector comprising the DNA molecule of claim 2 or a DNA molecule encoding a protein, wherein said protein comprises the amino acid sequence represented by SEQ ID NO: 2.

6. The expression vector according to claim 5, wherein the expression vector is a plasmid.

7. An isolated host cell transformed by the expression vector of claim 5.

8. A method for producing a protein comprising introducing the expression vector of claim 5 into an isolated host cell and culturing the cell to produce the protein.

9. The method of claim 8, wherein the expression vector is selected from the group consisting of a plasmid vector, a phage vector, and a virus vector.

10. The method of claim 8, wherein the isolated host cell is selected from the group consisting of a bacterial cell, a yeast cell, an insect cell, and a mammalian cell.

11. The method of claim 8, wherein the expression vector further comprises a promoter upstream of the DNA molecule.

12. The method of claim 11, wherein the promoter is selected form the group consisting of SV promoter, LTR promoter, and elongation 1α promoter.