HUMAN GLYCINE TRANSPORTER TYPE 2

Abstract: Nucleic acids and proteins deerived from sequences of the human glycine transporter type 2 are described.

Description: HUMAN GLYCINE TRANSPORTER TYPE 2 The present invention relates to nucleic acids encoding human glycine transporter type 2 (GlyT2) molecules, to proteins encoded by such nucleic acids, to methods of characterizing GlyT2-active compounds, to uses of DNA and RNA nucleotide probes directed to nucleic acids encoding the GlyT2 transporter, to uses of antisense molecules to inhibit GlyT2 expression, to uses of the GlyT2 protein to generate GlyT2-specific antibodies, and to screening methods using GlyT2-expressing cell lines, and to the field of drug discovery. BACKGROUND The termination of synaptic transmission in the central nervous system (CNS) involves either the enzymatic inactivation of neurotransmitters, or their uptake into pre-synaptic terminals or surrounding glial cells (Amara, S.G. and Kuhar, M.J., Annu. Rev. Neurosci. 16:73-93 (1998); Malandro, M.S. and Kilberg, M.S. , Annu. Rev. Biochem. 65:305-336 (1996)). High-affinity, membrane- associated transporters typically mediate the rapid removal of neurotransmitters from the synaptic cleft, with uptake across a concentration gradient being thermodynamically coupled to transmembrane ion gradients (Kanner, B.I., Curr. Opin. Cell Biol. 7:735-738 (1989)). Neurotransmitter transporters can be separated into two structurally- distinct families. One family mediates Na + -dependent excitatory amino acid uptake (Kanai, Y. and Hediger, M.A. , Nature 360:A61-A11 (1992); Pines, G., et al, Nature 560:464-466 (1993); Storck, et al, Proc. Natl. Acad. Sci. (USA) 89: 10855- 10859 (1992)). The other family contains several members involved in the Na + / Cl ' -dependent transport of a host of other neurotransmitters, including GAB A (Borden, L.A., et al, J. Biol. Chem. 267:21096-21104 (1992)); catecholamines (Pacholczyk, T., et al, Nature 550:350-354 (1991)); glycine (Kim, et al, Mol. Pharm. 45:608-17 (1994); Liu, Q.R., et al, FEBS Lett. 505: 110-114 (1992a)); proline (Fremeau, R.T., et al, Neuron S:915-926 (1992)) and taurine (Liu, Q.R., et al, Proc. Natl. Acad. Sci. (USA) 89: 12145-12149 (1992b)). The gene products for most of the latter family have recently been cloned and sequenced. These transporters demonstrate 40-50% amino acid similarity and likely exhibit structural conservation, as membrane topology analysis predicts twelve putative transmembrane regions (Guastella, J., et al, Science 249: 1303-1306 (1990); Smith, K.E., et al, Neuron 8:921-935 (1992)). The high homology within these transmembrane regions has facilitated the design of degenerate primers for the cloning of additional members of the transporter superfamily using PCR-based technologies (Borowsky, B., et al, Neuron 70:851-863 (1993); Hoffman, B.J., et al, Science 254:519-580 (1991)). Glycine is a major inhibitory neurotransmitter in the spinal cord, brainstem and retina, where it exerts its effects on the strychnine-sensitive glycine receptors (Betz, H., et al, Ann. N. Y. Acad. Sci., 207: 109-115 (1993); Betz, H., et al, Q. Rev. Biophys. 25:381-394 (1992)). In addition, glycine acts as a co-agonist with glutamate at the NMDA receptor (Kemp, J.A. and Leeson, P.D. , Trends. Pharmacol. Sci. 14:20-25 (1993); Benveniste, M., et al, J. Physiol. (London) 423:333-357 (1990)). Synaptic glycine concentrations are controlled by Na + /Cl - dependent high-affinity transporters found at nerve-terminals and glial cells (Johnston, G.A.R. and Iverson, L.L., /. Neurochem. 75: 1951-1961 (1971); Fedele E. and Foster A.C., Brain Res. 572: 154-163 (1992)). Two distinct glycine transporters, GlyTl (Smith, K.E., et al, Neuron 8:921-935 (1992); Liu, Q.R., et al, FEBS Lett. 505: 110-114 (1992a); Guastella, J., et al, Proc. Natl Acad. Sci. (USA) 59:7189-7193 (1992)) and GlyT2 (Liu, Q.R., et al, J. Biol. Chem. 265:22802-22806 (1993)), have been isolated, and share approximately 50% identity at both the nucleotide and amino acid levels. Localization of GlyTl and GlyT2 by in situ hybridization techniques reveals distinct patterns of expression in the CNS (Liu, Q.R., et al, J. Biol. Chem. 265:22802-22806 (1993); Zafra, F., et al, J. Neurosci. 75:3952-3969 (1995)). GlyTl is expressed in the hippocampal and cortical regions of the brain, as well as in the spinal cord and brainstem regions. In contrast, GlyT2 is expressed primarily in the spinal cord and cerebellum, and is absent in the hippocampal and cortical regions. Based on their patterns of expression, GlyTl is thought to co-localize with NMDA receptors, while GlyT2 expression mimics that of strychnine-sensitive glycine receptors (Jursky and Nelson, . Neurochem. 67:336-344 (1996); Liu, Q.R., et al, J. Biol. Chem. 265:22802-22806 (1993)). The GlyTl sequence has been determined for a number of species, including rat (Guastella, J., et al, Proc. Natl. Acad. Sci. (USA) 59:7189-7193 (1992)), mouse (Liu, Q.R., et al, FEBS Lett. 505: 110-114 (1992a)) and human (Kim, K-M., et al, Mol. Pharm. 45:608-17 (1994)). Three alternatively spliced forms of the human transporter have been identified (GlyTl a-c) which differ in their amino-terminal sequences (Guastella, J., et al, Proc. Natl. Acad. Sci. (USA) 59:7189-7193 (1992); Liu, Q.R., et al, FEBS Lett. 505: 110-114 (1992a); Liu, Q.R., et al, J. Biol. Chem. 265:22802-22806 (1993); Smith, K.E., et al, Neuron 5:927-935 (1992); Borowsky, B., et al, Neuron 70:851-863 (1993); Kim, K-M., et al, Mol. Pharm. 45:608-17 (1994)). The rat GlyT2 sequence has been published (Liu, Q.R., et al, J. Biol. Chem. 265:22802-22806 (1993)), and it also exhibits alternatively spliced forms (GlyT2a and GlyT2b) (Ponce, J., et al, Neurosci. Lett. 242:25-28 (1998)). Recently, two full-length clones described as ρHGT2-a and pHGT2-b of human GlyT2 have been constructed (WO98/07854; PCT/US97/ 14637). The precise regulation of synaptic glycine concentrations in the CNS is a very important process because glycine is involved in both excitatory and inhibitory neurotransmission (Betz, H., et al, Ann. N. Y. Acad. Sci. , 207: 109-115 (1993); Benveniste, M., et al, J. Physiol. (London) 428:333-351 (1990)). Glycine transporters are likely to be critical to this process. Compounds able to modulate glycine transporter function (i.e., that inhibit or activate glycine transporter) would be expected to provide a wide variety of therapeutic benefits. For example, glycine receptor inhibition is known to result in pain transmission (Yaksh, Pain. 111-123, (1989)). Therefore, compounds that inhibit GlyT2 transporter activity may increase the activity of neurons having strychnine-sensitive glycine receptors via increasing synaptic levels of glycine, thus diminishing the transmission of pain- related (i.e., nociceptive) information in the spinal cord, which has been shown to be mediated by these receptors. Further, because glycine receptor malfunction is known to play a role in muscle spasticity (Becker, FASEB J. 4:2767-2775 (1990)), compounds that inhibit GlyT2 transporter activity and lead to enhanced inhibitory glycinergic transmission through strychnine-sensitive glycine receptors in the spinal cord can be used to decrease muscle hyperactivity. Such compounds are useful in treating diseases or conditions associated with increased muscle contraction, such as muscle spasticity, myoclonus (which relates to rapid muscle spasms) and epilepsy. Spasticity that may be treated via modulation of glycine receptors is associated with epilepsy, stroke, head trauma, multiple sclerosis, spinal cord injury, dystonia, and other conditions of illness and injury of the nervous system. SUMMARY OF THE INVENTION The present invention provides novel human glycine transporter type 2 (GlyT2) molecules. Nucleic acids are provided comprising nucleic acid sequences that encode proteins that have glycine transport activity and that have one or more of the following amino acids: (1) serine at a position corresponding to amino acid 24, (2) tryptophan at a position corresponding to amino acid 74, (3) glycine at a position corresponding to amino acid 155, (4) aspartic acid at a position corresponding to amino acid 188, (5) leucine at a position corresponding to amino acid 362, (6) alanine at a position corresponding to amino acid 431, or (7) serine at a position corresponding to amino acid 582, of SEQ ID NO: 124. A preferred GlyT2 sequence comprises the amino acid sequence of SEQ ID NO: 120. The present invention provides nucleic acids wherein the nucleic acid sequence is that of bases 1 to 2391 of SEQ ID NO: 123 but having one or more of the following nucleotides: (1) A at position 70, (2) T at position 220, (3) G at position 463, (4) G at position 562, (5) T at position 1085, (6) C at position 1292, (7) A at position 1299, (8) T at position 1617, or (9) T at position 1744. A preferred GlyT2 sequence comprises the nucleic acid sequence of SEQ ID NO: 119. Nucleic acids are also provided comprising nucleic acid sequences that encode proteins that have glycine transport activity, that have one or more of the seven amino acids listed above and that additionally have one or more of the following amino acids: (1) leucine at a position corresponding to amino acid 26, (2) leucine at a position corresponding to amino acid 75, (3) valine at a position corresponding to amino acid 89, (4) glycine at a position corresponding to amino acid 102, (5) glutamic acid at a position corresponding to amino acid 174, (6) proline at a position corresponding to amino acid 195, (7) glycine at a position corresponding to amino acid 199, (8) leucine at a position corresponding to amino acid 249, (9) proline at a position corresponding to amino acid 306, (10) glutamic acid at a position corresponding to amino acid 419, (11) asparagine at a position corresponding to amino acid 442, (12) lysine at a position corresponding to amino acid 455, (13) cysteine at a position corresponding to amino acid 458, (14) proline at a position corresponding to amino acid 485, (15) arginine at a position corresponding to amino acid 493, or (16) glutamic acid at a position corresponding to amino acid 650, of SEQ ID NO: 124. The present invention also provides nucleic acids wherein the nucleic acid sequence is that of bases 1 to 2391 of SEQ ID NO: 119, 121 or 123 but having one or more of the nine nucleotides listed above but additionally having one or more of the following nucleotides: (a) T at position 77, (b) T at position 220, (c) T at position 244, (d) T at position 266, (e) G at position 304, (f) A at position 521, (g) C at position 583, (h) G at position 596, (i) G at position 678, (j) C at position 681, (k) T at position 745, (1) C at position 750, (m) T at position 765, (n) C or A at position 777, (o) G at position 867, (p) C at position 917, (q) A at position 1256, (r) C at position 1292, (s) A at position 1325, (t) A at position 1364, (u) C at position 1374, (v) A at position 1392, (w) C at position 1454, (x) G at position 1478, (y) C at position 1854, (z) A at position 1949, (aa) C at position 1959, or (bb) C at position 2130. The present invention also provides nucleic acids comprising nucleic acid sequences that encode proteins that have glycine transport activity and that have one or more of the following amino acids: (1) serine at a position corresponding to amino acid 24, (2) tryptophan at a position corresponding to amino acid 74, (3) glycine at a position corresponding to amino acid 155, (4) aspartic acid at a position corresponding to amino acid 188, (5) leucine at a position corresponding to amino acid 362, (6) alanine at a position corresponding to amino acid 431, or (7) serine at a position corresponding to amino acid 582, of SEQ ID NO: 124, and that additionally have one or more of the following amino acids: (1) phenylalanine at a position corresponding to amino acid 124, (2) asparagine at a position corresponding to amino acid 279, (3) glycine at a position corresponding to amino acid 393, (4) asparagine at a position corresponding to amino acid 457, (5) asparagine at a position corresponding to amino acid 463 , (6) tyrosine at a position corresponding to amino acid 610, (7) valine at a position corresponding to amino acid 611, (8) serine at a position corresponding to amino acid 733, (9) valine at a position corresponding to amino acid 735, (10) leucine at a position corresponding to amino acid 245, (11) leucine at a position corresponding to amino acid 305, (12) isoleucine at a position corresponding to amino acid 366, or (13) proline at a position corresponding to amino acid 400, of SEQ ID NO: 124. The present invention also provides nucleic acids wherein the nucleic acid sequence is that of bases 1 to 2391 of SEQ ID NO: 123 but having one or more of the following nucleotides: (1) A at position 70, (2) T at position 220, (3) G at position 463, (4) G at position 562, (5) T at position 1085, (6) C at position 1292, (7) A at position 1299, (8) T at position 1617, or (9) T at position 1744, but additionally having one or more of the following nucleotides: (a) C at position 6, (b) T at position 371, (c) T at position 571, (d) A at position 836, (e) G at position 1116, (f) G at position 1177, (g) C at position 1371, (h) A at position 1387, (i) A at position 1829, (j) G at position 1831, (k) C at position 2198, (1) G at position 2203, (m) G at position 342, (n) C at position 733, (o) C at position 913, (p) A at position 951, (q) T at position 1097, (r) C at position 1199, (s) T at position 352, or (t) A at position 2103. The present invention also provides nucleic acids encoding glycine transporters having at least about 96% sequence identity with the protein sequence of SEQ ID NO: 122 or with a sequence corresponding to the protein sequence of SEQ ID NO: 122 except that it has one or more of the following amino acid substitutions: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Val, (5) Ser 102 to Gly, (6) Val 174 to Glu, (7) Ser 195 to Pro, (8) Asp 199 to Gly, (9) Val 249 to Leu, (10) Leu 306 to Pro, (11) Gly 419 to Glu, (12) Thr 442 to Asn, (13) Thr 455 to Lys, (14) Trp 458 to Cys, (15) Leu 485 to Pro, (16) Lys 493 to Arg, or (17) Val 650 to Glu. Preferably, the sequence identity is at least about 97%, more preferably at least about 98% , yet more preferably at least about 99%, yet more preferably at least about 99.5% . In an embodiment of the invention, the sequence identity is 100% . Preferably, the encoded glycine transporter has no more than four amino acid differences in the region from amino acid 200 to 797 of reference protein sequence, where the reference sequence is SEQ ID NO: 122 or of a sequence corresponding to the protein sequence of SEQ ID NO: 122 except that it has one of the substitutions described above. More preferably, the encoded glycine transporter has no more than two such differences. The present invention also provides vectors comprising the nucleic acids described above. In one embodiment, the vector is effective to express a glycine transporter mRNA in at least one of a eukaryotic cell or a bacterial cell. In another embodiment of the invention, the vector is effective to express the mRNA in at least one of a mammalian cell, a yeast cell or an avian cell. The invention further provides an isolated glycine transporter derived from transformed cells according to the present invention, the transporter comprising the amino acid sequence encoded by an above-described nucleic acid or one to two contiguous portions of amino acid sequence encoded by such a nucleic acid, wherein the protein has glycine transporter activity and differs in sequence from the aligned segments of the GlyT2 transporter sequences described by WO98/07854 (PCT/US97/14637) (human) or Liu, et al , J. Biol. Chem. 265:22802-22808 (1993) (rat). Contiguous sequence as used herein, refers to uninterrupted portions of the relevant reference nucleic acid or amino acid sequence. Preferably, the glycine transporter protein of the present invention differs in sequence by at least two amino acids, more preferably, at least four amino acids, from the aligned segments of GlyT2 transporter sequences described by WO98/07854 (PCT/US97/14637) (human) or