NOVEL IRAP-BP POLYPEPTIDE AND NUCLEIC ACID MOLECULES AND USES THEREFOR

Abstract: Novel IRAP-BP polypeptides, proteins, and nucleic acid molecules are disclosed. In addition to isolated, full-length IRAP-BP polypeptides, the invention further provides isolated IRAP-BP fusion proteins, antigenic peptides and anti-IRAP-BP antibodies. The invention also provides IRAP-BP nucleic acid molecules, recombinant expression vectors containing a nucleic acid molecule of the invention, host cells into which the expression vectors have been introduced and non-human transgenic animals in which an IRAP-BP gene has been introduced or disrupted. Diagnostic, screening and therapeutic methods utilizing compositions of the invention are also provided.

Description: NONEL IRAP-BP POLYPEPTIDE AND NUCLEIC ACID MOLECULES AND USES THEREFOR Government Rights This invention was made at least in part with Government Support under Contract Number DK30425 awarded by the National Institutes of Health. The government has certain rights in this invention. Background of the Invention The regulation of blood glucose levels by insulin is acheived mainly by increased glucose transport exclusively into adipose and skeletal muscle tissue; De Fronzo et al. (1981) Diabetes 30:1000-1007 and James et al. (1985) Am. J. Physiol 248:E567-E574. These are the only two tissues that express a specific isoform of the glucose transporter, GLUT4, which mediates the hormonal effect of insulin (for reviews of glucose transporter isoforms and their expression, see Deveskar and Mueckler (1992) Pediatr. Res. 31 :1-13; Bell et al. (1993) J. Biol. Chem. 268:3352-3356; and Baldwin (1993) Biochim. Biophys. Acta 1154:17-49). The mechanism of glucose transport activation by insulin is the hormone-dependent enhancement of the rate of GLUT4 translocation from intracellular storage vesicles to the plasma membrane in such a way that the concentration of the transporter on the cell surface increases 10- to 40-fold, depending on the cell type and method of measurement (Zorzono et al. (1989) J. Biol. Chem. 264:12358-12363; Holman et al. (1990) J Biol Chem. 265:18172-18179; Slot et al. (1991) J Biol. Chem. 113:123-135; Slot et al. (1991) Proc. Natl Acad. Sci. USA 88:7815-7819; and Smith et al. (1991) Proc. Natl Acad. Sci. USA 88:6893-6897). Glucose uptake is increased proportionally to the increment of GLUT4 molecules in the plasma membrane, suggesting that redistribution of transporters is the main, if not only, mechanism that accounts for this effect, Kandror and Pilch (1994) Proc. Nat'l Acad. Sci. USA 91 :8017-8021. It is believed that GLUT4 recycles in cells as a constituent of tissue-specific secretory-like microsomal structures, known as "GLUT4-containing vesicles". In addition to GLUT 4, these vesicles have also been determined to include phosphatidylinositol 4-kinase, Del Vecchio and Pilch (1991) J. Biol. Chem. 266:13278- 13283; Vesicle-associated membrane proteins ("VAMPS"), Cain et al. (1992) J. Biol. Chem. 267:11681-11684; Secretory component-associated membrane proteins ("SCAMPS"), Thoidis et al. (1993) J. Biol. Chem. 268:11691-11696; and Laurie et al. (1993) J. Biol. Chem. 268:19110-19117; and low molecular weight GTP-binding proteins of the Rab family, Cormont et al. (1993) J. Biol. Chem. 268:19491-19497. In addition to the proteins enumerated above, a novel zinc-dependent protease named insulin-responsive aminopeptidase ("IRAP") has been identified and characterized as an important component of GLUT4-containing vesicles (designated previously as gpl60, Kandror and Pilch (1994) Proc. Nat'l Acad. Sea USA 91:8017-8021; Kandror et al. (1994) J. Biol. Chem. 269:30777-30780; and vpl65, Keller et al. (1995) J. Biol. Chem. 270:23612-23618. Structurally, IRAP contains a 109-amino acid amino-terminal end which projects into the cytoplasm, a single 22-amino acid transmembrane domain, and a large catalytic domain within the lumen of the vesicle which is responsible for the protein's enzymatic activity, Keller et al. (1995) J. Biol. Chem. 270:23612-23618. In the basal state, IRAP is primarily located intracellularly, like GLUT4, but is markedly translocated to the cell surface in response to insulin, Mastick et al. (1994) J. Biol. Chem. 269:6089-6092; Kandror and Pilch (1994) Proc. Nat'l Acad. Sea USA 91:8017- 8021; Ross et al. (1996) J. Biol. Chem. 271 :3328-3332; and Ross et al. (1997) Biochem. Biophys. Res. Commun. (1997) 239:247-251. Furthermore, it has been suggested that the amino terminus of IRAP, which contains two dileucine motifs and several acidic regions similar to those that occur in GLUT4, functions in the regulation of intracellular trafficking and retention of GLUT4; Waters et al. (1997) J. Biol. Chem. 272:23323- 23327. Given the important role of IRAP in regulating insulin-responsive translocation of the glucose transporter, GLUT4, there exists a need for determining the mechanism of action of IRAP, for identifying novel proteins with which IRAP interacts, as well as for identifying modulators of such molecules for use in regulating a variety insulin-sensitive cellular responses. Summary of the Invention The present invention is based, at least in part, on the discovery of nucleic acid molecules which encode a novel family of proteins which bind to and/or regulate the insulin-responsive aminopeptidase ("IRAP"), referred to herein as the IRAP -binding proteins ("IRAP-BP" family or "IRAP-BP polypeptides"). The IRAP-BP molecules of the present invention as well as IRAP-BP modulators, are useful in regulating a variety of cellular processes, in particular, insulin-responsive processes. Accordingly, in one aspect, this invention provides isolated nucleic acid molecules encoding IRAP-BP polypeptides or biologically active portions thereof, as well as nucleic acid fragments suitable as primers or hybridization probes for the detection of IRAP-BP-encoding nucleic acids. In one embodiment, an IRAP-BP nucleic acid molecule is 60% homologous to the nucleotide sequence shown in SEQ ID NO:l, the nucleotide sequence shown in SEQ ID NO:4, or complement thereof. In a preferred embodiment, an isolated IRAP-BP nucleic acid molecule has the nucleotide sequence shown SEQ ID NO:3, or a complement thereof. In another embodiment, an IRAP-BP nucleic acid molecule further comprises nucleotides 1-689 of SEQ ID NO:l. In another embodiment, an IRAP-BP nucleic acid molecule further comprises nucleotides 2187-2947 of SEQ ID NO:l. In another preferred embodiment, an isolated IRAP-BP nucleic acid molecule has the nucleotide sequence shown in SEQ ID NO: 1. In another preferred embodiment, an isolated IRAP-BP nucleic acid molecule has the nucleotide sequence shown SEQ ID NO:7, or a complement thereof. In another preferred embodiment, an isolated IRAP-BP nucleic acid molecule has the nucleotide sequence shown SEQ ID NO:6, or a complement thereof. In another embodiment, an IRAP-BP nucleic acid molecule further comprises nucleotides 1-183 of SEQ ID NO:4. In another embodiment, an IRAP-BP nucleic acid molecule further comprises nucleotides 1684-2064 of SEQ ID NO:4. In another preferred embodiment, an isolated IRAP-BP nucleic acid molecule has the nucleotide sequence shown in SEQ ID NO:4. In another embodiment, an IRAP-BP nucleic acid molecule includes a nucleotide sequence encoding a protein having an amino acid sequence sufficiently homologous to the amino acid sequence of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. In another preferred embodiment, an IRAP-BP nucleic acid molecule includes a nucleotide sequence encoding a protein having an amino acid sequence at least 60% homologous to the amino acid sequence of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. In yet another preferred embodiment, an IRAP-BP nucleic acid molecule includes a nucleotide sequence encoding a protein having the amino acid sequence of SEQ ID NO: 2, SEQ ID NO:5, or SEQ ID NO:8. In another embodiment, an isolated nucleic acid molecule of the present invention encodes an IRAP-BP polypeptide which includes a helix-turn-helix domain. In yet another embodiment, an IRAP-BP nucleic acid molecule encodes an IRAP-BP polypeptide and is a naturally occurring nucleotide sequence. Another embodiment of the invention features IRAP-BP nucleic acid molecules which specifically detect nucleic acid molecules which encode IRAP-BP polypeptides relative to nucleic acid molecules encoding non-IRAP-BP polypeptides. For example, in one embodiment, an IRAP-BP nucleic acid molecule is at least 550 nucleotides in length and hybridizes under stringent conditions to a nucleic acid molecule comprising the nucleotide sequence shown in SEQ ID NO:l, the nucleotide sequence shown in SEQ ID NO:4, or a complement thereof. Another embodiment of the invention provides an isolated nucleic acid molecule which is antisense to the coding strand of an IRAP-BP nucleic acid. Another aspect of the invention provides a vector comprising an IRAP-BP nucleic acid molecule. In certain embodiments, the vector is a recombinant expression vector. In another embodiment, the invention provides a host cell containing a vector of the invention. The invention also provides a method for producing an IRAP-BP polypeptide by culturing in a suitable medium, a host cell of the invention containing a recombinant expression vector such that an IRAP-BP polypeptide is produced. Another aspect of this invention features isolated or recombinant IRAP-BP polypeptides. In one embodiment, an isolated IRAP-BP polypeptide comprises at least a helix-turn-helix domain. In another embodiment, an isolated IRAP-BP polypeptide has an amino acid sequence sufficiently homologous to the amino acid sequence of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. In a preferred embodiment, an IRAP-BP polypeptide has an amino acid sequence at least about 60% homologous to the amino acid sequence of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. In another embodiment, an IRAP-BP polypeptide has the amino acid sequence of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. In a preferred embodiment, an IRAP-BP polypeptide interacts with an IRAP polypeptide. Another embodiment of the invention features an isolated IRAP-BP polypeptide which is encoded by a nucleic acid molecule having a nucleotide sequence at least about 60%> homologous to a nucleotide sequence of SEQ ID NO : 1 , or a complement thereof. Another embodiment of the invention features an isolated IRAP-BP polypeptide which is encoded by a nucleic acid molecule having a nucleotide sequence at least about 60% homologous to a nucleotide sequence of SEQ ID NO:4, or a complement thereof. This invention further features an isolated IRAP-BP polypeptide which is encoded by a nucleic acid molecule having a nucleotide sequence which hybridizes under stringent hybridization conditions to a nucleic acid molecule comprising the nucleotide sequence of SEQ ID NO: 1 , SEQ ID NO:4, or a complement thereof The IRAP-BP polypeptides of the present invention, or biologically active portions thereof, can be operatively linked to a non-IRAP-BP polypeptide to form IRAP-BP fusion proteins. The invention further features antibodies that specifically bind IRAP-BP polypeptides, such as monoclonal or polyclonal antibodies. In addition, the IRAP-BP polypeptides or biologically active portions thereof can be incoφorated into pharmaceutical compositions, which optionally include pharmaceutically acceptable carriers. In another aspect, the present invention provides a method for detecting IRAP- BP expression in a biological sample by contacting the biological sample with an agent capable of detecting an IRAP-BP nucleic acid molecule, protein or polypeptide such that the presence of an IRAP-BP nucleic acid molecule, protein or polypeptide is detected in the biological sample. In another aspect, the present invention provides a method for detecting the presence of IRAP-BP activity in a biological sample by contacting the biological sample with an agent capable of detecting an IRAP-BP activity such that the presence of IRAP- BP activity is detected in the biological sample. In another aspect, the invention provides a method for modulating IRAP-BP activity comprising contacting the cell with an agent that modulates IRAP-BP activity such that IRAP-BP activity in the cell is modulated. In one embodiment, the agent inhibits IRAP-BP activity. In another embodiment, the agent stimulates IRAP-BP activity. In one embodiment, the agent is an antibody that specifically binds to an IRAP-BP polypeptide. In another embodiment, the agent modulates expression of IRAP-BP by modulating transcription of an IRAP-BP gene or translation of an IRAP- BP mRNA. In yet another embodiment, the agent is a nucleic acid molecule having a nucleotide sequence that is antisense to the coding strand of an IRAP-BP mRNA or an IRAP-BP gene. In one embodiment, the methods of the present invention are used to treat a subject having a disorder characterized by aberrant IRAP-BP polypeptide or nucleic acid expression or activity by administering an agent which is an IRAP-BP modulator to the subject. In one embodiment, the IRAP-BP modulator is an IRAP-BP polypeptide. In another embodiment, the IRAP-BP modulator is an IRAP-BP nucleic acid molecule. In yet another embodiment, the IRAP-BP modulator is a peptide, peptidomimetic, or other small molecule. In a preferred embodiment, the disorder characterized by aberrant IRAP-BP polypeptide or nucleic acid expression is insulin resistance or diabetes. The present invention also provides a diagnostic assay for identifying the presence or absence of a genetic alteration characterized by at least one of (i) aberrant modification or mutation of a gene encoding an IRAP-BP polypeptide; (ii) mis- regulation of said gene; and (iii) aberrant post-translational modification of an IRAP-BP polypeptide, wherein a wild-type form of said gene encodes an protein with an IRAP- BP activity. In another aspect the invention provides a method for identifying a compound that binds to or modulates the activity of an IRAP-BP polypeptide. In one embodiment, the invention provides a method for identifying a compound which binds to an IRAP-BP polypeptide which involves contacting the IRAP-BP polypeptide, or a cell expressing the IRAP-BP polypeptide with a test compound and determining whether the IRAP-BP polypeptide binds to the test compound. In another embodiment, the invention provides a method for identifying a compound which modulates the activity of an IRAP-BP polypeptide which involves contacting an IRAP-BP polypeptide with a test compound, and determining the effect of the test compound on the activity of the polypeptide to thereby identify a compound which modulates