Liu, et al, J. Biol Chem. 265:22802-22808 (1993) (rat). Preferably, the contiguous sequences comprise at least about 600 amino acids, more preferably at least about 700 amino acids, more preferably at least about 750 amino acids. In one embodiment, the transporter protein comprises all of the protein sequence encoded by the above-described nucleic acid. Preferably, the transporter protein comprises the amino acid sequence set forth in the protein sequence of SEQ ID NO: 122 or a sequence corresponding to the protein sequence of SEQ ID NO: 122 except that it has one or more of the following amino acid substitutions: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Val, (5) Ser 102 to Gly, (6) Val 174 to Glu, (7) Ser 195 to Pro, (8) Asp 199 to Gly, (9) Val 249 to Leu, (10) Leu 306 to Pro, (11) Gly 419 to Glu, (12) Thr 442 to Asn, (13) Thr 455 to Lys, (14) Trp 458 to Cys, (15) Leu 485 to Pro, (16) Lys 493 to Arg, or (17) Val 650 to Glu, or an amino acid sequence comprising one to two contiguous portions of these sequences. The present invention also provides a nucleic acid encoding a transporter protein having at least about 99.5% sequence identity with all or one to two contiguous portions of the amino acid sequence of SEQ ID NO: 122 or with one to two continuous portions of an amino acid sequence corresponding to the protein sequence of SEQ ID NO: 122 except that it has one or more of the following substitutions: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Val, (5) Ser 102 to Gly, (6) Val 174 to Glu, (7) Ser 195 to Pro, (8) Asp 199 to Gly, (9) Val 249 to Leu, (10) Leu 306 to Pro, (11) Gly 419 to Glu, (12) Thr 442 to Asn, (13) Thr 455 to Lys, (14) Trp 458 to Cys, (15) Leu 485 to Pro, (16) Lys 493 to Arg, or (17) Val 650 to Glu, wherein the encoded protein has glycine transporter activity. Preferably, the contiguous sequences comprise at least about 600 amino acids, more preferably at least about 700 amino acids, more preferably at least about 750 amino acids. The invention also provides a vector comprising this nucleic acid. In one embodiment, the vector is effective to express a glycine transporter mRNA in at least one of a eukaryotic cell or a prokaryotic cell such as a bacterial cell. In another embodiment of the invention, the vector is effective to express the mRNA in at least one of a yeast cell, a mammalian cell or an avian cell. The present invention additionally provides a cell comprising a first extrinsically-derived nucleic acid according to the first embodiment or a second extrinsically-derived nucleic acid encoding a transporter protein having at least about 99.5% sequence identity with one to two contiguous portions of the protein sequence of SEQ ID NO: 122 or of a sequence corresponding to the protein sequence of SEQ ID NO: 122 except that it has one or more of the following substitutions: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Val, (5) Ser 102 to Gly, (6) Val 174 to Glu, (7) Ser 195 to Pro, (8) Asp 199 to Gly, (9) Val 249 to Leu, (10) Leu 306 to Pro, (11) Gly 419 to Glu, (12) Thr 442 to Asn, (13) Thr 455 to Lys, (14) Trp 458 to Cys, (15) Leu 485 to Pro, (16) Lys 493 to Arg, or (17) Val 650 to Glu, wherein the encoded protein has glycine transporter activity. In one embodiment, the cell expresses a glycine transporter from the nucleic acid. Preferably, the nucleic acid is functionally associated with a promoter that is operative in the cell. In an embodiment of the invention, the promoter is an inducible promoter. The present invention also provides a method of producing a glycine transporter comprising growing cells as described in the previous paragraphs. This method can further comprise at least one of (a) isolating membranes from said cells, which membranes comprise the glycine transporter or (b) extracting protein fraction from the cells, which fraction comprises the glycine transporter. The present invention provides a method for characterizing a bioactive agent for treatment of a nervous system disorder or condition or for identifying bioactive agents for treatment of a nervous system disorder or condition, the method comprising (a) providing a first assay composition comprising (i) a cell as described above or (ii) an isolated glycine transporter protein comprising the amino acid sequence encoded by the first or second extrinsically-derived nucleic acids described above, (b) contacting the first assay composition with the bioactive agent or a prospective bioactive agent, and measuring the amount of glycine transport exhibited by the assay composition. Preferably, the method further comprises comparing the amount of glycine transport exhibited by the first assay composition with the amount of glycine transport exhibited by a second such assay composition that is treated the same as the first assay composition except that it is not contacted with the bioactive agent or prospective bioactive agent. The method can be used for characterizing bioactive agents where the nervous system disorder or condition is one of the group including, but not limited to, (a) pain, (b) spasticity, (c) myoclonus, (d) muscle spasm, (e) muscle hyperactivity or (f) epilepsy. In a preferred embodiment, the spasticity for which the bioactive agent is characterized is associated with stroke, head trauma, neuronal cell death, multiple sclerosis, spinal cord injury, dystonia, Huntington's disease or amyotrophic lateral sclerosis. The invention further provides a nucleic acid that hybridizes with a reference nucleic acid sequence which is SEQ ID NO: 121 or a sequence that varies from the nucleic acid sequence of SEQ ID NO: 121 by having one or more of the following substitutions: (a) C 77 →T, (b) C 220 →T, (c) C 224 →T, (d) C 266 →T, (e) A 304 →G, (f) T 521 →A, (g) T 583 →C, (h) A 596 →G, (i) A 678 →G, (j) T 681 →C, (k) G 745 →T, (1) G 750 →C, (m) C 765 →T, (n) G 777 →C, (o) A 867 →G, (p) T 917 →C, (q) G 1256 →A, (r) T 1292 →C, (s) C 1325 →A, (t) C 136 →A, (u) G 1374 →C, (v) C 1392 →A, (w) T 1454 →C, (x) A 1478 →G, (y) T 1854 →C, (z) T 1949 →A, (aa) T 1959 →C, or (bb) T 2130 →C, under conditions of sufficient stringency to exclude hybridizations with (a) the sequence for a rat GlyT2 transporter or (b) the sequence for a mammalian GlyTl transporter. Preferably, the nucleic acid sequence is at least about 18 nucleotides in length and has at least about 95% sequence identity with a sequence embedded in the reference nucleic acid sequence. Preferably the nucleic acid sequence is at least about 40 nucleotides in length, more preferably at least about 100 nucleotides in length. Preferably the nucleic acid sequence has at least about 97% sequence identity with the above-recited reference sequence, more preferably 99% sequence identity. Preferably, the nucleic acid is a PCR primer and the stringent conditions are PCR conditions effective to amplify a human GlyT2 sequence but not amplify (a) the sequence for a rat or mouse GlyT2 transporter or (b) the sequence for a mammalian GlyTl transporter. Further, the invention provides a nucleic acid of at least about 18 nucleotides in length comprising a contiguous sequence from the coding or non-coding strand of a human GlyT2 gene or cDNA, wherein the contiguous sequence has at least 1 sequence difference when compared with the rat GlyT2 gene sequence that aligns with the contiguous sequence. Preferably the nucleic acid sequence is at least about 40 nucleotides in length, more preferably at least about 100 nucleotides in length. Preferably, the contiguous sequence has at least two differences, more preferably 3 differences when compared with the rat GlyT2 gene sequence that aligns with the contiguous sequence. Still further, the invention provides an antisense molecule comprising a contiguous sequence from a coding or non-coding strand of a human gene or cDNA for GlyT2 which is effective when administered to a cell, tissue, organ or animal to reduce the expression of GlyT2 in the cell or in a cell of the tissue, organ or animal, wherein the contiguous sequence has at least 1 sequence difference when compared with the rat GlyT2 gene sequence that aligns with said contiguous sequence. Preferably, the contiguous sequence has at least two differences, more preferably 3 differences when compared with the rat GlyT2 gene sequence that aligns with the contiguous sequence. Antisense molecule is used herein to refer to a molecule designed to bind genomic DNA or mRNA to interfere in transcription or translation, including interfering with mRNA stability. Preferably, the contiguous sequence is at least about 15 nucleotides in length. Preferably, the contiguous stretch is included in the coding or non-coding strand of the nucleic acid sequence of SEQ ID NO: 121. The invention further provides an expression vector comprising such an antisense molecule. The invention also provides a method of reducing GlyT2 expression in a tissue or cell comprising applying to the tissue or cell an amount of such an antisense molecule effective to reduce GlyT2 expression. An amount of an expression vector for expressing such an antisense molecule in a tissue or cell effective to reduce GlyT2 expression is also provided. Alternatively, the invention provides a method of treating a nervous system disorder or condition comprising applying to a tissue or cell of a human patient with a nervous system disorder or condition an amount of such an antisense molecule effective to treat such nervous system disorder or condition or an amount of an expression vector for expressing such an antisense molecule in a tissue or cell effective to treat such nervous system disorder or condition. Further, the invention provides a method for detecting whether an animal has autoimmune antibodies against a glycine transporter, the method comprising contacting an antibody preparation from the animal or a body fluid from the animal with a polypeptide antigen comprising a glycine transporter or derived from the glycine transporter. Preferably, the polypeptide antigen comprises a contiguous sequence encoded by the protein sequence of SEQ ID NO: 122 or with a sequence corresponding to the protein sequence of SEQ ID NO: 122 except that it has one or more of the following substitutions: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Val, (5) Ser 102 to Gly, (6) Val 174 to Glu, (7) Ser 195 to Pro, (8) Asp 199 to Gly, (9) Val 249 to Leu, (10) Leu 306 to Pro, (11) Gly 419 to Glu, (12) Thr 442 to Asn, (13) Thr 455 to Lys, (14) Tip 458 to Cys, (15) Leu 485 to Pro, (16) Lys 493 to Arg, or (17) Val 650 to Glu. Preferably, the contiguous sequence is at least about six amino acids in length, more preferably at least about ten amino acids in length, still more preferably at least about fifteen amino acids in length. In one embodiment of the invention, the peptide antigen is selective for antibodies against either a GlyTl transporter or a GlyT2 transporter. Still further, the invention provides the use of a GlyT2 transporter protein to generate GlyT2-specific antibodies. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows the assembly of an hGlyT2 transporter cDNA sequence using six isolated fragments. These fragments were assembled using restriction sites at their termini. The fragments were assembled as follows: A was ligated to B (Ncol site), C was ligated to D (S el site), CD was ligated to E (Sacl site), CDE was ligated to AB (Pin Al site), and F was ligated to ABCDE (Pstl site). The final construct was digested with HinDlll and EcoRI and was subcloned into pcDNA3 for expression studies. Fig. 2 shows sequences of hGlyT2 variants. Fig. 2A shows the sequence of the 5'-insert hGlyT2 variant. This variant contains an alternative 5'-end as shown (hGlyT2 (5'-insert)). Regions exhibiting homology with an hGlyT2 sequence are shown underlined in bold. The first 210 bases of this sequence are unlike any in the current database of sequences. This divergent region is followed by 135 bases of sequence matching an hGlyT2 sequence between nucleotides 541-675. After this region of homology there is a 172-base insert containing in-frame stop codons (boxed residues). Following this region, the sequence exhibits homology to the remainder of an hGlyT2 sequence from nucleotide 679. The predicted start methionine for this variant is shown. Fig. 2B shows the sequence of the 3'-deletion variant. This 3'-deletion variant (hGlyT2(3'-deletion)) is shown compared to an hGlyT2 sequence. This variant is missing amino acid residues 496-509, and when expressed has a valine followed by a proline in the place of amino acid residues 510-511. This deletion is found in the intracellular loop between putative transmembrane domains TM6 and TM7. The translated sequence for an hGlyT2 sequence is shown on top, while that of the deletion is shown on the bottom. Fig. 3 shows the expression of hGlyT2 transporters in COS-7 cells. hGlyT2 transporter constructs were transfected into COS-7 cells and uptake of [ 3 H] -glycine was measured for each. Control cells transfected with green fluorescent protein (GFP) and viewed under the fluorescent microscope were used to estimate transfection efficiency. The total uptake (open bars) and Cr-free (low affinity) uptake (hatched bars) are shown for a full-length hGlyT2, a truncated hGlyT2, and variants hGlyT2/5I, hGlyT2/3D, and hGlyT2/5I-3D (see, Example 2). Uptake mediated by a full-length hGlyT2 transporter was 10 fold higher than the GFP-transfected cells. The truncated transporter was only 40% as efficient as full- length hGlyT2 at mediating high-affinity glycine uptake. Variant hGlyT2/5I demonstrated a 1.6 fold increase in uptake compared to plasmid transfected control, while the two variants containing the 3 '-deletion did not exhibit functional transporter activity (see, Example 4). Fig. 4 shows kinetics of glycine uptake into COS-7 cells. Values from plasmid-transfected controls were subtracted from corresponding transporter- transfected data. Results shown are the average of three independent experiments, where each glycine concentration was assayed in triplicate. Kinetic analyses of the transport mediated by a full-length transporter (Fig. 4A) and a truncated form (Fig. 4B) of GlyT2 are shown. The Eadie-Hofstee analysis is depicted in each inset. Fig. 5 shows the effect of sarcosine on glycine uptake by COS-7 cells transfected with an hGlyT2. The effect of increasing concentrations of sarcosine on the glycine uptake mediated by a full-length (•) and a truncated form of an hGlyT2 (■) are shown. Uptake is depicted as a percent of the uptake mediated by transporter-transfected cells without sarcosine present. The full-length and truncated transporters were insensitive to sarcosine inhibition. DETAILED DESCRIPTION OF THE INVENTION The GlyT2 transporter nucleic acid sequences which are the sequences of SEQ ID NOS: 119 and 121 or the corresponding encoded protein sequences of SEQ ID NOS: 120 and 122 are analogues of human GlyT2 transporter sequences disclosed in WO98/07854 (PCT/US97/14637) and rat GlyT2 transporter sequences disclosed in Lui, Q.R., et al, J.Biol. Chem. 265:22802-22808 (1993). The GlyT2 nucleic acid sequence of SEQ ID NO: 119 is the nucleic acid sequence of a full length human GlyT2 transporter cDNA prepared by ligation of six PCR- amplified cDNA fragments (fragments A-F of Fig. 1) spanning a full length human GlyT2 transporter. The GlyT2 nucleic acid sequence of SEQ ID NO: 121 represents the consensus nucleic acid sequence of a full length human GlyT2 transporter, wherein the consensus sequence is derived by sequence analysis of at least eight clones each of various PCR-amplified fragments (fragments A-F of Fig. 1), which span the sequence of a full length GlyT2 transporter. The additional nucleic acid sequences and corresponding encoded protein sequences set forth in SEQ ID NOS:22 to 118 represent the sequences of eight clones of each of fragments A-F. These nucleic acid sequences represent further variations from the consensus sequence and most likely reflect allelic variations occurring within the cDNAs derived from the pooled mRNA samples used to generate these clones. In total, the various nucleic acid sequences encoding the human GlyT2 transporter that have been isolated herein display the following sequence variations: 1 WO98/07854 (PCT/US97/ 14637) 2 Liu, et al, J. Biol. Chem. 268: 22802-22808 (1993) Irrespective of the source of this variation, the point variations in amino acid sequences, excepting the insertion of a stop codon, are believed not to adversely affect the functioning of GlyT2 transporter activity. The above-described variations primarily reflect variations between human individuals. The material used to generate the nucleic acid sequences described above generally comprised three pools of 69, 92 and 92 individuals as well as a cell line derived from an individual. The use of pooled source material, together with the number of conservative or silent substitutions, support the conclusion that the variations are reflective of human-derived variations. The relationship between the human nucleotide sequence of SEQ ID NO: 121 and the rat nucleotide sequence for GlyT2, and between the sequences that they encode, is set forth in the tables below. The relatedness values or sequence identity as set forth in these tables was determined using the LASERGENE computer program (DNASTAR, Inc., Madison, WI). The relationship between the human GlyT2 transporter nucleic acid sequence of SEQ ID NO: 119 and the rat nucleotide sequence for the GlyT2 transporter, and their corresponding protein sequences was determined. This human GlyT2 transporter sequence exhibited very high homology with the rat GlyT2 sequence (Liu, Q.R., et al, J. Biol. Chem. 265:22802-22806 (1993)), with the overall homology between the two species being about 88% and about 93% at the nucleotide and amino acid levels, respectively. This human GlyT2 transporter sequence and the sequences of other members of the amino acid transporter family shared approximately 50% amino acid homology. The greatest diversity between the rat and human GlyT2 proteins is within the first 190 amino acids, where only about 74% conservation is observed. Sequence identity, as known in the art, is a relationship between two or more polypeptide sequences or two or more polynucleotide sequences, as determined by comparing the sequences, particularly, as determined by the match between strings of such sequences. Sequence identity or percent identity or percent homology is readily calculated by known methods (see, e.g., Computational Molecular Biology. Lesk, A.M., ed., Oxford University Press, New York (1988)). Biocomputing: Informatics and Genome Projects. Smith, D.W., ed. , Academic Press, New York (1993); Computer Analysis of Sequence Data. Part I. Griffin A.M. and Griffin, H.G., eds., Humana Press, New Jersey (1994); Sequence Analysis in Molecular Biology, von Heinje, G. , Academic Press (1987); and Sequence Analysis Primer. Gribskov, M. and Devereux, J., eds., M Stockton Press, New York (1991)). While there exists a number of methods to measure identity between two sequences, the term is well known to skilled artisans (see, e.g., Sequence Analysis in Molecular Biology, supra; Sequence Analysis Primer. supra; and Carillo, H. and Lipman, D. , SIAM, J. Applied Math. 45: 1073 (1988)). Methods commonly employed to determine identity between sequences include, but are not limited to, those disclosed in Carillo, H. and Lipman, D., supra, or in Needleman and Wunsch, J. Mol. Biol, 45:443-445 (1970), wherein the parameters are as set in version 2 of DNASIS (Hitachi Software Engineering Co. , San Bruno, CA). Computer programs for determining identity are publicly available. Preferred computer program methods to determine identity between two sequences include, but are not limited to, LASERGENE program (DNASTAR, Inc. , Madison, WI); GCG program package (Devereux, J. , et al, Nucleic Acids Research 12(1):3S1 (1984)); BLASTP, BLASTN and FASTA (Altschul, S.F., et al, J. Mol. Biol 275:403-410 (1990)). The BLAST X program is publicly available from NCBI (blast@ncbi . nlm . nih . ov) and other sources (BLAST Manual. Altschul, S. , et al, NCBI NLM NIH, Bethesda, MD 20894; Altschul, S., et al, supra). Human GlyT2 transporter sequences described herein differ from the pHGT2-a and pHGT2-b human GlyT2 transporter sequences described in WO98/07854 (PCT/US97/ 14637). Differences are represented in the nucleic acid and amino acid sequences, respectively, as: (1) A 70 /Ser 24 , (2) T 220 /Trp 74 , (3) G 463 /Gly 155 , (4) G 562 /Asp 188 , (5) T 1085 /Leu 362 , (6) C ,292 /Ala 431 , and (7) T 1744 /Ser 582 . Differences are also represented in the allelic variations, for the following amino acids: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Val, (5) Val 174 to Glu, (6) Ser 195 to Pro, (7) Asp 199 to Gly, (8) Val 249 to Leu, (9) Leu 306 to Pro, (10) Gly 419 to Glu, (11) Thr 442 to Asn, (12) Thr 455 to Lys, (13) Trp 458 to Cys, (14) Leu 485 to Pro, (15) Lys 493 to Arg, or (16) Val 650 to Glu; and for the following nucleotides: (a) C 77 →T, (b) C 220 →T, (c) C 224 →T, (d) C 266 →T, (e) T 521 →A, (f) T 583 →C, ( g) A 596 →G, (h) A 678 →G, (i) T 681 →C, (j) G 745 →T, (k) G 750 →C, (1) C 765 →T, (m) G 777 →C, (n) A 867 →G, (o) T 917 →C, (p) G 1256 →A, (q) T 1292 →C, (r) C 1299 →A, (s) C 13 5 →A, (t) C 1364 →A, (u) G I374 →C, (v) C 1392 →A, (w) T 1454 →C, (x) A 1478 →G, (y) A 1617 →C, (z) A 1744 →T, (aa) T 1854 →C, (bb) T 1949 →A, (cc) T 1959 →C, or (dd) T 2130 →C. Nucleic Acid-encoding glvcine transporter To construct non-naturally occurring glycine transporter-encoding nucleic acids, the native sequences can be used as a starting point and modified to suit particular needs. For instance, the sequences can be mutated to incorporate useful restriction sites. See, Maniatis, et al, Molecular Cloning, a Laboratory Manual. Cold Spring Harbor Press (1989). Such restriction sites can be used to create cassettes, or regions of nucleic acid sequence that are facilely substituted using restriction enzymes and ligation reactions. The cassettes can be used to substitute synthetic sequences encoding mutated glycine transporter amino acid sequences (see, e.g., Goeddel, et al, Proc. Natl Acad. Sci. USA, 76: 106-110 (1979)). For recombinant expression purposes, codon usage preferences for the organism in which such a nucleic acid is to be expressed are advantageously considered in designing a synthetic glycine transporter-encoding nucleic acid. For example, a nucleic acid sequence incorporating prokaryotic codon preferences can be designed from a mammalian-derived sequence using a software program such as Oligo-4, available from National Biosciences, Inc. (Plymouth, MN). The nucleic acid sequence embodiments of the invention are preferably deoxyribonucleic acid sequences, preferably double-stranded deoxyribonucleic acid sequences. However, they can also be ribonucleic acid sequences. Numerous methods are known to delete sequence from or mutate nucleic acid sequences that encode a protein and to confirm the function of the proteins encoded by those deleted or mutated sequences. Accordingly, the invention also relates to a mutated or deleted version of a human nucleic acid sequence that encodes a protein that retains the ability to bind specifically to glycine and to transport glycine across a membrane. These analogs can have N-terminal, C-terminal or internal deletions, so long as GlyT2 function is retained. The remaining human GlyT2 protein sequence will preferably have no more than about 4 amino acid variations, preferably no more than 2 amino acid variations, more preferably no more than 1 amino acid variation, relative to the protein sequence of SEQ ID NO: 120 or 122 except that it has one or more of the following substitutions: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Val, (5) Ser 102 to Gly, (6) Val 174 to Glu, (7) Ser 195 to Pro, (8) Asp 199 to Gly, (9) Val 249 to Leu, (10) Leu 306 to Pro, (11) Gly 419 to Glu, (12) Thr 442 to Asn, (13) Thr 455 to Lys, (14) Trp 458 to Cys, (15) Leu 485 to Pro, (16) Lys 493 to Arg, or (17) Val 650 to Glu. More preferably, the variations are relative to the protein sequence of SEQ ID NO: 120 or 122, still more preferably SEQ ID NO: 120. In one preferred embodiment, the protein embodiments of the invention are defined relative to the protein sequence of SEQ ID NO: 120 or 122 or with a sequence corresponding to the protein sequence of SEQ ID NO: 120 or 122 except that it has one or more of the following substitutions: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Val, (5) Ser 102 to Gly, (6) Val 174 to Glu, (7) Ser 195 to Pro, (8) Asp 199 to Gly, (9) Val 249 to Leu, (10) Leu 306 to Pro, (11) Gly 419 to Glu, (12) Thr 442 to Asn, (13) Thr 455 to Lys, (14) Trp 458 to Cys, (15) Leu 485 to Pro, (16) Lys 493 to Arg, or (17) Val 650 to Glu. The point variations are preferably conservative point variations. Preferably, the analogs or variants will have at least about 96% sequence identity, preferably at least about 97% , more preferably at least about 98% , still more preferably at least about 99% , yet still more preferably at least about 99.5% to the protein sequence of SEQ ID NO: 122 or with a sequence corresponding to the protein sequence of SEQ ID NO: 122 except that it has one or more of the following substitutions: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Val, (5) Ser 102 to Gly, (6) Val 174 to Glu, (7) Ser 195 to Pro, (8) Asp 199 to Gly, (9) Val 249 to Leu, (10) Leu 306 to Pro, (11) Gly 419 to Glu, (12) Thr 442 to Asn, (13) Thr 455 to Lys, (14) Trp 458 to Cys, (15) Leu 485 to Pro, (16) Lys 493 to Arg, or (17) Val 650 to Glu. More preferably, the variations are relative to the protein sequence of SEQ ID NO: 120 or 122, still more preferably SEQ ID NO: 120. Mutational and deletional approaches can be applied to all of the nucleic acid sequences of the invention that express human GlyT2 proteins. As discussed above, conservative mutations are preferred. Such conservative mutations include mutations that switch one amino acid for another within one of the following groups: 1. Small aliphatic, nonpolar or slightly polar residues: Ala, Ser, Thr, Pro and Gly; 2. Polar, negatively charges residues and their amides: Asp, Asn, Glu and Gin; 3. Polar, positively charged residues: His, Arg and Lys; 4. Large aliphatic, nonpolar residues: Met, Leu, He, Val and Cys; and 5. Aromatic residues: Phe, Tyr and Trp. A preferred listing of conservative variations is the following: The types of variations selected may be based on the analysis of the frequencies of amino acid variations between homologous proteins of different species developed by Schulz, et al , Principles of Protein Structure. Springer-Verlag (1978) on the analyses of structure-forming potentials developed by Chou and Fasman, Biochemistry 13:211 (1974) and Adv. Enzymol. 47:45-149 (1978) and on the analysis of hydrophobicity patterns in proteins developed by Eisenberg, et al, Proc. Natl. Acad. Sci. USA 57: 140-144 (1984), Kyte & Doolittle, /. Mol Biol. 775: 105-132 (1981) and Goldman, et al, Ann. Rev. Biophys. Chem. 75:321-353 (1986). Since the identified point variations which create amino acid substitutions between the various human GlyT2 mRNAs identified herein are believed to be useful in creating functional GlyT2 molecules, proteins that incorporate all combinations of these point variations are believed to be functional. These variations are within the present invention. For the purposes of this application, a nucleic acid of the invention is purified or isolated if it has been separated from other macromolecules of the cell or tissue from which it is derived. Preferably, the composition containing the nucleic acid is at least about 10-fold enriched, with respect to nucleic acid content, over the composition of the source cells. Preferably, the nucleic acid is substantially pure, meaning purity of at least about 60% w/w with respect to other nucleic acids, more preferably about 80%, still more preferably about 90% , yet more preferably about 95 % . Hybridization Probes It will be recognized that many deletional or mutational analogs of nucleic acid sequences for a glycine transporter will be effective hybridization probes for glycine transporter-encoding nucleic acid. Accordingly, the invention relates to nucleic acid sequences that hybridize with such glycine transporter- encoding nucleic acid sequences under stringent conditions. Preferably, the nucleic acid sequence hybridizes with the nucleic acid sequence of SEQ ID NO: 121 or with a nucleic acid sequence that varies therefrom by one or more of the following substitutions: (a) C 77 →T, (b) C 220 →T, (c) C 224 →T, (d) C 266 →T, (e) A 304 →G, (f) T 521 →A, (g) T 583 →C, (h) A 596 →G, (i) A 678 →G, (j) T 681 →C, (k) G 745 →T, (1) G 750 →C, (m) C 765 →T, (n) G 777 →C, (o) A 867 →G, (p) T 917 →C, (q) G 1256 →A, (r) T 1292 →C, (s) C ,3 5 →A, (t) C 1364 →A, (u) G 1374 →C, (v) C 1392 →A, (w) T 1454 →C, (x) A 1478 →G, (y) T 1854 →C, (z) T 1949 →A, (aa) T ,959 →C, or (bb) T 2130 →C. In one embodiment, the nucleic acid (or the functional equivalent) embodiments of the invention are defined relative to the nucleic acid sequence of SEQ ID NO: 121 or with a nucleic acid sequence that varies therefrom by one or more of the following substitutions: (a) C 77 →T, (b) C 220 →T, (c) C 224 →T, (d) C 266 →T, (e) A 304 →G, (f) T 521 →A, (g) T 583 →C, (h) A 596 →G, (i) A 678 →G, (j) T 681 →C, (k) G 745 →T, (1) G 750 →C, (m) C 765 →T, (n) G 777 →C, (o) A 867 →G, (p) T 917 →C, (q) G 1256 →A, (r) T 1292 →C, (s) C 1325 →A, (t) C 1364 →A, (u) G 1374 →C, (v) C ]392 →A, (w) T 1454 →C, (x) A 1478 →G, (y) T ,854 →C, (z) T 1949 →A, (aa) T 1959 →C, or (bb) T 2130 →C. Stringent conditions refers to conditions that allow for the hybridization of substantially related nucleic acid sequences. For instance, such conditions will generally allow hybridization of sequence with at least about 85% sequence identity, preferably with at least about 90% sequence identity, more preferably with at least about 95% sequence identity. Such hybridization conditions are described by Sambrook, et al, Molecular Cloning: A Laboratory Manual. 2 nd ed., Cold Spring Harbor Press (1989). Hybridization conditions and probes can be adjusted in well-characterized ways to achieve selective hybridization of human- derived probes. Nucleic acid molecules that will hybridize to a glycine transporter- encoding nucleic acid under stringent conditions can be identified functionally, using methods outlined above, or by using for example the hybridization rules reviewed in Sambrook, et al, Molecular Cloning: A Laboratory Manual. 