the activity of the IRAP-BP polypeptide. Other features and advantages of the invention will be apparent from the following detailed description and claims. Brief Description of the Drawings Figure 1 depicts the cDNA sequence and predicted amino acid sequence of murine IRAP-BP- 1. Figure 1A-D depicts the nucleotide sequence of murine IRAP-BP corresponding to nucleic acids 1 to 2947 of SEQ ID NO:l. Figure 1D-E depicts a hypothetical translation product of the murine IRAP-BP cDNA sequence. The murine IRAP-BP amino acid sequence is predicted to begin at residue 230 and end at residue 728, corresponding to amino acids 1 to 499 of SEQ ID NO:2. Figure 2 depicts the cDNA sequence and predicted amino acid sequence of human IRAP-BP- 1. Figure 2A-C depicts the nucleotide sequence of human IRAP-BP corresponding to nucleic acids 1 to 2064 of SEQ ID NO:4. Figure 2D depicts a hypothetical translation product of the IRAP-BP cDNA sequence. The human IRAP- BP amino acid sequence is predicted to begin at residue 62 and end at residue 561, corresponding to amino acids 1 to 500 of SEQ ID NO:5. Figure 3 depicts an alignment of the nucleic acid sequences of murine IRAP-BP (SEQ ID NO: 1 , top) and human IRAP-BP (SEQ ID NO:4, bottom) as well as an alignment of the hypothtical amino acid sequence of murine IRAP-BP (top) with the hypothtical amino acid sequence of human IRAP-BP (bottom). Figure 3A-C depicts a nucleic acid alignment performed using the BESTFIT program which is part of the GCG software package, Gap Weight of 50, Length Weight of 3. Figure 3D depicts an amino acid alignment performed using the same software package, Gap Weight of 12, Length Weight of 4. Figure 4 depicts an alignment of the amino acid seqeunces of murine IRAP-BP (SEQ ID NO:2, top) and human IRAP-BP (SEQ ID NO:5, bottom). The amino acid alignment was generated using the ALIGN algorithm, Gap Penalties = -12/-2. Detailed Description of the Invention The present invention is based on the discovery of novel molecules, referred to herein as IRAP-BP polypeptide and nucleic acid molecules, which comprise a family of molecules having certain conserved structural and functional features. The term "family" when referring to the protein and nucleic acid molecules of the invention is intended to mean two or more proteins or nucleic acid molecules having a common structural domains and having sufficient amino acid or nucleotide sequence homology as defined herein. Such family members can be naturally occurring and can be from either the same or different species. For example, a family can contain a first protein of human origin, as well as other, distinct proteins of human origin or alternatively, can contain homologues of non-human origin. Members of a family may also have common functional characteristics. In one embodiment, the IRAP-BP polypeptides of the present invention are proteins having an amino acid sequence of about 250-750, preferably about 300-700, more preferably about 350-650, even more preferably about 400-600, and even more preferably about 450-550 amino acids in length. In another embodiment, the IRAP-BP polypeptides of the present invention are proteins having an amino acid sequence of about 475-975, preferably about 525-925, more preferably about 575-875, even more preferably about 625-825, and even more preferably about 675-775 amino acid residues in length. In one embodiment, IRAP-BP molecules of the present invention include at least one "helix-turn-helix" domain. Preferred IRAP-BP molecules of the present invention have an amino acid sequence sufficiently homologous to the amino acid sequence of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. As used herein, the term "sufficiently homologous" refers to a first amino acid or nucleotide sequence which contains a sufficient or minimum number of identical or equivalent (e.g., an amino acid residue which has a similar side chain) amino acid residues or nucleotides to a second amino acid or nucleotide sequence such that the first and second amino acid or nucleotide sequences share common structural domains and/or a common functional activity. For example, amino acid or nucleotide sequences which share common structural domains have at least about 50% homology, preferably 60% homology, more preferably 70%-80%, and even more preferably 90-95%, 98% or more homology across the amino acid sequences of the domains and contain at least one and preferably two structural domains, are defined herein as sufficiently homologous. Furthermore, amino acid or nucleotide sequences which share at least 50%, preferably 60%, more preferably 70-80, 90- 95% or 98%> or more homology and share a common functional activity are defined herein as sufficiently homologous. As used interchangeably herein, an "IRAP-BP activity", "biological activity of IRAP- BP" or "functional activity of IRAP-BP", refers to an activity exerted by an IRAP-BP polypeptide or nucleic acid molecule on an IRAP-BP responsive cell as determined in vivo, or in vitro, according to standard techniques. In one embodiment, an IRAP-BP activity is a direct activity, such as an association with an IRAP-BP-target molecule. As used herein, a "target molecule" or "binding partner" is a molecule with which an IRAP-BP polypeptide binds or interacts in nature, such that IRAP-BP-mediated function is acheived. An IRAP-BP target molecule can be a non-IRAP-BP molecule or an IRAP-BP polypeptide of the present invention. In an exemplary embodiment, an IRAP-BP target molecule is an IRAP protein or polypeptide (e.g., a human IRAP protein). Alternatively, an IRAP-BP activity is an indirect activity, such as an intracellular activity mediated by interaction of the IRAP-BP polypeptide with an IRAP polypeptide (e.g., regulation of vesicle trafficking). In a preferred embodiment, an IRAP-BP activity is at least one or more of the following activities: (i) interaction with an intracellular non-IRAP-BP molecule, for example, an IRAP-BP target molecule (e.g., IRAP and/or GLUT4); (ii) binding to an intracellular non-IRAP-BP molecule, for example, and IRAP-BP target molecule (e.g., IRAP and/or GLUT4); (iii) recognition of a trafficking motif within an intracellular non- IRAP-BP molecule, for example, and IRAP-BP target molecule (e.g., IRAP and/or GLUT4); and (iv) binding to a trafficking motif within an intracellular non-IRAP-BP molecule, for example, and IRAP-BP target molecule (e.g., IRAP and/or GLUT4). In yet another preferred embodiment, an IRAP-BP activity is at least one or more of the following activities: (1) modulation of translocation of an IRAP-BP target molecule; (2) modulation of GLUT4 translocation (e.g., exocytosis); (3) modulation of IRAP translocation (e.g., exocytosis); (4) modulation of sorting of an IRAP-BP target molecule; (5) modulation of sorting of IRAP and/or GLUT4; (6) modulation of retention of an IRAP-BP target molecule; (7) modulation of retention of IRAP and/or GLUT4 (e.g., within a specialized intracellular compartment, for example, the endocytic recycling compartment; (8) modulation of the entry of an IRAP-BP target molecule into recycling vesicles; (9) modulation of entry of IRAP and/or GLUT4 into recycling vesicles; (10) regulation of intracellular trafficking and/or retention of an IRAP-BP target molecule; (11) regulation of intracellular trafficking and/or retention of IRAP and/or GLUT4; and (12) regulation of intracellular trafficking of GLUT4- and/or IRAP- containing vesicles. Accordingly, another embodiment of the invention features isolated IRAP-BP polypeptides having an IRAP-BP activity. Preferred IRAP-BP polypeptides have at least a helix-turn-helix domain and an IRAP-BP activity. In another preferred embodiment, an IRAP-BP polypeptide has a helix-turn-helix domain, an IRAP-BP activity, and an amino acid sequence sufficiently homologous to an amino acid sequence of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. The human IRAP-BP- 1 cDNA, which is at least approximately 2064 nucleotides in length, encodes a protein which is approximately 500 amino acid residues in length. The murine IRAP-BP- 1 cDNA, which is approximately 2947 nucleotides in length, encodes approximately 499 amino acid residues of the murine IRAP-BP- 1 protein. Various aspects of the invention are described in further detail in the following subsections: I. Isolated Nucleic Acid Molecules One aspect of the invention pertains to isolated nucleic acid molecules that encode IRAP-BP polypeptides or biologically active portions thereof, as well as nucleic acid fragments sufficient for use as hybridization probes to identify IRAP-BP-encoding nucleic acids (e.g., IRAP-BP mRNA) and fragments for use as PCR primers for the amplification or mutation of IRAP-BP nucleic acid molecules. As used herein, the term "nucleic acid molecule" is intended to include DNA molecules (e.g., cDNA or genomic DNA) and RNA molecules (e.g., mRNA) and analogs of the DNA or RNA generated using nucleotide analogs. The nucleic acid molecule can be single-stranded or double- stranded, but preferably is double-stranded DNA. An "isolated" nucleic acid molecule is one which is separated from other nucleic acid molecules which are present in the natural source of the nucleic acid. Preferably, an "isolated" nucleic acid is free of sequences which naturally flank the nucleic acid (i.e., sequences located at the 5' and 3' ends of the nucleic acid) in the genomic DNA of the organism from which the nucleic acid is derived. For example, in various embodiments, the isolated IRAP-BP nucleic acid molecule can contain less than about 5 kb, 4kb, 3kb, 2kb, 1 kb, 0.5 kb or 0.1 kb of nucleotide sequences which naturally flank the nucleic acid molecule in genomic DNA of the cell from which the nucleic acid is derived. Moreover, an "isolated" nucleic acid molecule, such as a cDNA molecule, can be substantially free of other cellular material, or culture medium when produced by recombinant techniques, or substantially free of chemical precursors or other chemicals when chemically synthesized. A nucleic acid molecule of the present invention, e.g., a nucleic acid molecule having the nucleotide sequence of SEQ ID NO:l, or SEQ ID NO:4, or a portion thereof, can be isolated using standard molecular biology techniques and the sequence information provided herein. Using all or portion of the nucleic acid sequence of SEQ ID NO:l or the nucleotide sequence of SEQ ID NO:4 as a hybridization probe, IRAP-BP nucleic acid molecules can be isolated using standard hybridization and cloning techniques (e.g., as described in Sambrook, J., Fritsh, E. F., and Maniatis, T. Molecular Cloning: A Laboratory Manual. 2nd, ed., Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, 1989). Moreover, a nucleic acid molecule encompassing all or a portion of SEQ ID NO: 1 or SEQ ID NO:4 can be isolated by the polymerase chain reaction (PCR) using synthetic oligonucleotide primers designed based upon the sequence of SEQ ID NO: 1 or SEQ ID NO:4. A nucleic acid of the invention can be amplified using cDNA, mRNA or alternatively, genomic DNA, as a template and appropriate oligonucleotide primers according to standard PCR amplification techniques. The nucleic acid so amplified can be cloned into an appropriate vector and characterized by DNA sequence analysis. Furthermore, oligonucleotides corresponding to IRAP-BP nucleotide sequences can be prepared by standard synthetic techniques, e.g., using an automated DNA synthesizer. In a preferred embodiment, an isolated nucleic acid molecule of the invention comprises the nucleotide sequence shown in SEQ ID NO:l. The sequence of SEQ ID NO:l corresponds to the murine IRAP-BP-1 cDNA. This cDNA comprises sequences encoding the murine IRAP-BP-1 protein (i.e., "the coding region", from nucleotides 690-2186), as well as 5' untranslated sequences (nucleotides 1-689) and 3' untranslated sequences (nucleotides 2187-2947). Alternatively, the nucleic acid molecule can comprise only the coding region of SEQ ID NO:l (e.g., nucleotides 690-2186, corresponding to SEQ ID NO:3). In another preferred embodiment, an isolated nucleic acid molecule of the invention comprises the nucleotide sequence shown in SEQ ID NO:4. The sequence of SEQ ID NO:l corresponds to the human IRAP-BP-1 cDNA. This cDNA comprises sequences encoding the human IRAP-BP-1 protein (i.e., "the coding region", from nucleotides 184-1683), as well as 5' untranslated sequences (nucleotides 1-183) and 3' untranslated sequences (nucleotides 1684-2064). Alternatively, the nucleic acid molecule can comprise only the coding region of SEQ ID NO:l (e.g., nucleotides 184- 1683, corresponding to SEQ ID NO:6). In another embodiment, an isolated nucleic acid molecule of the invention comprises the nucleotide sequence shown in SEQ ID NO:l comprising sequences encoding a murine IRAP-BP protein (i.e., "a coding region", from nucleotides 2-2186), as well as 3' untranslated sequences (nucleotides 2187-2947). Alternatively, the nucleic acid molecule can comprise only a coding region of SEQ ID NO:l (e.g., nucleotides 2- 2186, corresponding to SEQ ID NO:7). In another preferred embodiment, an isolated nucleic acid molecule of the invention comprises a nucleic acid molecule which is a complement of the nucleotide sequence shown in SEQ ID NO:l or SEQ ID NO:4, or a portion of any of these nucleotide sequences. A nucleic acid molecule which is complementary to the nucleotide sequence shown in SEQ ID NO:l or SEQ ID NO:4 is one which is sufficiently complementary to the nucleotide sequence shown in SEQ ID NO:l or SEQ ID NO:4 such that it can hybridize to the nucleotide sequence shown in SEQ ID NO: 1 or SEQ ID NO:4, thereby forming a stable duplex. In still another preferred embodiment, an isolated nucleic acid molecule of the present invention comprises a nucleotide sequence which is at least about 60-65%, preferably at least about 10-15%, more preferable at least about 80-85%, and even more preferably at least about 90-95%, 98% or more homologous to the nucleotide sequences shown in SEQ ID NO:l, SEQ ID NO:4, or a portion of any of these nucleotide sequences. Moreover, the nucleic acid molecule of the invention can comprise only a portion of the nucleic acid sequence of SEQ ID NO:l or SEQ ID NO:4, for example a fragment which can be used as a probe or primer or a fragment encoding a biologically active portion of an IRAP-BP polypeptide. The nucleotide sequence determined from the cloning of the IRAP-BP-1 genes allows for the generation of probes and primers designed for use in identifying and/or cloning other IRAP-BP family members, as well as IRAP-BP homologues from other species. The probe/primer typically comprises substantially purified oligonucleotide. The oligonucleotide