2 nd ed., Cold Spring Harbor Press (1989). Without limitation, examples of the uses for hybridization probes include: histochemical uses such as identifying tissues that express the human GlyT2 transporter, measuring mRNA levels, for instance to identify a sample's tissue type or to identify cells that express abnormal levels of glycine transporter, and detecting polymorphisms in the glycine transporter gene. RNA hybridization procedures are described in Maniatis, et al, Molecular Cloning, a Laboratory Manual. Cold Spring Harbor Press (1989). PCR Primers Rules for designing polymerase chain reaction (PCR) primers are now established, as reviewed by PCR Protocols. Cold Spring Harbor Press (1991). Degenerate primers, i.e. , preparations that are heterogeneous at given sequence locations, can be designed to amplify nucleic acid sequences that are highly homologous to, but not identical to, a human GlyT2 nucleic acid. Strategies are now available that allow for only one of the primers to be required to specifically hybridize with a known sequence. See, Froman, et al, Proc. Natl Acad. Sci. USA 55:8998 (1998) and Loh, et al, Science 243:211 (1989). For example, appropriate nucleic acid primers can be ligated to the nucleic acid sought to be amplified or provide the hybridization partner for one of the primers. In this way, only one of the primers need be based on the sequence of the nucleic acid sought to be amplified. PCR methods of amplifying nucleic acid will utilize at least two primers. One of these primers will be capable of hybridizing to a first strand of the nucleic acid to be amplified and of priming enzyme-driven nucleic acid synthesis in a first direction. The other will be capable of hybridizing the reciprocal sequence of the first strand (if the sequence to be amplified is single stranded, this sequence will initially be hypothetical, but will be synthesized in the first amplification cycle) and of priming nucleic acid synthesis from that strand in the direction opposite the first direction and towards the site of hybridization for the first primer. Conditions for conducting such amplifications, particularly under preferred stringent hybridization conditions, are well known. See, e.g., PCR Protocols. Cold Spring Harbor Press (1991). Vectors A suitable expression vector is capable of fostering expression of the included GlyT2 encoding DNA in a host cell, which can be eukaryotic, fungal or prokaryotic. Suitable expression vectors include pRc/CMV (Invitrogen, San Diego, CA), pRc/RDV (Invitrogen), pcDNA3 (Invitrogen), Zap Express Vector (Stratagene Cloning Systems, La Jolla, CA), pBk/CMV or pBk-RSV vectors (Stratagene), Bluescript II SK +/- Phagemid Vectors (Stratagene), LacSwitch (Stratagene), pMAM and pMAM neo (Clontech, Palo Alto, CA), pKSVIO (Pharmacia, Piscataway, NJ), pCRscript (Stratagene) and pCR2.1 (Invitrogen), among others. Useful yeast expression systems include, for example, pYEUra3 (Clontech). Useful baculovirus vectors include several viral vectors from Invitrogen (San Diego, CA) such as pVL1393, pVL1392, pBluBac2, pBluBacHis A, B or C, and pbacPACό (from Clontech). Cells According to the invention, a glycine transporter is preferably expressed in a mammalian cell line, preferably a transformed or transfected cell line with an established cell culture history. The invention further provides a cell transformed or transfected with a vector or nucleic acid sequence encoding a human GlyT2 transporter as described above. In one embodiment, the cell is a bacterial or eukaryotic cell. Preferably, the cell is a mammalian cell including COS-1, COS-7, LM(tk " ), HeLa, HEK293, CHO, Rat-1 and NIH3T3. More preferably, the cell is a cell from the COS-7 cell line. Still other cells that can be used include insect cells such as drosophila cells, fish cells, amphibian cells and reptilian cells. According to the present invention, a cell is provided comprising an extrinsically-derived nucleic acid encoding a GlyT2 transporter protein that has glycine transport activity. Extrinsically-derived nucleic acids are nucleic acids found in a cell that were introduced into the cell, a parent or ancestor of the cell, or a transgenic animal from which the cell is derived through a recombinant technology. Also contemplated by the invention is an extrinsically-derived nucleic acid that does not encode a GlyT2 transporter protein but rather encodes sequences, including targeting, regulatory, exon and splice-donor sequences, that when such sequences are introduced via a DNA construct into a cell not normally expressing a GlyT2 transporter protein, a GlyT2 transporter protein from a gene endogenous to the cell is expressed (see, e.g. , U.S. Patent Nos. 5,733,761; 5,733,746; 5,641,670). Human GlvT2 Transporter The invention further provides for a purified or isolated human GlyT2 transporter encoded by any of the nucleic acids of the invention purified or isolated to at least about 80% with respect to proteins, more preferably at least about 90% , most preferably at least about 95 % . These purities may be achieved by a variety of protein purification methods including, but not limited to, those described below, with a lysate of a recombinant cell according to the invention. The human GlyT2 variants of the above paragraphs can be used to create organisms or cells that produce human GlyT2 activity. Purification methods, including associated molecular biology methods, are described below. Method of Producing Glycine Transporter One simplified method of isolating polypeptides synthesized by an organism under the direction of one of the nucleic acids of the invention is to recombinantly express a fusion protein wherein the fusion partner is facilely affinity purified. For instance, the fusion partner can be glutathione S-transferase, which is encoded on commercial expression vectors (e.g. , vector pGEX4T3, available from Pharmacia, Piscataway, NJ). The fusion protein can then be purified on a glutathione affinity column (for instance, that available from Pharmacia, Piscataway, New Jersey). Additional fusion partners are available for example in various expression vectors sold by Invitrogen (Carlsbad, CA). Of course, the recombinant polypeptides can be affinity purified without such a fusion partner using an appropriate antibody that binds to GlyT2. Methods of producing such antibodies are available to those of ordinary skill in light of the ample description herein of GlyT2 expression systems and known antibody production methods (see, e.g. , Ausubel, et al , Short Protocols in Molecular Biology. John Wiley & Sons, New York (1992)). If fusion proteins are used, the fusion partner can be removed by partial proteolytic digestion approaches that preferentially attack unstructured regions such as the linkers between the fusion partner and GlyT2. The linkers can be designed to lack structure, for instance using the rules for secondary structure forming potential developed, for instance, by Chou and Fasman. Biochemistry 73:211 (1974) and Chou and Fasman, Adv. In Enzymol 47:45-147 (1978). The linker can also be designed to incorporate protease target amino acids, such as, arginine and lysine residues, the amino acids that define the sites cleaved by trypsin, or such as a target sequence for enterokinase, for example AspAspAspAspLys, which is cleaved after the lysine residue. To create the linkers, standard synthetic approaches for making oligonucleotides can be employed together with standard subcloning methodologies. Other fusion partners besides GST can be used. Procedures that utilize eukaryotic cells, particularly mammalian cells, are preferred since these cells will post-translationally modify the protein to create molecules highly similar to or functionally identical to native proteins. Additional purification techniques can be applied, including without limitation, preparative electrophoresis, FPLC (Pharmacia, Uppsala, Sweden), HPLC (e.g., using gel filtration, reverse-phase or mildly hydrophobic columns), gel filtration, differential precipitation (for instance, salting out precipitations), ion- exchange chromatography and affinity chromatography. Because GlyT2 is a membrane protein, which by analogy to related transporter proteins is believed to have twelve transmembrane sequences, isolation methods will often utilize detergents, generally non-ionic detergents, to maintain the appropriate secondary and tertiary structure of the protein (see, e.g., Lopez- Corcuera, et al , J. Biol. Chem. 266:24809-24814 (1991)). For a description of methods for re-integrating a solubilized transporter into a membrane (see, e.g., Lopez-Corcuera, et al. , J. Biol Chem. 266:24809-24814 (1991)). The isolation of GlyT2 can comprise isolating membranes from cells that have been transformed to express GlyT2. Preferably, such cells express GlyT2 in sufficient copy number such that the amount of GlyT2 in a membrane fraction is at least about 10-fold higher than that found in comparable membranes from cells that naturally express GlyT2, more preferably the amount is at least about 100-fold higher. Preferably, the protein is substantially pure, meaning a purity of at least 60%wt/wt with respect to other proteins. For the purposes of this application, GlyT2 is purified or isolated if it has been separated from other proteins or other macromolecules of the cell or tissue from which it is derived. Preferably, the composition containing GlyT2 is at least about 10-fold enriched, preferably at least about 100-fold, with respect to protein content, over the composition of the source cells. Expression of GlyTl bv RNA Insertion It will be recognized that human GlyT2 can be expressed by the simple method of inserting mRNA into a cell. RNA for these uses can be prepared by sub-cloning the nucleic acid encoding a protein with GlyT2 activity into a vector containing a promoter for high efficiency in vitro transcription, such as a SP6 or T7 RNA polymerase promoter. RNA production from the vector can be conducted, for instance, with the method described in Ausubel, et al. , Short Protocols in Molecular Biology. John Wiley & Sons, New York (1992) pp. 10-63 to 10-65. Insertion of RNA into Xenopus-derived oocytes is described, for instance, in Liu, et al. FEBS Letters 305: 110-114 (1992) and Bannon, et al , J. Neurochem 54:706-708 (1990). Alternatively, it will be recognized that human GlyT2 can be expressed by the simple method of inserting mRNA into an in vitro translation system, which can be a membrane-containing translation system. Expression of proteins in vitro is described, for instance, in Ausubel, et al , Short Protocols in Molecular Biology. John Wiley & Sons, New York (1992) pp. 10-63 to 10-65. See, also, Guastella, et al , Science 249: 1303-1306 (1990) (in vitro expression of a transporter). The use of subcellular membranous material to produce membrane proteins in vitro is described in Walter and Blobel, Meth. Enzymol. 96:84 (1983) (for rabbit reticulocyte translation system) and Spiess and Lodish, Cell 44: 111 (1986) (for wheat germ translation system). Method of Characterizing or Identifying Agent A method for the analysis of or screening for a bioactive agent for treatment of a disease or condition associated with a nervous system disorder or condition comprises culturing separately first and second cells, wherein the first and second cells are preferably of the same species, more preferably of the same strain thereof, and comprise an exogenous nucleic acid encoding a glycine transporter as described herein. The nervous system disorders or conditions for which the agent can be used for treatment include, but are not limited to, (a) pain, (b) myoclonus, (c) muscle spasm, (d) muscle hyperactivity, (e) epilepsy or (f) spasticity such as that associated with stroke, head trauma, neuronal cell death, multiple sclerosis, spinal cord injury, dystonia, Huntington's disease or amyotrophic lateral sclerosis. In this method, the first cell is contacted with the bioactive agent or a prospective bioactive agent, which is preferably a compound, such as a peptide or an organic compound in the presence of glycine, which preferably incorporates a radioisotope, such as 3 H or 14 C. The contacted first cell is then tested for enhancement or inhibition of glycine transport into the first cell as compared to glycine transport into the second cell that was not contacted with the compound (i.e., the control cell). Such analysis or screening preferably includes activities of finding, learning, discovering, determining, identifying, or ascertaining. Alternatively, the assay can utilize a composition comprising an isolated GlyT2 transporter in place of cells. Preferably, such preparation of isolated transporter will comprise membrane or lip id bilayer, preferably in vesicles, which vesicles have an inside and an outside across which transport can be measured. See, for example, Kanner, Biochemistry 17: 1207-1211, 1978. A bioactive agent is a substance such as a chemical that can act on a cell, virus, tissue organ or organism, including but not limited to drugs (i.e. , pharmaceuticals) to create a change in the functioning of the cell, virus, organ or organism. Preferably, the organism is a mammal, more preferably a human. In a preferred embodiment of the invention, the method of identifying bioactive agents of the invention is applied to organic molecules having molecular weight of about 1500 or less. A prospective bioactive agent is a substance which is being tested by the screening method of the invention to determine if it affects glycine transport. A bioactive agent is an enhancer of glycine transport uptake if at the end of the test the amount of intracellular, intravesicle or otherwise transported glycine is greater in the agent-contacted composition than in the non-agent- contacted composition; conversely, a bioactive agent is an inhibitor of glycine transport if the amount of intracellular or intravesicle glycine is greater in the non- agent-contacted composition as compared to the other. Preferably, the difference in glycine uptake between a tested first composition and a control second composition is at least about two-fold; more preferably, the difference is at least about five-fold; most preferably, the difference is at least about ten-fold or greater. A bioactive agent that is an inhibitor or an enhancer with respect to the GlyT2 transporter may have a neutral or opposite effect with another glycine transporter, such as one of the GlyTl transporters. Preferred bioactive agents have specificity to enhance or inhibit the GlyT2 transporter and have neutral or negligible effect on other glycine transporters. Preferably, a bioactive agent has at least an order of magnitude greater potency, reflected in a concentration dependent parameter such as the IC50 value, in inhibiting or activating glycine uptake mediated by the GlyT2 transporter as compared to its effect on the second glycine transporter. More preferred agents have greater potencies of at least about 100-fold for one of the glycine transporters as compared to the other. The bioactive agent can be any compound, material, composition, mixture, or chemical, that can be presented to a glycine transporter in a form that allows for the agent to diffuse so as to contact the transporter. Such bioactive agents include but are not limited to polypeptides preferably of two up to about 25 amino acids in length, more preferably from two to about ten, yet more preferably from two to about five amino acids in length. Other suitable bioactive agents in the context of the present invention include small organic compounds, preferably of molecular weight between about 100 daltons and about 5,000 daltons, and are composed of such functionalities as alkyl, aryl, alkene, alkyne, halo, cyano and other groups, including heteroatoms or not. Such organic compounds can be carbohydrates, including simple sugars, amino or imino acids, nucleic acids, steroids, and others. The chemicals tested as prospective bioactive agents can be prepared using combinatorial chemical processes known in the art or conventional means for chemical synthesis. Preferably, bioactive agents are useful as drugs for treatment of nervous system disorders or conditions. Some compounds that inhibit GlyTl or GlyT2 mediated transport also bind to the glycine binding site on the strychnine-sensitive receptor, or to the glycine binding site on the NMDA receptor. Such binding to the strychnine- sensitive receptor can be identified by a binding assay whereby, for example, radiolabeled strychnine is placed in contact with a preparation of strychnine- sensitive receptors, such as can be prepared from a membrane fraction from spinal cord or brain stem tissue. A membrane fraction can be prepared using conventional means, including, for example, methods of homogenization and centrifugation. Such binding to the NMDA receptor can be identified by a binding assay whereby, for example, radiolabeled glycine is placed in contact with a preparation of NMDA receptors, such as can be prepared from a membrane fraction from neuronal cells or brain tissue. Grimwood, et al , Molec. Pharmacol. 47:923-930 (1992). The NMDA receptors located in such membranes are treated using mild detergent, such as about 0.1 % to about 0.5% saponin, to remove any endogenous glycine or glutamate. The ligand used in such a binding assay is radiolabeled with any detectable isotope, such as radioactive isotopes of carbon or hydrogen. Specific binding of the radiolabeled ligand is then determined by subtracting the radioactivity due to non-specific binding from that which is due to total (i.e. , specific and non-specific) binding of the radiolabeled ligand. The radioactivity due to non-specific binding is determined by measuring the amount of radiolabel associated with a strychnine-sensitive or NMDA receptor-containing membrane fraction that has been contacted with both radiolabeled ligand and a significant excess of non-radiolabeled ligand, such as 100-fold excess. The radioactivity due to total binding of the radiolabeled ligand is determined by measuring the amount of radiolabel bound to the receptor preparation in the absence of non-radiolabeled ligand. For the NMDA receptor, one can also measure binding to the glycine site on the receptor using labeled analogs of amino acids, such as, for example, dichlorokynurenic acid or L-689,560. See, for example, Grimwood, et al. , Molecular Pharmacol. 49:923-930 (1992). Functional ion-flux assays are used to measure the effect of compounds identified by the present invention in enhancing or inhibiting calcium flux (for NMDA receptor preparations) or chloride flux (for strychnine-sensitive receptor preparations). This test is performed on cell cultures that have membrane- bound NMDA receptors or strychnine-sensitive receptors and glycine transporters. Such cells include neuronal cells generally, including those of the brain stem and spinal cord, and cell lines derived therefrom, and any other cell that has been induced or transfected to express NMDA receptors or strychnine-sensitive receptors. Calcium used in such a test is commonly the 45 Ca isotope, although other calcium measuring techniques can be used as well, such as calcium-associated fluorescence, which can be fluorescence associated with a calcium chelator, and the like. Chloride used in such a test usually includes the isotope 36 C1. By whatever method the calcium or chloride is monitored, ion flux can be enhanced or inhibited as a result of the discrete addition of a bioactive agent of the present invention. An advantage of this system is that it allows one to monitor the net effect on NMDA receptor or strychnine-sensitive receptor function of a compound that interacts with both the glycine site on a receptor and on a glycine transporter. GlyT2 inhibitors that are also strychnine-sensitive receptor agonists act in the above-described indications by increasing glycine concentrations at the strychnine-sensitive receptor-expressing synapses via inhibition of the glycine transporter, and via directly enhancing strychnine-sensitive receptor activity. Glycine transporter inhibitors that are also strychnine-sensitive receptor antagonists can nonetheless retain activity in treating these indications, for example if the increase in glycine due to glycine transport inhibition prevails over the strychnine- sensitive receptor antagonism. Where the strychnine-sensitive receptor antagonist activity prevails over the effect of increased extracellular glycine resulting from inhibition of the glycine transporter, these compounds are useful in treating conditions associated with decreased muscle activity such as myasthenia gravis. As discussed above, the bioactive agents of the invention can have a number of pharmacological actions. The relative effectiveness of the compounds can be assessed in a number of ways, including the following: 1. Comparing the activity mediated through GlyTl and GlyT2 transporters. This testing identifies bioactive agents (a) that are more active against GlyTl transporters and thus more useful in treating or preventing schizophrenia, increasing cognition and enhancing memory or (b) that are more active against GlyT2 transporters and thus more useful in treating or preventing epilepsy, pain or spasticity. 2. Testing for strychnine-sensitive receptor or NMDA receptor binding. This test establishes whether there is sufficient binding at this site to warrant further examination of the pharmacological effect of such binding. 3. Testing the activity of the compounds in enhancing or diminishing ion fluxes in primary tissue culture, for example chloride ion fluxes mediated by strychnine-sensitive receptors or calcium ion fluxes mediated by NMDA receptors. A bioactive agent that increases ion flux either (a) has little or no antagonist activity at the strychnine-sensitive receptor and should not affect the potentiation of glycine activity through GlyT2 transporter inhibition or (b), if marked increases are observed over results with comparative GlyT2 inhibitors that have little direct interaction with strychnine-sensitive receptors, then the agent is a receptor agonist. In some cases, the agent analysis method of the invention will be used to characterize whether a bioactive agent is useful in treating an indication in which NMDA receptors and GlyTl transporters are implicated. In this case, generally, a lower measure of activity with respect to strychnine-sensitive receptors and GlyT2 transporters is more desirable. Antisense Therapies One aspect of the present invention is directed to the use of antisense nucleic acid to treat neurological indications such as those identified above, including, but not limited to, muscle spasticity, stroke, epilepsy and multiple sclerosis. Antisense therapy involves the use of an antisense molecule designed to bind mRNA coding for a GlyT2, which will stop or inhibit the translation of the mRNA, or to bind to the GlyT2 gene to interfere with its transcription. Methods for designing nucleotide sequences that bind genomic DNA to interfere with transcription are known in the art (see, e.g., Helene, Anti-Cancer Drug Design 6:569 (1991)). Once the sequence of the mRNA sought to be bound is known, an antisense molecule is designed that binds to the sense strand by the Watson-Crick base-pairing rules to form a duplex structure analogous to the DNA double helix. Gene Regulation: Biology of Antisense RNA and DNA. Erikson and Ixzant, eds., Raven Press, New York (1991); Helene, Anti-Cancer Drug Design 6:569 (1991); Crooke, Anti-Cancer Drug Design 6:609 (1991). Efficient introduction of antisense molecules into cells so as to effectively interfere with the translation of the targeted mRNA or the function of DNA remains a significant barrier to successful antisense therapy. One method that has been employed to overcome this problem is to covalently modify the 5' or the 3' end of the antisense polynucleic acid molecule with hydrophobic substituents. Such nucleic acids modified with hydrophobic substituents generally gain access to the cells interior with greater efficiency (see, e.g., Boutorin, et al, FEBS Lett. 23: 1382-1390 (1989) and Shea, et al, Nucleic Acids Res. 75:3777-3783 (1990)). Other methods include modifying the phosphate backbone of the antisense molecules to remove or diminish negative charge (see, e.g., Agris, et al, Biochemistry 25:6268 (1986); Cazenave and Helene in Antisense Nucleic Acids and Proteins: Fundamentals and Applications. Mol and Van der Krol, eds., p. 47 et seq. , Marcel Dekker, New York (1991)) or modifying the purine or pyrimidine bases (see, e.g., Cazenave and Helene in Antisense Nucleic Acids and Proteins: Fundamentals and Applications. Mol and Van der Krol, eds. , p. 47 et seq. , Marcel Dekker, New York (1991); Milligan, et al, in Gene Therapy For Neoplastic Diseases. Huber and Laso, eds., p. 228 et seq., New York Academy of Sciences, New York (1994)). Additional methods to overcome the cell penetration barrier include incorporating antisense polynucleic acid sequences into so-called over- expression vectors that can be inserted into the cell in low copy number, but which in the cell can direct the cellular machinery to synthesize more substantial amounts of antisense polynucleotide molecules (see, e.g. , Farhood, et al, Ann. N. Y. Acad. Sci. 716:23 (1994)). This strategy includes the use of recombinant viruses that have an expression site into which the antisense sequence has been incorporated (see, e.g., Boris-Lawrie and Temin, Ann. N. Y. Acad. Sci. 716:59 (1994)). Yet another method includes increasing membrane permeability by neutralizing the negative charges on antisense molecules or other nucleic acid molecules with polycations (see, e.g., Wu and Wu, Biochemistry 27:887-892 (1988) and Behr, et al, Proc. Natl. Acad. Sci. U.S.A. 56:6982-6986 (1989)). Gene therapy such as antisense therapy may involve the introduction of antisense expression vectors into host cells. For example, a DNA vector capable of directing the synthesis of a protein missing from the cell or useful to the cell or organism when expressed in greater amounts may be incorporated into one or more cell types of an organism. Methods for introducing DNA into a cell to produce a new protein or a greater amount of a protein are called transfection methods and are commonly known in the art. See, generally, Neoplastic Diseases. Huber and Lazo, eds. , New York Academy of Science, New York (1994); Fiegner, Adv. Drug Deliv. Reb. 5: 163 (1990); McLachlin, et al , Progr. Nucl. Acids. Res. Mol. Biol. 35:91 (1990); Karlsson, S. Blood 75:2481 (1991); Einerhand and Valerio, Curr. Top. Microbiol. Immunol. 777:217-235 (1992); Makdisi, et al, Prog. Liver Dis. 10: 1 (1992); Litzinger and Huang, Biochem. Biophys. Acta. 7773:201 (1992); Morsy, et al, J.A.M.A. 270:2338 (1993); Dorudi, et al, British J. Surgery, 80:566 (1993). Other general methods of incorporating nucleic acids into cells include calcium phosphate precipitation (Graham and Van der Eb, Virology 52:456 (1983)), coincubation of nucleic acid, DEAE-dextran and cells (Sompayrac and Danna, Proc. Natl Acad. Sci. 72:7575 (1981)), electroporation of cells in the presence of nucleic acid (Potter, et al , Proc. Natl. Acad. Sci. 57:7161-7165 (1984)), incorporating nucleic acid into virus coats to create transfection vehicles (Gitman. et al, Proc. Natl Acad. Sci. U.S.A. 52:7309-7313 (1985)) and incubating cells with nucleic acids incorporated into liposomes (Wang and Huang, Proc. Natl Acad. Sci. 54:7851-7855 (1987)). Another approach to gene therapy including antisense therapy is to incorporate the nucleic acid of interest into a virus, such as a herpes virus, adenovirus, parvovirus or a retro virus for later infection into the host cells. See, e.g. , Alki, et al, Nature Genetics 3:224 (1993). Nucleic acid compositions of the invention can be, for example, administered orally, topically, rectally, nasally, vaginally, by inhalation, for example by use of an aerosol, or parenterally, e.g., intramuscularly, subcutaneously, intraperitoneally, intraventricularly, or intravenously. The nucleic acid compositions can be administered alone, or they can be combined with a pharmaceutically-acceptable carrier or excipient according to standard pharmaceutical practice. For the oral mode of administration, the nucleic acid compositions can be used in the form of tablets, capsules, lozenges, troches, powders, syrups, elixers, aqueous solutions and suspensions, and the like. In the case of tablets, carriers that can be used include lactose, sodium citrate and salts of phosphoric acid. Various disintegrants such as starch, and lubricating agents such as magnesium stearate, sodium lauryl sulfate and talc, are commonly used in tablets. For oral administration in capsule form, useful diluents are lactose and high molecular weight polyethylene glycols. When aqueous suspensions are required for oral use, the nucleic acid compositions can be combined with emulsifying and suspending agents. If desired, certain sweetening and/or flavoring agents can be added. For parenteral administration, sterile solutions of the conjugate are usually prepared, and the pH of the solutions are suitably adjusted and buffered. For intravenous use, the total concentration of solutes should be controlled to render the preparation isotonic. For ocular administration, ointments or droppable liquids may be delivered by ocular delivery systems known to the art such as applicators or eye droppers. Such compositions can include mucomimetics such as hyaluronic acid, chondroitin sulfate, hydroxypropyl methylcellulose or poly (vinyl alcohol), preservatives such as sorbic acid, EDTA or benzylchromium chloride, and the usual quantities of diluents and/or carriers. For pulmonary administration, diluents and/or carriers will be selected to be appropriate to allow the formation of an aerosol. Generally, the nucleic acid compositions will be administered in an effective amount (e.g. , prophylactically or therapeutically effective amount). For pharmaceutical uses, an effective amount is an amount effective to either (1) prevent, delay or reduce the symptoms of the indication sought to be treated or (2) induce a pharmacological change relevant to treating or preventing the indication sought to be treated. For viral gene therapy vectors, antisense oligonucleotide dosages will generally be from about 1 μg to about 100 mg of nucleic acid per kg of body mass. Autoimmune Disorders Autoimmune disorders whereby antibodies are produced against glycine transporters can be expected to be associated with disease states. For example, for the GlyT2 transporters such disorders can be expected to be associated with decreased muscle activity, for instance, decreased muscle activity that clinically presents much like myasthenia-gravis, or to be associated with decreased pain perception. See, for an example of a disease caused by autoantibodies to a molecule involved in neurotransmission (glutamic acid decarboxylase), Nathan, et al, Neurosci. Res. 40: 134-137 (1995). The presence of these antibodies can be measured by established immunological methods using protein sequences obtained from the nucleic acids described herein or the related glycine transporters reported elsewhere (see, e.g., Kim, et al, Mol. Pharmacol. 45:608-617 (1994) and Liu, et al, J. Biol. Chem. 265:22802-22808 (1992)). Such immunological methods are described, for example, in Ausubel, et al, Short Protocols in Molecular Biology. John Wiley & Sons, New York (1992). Use of GlvT2 Transporter Protein to Generate GlvT2-specific Antibodies For the purpose of the present invention, an immunoglobulin, such as an antibody, is specific, reactive with or binds to an antigen if it binds or is capable of binding a human GlyT2 transporter as determined by standard antibody- antigen or ligand-receptor assays. Examples of such assays include competitive assays, immunocytochemistry assays, saturation assays, or standard immunoassays such as ELISA, RIA, and flow cytometric assays. This definition of specificity also applies to single heavy and/or light chains, CDRs, fusion proteins, or fragments of heavy and/or light chains, which bind human GlyT2 transporter alone or are capable of binding human GlyT2 transporter if properly incorporated into immunoglobulin conformation with complementary variable regions and constant regions as appropriate. Polyclonal antibodies against a human GlyT2 transporter can be prepared by immunizing an animal with a human GlyT2 transporter or an immunogenic portion thereof. Means for immunizing animals for the production of antibodies are well known in the art. By way of example, a mammal can be injected with an inoculum that includes GlyT2. GlyT2 can be included in an inoculum alone or conjugated to a carrier protein such as keyhole limpet hemocyanin (KLH). GlyT2 can be suspended, as is well known in the art, in an adjuvant to enhance its immunogenicity. Sera containing immunologically active antibodies are then produced from the blood of such immunized animals using standard procedures well known in the art. The identification of antibodies that immunoreact specifically with GlyT2 is made by exposing sera suspected of containing such antibodies to GlyT2 to form a conjugate between antibodies and hGlyT2. The existence of the conjugate is then determined using standard procedures well known in the art. GlyT2 can also be used to prepare monoclonal antibodies against GlyT2 and used as a screening assay to identify such monoclonal antibodies. Monoclonal antibodies are produced from hybridomas prepared in accordance with standard techniques such as that described by Kohler, et al, Nature, 256:495, (1975). Briefly, a suitable mammal, e.g., BALB/c mouse, is immunized by injection with GlyT2. After a predetermined period of time, splenocytes are removed from the mouse and suspended in a cell culture medium. The splenocytes are then fused with an immortal cell line to form a hybridoma. The formed hybridomas