typically comprises a region of nucleotide sequence that hybridizes under stringent conditions to at least about 12, preferably about 25, more preferably about 40, 50 or 75 consecutive nucleotides of a sense sequence of SEQ ID NO:l or SEQ ID NO:4, of an anti-sense sequence of SEQ ID NO:l or SEQ ID NO:4, or of a naturally occurring mutant of SEQ ID NO:l or SEQ ID NO:4. In an exemplary embodiment, a nucleic acid molecule of the present invention comprises a nucleotide sequence which is greater than 550 , preferably 551 -600, more preferably 601-650, more preferably 651-700, and even more preferably 701-800, 801- 900, 901-1000, 1001-1500, or 1501-2000 nucleotides in length and hybridizes under stringent hybridization conditions to a nucleic acid molecule of SEQ ID NO:l or SEQ ID NO:4. Probes based on the IRAP-BP nucleotide sequences can be used to detect transcripts or genomic sequences encoding the same or homologous proteins. In preferred embodiments, the probe further comprises a label group attached thereto, e.g., the label group can be a radioisotope, a fluorescent compound, an enzyme, or an enzyme co-factor. Such probes can be used as a part of a diagnostic test kit for identifying cells or tissue which misexpress an IRAP-BP polypeptide, such as by measuring a level of an IRAP-BP-encoding nucleic acid in a sample of cells from a subject e.g., detecting IRAP- BP mRNA levels or determining whether a genomic IRAP-BP gene has been mutated or deleted. A nucleic acid fragment encoding a "biologically active portion of an IRAP-BP polypeptide" can be prepared by isolating a portion of the nucleotide sequence of SEQ ID NO:l or SEQ ID NO:4 which encodes a polypeptide having an IRAP-BP biological activity (the biological activities of the IRAP-BP polypeptides have previously been described), expressing the encoded portion of the IRAP-BP polypeptide (e.g., by recombinant expression in vitro) and assessing the activity of the encoded portion of the IRAP-BP polypeptide. The invention further encompasses nucleic acid molecules that differ from the nucleotide sequence shown in SEQ ID NO:l or SEQ ID NO:4 due to degeneracy of the genetic code and thus encode the same IRAP-BP polypeptides as those encoded by the nucleotide sequence shown in SEQ ID NO: 1 or SEQ ID NO:4. In another embodiment, an isolated nucleic acid molecule of the invention has a nucleotide sequence encoding a protein having an amino acid sequence shown in SEQ ID NO:2 or SEQ ID NO:5. In addition to the IRAP-BP nucleotide sequences shown in SEQ ID NO:l or SEQ ID NO:4, it will be appreciated by those skilled in the art that DNA sequence polymorphisms that lead to changes in the amino acid sequences of the IRAP-BP polypeptides may exist within a population (e.g., the human population). Such genetic polymorphism in the IRAP-BP genes may exist among individuals within a population due to natural allelic variation. As used herein, the terms "gene" and "recombinant gene" refer to nucleic acid molecules comprising an open reading frame encoding an IRAP-BP polypeptide, preferably a mammalian IRAP-BP polypeptide. Such natural allelic variations can typically result in 1-5% variance in the nucleotide sequence of an IRAP-BP gene. Any and all such nucleotide variations and resulting amino acid polymorphisms in IRAP-BP genes that are the result of natural allelic variation and that do not alter the functional activity of an IRAP-BP polypeptide are intended to be within the scope of the invention. Moreover, nucleic acid molecules encoding other IRAP-BP family members and thus which have a nucleotide sequence which differs from the IRAP-BP-1 sequences of SEQ ID NO: 1 or SEQ ID NO:4 are intended to be within the scope of the invention. For example, an IRAP-BP-2 cDNA can be identified based on the nucleotide sequence of human IRAP-BP-1 or murine IRAP-BP-1. Moreover, nucleic acid molecules encoding IRAP-BP polypeptides from different species, and thus which have a nucleotide sequence which differs from the IRAP-BP sequences of SEQ ID NO:l or SEQ ID NO:4 are intended to be within the scope of the invention. For example, an rat IRAP-BP cDNA can be identified based on the nucleotide sequence of a human or murine IRAP- BP. Nucleic acid molecules corresponding to natural allelic variants and homologues of the IRAP-BP cDNAs of the invention can be isolated based on their homology to the IRAP-BP nucleic acids disclosed herein using the cDNAs disclosed herein, or a portion thereof, as a hybridization probe according to standard hybridization techniques under stringent hybridization conditions. Accordingly, in another embodiment, an isolated nucleic acid molecule of the invention is at least 15 nucleotides in length and hybridizes under stringent conditions to the nucleic acid molecule comprising the nucleotide sequence of SEQ ID NO:l or SEQ ID NO:4. In another embodiment, the nucleic acid is at least 30, 50, 100, 250 or 500 nucleotides in length. As used herein, the term "hybridizes under stringent conditions" is intended to describe conditions for hybridization and washing under which nucleotide sequences at least 60% homologous to each other typically remain hybridized to each other. Preferably, the conditions are such that sequences at least about 70%, more preferably at least about 80%, even more preferably at least about 85% or 90% homologous to each other typically remain hybridized to each other. Such stringent conditions are known to those skilled in the art and can be found in Current Protocols in Molecular Biology, John Wiley & Sons, N.Y. (1989), 6.3.1-6.3.6. A preferred, non- limiting example of stringent hybridization conditions are hybridization in 6X sodium chloride/sodium citrate (SSC) at about 45C, followed by one or more washes in 0.2 X SSC, 0.1% SDS at 50-65°C. Preferably, an isolated nucleic acid molecule of the invention that hybridizes under stringent conditions to the sequence of SEQ ID NO:l or SEQ ID NO:4 corresponds to a naturally-occurring nucleic acid molecule. As used herein, a "naturally-occurring" nucleic acid molecule refers to an RNA or DNA molecule having a nucleotide sequence that occurs in nature (e.g., encodes a natural protein). In addition to naturally-occurring allelic variants of the IRAP-BP sequences that may exist in the population, the skilled artisan will further appreciate that changes can be introduced by mutation into the nucleotide sequences of SEQ ID NO:l or SEQ ID NO:4, thereby leading to changes in the amino acid sequence of the encoded IRAP-BP polypeptides, without altering the functional ability of the IRAP-BP polypeptides. For example, nucleotide substitutions leading to amino acid substitutions at "non-essential" amino acid residues can be made in the sequence of SEQ ID NO: 1 or SEQ ID NO:4. A "non-essential" amino acid residue is a residue that can be altered from the wild-type sequence of IRAP-BP (e.g., the sequence of SEQ ID NO:2) without altering the biological activity, whereas an "essential" amino acid residue is required for biological activity. For example, amino acid residues that are conserved among the IRAP-BP polypeptides of the present invention, are predicted to be particularly unamenable to alteration. Moreover, amino acid residues that are defined by the helix-turn-helix domain are particularly unamenable to alteration. Accordingly, another aspect of the invention pertains to nucleic acid molecules encoding IRAP-BP polypeptides that contain changes in amino acid residues that are not essential for activity. Such IRAP-BP polypeptides differ in amino acid sequence from SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8, yet retain biological activity. In one embodiment, the isolated nucleic acid molecule comprises a nucleotide sequence encoding a protein, wherein the protein comprises an amino acid sequence at least about 60% homologous to the amino acid sequence of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. Preferably, the protein encoded by the nucleic acid molecule is at least about 65-70% homologous to SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8, more preferably at least about 75-80% homologous to SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8, even more preferably at least about 85-90% homologous to SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8, and even more preferably at least about 95%, 98% or more homologous to SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. An isolated nucleic acid molecule encoding an IRAP-BP polypeptide homologous to the protein of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8 can be created by introducing one or more nucleotide substitutions, additions or deletions into the nucleotide sequence of SEQ ID NO:l or SEQ ID NO:4 or such that one or more amino acid substitutions, additions or deletions are introduced into the encoded protein. Mutations can be introduced into SEQ ID NO: 1 or SEQ ID NO:4 by standard techniques, such as site-directed mutagenesis and PCR-mediated mutagenesis. Preferably, conservative amino acid substitutions are made at one or more predicted non-essential amino acid residues. A "conservative amino acid substitution" is one in which the amino acid residue is replaced with an amino acid residue having a similar side chain. Families of amino acid residues having similar side chains have been defined in the art. These families include amino acids with basic side chains (e.g., lysine, arginine, histidine), acidic side chains (e.g., aspartic acid, glutamic acid), uncharged polar side chains (e.g., glycine, asparagine, glutamine, serine, threonine, tyrosine, cysteine), nonpolar side chains (e.g., alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan), beta-branched side chains (e.g., threonine, valine, isoleucine) and aromatic side chains (e.g., tyrosine, phenylalanine, tryptophan, histidine). Thus, a predicted nonessential amino acid residue in an IRAP-BP polypeptide is preferably replaced with another amino acid residue from the same side chain family. Alternatively, in another embodiment, mutations can be introduced randomly along all or part of an IRAP-BP coding sequence, such as by saturation mutagenesis, and the resultant mutants can be screened for IRAP-BP biological activity to identify mutants that retain activity. Following mutagenesis of SEQ ID NO:l or SEQ ID NO:4 or the encoded protein can be expressed recombinantly and the activity of the protein can be determined. In a preferred embodiment, a mutant IRAP-BP polypeptide can be assayed for the ability to (1) interact with an intracellular non-IRAP-BP molecule, for example, and IRAP-BP target molecule (e.g., IRAP and/or GLUT4); (2) bind to an intracellular non- IRAP-BP molecule, for example, and IRAP-BP target molecule (e.g., IRAP and/or GLUT4); (3) recognize a trafficking motif within an intracellular non-IRAP-BP molecule, for example, and IRAP-BP target molecule (e.g., IRAP and/or GLUT4); (4) bind to a trafficking motif within an intracellular non-IRAP-BP molecule, for example, and IRAP-BP target molecule (e.g., IRAP and/or GLUT4); (5) modulate translocation of an IRAP-BP target molecule; (6) modulate GLUT4 translocation (e.g., exocytosis); (7) modulate IRAP translocation (e.g., exocytosis); (8) modulate sorting of an IRAP-BP target molecule; (9) modulate sorting of IRAP and/or GLUT4; (10) modulate retention of an IRAP-BP target molecule; (11) modulate retention of IRAP and/or GLUT4 (e.g., within a specialized intracellular compartment, for example, the endocytic recycling compartment; (12) modulate the entry of an IRAP-BP target molecule into recycling vesicles; (13) modulate entry of IRAP and/or GLUT4 into recycling vesicles; (15) regulate intracellular trafficking and/or retention of an IRAP-BP target molecule; (16) regulate intracellular trafficking and/or retention of IRAP and/or GLUT4; or (77) regulate intracellular trafficking of GLUT4- and/or IRAP-containing vesicles. In addition to the nucleic acid molecules encoding IRAP-BP polypeptides described above, another aspect of the invention pertains to isolated nucleic acid molecules which are antisense thereto. An "antisense" nucleic acid comprises a nucleotide sequence which is complementary to a "sense" nucleic acid encoding a protein, e.g., complementary to the coding strand of a double-stranded cDNA molecule or complementary to an mRNA sequence. Accordingly, an antisense nucleic acid can hydrogen bond to a sense nucleic acid. The antisense nucleic acid can be complementary to an entire IRAP-BP coding strand, or to only a portion thereof. In one embodiment, an antisense nucleic acid molecule is antisense to a "coding region" of the coding strand of a nucleotide sequence encoding IRAP-BP. The term "coding region" refers to the region of the nucleotide sequence comprising codons which are translated into amino acid residues (e.g., the coding region of murine IRAP-BP-1 corresponds to SEQ ID NO:3). In another embodiment, the antisense nucleic acid molecule is antisense to a "noncoding region" of the coding strand of a nucleotide sequence encoding IRAP- BP. The term "noncoding region" refers to 5' and 3' sequences which flank the coding region that are not translated into amino acids (i.e., also referred to as 5' and 3' untranslated regions). Given the coding strand sequences encoding IRAP-BP disclosed herein (e.g., SEQ ID NO: 3 and SEQ ID NO: 6), antisense nucleic acids of the invention can be designed according to the rules of Watson and Crick base pairing. The antisense nucleic acid molecule can be complementary to the entire coding region of IRAP-BP mRNA, but more preferably is an oligonucleotide which is antisense to only a portion of the coding or noncoding region of IRAP-BP mRNA. For example, the antisense oligonucleotide can be complementary to the region surrounding the translation start site of IRAP-BP mRNA. An antisense oligonucleotide can be, for example, about 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50 nucleotides in length. An antisense nucleic acid of the invention can be constructed using chemical synthesis and enzymatic ligation reactions using procedures known in the art. For example, an antisense nucleic acid (e.g., an antisense oligonucleotide) can be chemically synthesized using naturally occurring nucleotides or variously modified nucleotides designed to increase the biological stability of the molecules or to increase the physical stability of the duplex formed between the antisense and sense nucleic acids, e.g., phosphorothioate derivatives and acridine substituted nucleotides can be used. Examples of modified nucleotides which can be used to generate the antisense nucleic acid include 5-fluorouracil, 5-bromouracil, 5-chlorouracil, 