are grown in cell culture and screened for their ability to produce a monoclonal antibody against GlyT2. Immunoglobulins of the present invention may be monoclonal antibodies (hereinafter referred to as MoAbs) of the IgM or IgG isotype of murine, human or other mammalian origin. Most preferably, such a MoAb is reactive with the human GlyT2 transporter protein. A variety of methods for producing human- engineered MoAbs are known in the art (see, e.g., Antibody Engineering. (2 nd Edition), Borrebaeck, C.A.K. , ed., Oxford University Press, New York (1995)) Other forms of immunoglobulins may be produced by methods well-known to those skilled in the art, such as by chromatographic purification of polyclonal sera to produce substantially monospecific antibody populations. Monoclonal antibodies specifically directed against human GlyT2 transporter antigen may be obtained by using combinations of immunogens and screening antigens which have only the human GlyT2 transporter antigen in common or by a screening assay designed to be specific for only anti-hGlyT2 transporter monoclonals. For example, production of monoclonal antibodies directed against human GlyT2 transporter may be accomplished by (1) immunization with human GlyT2 transporter antigen followed by screening of the resultant hybridomas for reactivity against a non-human cell line transfected with human GlyT2 transporter (constructed in a manner similar to that described in Nishimura, et al, Eur. J. Immunol, 75:747-753 (1988); (2) immunization with a non-human cell line transfected with human GlyT2 transporter followed by screening of the resultant hybridomas for reactivity against a human cell line expressing the human GlyT2 transporter antigen; (3) immunization with human or non-human cell lines expressing human hGlyT2 transporter followed by screening of the resultant hybridomas for ability to block reactivity of existing anti- hGlyT2 transporter monoclonals with a human cell line; (4) immunization with human or non-human cell lines expressing human GlyT2 transporter followed by screening of the resultant hybridomas for reactivity with purified native or recombinant human GlyT2 transporter antigen; or (5) immunization with a recombinant derivative of the human GlyT2 transporter antigen followed by screening of the resultant hybridomas for reactivity against a human cell line expressing human GlyT2 transporter. Monoclonal antibodies directed against GlyT2 may also be screened for reactivity or lack of reactivity to human GlyTl transporter and/or rat GlyT2 transporter. The generation of human MoAbs to a human antigen is also known in the art (see, e.g., Koda, et al, Hum. Antibod. Hybridomas l(l): 15-22 (1990)). Generation of such MoAbs may be difficult with conventional techniques. Thus, it may be desirable to modify the antigen binding regions of the non-human antibodies, e.g. , the F(ab')2 or hypervariable regions (CDRs), and fuse them to human constant regions (Fc) or framework regions by recombinant DNA techniques to produce substantially human molecules. Alternatively, one may isolate DNA sequences which encode a human MoAb or portions thereof which specifically bind to a human GlyT2 transporter by screening a DNA library from human B cells according to the general protocols outlined by Huse, et al, Science 246: 1275-1281 (1989) and Marks, et al, J. Mol. Biol. 222:581-597 (1991), and then cloning and amplifying the sequences which encode the antibody (or binding fragment) of the desired specificity. In addition to the immunoglobulins specifically described herein, other substantially homologous modified immunoglobulins may be readily designed and manufactured utilizing various recombinant DNA techniques known to those skilled in the art. Modifications of the immunoglobulin genes may be readily accomplished by a variety of well-known techniques, such as site-directed mutagenesis (see, Giliman, et al, Gene 5:81-97 (1979); Roberts, et al, Nature 325:731-734 (1987)). Also, modifications which affect the binding affinity antibody may be selected using the general protocol outlined by Marks, et al, J. Biol. Chem., 267: 16007-16010 (1992). Use of GlyT2-specific Antibodies for Immunohistochemical and Diagnostic Purposes The invention also provides for the use of the human GlyT2-specific antibodies generated above for a variety of immunohistochemical and diagnostic purposes, including but not limited to competitive and noncompetitive assay systems, RIA, ELISA, flow cytometry, immunoblotting and simple immunocytochemical staining. In some competition assays, the ability of an immunoglobulin, antibody, or peptide fragment to bind an antigen is determined by detecting the ability of the immunoglobulin, antibody, or peptide to compete with a compound known to bind the antigen. Numerous types of competitive assays are known and are discussed herein. Alternatively, assays which measure binding of a test compound in the absence of an inhibitor may also be used. For instance, the ability of a molecule or other compound to bind human GlyT2 transporter may be detected by labeling the molecule of interest directly, or it may be unlabelled and detected indirectly using various sandwich assay formats. Numerous types at binding assays such as competitive binding assays are known (see, e.g., U.S. Patent No. 3,376,110; U.S. Patent No. 4,016,043; and Harlow, et al, Antibodies: A Laboratory Manual. Cold Spring Harbor Publications, N.Y. (1998)). Assays for measuring binding of a test compound to one component alone rather than using a competition assay are also available. For instance, immunoglobulins may be used to identify the presence of a human GlyT2 transporter. Standard procedures for monoclonal antibody assays, such as ELISA, may be used. For a review of various signal producing systems which may be used, see, U.S. Patent No. 4,391,904. Other assay formats may involve the detection of the presence or absence of various physiological or chemical changes which result from an antigen-antibody interaction (see, Receptor-Effector Coupling-A Practical Approach. Hulme, ed. , IRL Press, Oxford (1990)). EXAMPLE 1 Human Glycine Type 2 Transporter (hGlyT2) Cloning A. First Strand Synthesis and PCR of hGlvT2 To create the cDNA used in cloning hGlyT2, first strand synthesis was accomplished using the alternate protocol (for GC rich templates) described in the 5'-RACE system (Gibco/Life Technologies). Briefly, human spinal cord poly- A + mRNA (50 ng) (Clontech, Palo Alto, CA) was added to 3 μg of random primers (Gibco BRL) and the resulting mixture was denatured for 10 min. at 70 °C. After the addition of Superscript II™ Reverse Transcriptase, reactions were allowed to proceed at room temperature for 5 min. , followed by incubation at 45 °C for 50 min. PCR amplification of the cDNA reaction was accomplished using the Expand™ High Fidelity PCR system (Boehringer Mannheim Biochemicals). This system contains a mixture of TAQ and Pwo DNA polymerases, the latter of which possesses a proof-reading function to minimize any PCR-induced sequence errors. Amplification occurred under the following conditions: 94 °C, 2.00 min. (pre- dwell); [94 °C, 40 sec, 58 °C, 40 sec , 72 °C, 40 se ] for 10 cycles; [94 °C, 40 sec, 58 °C, 40 sec, 72 °C, 40 sec. (+ 20 sec./ cycle)] for 25 cycles; 72 °C, 10 min. ; 4 °C hold. PCR products were typically subcloned into the vector p-AMP-1 using the uracil degycosylase CLONEAMP™ system. All sequences were verified by cycle sequencing (Epicentre Technologies, Madison, WI) using vector-derived SP6 (forward) and T7 (reverse) primers. Primers used in sequencing were end- labeled with T4 kinase (New England Biolabs, Beverly, MA) using γ- 2 P-dATP from NEN Research Products/Dupont (Boston, MA). The full-length clones were sequenced using the SP6 and T7 primers in addition to ten hGlyT2-specific primers spaced evenly across the sequence. B. Cloning of the hGlvT2 Region TM2 to TM10 Because transmembrane regions tend to exhibit the greatest conservation between different species at the amino acid level, degenerate primers (Genemed Biotechnologies, South San Francisco, CA) were designed based on rat GlyT2 sequence in the putative transmembrane (TM) regions 2, 5 and 10. The nucleotide sequences of all of the primers (A-U corresponding to SEQ ID NOSJ- 21) used in PCR cloning are shown as follows: Primer A (Sense): 5'- CUACUACUACUAGGCGCCTTCCTSATCC CNTAYYTSATGATG- 3' (SEQ ID NOJ; based on nt 892-921 of rat sequence) Primer B (Anti-sense): 5'- CAUCAUCAUCAUCACGTAGGGGAAS GTGGCSGTRAARTASAC- 3' (SEQ ID NO:2; based on nt 1506- 1477 of rat sequence) Primer C (Sense): 5' -CUACUACUACUAGTGTACTTCACGGCCAC STTYCCNTAYG- 3' (SEQ ID NO:3; based on nt 1477-1504 of rat sequence) Primer D (Anti-sense): 5' - CAUCAUCAUCAUGGCTGGAABCCRA TCATCATYTCDATRTC - 3' (SEQ ID NO :4; based on nt 2231- 2203 of rat sequence) Primer E (Sense): 5' - CUACUACUACUACTCGAGGGGATCTCCT ATGTGTATGGC - 3' (SEQ ID NO:5; based on nt 2158-2184 of rat sequence) Primer F (Anti-sense): 5' - CAUCAUCAUCAUAGATCTAGCACTG GGTGCCCAGTTCSARRTCYTTSACNGG -3' (SEQ ID NO:6; based on nt 2609-2571 of rat sequence) Primer G (Anti-sense): 5' - CAUCAUCAUCAUCACAGAACACCGG TCCCTGGCTGGC -3' (SEQ ID NO:7; based on nt 999-976 of rat sequence) Primer H (Sense): 5' - CUACUACUACUAACTGGTCCAGCAAACT GGACTTCATCCTGT -3' (SEQ ID NO: 8; based on nt 791-820 of rat sequence) Primer I (Anti-sense): 5' - CAUCAUCAUCAUGATGATCAGCATCG CGATGCC - 3' (SEQ ID NO:9; based on nt 1054-1030 of rat sequence) Primer J (Anti-sense): 5' - CAGAGATGATCAGCATCGCG - 3' (SEQ ID NO: 10; based on nt 1054-1030 of rat sequence) Primer K (Anti-sense): 5' - GGTAATCCAGCCAGAGCCAG -3' (SEQ ID NO: 11 ; based on nt 941-922of rat sequence) Primer L (Anti-sense): 5' - CAUCAUCAUCAUGCCTTATCCTCATC CCCTTGCTCGTCCTC - 3' (SEQ ID NO: 12; based on nt 782-754 of rat sequence) Primer M (Sense): 5' - CUACUACUACUACTGTTGTCCGCATCAG ACATG -3' (SEQ ID NO: 13; based on nt 189-210 of rat sequence) Primer N (Anti-sense): 5' - CAUCAUCAUCAUAGGCCTGCGCGCT TTGCGCCTCTCT -3' (SEQ ID NO: 14; based on nt 529-505 of rat sequence) Primer O (Sense): 5' - CUACUACUACUAATTTAGGTGACACTA TAG -3' (SEQ ID NO: 15) Primer P (Anti-sense): 5' - TTTGCTGGACCAGTTCCCTCGGGCC TTATCCTCATCCCCTTG - 3' (SEQ ID NO: 16; based on nt 804- 763 of rat sequence) Primer Q (Anti-sense): 5' -AUCAUCAUCAUCCATGGACAGGATG AAGTCCAGTTTGCTGGACCAGTTCCCTC-3' (SEQ ID NO: 17; based on nt 785-726 of rat sequence) Primer R (Anti-sense): 5' - GCCTTCCACACAGACACCGGTCCCT GGCTGGCAAACTGGCCC - 3' (SEQ ID NO: 18; based on nt 1007-965 of rat sequence) Primer S (Sense): 5'-CUACUACUACUAGAGCTCGTGGGGATCTC CTATGTGTATGGC -3' (SEQ ID NO: 19; based on nt 2155-2184 of rat sequence) Primer T (Anti-sense): 5' - CAUCAUCAUCAUTAATACGACTCAC TATAGGG -3' (SEQ ID NO:20) Primer U (Sense): 5' - UACUACUACUAGTACTAGTGATCCTCCT CATCCGAGG -3' (SEQ ID NO:21; based on nt 1532-1504 of rat sequence) In the above listed primer sequences, Y refers to C and T, R refers to A and G, B refers to G, T and C and I refers to Inosine. The PCR isolation of the first fragments of hGlyT2 was accomplished using two sets of primers encoding the rat peptide sequence (Liu, Q.R., et al, J. Biol. Chem. 265:22802-22806 (1993)) that were degenerate at the nucleic acid level. The region between TM2 and TM5 was amplified using primers A and B. Likewise, the region between TM5 and TM10 was amplified using primers C and D. After obtaining the partial sequence of the human GlyT2 transporter, the gene-specific primer E and primer F (a degenerate 3 '-primer against the final 40 bases of the rat GlyT2 coding sequence) were used to amplify the 3'-end of the hGlyT2 coding sequence. C. Cloning of the 5'-end of hGlvT2 A primer which encoded the sequence upstream of TM1 (primer H) was used with primer G to PCR-amplify the human sequence corresponding to rat GlyT2 bases 791-999. The additional 100 bases allowed the sequence 5' to TM1 to be used for gene-specific primer design. The Gibco/ BRL 5'-RACE System was used to clone the additional sequence at the 5'-end of the transporter sequence. A non-coding, gene-specific primer 3' to TM2 (primer I) was used in the reverse transcription of human spinal cord poly A + mRNA. The cDNA product was then tailed with poly-dC nucleotides using terminal transferase. The first round of PCR amplification used an additional non-coding primer immediately 5' to primer I (primer J) and the kit supplied primer (AAP). Using the product of the first round of PCR as a template, two additional nested amplifications were performed. The gene-specific primer K and primer AUAP (Gibco/BRL) (Round 2), and then primers L and UAP (Gibco/BRL) (Round 3), were used in PCR to yield a 5'-RACE product. The largest products from the final round of amplification contained approximately 200 fewer bases of coding sequence than rat GlyT2 transporter. The 5'-RACE products were subcloned into pAMP-1, as described previously. The entire 5'-end of the hGlyT2 coding sequence was ultimately isolated using a non-degenerate primer against bases 189-210 of the rat 5'- untranslated region (Primer M) in conjunction with primer N. The complete sequence was cloned from a sample of spinal cord poly A + mRNA which contained mRNA from a pool of 69 individuals (lot number 6120295, Clontech Laboratories Inc.). In order to obtain an accurate consensus sequence for hGlyT2, the entire sequence was re-amplified from a different lot of spinal cord mRNA (lot 7050107), which represented a pool of 92 individuals. In addition, each of the six fragments (Fig. 1) of the transporter were re-amplified in three independent PCR reactions from two additional sources of poly A+ mRNA. The first set of six fragments was cloned from a different aliquot of human spinal cord poly A+ mRNA (lot 7050107, purchased several months later), and the second set of fragments was cloned from poly A+ mRNA isolated from the astrocytoma cell line U373MG. The resulting 48 sequences (SEQ ID NOS:22/23 to 117/118) were used to generate the consensus sequence shown in SEQ ID NOJ21/122. EXAMPLE 2 Transporter Construct Assembly A detailed outline of the assembly of the different hGlyT2 fragments is shown in Fig. 1. The hGlyT2 transporter coding sequence was assembled from six different PCR product fragments (numbering corresponds to the rat GlyT2 sequence): 410-822 (A), 791-1050 (B), 892-1506 (C), 1477-2208 (D), 2157-2607 (E), 189-529 (F). These six isolated PCR fragments encode the entire hGlyT2 transporter, and were assembled using five restriction sites as shown in Fig. 1. At least eight independent clones of each fragment were sequenced to verify the hGlyT2 coding sequence [fragment: A (SEQ ID NOS:22/23 to 36/37), B (SEQ ID NOS:38/39 to 52/53), C (SEQ ID NOS:54/55 to 69/70), D (SEQ ID NOS:71/72 to 85/86), E (SEQ ID NOS:87/88 to 101/102), and F (SEQ ID NOS: 103/104 to 117/118)]. Fragment A and B were ligated together using a Ncol restriction site that was found at position 822. Because the sequences of fragments A and B did not overlap, an Ncol site was introduced into fragment A by two rounds of PCR extension using primer sets O and P, and O and Q, respectively (Fig. 1). Fragments AB and C were ligated together using a Pin Al restriction site. Although this site was found in fragment C, it had to be introduced into the AB sequence (using primer R, Fig. 1) due to an allelic variation in AB at base 782. The product of the ligation of PinAl digested AB and C was named fragment ABC. Because fragment E contained only half of the Sacl site, the first three bases were added to the 5'-end using primers S and T. Fragment D was amplified with primers U and D. Fragments D and E were digested with Sa and ligated. The resulting product (DE) was subcloned into pAMP-1. The plasmid containing ABC was digested with HinDϊll and Spel and the resulting 1100 base- pair product was ligated into H -DIII/S d-digested plasmid containing DE, creating the truncated form of hGlyT2 (nucleotides 410-2607). Finally, the 5'-end was added by combining fragments ABCDE and F (Fig. 1). Both fragments were digested with Pstl and ligated together. The ligation reaction was then PCR amplified with primers M and I, and the resulting construct containing bases 189- 1054 was subcloned back into HinDllll P AI-digested hGlyT2 (410-2607). The complete hGlyT2 transporter sequence (189-2607) and the truncated form (470- 2607) were subcloned into the EcoRI/H -DIII sites of pCDΝA3 (Invitrogen, Palo Alto, CA) for expression in mammalian cells. Because the original cDNA was derived from three different lots of pooled mRNA samples, as well as from a single cell line, numerous allelic variations were identified in the complete sequence. The variations identified were as follows: (a) C 77 →T, (b) C 220 →T, (c) C 224 →T, (d) C 266 →T, (e) A 304 →G, (f) T 521 →A, (g) T 583 →C, (h) A 596 →G, (i) A 678 →G, (j) T 681 →C, (k) G 745 →T, (1) G 750 →C, (m) C 765 →T, (n) G 777 →C or A, (o) A 867 →G, (p) T 917 →C, (q) G 1256 →A, (r) T 1292 →C, (s) C 1299 →A, (t) C 1325 →A, (u) C 1364 →A, (v) G I374 →C, (w) C 1392 →A, (x) T ,454 →C, (x) A ,478 →G, (y) A I617 →T, (z) A 1744 →T, (aa) T 1854 →C, (bb) T ,949 →A, (cc) T 1959 →C, or (dd) T 2130 →C. These allelic variants encode the following amino acid substitutions: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Nal, (5) Ser 102 to Gly, (6) Val 174 to Glu, (7) Ser 195 to Pro, (8) Asp 199 to Gly, (9) Val 249 to Leu, (10) Leu 306 to Pro, (11) Gly 419 to Glu, (12) Thr 442 to Asn, (13) Thr 455 to Lys, (14) Trp 458 to Cys, (15) Leu 485 to Pro, (16) Lys 493 to Arg, or (17) Val 650 to Glu. The human GlyT2 transporter exhibited very high homology with the rat GlyT2 sequence (Liu, Q.R., et al, J. Biol. Chem. 265:22802-22806 (1993)) (Fig. 2), with the overall homology between the two species being 88% and 93 % at the nucleotide and amino acid levels, respectively (Fig. 3). The human GlyT2 transporter and other members of the amino acid transporter family shared approximately 50% amino acid homology. The greatest diversity between the rat and human GlyT2 proteins is within the first 190 amino acids, where only 74% conservation is observed. The 5'-untranslated regions of the rat and human GlyT2 transporters are very highly conserved. Sequencing of the human construct revealed that the first 52 bases (20 5 '-untranslated and 32 coding sequence) of the human cDΝA sequence are identical to the rat sequence. This 52-base region does not exhibit homology to any other sequence currently found in Genbank, and thus may represent a regulatory sequence that is unique to GlyT2. The next 100 bases of coding sequence exhibit only 66% identity between the two species. Two splice forms of hGlyT2 were isolated during the cloning of the GlyT2 transporter (Fig. 2). The first variant was characterized by a divergent 5'- end, followed by bases 541-675 of the hGlyT2 sequence (Fig. 2A). After base 675, a 172 base-pair insert containing stop codons in all three reading frames was present. This was followed by the rest of the transporter sequence. Expression of this variant predicts a start codon at methionine-235, thus forming a transporter lacking the first 234 amino acids (hGlyT2/5I (5'-insert)). A second variant contained a 42 base-pair deletion in the region between bases 1488-1530 (hGlyT2- 3D (3 '-deletion)) (Fig. 2B). This variant lacks 14 amino acids in the intracellular loop between putative transmembrane regions TM6 and TM7. Both variants (hGlyT2/5I and hGlyT2/3D) were assembled and expressed in the same manner as the full-length sequence. In addition, a construct containing both variations (hGlyT2/5I-3D (5'-insert/3'-deletion)) was also assembled and expressed. In addition, a truncated form of hGlyT2 was constructed with nucleotides 221-2391 of the hGlyT2 sequence. Expression of this construct predicts a start codon at methionine- 154, thus forming a transporter lacking the first 153 amino acids (truncated hGlyT2). EXAMPLE 3 Expression of hGIyT2 Transporters The hGlyT2 constructs described in Example 2 above, including full- length, truncated and variant splice forms of GlyT2, were transiently expressed in COS-7 cells using the Superfect™ system (Qiagen, Santa Clarita, CA). COS-7 cells were plated the day before transfection at 20,000 cells per well. DNA for transfection was prepared using the Qiagen Midiprep™ kit, and 1 μg of each of the forms of hGlyT2 was transfected into each well of a 24-well plate. Transfection was terminated 2-2.5 hours after adding the DNA complex to the cells by changing the media. Transfection efficiencies were monitored by transiently expressing a construct containing green fluorescent protein. The number of transfectants was typically 20-50% of the total cells. Standard growth conditions were DMEM, high glucose, 10% fetal bovine serum, penicillin G (100 U/mL), streptomycin sulfate (100 μg/mL), 5% CO 2 / humid atmosphere, 37 °C. Cells were typically grown for 48 hours after transfection before use in glycine uptake assays, as described in Example 4 with assay results shown in Fig. 3. EXAMPLE 4 Glycine Uptake Glycine uptake was essentially as previously (Kim, K-M., et al, Mol. Pharm. 45:608-17 (1994)). Briefly, the cells were washed three times with either glycine uptake buffer (GUB; 5 mM Tris, pH 7.4, 7.5 mM Hepes, 120 mM NaCI, 5.4 mM KCl, 1.2 mM CaCk, 1.2 mM MgSO 4 , 1 mM L-ascorbic acid, 5 mM D-Glucose), or chloride-free uptake buffer (same as GUB with NaCI substituted with 120 mM sodium acetate and KCl and CaCk omitted). [2- 3 H]- glycine (581 GBq/ mmol; Amersham, Arlington Heights, IL) was added to either GUB or Cr-free GUB at a concentration of 2 μCi/ mL (127 nM). Uptake of [ 3 H] glycine was allowed to proceed for 15 min. at 37 °C, after which the cells were washed three times with excess ice-cold PBS. The cells were solubilized in 0.5% Triton X-100 and the radioactivity was quantified by scintillation counting. The amount of protein per well was determined by the BCA reagent assay (Pierce). For the sarcosine inhibition studies, the cells were pretreated with the desired concentration of sarcosine (in either GUB or Cl -free GUB) for 15 min. prior to uptake. All conditions were carried out in triplicate, and the uptake was expressed as pmol glycine/ mg protein/ min. When completely assembled, the hGlyT2 transporter sequence mediates high-affinity glycine transport when transiently transfected into COS-7 cells (Fig. 3). HGlyT2-transfected cells showed a 7-10 fold increase in [ 3 H] glycine uptake relative to untransfected and mock-transfected COS-7 cells. Cl ' -dependent uptake represented 80-90% of the total uptake in the GlyT2 transfection experiments (Fig. 3). An Eadie-Hofstee transformation of the glycine uptake data with the transfected cells revealed a Km= 10 ± 0.4 μM and a Vmax = 190 ± 3 pmol glycine/mg protein/ min. (Figs. 4 A and 4B). Because the Km reported for rat GlyT2 was 13 μM (Liu, Q.R., et al, J. Biol. Chem. 265:22802-22806 (1993)), the glycine affinity for the human and rat GlyT2 transporters is nearly identical. Interestingly, truncating hGlyT2 such that the initial methionine is residue 154 also yielded an active transporter which exhibited a Km of 9 ± 1 μM , and a Vma of 67 ± 3 pmol glycine/mg protein/ min (Figs. 4A and 4B). These data suggest that the deletion of the first 153 amino acids has no effect on the glycine affinity of this transporter. As described in Example 2 above, two additional variants of hGlyT2 were identified during the PCR cloning (Fig. 2). The first was found in 3 out of 45 clones isolated, and was characterized by a truncated amino terminus and a 168 base-pair insertion in the region encoding the loop between transmembrane domains one and two (Fig. 2A). Sequence analysis of the insertion revealed stop codons in all three reading frames. As expected, expression of the transporter containing this insertion resulted in an insignificant increase in glycine uptake in COS-7 cells (Fig. 3). Thus, the loss of the first transmembrane region and cytoplasmic amino terminus yields a transporter that is unable to mediate high-affinity uptake of glycine. A second splice form was also isolated in one of 50 clones, and represented a 48 base-pair deletion between putative transmembrane regions 6 and 7 (Fig. 2B). This deletion did not affect the reading frame of the transporter, and effectively caused the deletion of 16 amino acids from this intracellular loop. However, this relatively small deletion resulted in a non-functional transporter (Fig. 3). A truncated form of hGlyT2 as discussed in Example 2 above, was expressed from a construct containing nucleotides 221-2391 of the human transporter sequence (Fig. 3). This truncated transporter, which has a predicted start methionine corresponding to Met- 154 of the human transporter, mediated high-affinity glycine uptake about one third as effectively as the full-length transporter (Figs. 3 and 4). Eadie-Hofstee analysis of the uptake mediated by this receptor revealed that the affinity of this transporter for glycine was identical to the full-length transporter (Figs. 4A and 4B). The differences in uptake for the two transporter types were accounted for in the Vmax values. Since the truncated form of the transporter mediates high-affinity uptake, the amino-terminus of hGlyT2 is not likely to be directly involved in the transport of the glycine molecule. Thus, this region may play a role in either membrane targeting of the transporter or may contribute to the optimal conformation of the transporter. Alternatively, there may be interactions of the extended N-terminus with intracellular regulatory proteins. Indeed, there are many potential sites of phosphorylation in the intracellular N- terminus of GlyT2 which are not present in other members of the transporter. Interestingly, this amino-terminal truncated form of hGlyT2 is predicted to have 44 amino acids prior to the start of the first putative transmembrane region (TMl). This is similar to the predicted 33 and 43 residues prior to TMl in hGlyTl (Kim, et al , (1994)) and proline transporters (Fremeau, et al , (1992)), respectively. The additional 153 amino acids at the amino-terminal end makes GlyT2 unique in the Na + /Cl " -dependent amino-acid transporter family. A pharmacological distinction between the GlyTl and GlyT2 transporters involves sarcosine sensitivity (Liu, Q.R., et al, Proc. Natl. Acad. Sci. (USA) 59: 12145-12149 (1992b); Kim, K-M., et al, Mol. Pharm. 45:608-17 (1994); Guastella, J., et al, Proc. Natl. Acad. Sci. (USA) 59:7189-7193 (1992)). Glycine uptake mediated by GlyTl is almost completely inhibited when the sarcosine concentration exceeds 200 μM, while rat GlyT2-mediated uptake is inhibited only by 10-20% at sarcosine concentrations of 1 mM (Liu, Q.R., et al, J. Biol. Chem. 265:22802-22806 (1993)). Glycine uptake mediated by both the hGlyT2 transporter and the truncated form of the transporter was evaluated at increasing sarcosine concentrations (Fig. 5). Both forms were relatively insensitive to sarcosine, with less than 15% reduction of uptake in the presence of ImM sarcosine. The human form is therefore comparable to the rat in regards to sarcosine inhibition. The nucleic acid or amino acid sequences referred to herein by SEQ ID NO: are as follows: 1 HSpC-1 source is Clontech spinal cord poly A + mRNA, lot number 6120295, (pool size = 69, age range = 22-70 years); HSpC-2 source is Clontech spinal cord poly A + mRNA, lot number 7050107, (pool size =92, age range = 16-75 years); HSpC-3 source is Clontech spinal cord poly A + mRNA, lot number 7050107, (pool size=92, age range = 16-75 years); U373MG source is U373MG astrocytoma cell line, poly A + mRNA. While this invention has been described with an emphasis upon preferred embodiments, it will be obvious to those of ordinary skill in the art that variations in the preferred devices and methods may be used and this it is intended that the invention may be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications encompassed within the spirit and scope of the invention as defined by the claims that follow.

Claims: WHAT IS CLAIMED IS: 1. A purified nucleic acid comprising a nucleic acid sequence that encodes a glycine transporter type 2 (GlyT2) amino acid sequence that has glycine transport activity and that has a serine at a position corresponding to amino acid 24 of SEQ ID NO: 124. 2. A purified nucleic acid comprising a nucleic acid sequence that encodes a GlyT2 amino acid sequence that has glycine transport activity and that has a tryptophan at a position corresponding to amino acid 74 of SEQ ID NO: 124. 3. A purified nucleic acid comprising a nucleic acid sequence that encodes a GlyT2 amino acid sequence that has glycine transport activity and that has a glycine at a position corresponding to amino acid 155 of SEQ ID NO: 124. 4. A purified nucleic acid comprising a nucleic acid sequence that encodes a GlyT2 amino acid sequence that has glycine transport activity and that has an aspartic acid at a position corresponding to amino acid 188 of SEQ ID NO: 124. 5. A purified nucleic acid comprising a nucleic acid sequence that encodes a GlyT2 amino acid sequence that has glycine transport activity and that has a leucine at a position corresponding to amino acid 362 of SEQ ID NO: 124. 6. A purified nucleic acid comprising a nucleic acid sequence that encodes a GlyT2 amino acid sequence that has glycine transport activity and that has an alanine at a position corresponding to amino acid 431 of SEQ ID NO: 124. 7. A purified nucleic acid comprising a nucleic acid sequence that encodes a GlyT2 amino acid sequence that has glycine transport activity and that has a serine at a position corresponding to amino acid 582 of SEQ ID NO: 124. 8. A purified nucleic acid comprising a nucleic acid sequence that encodes a GlyT2 amino acid sequence that has glycine transport activity and that has one or more of the following amino acids: (1) serine at a position corresponding to amino acid 24, (2) tryptophan at a position corresponding to amino acid 74, (3) glycine at a position corresponding to amino acid 155, (4) aspartic acid at a position corresponding to amino acid 188, (5) leucine at a position corresponding to amino acid 362, (6) alanine at a position corresponding to amino acid 431, or (7) serine at a position corresponding to amino acid 582, of SEQ ID NO: 124. 9. A purified nucleic acid according to any one of claims 1-8, wherein the encoded GlyT2 amino acid sequence is that of amino acids 1 to 797 of SEQ ID NO: 124 but having one or more of the following amino acid substitutions: (1) serine at position 24, (2) tryptophan at position 74, (3) glycine at position 155, (4) aspartic acid at position 188, (5) leucine at position 362, (6) alanine at a position corresponding to amino acid 431, or (7) serine at position 582. 10. A purified nucleic acid according to claim 9, wherein the encoded GlyT2 amino acid sequence comprises the amino acid sequence of SEQ ID NO: 122. 11. A purified nucleic acid according to claim 9, wherein the encoded GlyT2 amino acid sequence comprises the amino acid sequence of SEQ ID NO: 120. 12. A purified nucleic acid according to any one of claims 1-9, wherein the nucleic acid sequence is that of bases 1 to 2391 of SEQ ID NO: 123 but having one or more of the following nucleotide substitutions: (1) A at position 70, (2) T at position 220, (3) G at position 463, (4) G at position 562, (5) T at position 1085, (6) C at position 1292, (7) A at position 1299, (8) T at position 1617, or (9) T at position 1744. 13. A purified nucleic acid according to claim 12, wherein the nucleic acid sequence is that of SEQ ID NO: 121. 14. A purified nucleic acid according to claim 12, wherein the nucleic acid sequence is that of SEQ ID NO: 119. 15. A purified nucleic acid according to any one of claims 1-9, that additionally has one or more of the following amino acids: (1) leucine at a position corresponding to amino acid 26, (2) leucine at a position corresponding to amino acid 75, (3) valine at a position corresponding to amino acid 89, (4) glycine at a position corresponding to amino acid 102, (5) glutamic acid at a position corresponding to amino acid 174, (6) proline at a position corresponding to amino acid 195, (7) glycine at a position corresponding to amino acid 199, (8) leucine at a position corresponding to amino acid 249, (9) proline at a position corresponding to amino acid 306, (10) glutamic acid at a position corresponding to amino acid 419, (11) asparagine at a position corresponding to amino acid 442, (12) lysine at a position corresponding to amino acid 455, (13) cysteine at a position corresponding to amino acid 458, (14) proline at a position corresponding to amino acid 485, (15) arginine at a position corresponding to amino acid 493, or (16) glutamic acid at a position corresponding to amino acid 650, of SEQ ID NO: 124. 