5-iodouracil, hypoxanthine, xantine, 4-acetylcytosine, 5- (carboxyhydroxylmethyl) uracil, 5-carboxymethylaminomethyl-2-thiouridine, 5- carboxymethylaminomethyluracil, dihydrouracil, beta-D-galactosylqueosine, inosine, N6-isopentenyladenine, 1-methylguanine, 1-methylinosine, 2,2-dimethylguanine, 2- methyladenine, 2-methylguanine, 3-methylcytosine, 5-methylcytosine, N6-adenine, 7- methylguanine, 5-methylaminomethyluracil, 5-methoxyaminomethyl-2-thiouracil, beta- D-mannosylqueosine, 5'-methoxycarboxymethyluracil, 5-methoxyuracil, 2-methylthio- N6-isopentenyladenine, uracil-5-oxyacetic acid (v), wybutoxosine, pseudouracil, queosine, 2-thiocytosine, 5-methyl-2-thiouracil, 2-thiouracil, 4-thiouracil, 5- methyluracil, uracil-5- oxyacetic acid methylester, uracil-5-oxyacetic acid (v), 5-methyl- 2-thiouracil, 3-(3-amino-3-N-2-carboxypropyl) uracil, (acp3)w, and 2,6-diaminopurine. Alternatively, the antisense nucleic acid can be produced biologically using an expression vector into which a nucleic acid has been subcloned in an antisense orientation (i.e., RNA transcribed from the inserted nucleic acid will be of an antisense orientation to a target nucleic acid of interest, described further in the following subsection). The antisense nucleic acid molecules of the invention are typically administered to a subject or generated in situ such that they hybridize with or bind to cellular mRNA and/or genomic DNA encoding an IRAP-BP polypeptide to thereby inhibit expression of the protein, e.g., by inhibiting transcription and/or translation. The hybridization can be by conventional nucleotide complementarity to form a stable duplex, or, for example, in the case of an antisense nucleic acid molecule which binds to DNA duplexes, through specific interactions in the major groove of the double helix. An example of a route of administration of antisense nucleic acid molecules of the invention include direct injection at a tissue site. Alternatively, antisense nucleic acid molecules can be modified to target selected cells and then administered systemically. For example, for systemic administration, antisense molecules can be modified such that they specifically bind to receptors or antigens expressed on a selected cell surface, e.g., by linking the antisense nucleic acid molecules to peptides or antibodies which bind to cell surface receptors or antigens. The antisense nucleic acid molecules can also be delivered to cells using the vectors described herein. To achieve sufficient intracellular concentrations of the antisense molecules, vector constructs in which the antisense nucleic acid molecule is placed under the control of a strong pol II or pol III promoter are preferred. In yet another embodiment, the antisense nucleic acid molecule of the invention is an α-anomeric nucleic acid molecule. An α-anomeric nucleic acid molecule forms specific double-stranded hybrids with complementary RNA in which, contrary to the usual β-units, the strands run parallel to each other (Gaultier et al. (1987) Nucleic Acids. Res. 15:6625-6641). The antisense nucleic acid molecule can also comprise a 2'-o- methylribonucleotide (Inoue et al. (1987) Nucleic Acids Res. 15:6131-6148) or a chimeric RNA-DNA analogue (Inoue et al. (1987) FEBS Lett. 215:327-330). In still another embodiment, an antisense nucleic acid of the invention is a ribozyme. Ribozymes are catalytic RNA molecules with ribonuclease activity which are capable of cleaving a single-stranded nucleic acid, such as an mRNA, to which they have a complementary region. Thus, ribozymes (e.g., hammerhead ribozymes (described in Haselhoff and Gerlach (1988) Nature 334:585-591)) can be used to catalytically cleave IRAP-BP mRNA transcripts to thereby inhibit translation of IRAP- BP mRNA. A ribozyme having specificity for an IRAP-BP-encoding nucleic acid can be designed based upon the nucleotide sequence of an IRAP-BP-1 or IRAP-BP-2 cDNA disclosed herein (i.e., SEQ ID NO:l or SEQ ID NO:4). For example, a derivative of a Tetrahymena L-19 IVS RNA can be constructed in which the nucleotide sequence of the active site is complementary to the nucleotide sequence to be cleaved in an IRAP-BP- encoding mRNA. See, e.g., Cech et al. U.S. Patent No. 4,987,071; and Cech et al. U.S. Patent No. 5,116,742. Alternatively, IRAP-BP mRNA can be used to select a catalytic RNA having a specific ribonuclease activity from a pool of RNA molecules. See, e.g., Bartel, D. and Szostak, J.W. (1993) Science 261 :1411-1418. Alternatively, IRAP-BP gene expression can be inhibited by targeting nucleotide sequences complementary to the regulatory region of the IRAP-BP (e.g., the IRAP-BP promoter and/or enhancers) to form triple helical structures that prevent transcription of the IRAP-BP gene in target cells. See generally, Helene, C. (1991) Anticancer Drug Des. 6(6):569-84; Helene, C. et al. (1992) Ann. N. Y. Acad. Sci. 660:27-36; and Maher, L.J. (1992) Bioassays 14(12):807-15. II. Isolated IRAP-BP Polypeptides and Anti-IRAP-BP Antibodies One aspect of the invention pertains to isolated IRAP-BP polypeptides, and biologically active portions thereof, as well as polypeptide fragments suitable for use as immunogens to raise anti-IRAP-BP antibodies. In one embodiment, native IRAP-BP polypeptides can be isolated from cells or tissue sources by an appropriate purification scheme using standard protein purification techniques. In another embodiment, IRAP- BP polypeptides are produced by recombinant DNA techniques. Alternative to recombinant expression, an IRAP-BP polypeptide can be synthesized chemically using standard peptide synthesis techniques. An "isolated" or "purified" protein or biologically active portion thereof is substantially free of cellular material or other contaminating proteins from the cell or tissue source from which the IRAP-BP polypeptide is derived, or substantially free from chemical precursors or other chemicals when chemically synthesized. The language "substantially free of cellular material" includes preparations of IRAP-BP polypeptide in which the protein is separated from cellular components of the cells from which it is isolated or recombinantly produced. In one embodiment, the language "substantially free of cellular material" includes preparations of IRAP-BP polypeptide having less than about 30% (by dry weight) of non-IRAP-BP polypeptide (also referred to herein as a "contaminating protein"), more preferably less than about 20% of non-IRAP-BP polypeptide, still more preferably less than about 10% of non-IRAP-BP polypeptide, and most preferably less than about 5% non-IRAP-BP polypeptide. When the IRAP-BP polypeptide or biologically active portion thereof is recombinantly produced, it is also preferably substantially free of culture medium, i.e., culture medium represents less than about 20%, more preferably less than about 10%, and most preferably less than about 5% of the volume of the protein preparation. The language "substantially free of chemical precursors or other chemicals" includes preparations of IRAP-BP polypeptide in which the protein is separated from chemical precursors or other chemicals which are involved in the synthesis of the protein. In one embodiment, the language "substantially free of chemical precursors or other chemicals" includes preparations of IRAP-BP polypeptide having less than about 30% (by dry weight) of chemical precursors or non-IRAP-BP chemicals, more preferably less than about 20% chemical precursors or non-IRAP-BP chemicals, still more preferably less than about 10% chemical precursors or non-IRAP-BP chemicals, and most preferably less than about 5% chemical precursors or non-IRAP-BP chemicals. Biologically active portions of an IRAP-BP polypeptide include peptides comprising amino acid sequences sufficiently homologous to or derived from the amino acid sequence of the IRAP-BP polypeptide, e.g., the amino acid sequence shown in SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8, which include less amino acids than the full length IRAP-BP polypeptides, and exhibit at least one activity of an IRAP-BP polypeptide. Typically, biologically active portions comprise a domain or motif with at least one activity of the IRAP-BP polypeptide. A biologically active portion of an IRAP-BP polypeptide can be a polypeptide which is, for example, 50, 100, 150, 200, 250, 300, 350, 400, 450 or more amino acids in length. For example, in one embodiment, a biologically active portion of an IRAP-BP polypeptide comprises at least a helix-turn-helix domain. Moreover, other biologically active portions, in which other regions of the protein are deleted, can be prepared by recombinant techniques and evaluated for one or more of the functional activities of a native IRAP-BP polypeptide. In a preferred embodiment, the IRAP-BP polypeptide has an amino acid sequence shown in SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. In other embodiments, the IRAP-BP polypeptide is substantially homologous to SEQ ID NO:2, SEQ ID NO: 5, or SEQ ID NO: 8, and retains the functional activity of the protein of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8, yet differs in amino acid sequence due to natural allelic variation or mutagenesis, as described in detail in subsection I above. Accordingly, in another embodiment, the IRAP-BP polypeptide is a protein which comprises an amino acid sequence at least about 60% homologous to the amino acid sequence of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8 and retains the functional activity of the IRAP-BP polypeptides of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8, respectively. Preferably, the protein is at least about 65-70% homologous to SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8, more preferably at least about 75-80% homologous to SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8, even more preferably at least about 85-90% homologous to SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8, and most preferably at least about 95%, 98% or more homologous to SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. To determine the percent homology or identity of two amino acid sequences or of two nucleotide or amino acid sequences, the sequences are aligned for optimal comparison purposes (e.g., gaps can be introduced in the sequence of a first amino acid or nucleic acid sequence for optimal alignment with a second amino or nucleic acid sequence). In a preferred embodiment, the length of a reference sequence aligned for comparison purposes is at least 30%, preferably at least 40%, more preferably at least 50%, even more preferably at least 60%, and even more preferably at least 70%, 80%, or 90% of the length of the reference sequence (e.g., when aligning a second sequence to the IRAP-BP amino acid sequence of SEQ ID NO:2 having 499 amino acid residues, at least 150, preferably at least 200, more preferably at least 250, even more preferably at least 299, and even more preferably at least 349, 399 or 449 amino acid residues are aligned). The amino acid residues or nucleotides at corresponding amino acid positions or nucleotide positions are then compared. When a position in the first sequence is occupied by the same amino acid residue or nucleotide as the corresponding position in the second sequence, then the molecules are homologous at that position (i.e., as used herein amino acid or nucleic acid "homology" is equivalent to amino acid or nucleic acid "identity"). The percent homology between the two sequences is a function of the number of identical positions shared by the sequences (i.e., % homology = # of identical positions/total # of positions x 100). The comparison of sequences and determination of percent homology between two sequences can be accomplished using a mathematical algorithim. A preferred, non- limiting example of a mathematical algorithim utilized for the comparison of sequences is the algorithm of Karlin and Altschul (1990) Proc. Natl. Acad. Sci. USA 87:2264-68, modified as in Karlin and Altschul (1993) Proc. Natl. Acad. Sci. USA 90:5873-77. Such an algorithm is incoφorated into the NBLAST and XBLAST programs (version 2.0) of Altschul, et al. (1990) J. Mol. Biol. 215:403-10. BLAST nucleotide searches can be performed with the NBLAST program, score = 100, wordlength = 12 to obtain nucleotide sequences homologous to IRAP-BP nucleic acid molecules of the invention. BLAST protein searches can be performed with the XBLAST program, score = 50, wordlength = 3 to obtain amino acid sequences homologous to IRAP-BP polypeptide molecules of the invention. To obtain gapped alignments for comparison puφoses, Gapped BLAST can be utilized as described in Altschul et al., (1997) Nucleic Acids Research 25(17):3389-3402. When utilizing BLAST and Gapped BLAST programs, the default parameters of the respective programs (e.g., XBLAST and NBLAST) can be used. See http://www.ncbi.nlm.nih.gov. Another preferred, non-limiting example of a mathematical algorithim utilized for the comparison of sequences is the algorithm of Myers and Miller, CABIOS (1989). Such an algorithm is incoφorated into the ALIGN program (version 2.0) which is part of the GCG sequence alignment software package. When utilizing the ALIGN program for comparing amino acid sequences, a PAM120 weight residue table, a gap length penalty of 12, and a gap penalty of 4 can be used. The invention also provides IRAP-BP chimeric or fusion proteins. As used herein, an IRAP-BP "chimeric protein" or "fusion protein" comprises an IRAP-BP polypeptide operatively linked to a non-IRAP-BP polypeptide. A "IRAP-BP polypeptide" refers to a polypeptide having an amino acid sequence corresponding to IRAP-BP, whereas a "non-IRAP-BP polypeptide" refers to a polypeptide having an amino acid sequence corresponding to a protein which is not substantially homologous to the IRAP-BP polypeptide, e.g., a protein which is different from the IRAP-BP polypeptide and which is derived from the same or a different organism. Within an IRAP-BP fusion protein the IRAP-BP polypeptide can correspond to all or a portion of an IRAP-BP polypeptide. In a preferred embodiment, an IRAP-BP fusion protein comprises at least one biologically active portion of an IRAP-BP polypeptide. In another preferred embodiment, an IRAP-BP fusion protein comprises at least two biologically active portions of an IRAP-BP polypeptide. Within the fusion protein, the term "operatively linked" is intended to indicate that the IRAP-BP polypeptide and the non-IRAP-BP polypeptide are fused in-frame to each other. The non-IRAP-BP polypeptide can be fused to the N-terminus or C-terminus of the IRAP-BP polypeptide. For example, in one embodiment, the fusion protein is a GST-IRAP-BP fusion protein in which the IRAP-BP sequences are fused to the C-terminus of the GST sequences. Such fusion proteins can facilitate the purification of recombinant IRAP-BP. In another embodiment, the fusion protein is an IRAP-BP polypeptide containing a heterologous signal sequence at its N-terminus. For example, a native IRAP-BP signal sequence can be removed and replaced with a signal sequence from another protein. In certain host cells (e.g., mammalian host cells), expression and/or secretion of IRAP-BP can be increased through use of a heterologous signal sequence. The IRAP-BP fusion proteins of the invention can be incoφorated into pharmaceutical compositions and administered to a subject in vivo. The IRAP-BP fusion proteins can be used to affect the bioavailability of an IRAP-BP substrate. Use of IRAP-BP fusion proteins may be useful therapeutically for the treatment of respiratory disorders (e.g., asthma). Moreover, the IRAP-BP-fusion proteins of the invention can be used as immunogens to produce anti-IRAP-BP antibodies in a subject, to purify IRAP-BP ligands and in screening assays to identify molecules which inhibit the interaction of IRAP-BP with an IRAP-BP ligand. Preferably, an IRAP-BP chimeric or fusion protein of the invention is produced by standard recombinant DNA techniques. For example, DNA fragments coding for the different polypeptide sequences are ligated together in-frame in accordance with conventional techniques, for example by employing blunt-ended or stagger-ended termini for ligation, restriction enzyme digestion to provide for appropriate termini, filling-in of cohesive ends as appropriate, alkaline phosphatase treatment to avoid undesirable joining, and enzymatic ligation. In another embodiment, the fusion gene can be synthesized by conventional techniques including automated DNA synthesizers. Alternatively, PCR amplification of gene fragments can be carried out using anchor primers which give rise to complementary overhangs between two consecutive gene fragments which can subsequently be annealed and reamplified to generate a chimeric gene sequence (see, for example, Current Protocols in Molecular Biology, eds. Ausubel et al. John Wiley & Sons: 1992). Moreover, many expression vectors are commercially available that already encode a fusion moiety (e.g., a GST polypeptide). An IRAP-BP- encoding nucleic acid can be cloned into such an expression vector such that the fusion moiety is linked in-frame to the IRAP-BP polypeptide. The present invention also pertains to variants of the IRAP-BP polypeptides which function as either IRAP-BP agonists (mimetics) or as IRAP-BP antagonists. Variants of the IRAP-BP polypeptides can be generated by mutagenesis, e.g., discrete point mutation or truncation of an IRAP-BP polypeptide. An agonist of the IRAP-BP polypeptides can retain substantially the same, or a subset, of the biological activities of the naturally occurring form of an IRAP-BP polypeptide. An antagonist of an IRAP-BP polypeptide can inhibit one or more of the activities of the naturally occurring form of the IRAP-BP polypeptide by, for example, competitively inhibiting the protease activity of an IRAP-BP polypeptide. Thus, specific biological effects can be elicited by treatment with a variant of limited function. In one embodiment, treatment of a subject with a variant having a subset of the biological activities of the naturally occurring form of the protein has fewer side effects in a subject relative to treatment with the naturally occurring form of the IRAP-BP polypeptide. In one embodiment, variants of an IRAP-BP polypeptide which function as either IRAP-BP agonists (mimetics) or as IRAP-BP antagonists can be identified by screening combinatorial libraries of mutants, e.g., truncation mutants, of an IRAP-BP polypeptide for IRAP-BP polypeptide agonist or antagonist activity. In one embodiment, a variegated library of IRAP-BP variants is generated by combinatorial mutagenesis at the nucleic acid level and is encoded by a variegated gene library. A variegated library of IRAP-BP variants can be produced by, for example, enzymatically ligating a mixture of synthetic oligonucleotides into gene sequences such that a degenerate set of potential IRAP-BP sequences is expressible as individual polypeptides, or alternatively, as a set of larger fusion proteins (e.g., for phage display) containing the set of IRAP-BP sequences therein. There are a variety of methods which can be used to produce libraries of potential IRAP-BP variants from a degenerate oligonucleotide sequence. Chemical synthesis of a degenerate gene sequence can be performed in an automatic DNA synthesizer, and the synthetic gene then ligated into an appropriate expression vector. Use of a degenerate set of genes allows for the provision, in one mixture, of all of the sequences encoding the desired set of potential IRAP-BP sequences. Methods for synthesizing degenerate oligonucleotides are known in the art (see, e.g., Narang, S.A. (1983) Tetrahedron 39:3; Itakura et al. (1984) Annu. Rev. Biochem. 53:323; Itakura et al. (1984) Science 198:1056; Ike et al. (1983) Nucleic Acid Res. 11 :477. In addition, libraries of fragments of an IRAP-BP polypeptide coding sequence can be used to generate a variegated population of IRAP-BP fragments for screening and subsequent selection of variants of an IRAP-BP polypeptide. In one embodiment, a library of coding sequence fragments can be generated by treating a double stranded PCR fragment of an IRAP-BP coding sequence with a nuclease under conditions wherein nicking occurs only about once per molecule, denaturing the double stranded DNA, renaturing the DNA to form double stranded DNA which can include sense/antisense pairs from different nicked products, removing single stranded portions from reformed duplexes by treatment with S 1 nuclease, and ligating the resulting fragment library into an expression vector. By this method, an expression library can be derived which encodes N-terminal, C-terminal and internal fragments of various sizes of the IRAP-BP polypeptide. Several techniques are known in the art for screening gene products of combinatorial libraries made by point mutations or truncation, and for screening cDNA libraries for gene products having a selected property. Such techniques are adaptable for rapid screening of the gene libraries generated by the combinatorial mutagenesis of IRAP-BP polypeptides. The most widely used techniques, which are amenable to high through-put analysis, for screening large gene libraries typically include cloning the gene library into replicable expression vectors, transforming appropriate cells with the resulting library of vectors, and expressing the combinatorial genes under conditions in which detection of a desired activity facilitates isolation of the vector encoding the gene whose product was detected. Recrusive ensemble mutagenesis (REM), a new technique which enhances the frequency of functional mutants in the libraries, can be used in combination with the screening assays to identify IRAP-BP variants (Arkin and Yourvan (1992) PNAS 59:7811 -7815; Delgrave et al. (1993) Protein Engineering 6(3):327-331). In one embodiment, cell based assays can be exploited to analyze a variegated IRAP-BP library. For example, a library of expression vectors can be transfected into a cell line which ordinarily synthesizes IRAP-BP. The transfected cells are then cultured such that a particular mutant IRAP-BP is expressed and the effect of expression of the mutant on IRAP-BP activity in the cell can be detected, e.g., by any of a number of activity assays for native IRAP-BP polypeptide. Plasmid DNA can then be recovered from the cells which score for modulated IRAP-BP activity, and the individual clones further characterized. An isolated IRAP-BP polypeptide, or a portion or fragment thereof, can be used as an immunogen to generate antibodies that bind IRAP-BP using standard techniques for polyclonal and monoclonal antibody preparation. A full-length IRAP-BP polypeptide can be used or, alternatively, the invention provides antigenic peptide fragments of IRAP-BP for use as immunogens. The antigenic peptide of IRAP-BP comprises at least 8 amino acid residues of the amino acid sequence shown in SEQ ID NO:2 and encompasses an epitope of IRAP-BP such that an antibody raised against the peptide forms a specific immune complex with IRAP-BP. Preferably, the antigenic peptide comprises at least 10 amino acid residues, more preferably at least 15 amino acid residues, even more preferably at least 20 amino acid residues, and most preferably at least 30 amino acid residues. Preferred epitopes encompassed by the antigenic peptide are regions of IRAP-BP that are located on the surface of the protein, e.g., hydrophilic regions. An IRAP-BP immunogen typically is used to prepare antibodies by immunizing a suitable subject, (e.g., rabbit, goat, mouse or other mammal) with the immunogen. An appropriate immunogenic preparation can contain, for example, recombinantly expressed IRAP-BP polypeptide or a chemically synthesized IRAP-BP polypeptide. The preparation can further include an adjuvant, such as Freund's complete or incomplete adjuvant, or similar immunostimulatory agent. Immunization of a suitable subject with an immunogenic IRAP-BP preparation induces a polyclonal anti-IRAP-BP antibody response. Accordingly, another aspect of the invention pertains to anti-IRAP-BP antibodies. The term "antibody" as used herein refers to immunoglobulin molecules and immunologically active portions of immunoglobulin molecules, i.e., molecules that contain an antigen binding site which specifically binds (immunoreacts with) an antigen, such as IRAP-BP. Examples of immunologically active portions of immunoglobulin molecules include F(ab) and F(ab') 2 fragments which can be generated by treating the antibody with an enzyme such as pepsin. The invention provides polyclonal and monoclonal antibodies that bind IRAP-BP. The term "monoclonal antibody" or "monoclonal antibody composition", as used herein, refers to a population of antibody molecules that contain only one species of an antigen binding site capable of immunoreacting with a particular epitope of IRAP-BP. A monoclonal antibody composition thus typically displays a single binding affinity for a particular IRAP-BP polypeptide with which it immunoreacts. Polyclonal anti-IRAP-BP antibodies can be prepared as described above by immunizing a suitable subject with an IRAP-BP immunogen. The anti-IRAP-BP antibody titer in the immunized subject can be monitored over time by standard techniques, such as with an enzyme linked immunosorbent assay (ELISA) using immobilized IRAP-BP. If desired, the antibody molecules directed against IRAP-BP can be isolated from the mammal (e.g., from the blood) and further purified by well known techniques, such as protein A chromatography to obtain the IgG fraction. At an appropriate time after immunization, e.g., when the anti-IRAP-BP antibody titers are highest, antibody-producing cells can be obtained from the subject and used to prepare monoclonal antibodies by standard techniques, such as the hybridoma technique originally described by Kohler and Milstein (1975) Nature 256:495-497) (see also, Brown et al. (1981) J. Immunol. 127:539-46; Brown et al. (1980) J. Biol. Chem .255:4980-83; Yeh et al. (1976) PNAS 76:2927-31; and Yeh et al. (1982) Int. J. Cancer 29:269-75), the more recent human B cell hybridoma technique (Kozbor et al. (1983) Immunol Today 4:72), the EBV-hybridoma technique (Cole et al. (1985), Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc., pp. 77-96) or trioma techniques. The technology for producing monoclonal antibody hybridomas is well known (see generally R. H. Kenneth, in Monoclonal Antibodies: A New Dimension In Biological Analyses, Plenum Publishing Coφ., New York, New York (1980); E. A. Lerner (1981) Yale J. Biol. Med, 54:387-402; M. L. Gefter et al. (1977) Somatic Cell Genet. 3:231-36). Briefly, an immortal cell line (typically a myeloma) is fused to lymphocytes (typically splenocytes) from a mammal immunized with an IRAP-BP immunogen as described above, and the culture supernatants of the resulting hybridoma cells are screened to identify a hybridoma producing a monoclonal antibody that binds IRAP-BP. Any of the many well known protocols used for fusing lymphocytes and immortalized cell lines can be applied for the puφose of generating an anti-IRAP-BP monoclonal antibody (see, e.g., G. Galfre et al. (1977) Nature 266:55052; Gefter et al. Somatic Cell Genet. , cited supra; Lerner, Yale J. Biol. Med. , cited supra; Kenneth, Monoclonal Antibodies, cited supra). Moreover, the ordinarily skilled worker will appreciate that there are many variations of such methods which also would be useful. Typically, the immortal cell line (e.g., a myeloma cell line) is derived from the same mammalian species as the lymphocytes. For example, murine hybridomas can be made by fusing lymphocytes from a mouse immunized with an immunogenic preparation of the present invention with an immortalized mouse cell line. Preferred immortal cell lines are mouse myeloma cell lines that are sensitive to culture medium containing hypoxanthine, aminopterin and thymidine ("HAT medium"). Any of a number of myeloma cell lines can be used as a fusion partner according to standard techniques, e.g., the P3-NSl/l-Ag4-l, P3-x63-Ag8.653 or Sp2/O-Agl4 myeloma lines. These myeloma lines are available from ATCC. Typically, HAT-sensitive mouse myeloma cells are fused to mouse splenocytes using polyethylene glycol ("PEG"). Hybridoma cells resulting from the fusion are then selected using HAT medium, which kills unfused and unproductively fused myeloma cells (unfused splenocytes die after several days because they are not transformed). Hybridoma cells producing a monoclonal antibody of the invention are detected by screening the hybridoma culture supernatants for antibodies that bind IRAP-BP, e.g., using a standard ELISA assay. Alternative to preparing monoclonal antibody-secreting hybridomas, a monoclonal anti-IRAP-BP antibody can be identified and isolated by screening a recombinant combinatorial immunoglobulin library (e.g., an antibody phage display library) with IRAP-BP to thereby isolate immunoglobulin library members that bind IRAP-BP. Kits for generating and screening phage display libraries are commercially available (e.g., the Pharmacia Recombinant Phage Antibody System, Catalog No. 27- 9400-01; and the Stratagene SurfZAP™ Phage Display Kit, Catalog No. 240612). Additionally, examples of methods and reagents particularly amenable for use in generating and screening antibody display library can be found in, for example, Ladner et al. U.S. Patent No. 5,223,409; Kang et al. PCT International Publication No. WO 92/18619; Dower et al. PCT International Publication No. WO 91/17271; Winter et al. PCT International Publication WO 92/20791; Markland et al. PCT International Publication No. WO 92/15679; Breitling et al. PCT International Publication WO 93/01288; McCafferty et al. PCT International Publication No. WO 92/01047; Garrard et al. PCT International Publication No. WO 92/09690; Ladner et al. PCT International Publication No. WO 90/02809; Fuchs et al. (1991) Bio/Technology 9:1370-1372; Hay et al. (1992) Hum. Antibod. Hybridomas 3:81-85; Huse et al. (1989) Science 246:1275- 1281; Griffiths et al. (1993) EMBO J 12:725 -734; Hawkins et al. (1992) J. Mol. Biol. 226:889-896; Clarkson et al. (1991) Nature 352:624-628; Gram et al. (1992) PNAS 89:3576-3580; Garrad et al. (1991) Bio/Technology 9:1373-1377; Hoogenboom et al. (1991) Nuc. Acid Res. 19:4133-4137; Barbas et al. (1991) PNAS 88:7978-7982; and McCafferty et al. Nature (1990) 348:552-554. Additionally, recombinant anti-IRAP-BP antibodies, such as chimeric and humanized monoclonal antibodies, comprising both human and non-human portions, which can be made using standard recombinant DNA techniques, are within the scope of the invention. Such chimeric and humanized monoclonal antibodies can be produced by recombinant DNA techniques known in the art, for example using methods described in Robinson et al. International Application No. PCT/US86/02269; Akira, et al. European Patent Application 184,187; Taniguchi, M., European Patent Application 171,496; Morrison et al. European Patent Application 173,494; Neuberger et al. PCT International Publication No. WO 86/01533; Cabilly et al. U.S. Patent No. 4,816,567; Cabilly et al. European Patent Application 125,023; Better et al. (1988) Science 240:1041-1043; Liu et al. (1987) PNAS 84:3439-3443; Liu et al. (1987) J. Immunol. 