16. A purified nucleic acid according to any one of claims 1-9, that additionally has one or more of the following amino acids: (1) phenylalanine at a position corresponding to amino acid 124, (2) asparagine at a position corresponding to amino acid 279, (3) glycine at a position corresponding to amino acid 393, (4) asparagine at a position corresponding to amino acid 457, (5) asparagine at a position corresponding to amino acid 463 , (6) tyrosine at a position corresponding to amino acid 610, (7) valine at a position corresponding to amino acid 611, (8) serine at a position corresponding to amino acid 733, (9) valine at a position corresponding to amino acid 735, (10) leucine at a position corresponding to amino acid 245, (11) leucine at a position corresponding to amino acid 305, (12) isoleucine at a position corresponding to amino acid 366, or (13) proline at a position corresponding to amino acid 400, of SEQ ID NO: 124. 17. A purified nucleic acid according to claim 12, but additionally having has one or more of the following nucleotide substitutions: (a) T at position 77, (b) T at position 220, (c) T at position 244, (d) T at position 266, (e) G at position 304, (f) A at position 521, (g) C at position 583, (h) G at position 596, (i) G at position 678, 0) C at position 681, (k) T at position 745, (1) C at position 750, (m) T at position 765, (n) C or A at position 777, (o) G at position 867, (p) C at position 917, (q) A at position 1256, (r) C at position 1292, (s) A at position 1325, (t) A at position 1364, (u) C at position 1374, (v) A at position 1392, (w) C at position 1454, (x) G at position 1478, (y) C at position 1854, (z) A at position 1949, (aa) C at position 1959, or (bb) C at position 2130. 18. A purified nucleic acid according to claim 12, but additionally having one or more of the following nucleotide substitutions: (a) C at position 6, (b) T at position 371, (c) T at position 571, (d) A at position 836, (e) G at position 1116, (f) G at position 1177, (g) C at position 1371, (h) A at position 1387, (i) A at position 1829, (j) G at position 1831, (k) C at position 2198, (1) G at position 2203, (m) G at position 342, (n) C at position 733, (o) C at position 913, (p) A at position 951, (q) T at position 1097, (r) C at position 1199, (s) T at position 352, or (t) A at position 2103. 19. A vector comprising the nucleic acid sequence of any one of claims 1-8, 10-11, and 13-14. 20. A vector comprising the nucleic acid sequence of claim 9. 21. A vector comprising the nucleic acid sequence of claim 12. 22. A vector comprising the nucleic acid sequence of claim 15. 23. A vector comprising the nucleic acid sequence of claim 16. 24. A vector comprising the nucleic acid sequence of claim 17. 25. A vector comprising the nucleic acid sequence of claim 18. 26. A recombinant host cell containing the vector of claim 19. 27. A recombinant host cell containing the vector of claim 20. 28. A recombinant host cell containing the vector of claim 21. 29. A recombinant host cell containing the vector of claim 22. 30. A recombinant host cell containing the vector of claim 23. 31. A recombinant host cell containing the vector of claim 24. 32. A recombinant host cell containing the vector of claim 25. 33. A recombinant host cell containing the nucleic acid sequence of any one of claims 1-8, 10-11, and 13-14. 34. A recombinant host cell containing the nucleic acid sequence of claim 9. 35. A recombinant host cell containing the nucleic acid sequence of claim 12. 36. A recombinant host cell containing the nucleic acid sequence of claim 15. 37. A recombinant host cell containing the nucleic acid sequence of claim 16. 38. A recombinant host cell containing the nucleic acid sequence of claim 17. 39. A recombinant host cell containing the nucleic acid sequence of claim 18. 40. A method of making GlyT2 comprising expressing the GlyT2 in the recombinant host cell of claim 33. 41. A method of making GlyT2 comprising expressing the GlyT2 in the recombinant host cell of claim 34. 42. A method of making GlyT2 comprising expressing the GlyT2 in the recombinant host cell of claim 35. 43. A method of making GlyT2 comprising expressing the GlyT2 in the recombinant host cell of claim 36. 44. A method of making GlyT2 comprising expressing the GlyT2 in the recombinant host cell of claim 37. 45. A method of making GlyT2 comprising expressing the GlyT2 in the recombinant host cell of claim 38. 46. A method of making GlyT2 comprising expressing the GlyT2 in the recombinant host cell of claim 39. 47. A purified nucleic acid encoding a glycine transporter type 2 (GlyT2) having at least about 96% sequence identity with a reference sequence which is the protein sequence of SEQ ID NO: 122 or with a sequence corresponding to the protein sequence of SEQ ID NO: 122 except that it has one or more of the following amino acid substitutions: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Val, (5) Ser 102 to Gly, (6) Val 174 to Glu, (7) Ser 195 to Pro, (8) Asp 199 to Gly, (9) Val 249 to Leu, (10) Leu 306 to Pro, (11) Gly 419 to Glu, (12) Thr 442 to Asn, (13) Thr 455 to Lys, (14) Trp 458 to Cys, (15) Leu 485 to Pro, (16) Lys 493 to Arg, or (17) Val 650 to Glu. 48. The nucleic acid of claim 47, wherein the reference sequence is the protein sequence of SEQ ID NO: 122 or with a sequence corresponding to the protein sequence of SEQ ID NO: 122 except that it has one or more of the following amino acid substitutions: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Val, (5) Ser 102 to Gly, (6) Val 174 to Glu, (7) Ser 195 to Pro, (8) Asp 199 to Gly, (9) Val 249 to Leu, (10) Leu 306 to Pro, (11) Gly 419 to Glu, (12) Thr 442 to Asn, (13) Thr 455 to Lys, (14) Trp 458 to Cys, (15) Leu 485 to Pro, (16) Lys 493 to Arg, or (17) Val 650 to Glu. 49. The nucleic acid of claim 47, wherein the sequence identity is selected from the group of at least about 97 % , at least about 98 % and at least about 99% sequence identity. 50. The nucleic acid of claim 47, wherein the sequence identity is at least about 99.5% . 51. The nucleic acid of claim 47, wherein the nucleic acid encodes a GlyT2 having the reference sequence. 52. The nucleic acid of claim 47, comprising the nucleic acid sequence of SEQ ID NO: 121 or with a sequence that varies from the nucleic acid sequence of SEQ ID NO: 121 by having one or more of the following nucleotide substitutions: (a) C 77 →T, (b) C 220 →T, (c) C 224 →T, (d) C 266 →T, (e) A 304 →G, (f) T 521 →A, (g) T 583 →C, (h) A 596 →G, (i) A 678 →G, Q) T m →C, (k) G 745 →T, (1) G 750 →C, (m) C 765 →T, (n) G 777 →C, (o) A 867 →G, (p) T 917 →C, (q) G 1256 →A, (r) T 1292 →C, (s) C 1325 →A, (t) C 1364 →A, (u) G 137 →C, (v) C 1392 →A, (w) T 1454 →C, (x) A 1478 →G, (y) T 1854 →C, (z) T 1949 →A, (aa) T 1959 →C, or (bb) T 2130 →C. 53. The nucleic acid sequence of SEQ ID NO .121 or a sequence that varies from the nucleic acid sequence of SEQ ID NO: 121 by having one or more of the following nucleotide substitutions: C 220 →T, A 304 →G, or T 1292 →C. 54. A vector comprising the nucleic acid of claim 47 and an extrinsic promoter functionally associated therewith. 55. A nucleic acid encoding a glycine transporter protein having at least about 99.5% sequence identity with all or one to two contiguous portions of a reference amino acid sequence, wherein the reference sequence is SEQ ID NO: 122 or an amino acid sequence corresponding to the amino acid sequence of SEQ ID NO: 122 except that it has one or more of the following amino acid substitutions: (1) Pro 26 to Leu, (2) Arg 74 to Trp, (3) Pro 75 to Leu, (4) Ala 89 to Val, (5) Ser 102 to Gly, (6) Val 174 to Glu, (7) Ser 195 to Pro, (8) Asp 199 to Gly, (9) Val 249 to Leu, (10) Leu 306 to Pro, (11) Gly 419 to Glu, (12) Thr 442 to Asn, (13) Thr 455 to Lys, (14) Trp 458 to Cys, (15) Leu 485 to Pro, (16) Lys 493 to Arg, or (17) Val 650 to Glu. 56. The nucleic acid of claim 55, wherein the one to two contiguous sequence portions comprise amino acids selected from the group of at least about 600 amino acids, at least about 700 amino acids and at least about 750 amino acids. 57. A cell containing the nucleic acid of any one of claims 47-53 and 55-56. 58. A cell transformed with a vector according to claim 54. 59. A method of producing a glycine transporter comprising growing the cells of claim 57. 60. A method of producing a glycine transporter comprising growing the cells of claim 58. 61. The method of claim 59 further comprising at least one of (a) isolating membranes from the cells, which membranes comprise the glycine transporter or (b) extracting a protein fraction from the cells which fraction comprises the glycine transporter. 62. The method of claim 60 further comprising at least one of (a) isolating membranes from the cells, which membranes comprise the glycine transporter or (b) extracting a protein fraction from the cells which fraction comprises the glycine transporter. 63. A glycine transporter isolated from a cell according to claim 57. 64. A glycine transporter isolated from a cell according to claim 58. 65. A method for characterizing a bioactive agent for treatment of a nervous system disorder or condition or for identifying bioactive agents for treatment of a nervous system disorder or condition, the method comprising (a) providing a first assay composition comprising (i) a cell according to claim 57 or (ii) an isolated glycine transporter protein according to claim 63, (b) contacting the first assay composition with the bioactive agent or a prospective bioactive agent and measuring the amount of glycine transport exhibited by the assay composition. 66. A method for characterizing a bioactive agent for treatment of a nervous system disorder or condition or for identifying bioactive agents for treatment of a nervous system disorder or condition, the method comprising (a) providing a first assay composition comprising (i) a cell according to claim 58 or (ii) an isolated glycine transporter protein (ii) an isolated glycine transporter protein according to claim 64, (b) contacting the first assay composition with the bioactive agent or a prospective bioactive agent and measuring the amount of glycine transport exhibited by the assay composition. 67. The method of claim 65, further comprising comparing the amount of glycine transport exhibited by the first assay composition with the amount of glycine transport exhibited by a second such assay composition that is treated the same as the first assay composition except that it is not contacted with the bioactive agent or prospective bioactive agent. 68. The method of claim 66, further comprising comparing the amount of glycine transport exhibited by the first assay composition with the amount of glycine transport exhibited by a second such assay composition that is treated the same as the first assay composition except that it is not contacted with the bioactive agent or prospective bioactive agent. 69. The method of claim 65, wherein the nervous system disorder or condition is one of the group consisting of (a) pain, (b) spasticity, (c) myoclonus, (d) muscle spasm, (e) muscle hyperactivity or (f) epilepsy. 70. The method of claim 66, wherein the nervous system disorder or condition is one of the group consisting of (a) pain, (b) spasticity, (c) myoclonus, (d) muscle spasm, (e) muscle hyperactivity or (f) epilepsy. 71. The method of claim 69, wherein the spasticity is associated with stroke, head trauma, neuronal cell death, multiple sclerosis, spinal cord injury, dystonia, Huntington's disease or amyotrophic lateral sclerosis. 72. The method of claim 70, wherein the spasticity is associated with stroke, head trauma, neuronal cell death, multiple sclerosis, spinal cord injury, dystonia, Huntington's disease or amyotrophic lateral sclerosis. 73. A nucleic acid that hybridizes with a reference nucleic acid sequence which is SEQ ID NO: 121 or with a sequence that varies from the nucleic acid sequence of SEQ ID NO: 121 by having one or more of the following nucleotide substitutions: (a) C 77 →T, (b) C 220 →T, (c) C 2 4 →T, (d) C 266 →T, (e) A 304 →G, (f) T 521 →A, (g) T 583 →C, (h) A 596 →G, (i) A 678 →G, (j) T 681 →C, (k) G 745 →T, (1) G 750 →C, (m) C 765 →T, (n) G 777 →C, (o) A 867 →G, (p) T 917 →C, (q) G 1256 →A, (r) T 1292 →C, (s) C 1325 →A, (t) C 1364 →A, (u) G 1374 →C, (v) C 1392 →A, (w) T 1454 →C, (x) A 1478 →G, (y) T 1854 →C, (z) T 1949 →A, (aa) T 1959 →C, or (bb) T 2130 →C, under conditions of sufficient stringency to exclude hybridizations with (a) the sequence for a rat GlyT2 transporter or (b) the sequence for a mammalian GlyTl transporter. 74. The nucleic acid sequence of claim 73, wherein the nucleic acid is a PCR primer and the stringent conditions are PCR conditions effective to amplify a human Glyt2 sequence but not to amplify (a) the sequence for a rat GlyT2 transporter or (b) the sequence for a mammalian GlyTl transporter. 75. A nucleic acid of at least about eighteen nucleotides in length comprising a contiguous sequence from the coding or non-coding strand of a human GlyT2 gene or cDNA, wherein the contiguous sequence has at least 1 nucleotide difference when compared with the rat GlyT2 gene sequence that aligns with the contiguous sequence. 76. The antisense molecule of claim 73, wherein the contiguous stretch is included in the coding or non-coding strand of the nucleic acid sequence of SEQ ID NO: 121 or of a sequence that varies from the nucleic acid sequence of SEQ ID NO: 121 by having one or more of the following nucleotide substitutions: (a) C 77 →T, (b) C 220 →T, (c) C 224 →T, (d) C 266 →T, (e) A 304 →G, (f) T 521 →A, (g) T 583 →C, (h) A 96 →G, (i) A 678 →G, (j) T 681 →C, (k) G 745 →T, (1) G 750 →C, (m) C 765 →T, (n) G 777 →C, (o) A 867 →G, (p) T 917 →C, (q) G 1256 -→A, (r) T 1292 →C, (s) C 1325 →A, (t) C 1364 →A, (u) G 1374 →C, (v) C 1392 →A, (w) T 1454 →C, (x) A 1478 →G, (y) T 1854 →C, (z) T 1949 →A, (aa) T 1959 →C, or (bb) T 2130 →C. 77. An expression vector comprising the nucleic acid of claim 73. 78. A method of reducing GlyT2 expression in a tissue or cell comprising applying to the tissue or cell (a) a nucleic acid of claim 73 in an amount effective to reduce GlyT2 expression or (b) an expression vector for expressing the nucleic acid in the tissue or cell in an amount effective to reduce GlyT2 expression. 79. A method of treating a nervous system disorder or condition comprising applying to a tissue or cell of a human patient a nervous system disorder- or condition-treating effective amount of a nucleic acid of claim 73 or a nervous system disorder- or condition-treating effective amount of an expression vector for expressing the nucleic acid in the tissue or cell. 80. A method for detecting whether an animal has autoimmune antibodies against a glycine transporter, the method comprising contacting an antibody preparation from the animal or a body fluid from the animal with a polypeptide antigen comprising a glycine transporter or derived from the glycine transporter. 81. A method for preparing an antibody specific for human GlyT2 comprising immunizing an animal with an isolated glycine transporter protein according to claim 63. 82. A method for preparing an antibody specific for human GlyT2 comprising immunizing an animal with an isolated glycine transporter protein according to claim 64.

Inventors: BRUNDEN KURT R, BURGESS LOYD H, GALLAGHER MICHAEL J, BRUNDEN, KURT, R., BURGESS, LOYD, H., GALLAGHER, MICHAEL, J.
Applicants: BRUNDEN KURT R, BURGESS LOYD H, GALLAGHER MICHAEL J, GLIATECH INC, BRUNDEN, KURT, R., BURGESS, LOYD, H., GALLAGHER, MICHAEL, J., GLIATECH INC.
Application year: 1999
Main classification: C12N15
IPC codes: A61K31, A61K35, A61K48, A61P21, A61P25, C07K14, C07K16, C12N1, C12N5, C12N15, C12P21, C12Q1, C12R1, G01N33