139:3521-3526; Sun et al. (1987) PNAS 84:214-218; Nishimura et al. (1987) Cane. Res. 47:999-1005; Wood et al. (1985) Nature 314:446-449; and Shaw et al. (1988) J. Natl. Cancer Inst. 80:1553- 1559); Morrison, S. L. (1985) Science 229:1202-1207; Oi et al. (1986) BioTechniques 4:214; Winter U.S. Patent 5,225,539; Jones et al. (1986) Nature 321 :552-525; Verhoeyan et al. (1988) Science 239:1534; and Beidler et al. (1988) J. Immunol. 141 :4053-4060. An anti-IRAP-BP antibody (e.g., monoclonal antibody) can be used to isolate IRAP-BP by standard techniques, such as affinity chromatography or immunoprecipitation. An anti-IRAP-BP antibody can facilitate the purification of natural IRAP-BP from cells and of recombinantly produced IRAP-BP expressed in host cells. Moreover, an anti-IRAP-BP antibody can be used to detect IRAP-BP polypeptide (e.g., in a cellular lysate or cell supernatant) in order to evaluate the abundance and pattern of expression of the IRAP-BP polypeptide. Anti-IRAP-BP antibodies can be used diagnostically to monitor protein levels in tissue as part of a clinical testing procedure, e.g., to, for example, determine the efficacy of a given treatment regimen. Detection can be facilitated by coupling (i.e., physically linking) the antibody to a detectable substance. Examples of detectable substances include various enzymes, prosthetic groups, fluorescent materials, luminescent materials, bioluminescent materials, and radioactive materials. Examples of suitable enzymes include horseradish peroxidase, alkaline phosphatase, -galactosidase, or acetylcholinesterase; examples of suitable prosthetic group complexes include streptavidin/biotin and avidin/biotin; examples of suitable fluorescent materials include umbelliferone, fluorescein, fluorescein isothiocyanate, rhodamine, dichlorotriazinylamine fluorescein, dansyl chloride or phycoerythrin; an example of a luminescent material includes luminol; examples of bioluminescent materials include luciferase, luciferin, and aequorin, and examples of suitable radioactive material include I, I, S or H. HI. Recombinant Expression Vectors and Host Cells Another aspect of the invention pertains to vectors, preferably expression vectors, containing a nucleic acid encoding an IRAP-BP polypeptide (or a portion thereof). As used herein, the term "vector" refers to a nucleic acid molecule capable of transporting another nucleic acid to which it has been linked. One type of vector is a "plasmid", which refers to a circular double stranded DNA loop into which additional DNA segments can be ligated. Another type of vector is a viral vector, wherein additional DNA segments can be ligated into the viral genome. Certain vectors are capable of autonomous replication in a host cell into which they are introduced (e.g., bacterial vectors having a bacterial origin of replication and episomal mammalian vectors). Other vectors (e.g., non-episomal mammalian vectors) are integrated into the genome of a host cell upon introduction into the host cell, and thereby are replicated along with the host genome. Moreover, certain vectors are capable of directing the expression of genes to which they are operatively linked. Such vectors are referred to herein as "expression vectors". In general, expression vectors of utility in recombinant DNA techniques are often in the form of plasmids. In the present specification, "plasmid" and "vector" can be used interchangeably as the plasmid is the most commonly used form of vector. However, the invention is intended to include such other forms of expression vectors, such as viral vectors (e.g., replication defective retroviruses, adenoviruses and adeno-associated viruses), which serve equivalent functions. The recombinant expression vectors of the invention comprise a nucleic acid of the invention in a form suitable for expression of the nucleic acid in a host cell, which means that the recombinant expression vectors include one or more regulatory sequences, selected on the basis of the host cells to be used for expression, which is operatively linked to the nucleic acid sequence to be expressed. Within a recombinant expression vector, "operably linked" is intended to mean that the nucleotide sequence of interest is linked to the regulatory sequence(s) in a manner which allows for expression of the nucleotide sequence (e.g., in an in vitro transcription/translation system or in a host cell when the vector is introduced into the host cell). The term "regulatory sequence" is intended to includes promoters, enhancers and other expression control elements (e.g., polyadenylation signals). Such regulatory sequences are described, for example, in Goeddel; Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, CA (1990). Regulatory sequences include those which direct constitutive expression of a nucleotide sequence in many types of host cell and those which direct expression of the nucleotide sequence only in certain host cells (e.g., tissue-specific regulatory sequences). It will be appreciated by those skilled in the art that the design of the expression vector can depend on such factors as the choice of the host cell to be transformed, the level of expression of protein desired, etc. The expression vectors of the invention can be introduced into host cells to thereby produce proteins or peptides, including fusion proteins or peptides, encoded by nucleic acids as described herein (e.g., IRAP-BP polypeptides, mutant forms of IRAP-BP polypeptides, fusion proteins, etc.). The recombinant expression vectors of the invention can be designed for expression of IRAP-BP polypeptides in prokaryotic or eukaryotic cells. For example, IRAP-BP polypeptides can be expressed in bacterial cells such as E. coli, insect cells (using baculovirus expression vectors) yeast cells or mammalian cells. Suitable host cells are discussed further in Goeddel, Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, CA (1990). Alternatively, the recombinant expression vector can be transcribed and translated in vitro, for example using T7 promoter regulatory sequences and T7 polymerase. Expression of proteins in prokaryotes is most often carried out in E. coli with vectors containing constitutive or inducible promoters directing the expression of either fusion or non-fusion proteins. Fusion vectors add a number of amino acids to a protein encoded therein, usually to the amino terminus of the recombinant protein. Such fusion vectors typically serve three puφoses: 1) to increase expression of recombinant protein; 2) to increase the solubility of the recombinant protein; and 3) to aid in the purification of the recombinant protein by acting as a ligand in affinity purification. Often, in fusion expression vectors, a proteolytic cleavage site is introduced at the junction of the fusion moiety and the recombinant protein to enable separation of the recombinant protein from the fusion moiety subsequent to purification of the fusion protein. Such enzymes, and their cognate recognition sequences, include Factor Xa, thrombin and enterokinase. Typical fusion expression vectors include pGEX (Pharmacia Biotech Inc; Smith, D.B. and Johnson, K.S. (1988) Gene 67:31-40), pMAL (New England Biolabs, Beverly, MA) and pRIT5 (Pharmacia, Piscataway, NJ) which fuse glutathione S-transferase (GST), maltose E binding protein, or protein A, respectively, to the target recombinant protein. Purified fusion proteins can be utilized in IRAP-BP activity assays, (e.g., direct assays or competitive assays described in detail below), or to generate antibodies specific for IRAP-BP polypeptides, for example. In a preferred embodiment, an IRAP- BP fusion protein expressed in a retroviral expression vector of the present invention can be utilized to infect bone marrow cells which are subsequently transplanted into irradiated recipients. The pathology of the subject recipient is then examined after sufficient time has passed (e.g six (6) weeks). Examples of suitable inducible non-fusion E. coli expression vectors include pTrc (Amann et al., (1988) Gene 69:301-315) and pET 1 Id (Studier et al., Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, California (1990) 60-89). Target gene expression from the pTrc vector relies on host RNA polymerase transcription from a hybrid tφ-lac fusion promoter. Target gene expression from the pET 1 Id vector relies on transcription from a T7 gnlO-lac fusion promoter mediated by a coexpressed viral RNA polymerase (T7 gnl). This viral polymerase is supplied by host strains BL21(DE3) or HMS174(DE3) from a resident prophage harboring a T7 gnl gene under the transcriptional control of the lacUV 5 promoter. One strategy to maximize recombinant protein expression in E. coli is to express the protein in a host bacteria with an impaired capacity to proteolytically cleave the recombinant protein (Gottesman, S., Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, California (1990) 119-128). Another strategy is to alter the nucleic acid sequence of the nucleic acid to be inserted into an expression vector so that the individual codons for each amino acid are those preferentially utilized in E coli (Wada et al., (1992) Nucleic Acids Res. 20:2111-2118). Such alteration of nucleic acid sequences of the invention can be carried out by standard DNA synthesis techniques. In another embodiment, the IRAP-BP expression vector is a yeast expression vector. Examples of vectors for expression in yeast S. cerivisae include pYepSecl (Baldari, et al., (1987) Embo J. 6:229-234), pMFa (Kurjan and Herskowitz, (1982) Cell 30:933-943), pJRY88 (Schultz et al., (1987) Gene 54:113-123), pYES2 (Invitrogen Coφoration, San Diego, CA), and picZ (InVitrogen Coφ, San Diego, CA). Alternatively, IRAP-BP polypeptides can be expressed in insect cells using baculovirus expression vectors. Baculovirus vectors available for expression of proteins in cultured insect cells (e.g., Sf 9 cells) include the pAc series (Smith et al. (1983) Mol. Cell Biol. 3:2156-2165) and the pVL series (Lucklow and Summers (1989) Virology 170:31-39). In yet another embodiment, a nucleic acid of the invention is expressed in mammalian cells using a mammalian expression vector. Examples of mammalian expression vectors include pCDM8 (Seed, B. (1987) Nature 329:840) and pMT2PC (Kaufman et al. (1987) EMBO J. 6:187-195). When used in mammalian cells, the expression vector's control functions are often provided by viral regulatory elements. For example, commonly used promoters are derived from polyoma, Adenovirus 2, cytomegalovirus and Simian Virus 40. For other suitable expression systems for both prokaryotic and eukaryotic cells see chapters 16 and 17 of Sambrook, J., Fritsh, E. F., and Maniatis, T. Molecular Cloning: A Laboratory Manual. 2nd, ed., Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, 1989. In another embodiment, the recombinant mammalian expression vector is capable of directing expression of the nucleic acid preferentially in a particular cell type (e.g., tissue-specific regulatory elements are used to express the nucleic acid). Tissue- specific regulatory elements are known in the art. Non-limiting examples of suitable tissue-specific promoters include the albumin promoter (liver-specific; Pinkert et al. (1987) Genes Dev. 1 :268-277), lymphoid-specific promoters (Calame and Eaton (1988) Adv. Immunol. 43:235-275), in particular promoters of T cell receptors (Winoto and Baltimore (1989) EMBO J. 8:729-733) and immunoglobulins (Banerji et al. (1983) Cell 33:729-740; Queen and Baltimore (1983) Cell 33:741-748), neuron-specific promoters (e.g., the neurofilament promoter; Byrne and Ruddle (1989) PNAS 86:5473-5477), pancreas-specific promoters (Edlund et al. (1985) Science 230:912-916), and mammary gland-specific promoters (e.g., milk whey promoter; U.S. Patent No. 4,873,316 and European Application Publication No. 264, 166). Developmentally-regulated promoters are also encompassed, for example the murine hox promoters (Kessel and Gruss (1990) Science 249:374-379) and the α-fetoprotein promoter (Campes and Tilghman (1989) Genes Dev. 3:537-546). The invention further provides a recombinant expression vector comprising a DNA molecule of the invention cloned into the expression vector in an antisense orientation. That is, the DNA molecule is operatively linked to a regulatory sequence in a manner which allows for expression (by transcription of the DNA molecule) of an RNA molecule which is antisense to IRAP-BP mRNA. Regulatory sequences operatively linked to a nucleic acid cloned in the antisense orientation can be chosen which direct the continuous expression of the antisense RNA molecule in a variety of cell types, for instance viral promoters and/or enhancers, or regulatory sequences can be chosen which direct constitutive, tissue specific or cell type specific expression of antisense RNA. The antisense expression vector can be in the form of a recombinant plasmid, phagemid or attenuated virus in which antisense nucleic acids are produced under the control of a high efficiency regulatory region, the activity of which can be determined by the cell type into which the vector is introduced. For a discussion of the regulation of gene expression using antisense genes see Weintraub, H. et al., Antisense RNA as a molecular tool for genetic analysis, Reviews - Trends in Genetics, Vol. 1(1) 1986. Another aspect of the invention pertains to host cells into which a recombinant expression vector of the invention has been introduced. The terms "host cell" and "recombinant host cell" are used interchangeably herein. It is understood that such terms refer not only to the particular subject cell but to the progeny or potential progeny of such a cell. Because certain modifications may occur in succeeding generations due to either mutation or environmental influences, such progeny may not, in fact, be identical to the parent cell, but are still included within the scope of the term as used • herein. A host cell can be any prokaryotic or eukaryotic cell. For example, an IRAP-BP polypeptide can be expressed in bacterial cells such as E. coli, insect cells, yeast or mammalian cells (such as Chinese hamster ovary cells (CHO) or COS cells). Other suitable host cells are known to those skilled in the art. Vector DNA can be introduced into prokaryotic or eukaryotic cells via conventional transformation or transfection techniques. As used herein, the terms "transformation" and "transfection" are intended to refer to a variety of art-recognized techniques for introducing foreign nucleic acid (e.g., DNA) into a host cell, including calcium phosphate or calcium chloride co-precipitation, DΕAΕ-dextran-mediated transfection, lipofection, or electroporation. Suitable methods for transforming or transfecting host cells can be found in Sambrook, et al. (Molecular Cloning: A Laboratory Manual. 2nd, ed, Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, 1989), and other laboratory manuals. For stable transfection of mammalian cells, it is known that, depending upon the expression vector and transfection technique used, only a small fraction of cells may integrate the foreign DNA into their genome. In order to identify and select these integrants, a gene that encodes a selectable marker (e.g., resistance to antibiotics) is generally introduced into the host cells along with the gene of interest. Preferred selectable markers include those which confer resistance to drugs, such as G418, hygromycin and methotrexate. Nucleic acid encoding a selectable marker can be introduced into a host cell on the same vector as that encoding an IRAP-BP polypeptide or can be introduced on a separate vector. Cells stably transfected with the introduced nucleic acid can be identified by drug selection (e.g., cells that have incoφorated the selectable marker gene will survive, while the other cells die). A host cell of the invention, such as a prokaryotic or eukaryotic host cell in culture, can be used to produce (i.e., express) an IRAP-BP polypeptide. Accordingly, the invention further provides methods for producing an IRAP-BP polypeptide using the host cells of the invention. In one embodiment, the method comprises culturing the host cell of invention (into which a recombinant expression vector encoding an IRAP-BP polypeptide has been introduced) in a suitable medium such that an IRAP-BP polypeptide is produced. In another embodiment, the method further comprises isolating an IRAP-BP polypeptide from the medium or the host cell. The host cells of the invention can also be used to produce nonhuman transgenic animals. For example, in one embodiment, a host cell of the invention is a fertilized oocyte or an embryonic stem cell into which IRAP-BP-coding sequences have been introduced. Such host cells can then be used to create non-human transgenic animals in which exogenous IRAP-BP sequences have been introduced into their genome or homologous recombinant animals in which endogenous IRAP-BP sequences have been altered. Such animals are useful for studying the function and/or activity of an IRAP-BP and for identifying and/or evaluating modulators of IRAP-BP activity. As used herein, a "transgenic animal" is a non-human animal, preferably a mammal, more preferably a rodent such as a rat or mouse, in which one or more of the cells of the animal includes a transgene. Other examples of transgenic animals include non-human primates, sheep, dogs, cows, goats, chickens, amphibians, etc. A transgene is exogenous DNA which is integrated into the genome of a cell from which a transgenic animal develops and which remains in the genome of the mature animal, thereby directing the expression of an encoded gene product in one or more cell types or tissues of the transgenic animal. As used herein, a "homologous recombinant animal" is a non-human animal, preferably a mammal, more preferably a mouse, in which an endogenous IRAP-BP gene has been altered by homologous recombination between the endogenous gene and an exogenous DNA molecule introduced into a cell of the animal, e.g., an embryonic cell of the animal, prior to development of the animal. A transgenic animal of the invention can be created by introducing an IRAP-BP- encoding nucleic acid into the male pronuclei of a fertilized oocyte, e.g., by microinjection, retroviral infection, and allowing the oocyte to develop in a pseudopregnant female foster animal. The IRAP-BP-1 cDNA sequence, e.g., that of SEQ ID NO:4 or SEQ ID NO:6, can be introduced as a transgene into the genome of a non-human animal. Alternatively, a nonhuman homologue of a human IRAP-BP gene, such as a mouse IRAP-BP gene (e.g., SEQ ID NO:l or SEQ ID NO:3), can be used as a transgene. Alternatively, an IRAP-BP gene homologue, such as an IRAP-BP-2 gene can be isolated based on hybridization to the IRAP-BP cDNA sequences of SEQ ID NO: 1 or SEQ ID NO:4 (described further in subsection I above) and used as a transgene. Intronic sequences and polyadenylation signals can also be included in the transgene to increase the efficiency of expression of the transgene. A tissue-specific regulatory sequence(s) can be operably linked to an IRAP-BP transgene to direct expression of an IRAP-BP polypeptide to particular cells. Methods for generating transgenic animals via embryo manipulation and microinjection, particularly animals such as mice, have become conventional in the art and are described, for example, in U.S. Patent Nos. 4,736,866 and 4,870,009, both by Leder et al., U.S. Patent No. 4,873,191 by Wagner et al. and in Hogan, B., Manipulating the Mouse Embryo, (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1986). Similar methods are used for production of other transgenic animals. A transgenic founder animal can be identified based upon the presence of an IRAP-BP transgene in its genome and/or expression of IRAP-BP mRNA in tissues or cells of the animals. A transgenic founder animal can then be used to breed additional animals carrying the transgene. Moreover, transgenic animals carrying a transgene encoding an IRAP-BP polypeptide can further be bred to other transgenic animals carrying other transgenes. To create a homologous recombinant animal, a vector is prepared which contains at least a portion of an IRAP-BP gene into which a deletion, addition or substitution has been introduced to thereby alter, e.g., functionally disrupt, the IRAP-BP gene. The IRAP-BP gene can be a human gene (e.g., the cDNA of SEQ ID NO:4 or SEQ ID NO:6), but more preferably, is a non-human homologue of a human IRAP-BP gene. For example, a mouse IRAP-BP gene (SEQ ID NO:l) can be used to construct a homologous recombination vector suitable for altering an endogenous IRAP-BP gene in the mouse genome. In a preferred embodiment, the vector is designed such that, upon homologous recombination, the endogenous IRAP-BP gene is functionally disrupted (i.e., no longer encodes a functional protein; also referred to as a "knock out" vector). Alternatively, the vector can be designed such that, upon homologous recombination, the endogenous IRAP-BP gene is mutated or otherwise altered but still encodes functional protein (e.g., the upstream regulatory region can be altered to thereby alter the expression of the endogenous IRAP-BP polypeptide). In the homologous recombination vector, the altered portion of the IRAP-BP gene is flanked at its 5' and 3' ends by additional nucleic acid sequence of the IRAP-BP gene to allow for homologous recombination to occur between the exogenous IRAP-BP gene carried by the vector and an endogenous IRAP-BP gene in an embryonic stem cell. The additional flanking IRAP-BP nucleic acid sequence is of sufficient length for successful homologous recombination with the endogenous gene. Typically, several kilobases of flanking DNA (both at the 5' and 3' ends) are included in the vector (see e.g., Thomas, K.R. and Capecchi, M. R. (1987) Cell 51 :503 for a description of homologous recombination vectors). The vector is introduced into an embryonic stem cell line (e.g., by electroporation) and cells in which the introduced IRAP-BP gene has homologously recombined with the endogenous IRAP-BP gene are selected (see e.g., Li, E. et al. (1992) Cell 69:915). The selected cells are then injected into a blastocyst of an animal (e.g., a mouse) to form aggregation chimeras (see e.g., Bradley, A. in Teratocarcinomas and Embryonic Stem Cells: A Practical Approach, E.J. Robertson, ed. (IRL, Oxford, 1987) pp. 113-152). A chimeric embryo can then be implanted into a suitable pseudopregnant female foster animal and the embryo brought to term. Progeny harboring the homologously recombined DNA in their germ cells can be used to breed animals in which all cells of the animal contain the homologously recombined DNA by germline transmission of the transgene. Methods for constructing homologous recombination vectors and homologous recombinant animals are described further in Bradley, A. (1991) Current Opinion in Biotechnology 2:823-829 and in PCT International Publication Nos.: WO 90/11354 by Le Mouellec et al.; WO 91/01140 by Smithies et al.; WO 92/0968 by Zijlstra et al; and WO 93/04169 by Berns et al. In another embodiment, transgenic non-humans animals can be produced which contain selected systems which allow for regulated expression of the transgene. One example of such a system is the cre/loxP recombinase system of bacteriophage PI . For a description of the cre/loxP recombinase system, see, e.g., Lakso et al. (1992) PNAS 89:6232-6236. Another example of a recombinase system is the FLP recombinase system of Saccharomyces cerevisiae (O'Gorman et al. (1991) Science 251 :1351-1355. If a cre/loxP recombinase system is used to regulate expression of the transgene, animals containing transgenes encoding both the Cre recombinase and a selected protein are required. Such animals can be provided through the construction of "double" transgenic animals, e.g., by mating two transgenic animals, one containing a transgene encoding a selected protein and the other containing a transgene encoding a recombinase. Clones of the non-human transgenic animals described herein can also be produced according to the methods described in Wilmut, I. et al. (1997) Nature 385:810- 813. In brief, a cell, e.g., a somatic cell, from the transgenic animal can be isolated and induced to exit the growth cycle and enter G 0 phase. The quiescent cell can then be fused, e.g., through the use of electrical pulses, to an enucleated oocyte from an animal of the same species from which the quiescent cell is isolated. The recontructed oocyte is then cultured such that it develops to morula or blastocyte and then transferred to pseudopregnant female foster animal. The offspring borne of this female foster animal will be a clone of the animal from which the cell, e.g., the somatic cell, is isolated. IN. Pharmaceutical Compositions The IRAP-BP nucleic acid molecules, IRAP-BP polypeptides, anti-IRAP-BP antibodies, IRAP-BP ligands, and IRAP-BP modulators (also referred to herein as "active compounds") of the invention can be incoφorated into pharmaceutical compositions suitable for administration. Such compositions typically comprise the nucleic acid molecule, protein, antibody, or modulatory compound and a pharmaceutically acceptable carrier. As used herein the language "pharmaceutically acceptable carrier" is intended to include any and all solvents, dispersion media, coatings, antibacterial and antifungal agents, isotonic and absoφtion delaying agents, and the like, compatible with pharmaceutical administration. The use of such media and agents for pharmaceutically active substances is well known in the art. Except insofar as any conventional media or agent is incompatible with the active compound, use thereof in the compositions is contemplated. Supplementary active compounds can also be incoφorated into the compositions. A pharmaceutical composition of the invention is formulated to be compatible with its intended route of administration. Examples of routes of administration include parenteral, e.g., intravenous, intradermal, subcutaneous, oral (e.g., inhalation), transdermal (topical), transmucosal, and rectal administration. Solutions or suspensions used for parenteral, intradermal, or subcutaneous application can include the following components: a sterile diluent such as water for injection, saline solution, fixed oils, polyethylene glycols, glycerine, propylene glycol or other synthetic solvents; antibacterial agents such as benzyl alcohol or methyl parabens; antioxidants such as ascorbic acid or sodium bisulfite; chelating agents such as ethylenediaminetetraacetic acid; buffers such as acetates, citrates or phosphates and agents for the adjustment of tonicity such as sodium chloride or dextrose. pH can be adjusted with acids or bases, such as hydrochloric acid or sodium hydroxide. The parenteral preparation can be enclosed in ampoules, disposable syringes or multiple dose vials made of glass or plastic. Pharmaceutical compositions suitable for injectable use include sterile aqueous solutions (where water soluble) or dispersions and sterile powders for the extemporaneous preparation of sterile injectable solutions or dispersion. For intravenous administration, suitable carriers include physiological saline, bacteriostatic water, Cremophor EL™ (BASF, Parsippany, NJ) or phosphate buffered saline (PBS). In all cases, the composition must be sterile and should be fluid to the extent that easy syringability exists. It must be stable under the conditions of manufacture and storage and must be preserved against the contaminating action of microorganisms such as bacteria and fungi. The carrier can be a solvent or dispersion medium containing, for example, water, ethanol, polyol (for example, glycerol, propylene glycol, and liquid polyetheylene glycol, and the like), and suitable mixtures thereof. The proper fluidity can be maintained, for example, by the use of a coating such as lecithin, by the maintenance of the required particle size in the case of dispersion and by the use of surfactants. Prevention of the action of microorganisms can be achieved by various antibacterial and antifungal agents, for example, parabens, chlorobutanol, phenol, ascorbic acid, thimerosal, and the like. In many cases, it will be preferable to include isotonic agents, for example, sugars, polyalcohols such as manitol, sorbitol, sodium chloride in the composition. Prolonged absoφtion of the injectable compositions can be brought about by including in the composition an agent which delays absoφtion, for example, aluminum monostearate and gelatin. Sterile injectable solutions can be prepared by incoφorating the active compound (e.g., an IRAP-BP polypeptide or anti-IRAP-BP antibody) in the required amount in an appropriate solvent with one or a combination of ingredients enumerated above, as required, followed by filtered sterilization. Generally, dispersions are prepared by incoφorating the active compound into a sterile vehicle which contains a basic dispersion medium and the required other ingredients from those enumerated above. In the case of sterile powders for the preparation of sterile injectable solutions, the preferred methods of preparation are vacuum drying and freeze-drying which yields a powder of the active ingredient plus any additional desired ingredient from a previously sterile-filtered solution thereof. Oral compositions generally include an inert diluent or an edible carrier. They can be enclosed in gelatin capsules or compressed into tablets. For the puφose of oral therapeutic administration, the active compound can be incoφorated with excipients and used in the form of tablets, troches, or capsules. Oral compositions can also be prepared using a fluid carrier for use as a mouthwash, wherein the compound in the fluid carrier is applied orally and swished and expectorated or swallowed. Pharmaceutically compatible binding agents, and/or adjuvant materials can be included as part of the composition. The tablets, pills, capsules, troches and the like can contain any of the following ingredients, or compounds of a similar nature: a binder such as microcrystalline cellulose, gum tragacanth or gelatin; an excipient such as starch or lactose, a disintegrating agent such as alginic acid, Primogel, or corn starch; a lubricant such as magnesium stearate or Sterotes; a glidant such as colloidal silicon dioxide; a sweetening agent such as sucrose or saccharin; or a flavoring agent such as peppermint, methyl salicylate, or orange flavoring. For administration by inhalation, the compounds are delivered in the form of an aerosol spray from pressured container or dispenser which contains a suitable propellant, e.g., a gas such as carbon dioxide, or a nebulizer. Systemic administration can also be by transmucosal or transdermal means. For transmucosal or transdermal administration, penetrants appropriate to the barrier to be permeated are used in the formulation. Such penetrants are generally known in the art, and include, for example, for transmucosal administration, detergents, bile salts, and fusidic acid derivatives. Transmucosal administration can be accomplished through the use of nasal sprays or suppositories. For transdermal administration, the active compounds are formulated into ointments, salves, gels, or creams as generally known in the art. The compounds can also be prepared in the form of suppositories (e.g., with conventional suppository bases such as cocoa butter and other glycerides) or retention enemas for rectal delivery. In one embodiment, the active compounds are prepared with carriers that will protect the compound against rapid elimination from the body, such as a controlled release formulation, including implants and microencapsulated delivery systems. Biodegradable, biocompatible polymers can be used, such as ethylene vinyl acetate, polyanhydrides, polyglycolic acid, collagen, polyorthoesters, and polylactic acid. Methods for preparation of such formulations will be apparent to those skilled in the art. The materials can also be obtained commercially from Alza Coφoration and Nova Pharmaceuticals, Inc. Liposomal suspensions (including liposomes targeted to infected cells with monoclonal antibodies to viral antigens) can also be used as pharmaceutically acceptable carriers. These can be prepared according to methods known to those skilled in the art, for example, as described in U.S. Patent No. 4,522,811. It is especially advantageous to formulate oral or parenteral compositions in dosage unit form for ease of administration and uniformity of dosage. Dosage unit form as used herein refers to physically discrete units suited as unitary dosages for the subject to be treated; each unit containing a predetermined quantity of active compound calculated to produce the desired therapeutic effect in association with the required pharmaceutical carrier. The specification for the dosage unit forms of the invention are dictated by and directly dependent on the unique characteristics of the active compound and the particular therapeutic effect to be achieved, and the limitations inherent in the art of compounding such an active compound for the treatment of individuals. Toxicity and therapeutic efficacy of such compounds can be determined by standard pharmaceutical procedures in cell cultures or experimental animals, e.g., for determining the LD50 (the dose lethal to 50% of the population) and the ED50 (the dose therapeutically effective in 50% of the population). The dose ratio between toxic and therapeutic effects is the therapeutic index and it can be expressed as the ratio LD50/ED50. Compounds which exhibit large therapeutic indices are preferred. While compounds that exhibit toxic side effects may be used, care should be taken to design a delivery system that targets such compounds to the site of affected tissue in order to minimize potential damage to uninfected cells and, thereby, reduce side effects. The data obtained from the cell culture assays and animal studies can be used in formulating a range of dosage for use in humans. The dosage of such compounds lies preferably within a range of circulating concentrations that include the ED50 with little or no toxicity. The dosage may vary within this range depending upon the dosage form employed and the route of administration utilized. For any compound used in the method of the invention, the therapeutically effective dose can be estimated initially from cell culture assays. A dose may be formulated in animal models to achieve a circulating plasma concentration range that includes the IC50 (i.e., the concentration of the test compound which achieves a half-maximal inhibition of symptoms) as determined in cell culture. Such information can be used to more accurately determine useful doses in humans. Levels in plasma may be measured, for example, by high performance liquid chromatography. The nucleic acid molecules of the invention can be inserted into vectors and used as gene therapy vectors. Gene therapy vectors can be delivered to a subject by, for example, intravenous injection, local administration (see U.S. Patent 5,328,470) or by stereotactic injection (see e.g., Chen et al. (1994) PNAS 91 :3054-3057). The pharmaceutical preparation of the gene therapy vector can include the gene therapy vector in an acceptable diluent, or can comprise a slow release matrix in which the gene delivery vehicle is imbedded. Alternatively, where the complete gene delivery vector can be produced intact from recombinant cells, e.g., retroviral vectors, the pharmaceutical preparation can include one or more cells which produce the gene delivery system. The pharmaceutical compositions can be included in a container, pack, or dispenser together with instructions for administration. V. Uses and Methods of the Invention The nucleic acid molecules, proteins, protein homologues, and antibodies described herein can be used in at least one the following methods: a) screening assays; b) diagnostic assays and or prognostic assays; or c) methods of treatment (e.g., therapeutic and prophylactic). As described herein, an IRAP-BP polypeptide of the invention has one or more of the following activities: (i) interaction with an intracellular non-IRAP-BP molecule, for example, and IRAP-BP target molecule (e.g., IRAP and/or GLUT4); (ii) binding to an intracellular non-IRAP-BP molecule, for example, and IRAP-BP target molecule (e.g., IRAP and/or GLUT4); (iii) recognition of a trafficking motif within an intracellular non-IRAP-BP molecule, for example, and IRAP-BP target molecule (e.g., IRAP and/or GLUT4); and (iv) binding to a trafficking motif within an intracellular non-IRAP-BP molecule, for example, and IRAP-BP target molecule (e.g., IRAP and/or GLUT4), and can can thus be used in, for example, (1) modulation of translocation of an IRAP-BP target molecule; (2) modulation of GLUT4 translocation (e.g., exocytosis); (3) modulation of IRAP translocation (e.g., exocytosis); (4) modulation of sorting of an IRAP-BP target molecule; (5) modulation of sorting of IRAP and/or GLUT4; (6) modulation of retention of an IRAP-BP target molecule; (7) modulation of retention of IRAP and/or GLUT4 (e.g., within a specialized intracellular compartment, for example, the endocytic recycling compartment; (8) modulation of the entry of an IRAP-BP target molecule into recycling vesicles; (9) modulation of entry of IRAP and/or GLUT4 into recycling vesicles; (10) regulation of intracellular trafficking and/or retention of an IRAP-BP target molecule; (11) regulation of intracellular trafficking and/or retention of IRAP and/or GLUT4; and (12) regulation of intracellular trafficking of GLUT4- and/or IRAP-containing vesicles. The isolated nucleic acid molecules of the invention can be used, for example, to express IRAP-BP polypeptide (e.g., via a recombinant expression vector in a host cell in gene therapy applications), to detect IRAP-BP mRNA (e.g., in a biological sample) or a genetic alteration in an IRAP- BP gene, and to modulate IRAP-BP activity, as described further below. The IRAP-BP polypeptides can be used to treat disorders characterized by insufficient or excessive production of an IRAP-BP polypeptide and/or IRAP-BP target molecule. In addition, the IRAP-BP polypeptides can be used to screen drugs or compounds which modulate the IRAP-BP activity as well as to treat disorders characterized by insufficient or excessive production of IRAP-BP polypeptide or production of IRAP-BP polypeptide forms which have decreased or aberrant activity compared to IRAP-BP wild type protein. Moreover, the anti-IRAP-BP antibodies of the invention can be used to detect and isolate IRAP-BP polypeptides, regulate the bioavailability of IRAP-BP polypeptides, and modulate IRAP-BP activity. Screening Assays: The invention provides a method (also referred to herein as a "screening assay") for identifying modulators, i.e., candidate or test compounds or agents (e.g., peptides, peptidomimetics, small molecules or other drugs) which bind to IRAP-BP polypeptides, or have a stimulatory or inhibitory effect on, for example, IRAP-BP expression or IRAP-BP activity. Such modulators are useful, for example, in (1) the modulation of insulin sensitivity (e.g., enhancement of insulin sensitivity); (2) modulation of translocation of IRAP-BP target molecules (e.g., GLUT4); and (3) regulation of diabetes (e.g., Type II diabetes). In one embodiment, the invention provides assays for screening candidate or test compounds which bind to or modulate the activity of an IRAP-BP polypeptide or polypeptide or biologically active portion thereof. The test compounds of the present invention can be obtained using any of the numerous approaches in combinatorial library methods known in the art, including: biological libraries; spatially addressable parallel solid phase or solution phase libraries; synthetic library methods requiring deconvolution; the 'one-bead one-compound' library method; and synthetic library methods using affinity chromatography selection. The biological library approach is limited to peptide libraries, while the other four approaches are applicable to peptide, non-peptide oligomer or small molecule libraries of compounds (Lam, K.S. (1997) Anticancer Drug Des. 12: 145). Examples of methods for the synthesis of molecular libraries can be found in the art, for example in: DeWitt et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90:6909; Erb et al. (1994) Proc. Natl. Acad. Sci. USA 91:11422; Zuckermann et al. (1994). J. Med. Chem. 37:2678; Cho et al. (1993) Science 261:1303; Carrell et al. (1994) Angew. Chem. Int. Ed. Engl. 33:2059; Carell et al (1994) Angew. Chem. Int. Ed. Engl. 33:2061; and in Gallop et al. (1994) J. Med. Chem. 37:1233. Libraries of compounds may be presented in solution (e.g., Houghten (1992) Biotechniques 13:412-421), or on beads (Lam (1991) Nature 354:82-84), chips (Fodor (1993) Nature 364:555-556), bacteria (Ladner USP 5,223,409), spores (Ladner USP '409), plasmids (Cull et al. (1992) Proc Natl Acad Sci USA 89:1865-1869) or on phage (Scott and Smith (1990) Science 249:386-390); (Devlin (1990) Science 249:404-406); (Cwirla et al. (1990) Proc. Natl. Acad. Sci. 87:6378-6382); (Felici (1991) J Mol. Biol. 222:301-310); (Ladner supra.). In one embodiment, an assay is a cell-based assay in which a cell which expresses an IRAP-BP polypeptide, or biologically active portion thereof, is contacted with a test compound and the ability of the test compound to modulate the activity of the IRAP-BP polypeptide, or biologically active portion thereof, determined. The cell, for example, can be of mammalian origin or a yeast cell. The IRAP-BP polypeptide, for example, can be expressed heterologously or native to the cell. Determining the ability of the test compound to modulate the activity of an IRAP-BP polypeptide, or biologically active portion thereof, can be accomplished by assaying for any of the activities of an IRAP-BP polypeptide described herein. Determining the ability of the test compound to modulate the activity of an IRAP-BP polypeptide, or biologically active portion thereof, can also be accomplished by assaying for the activity of an IRAP- BP target molecule. In one embodiment, determining the ability of the test compound to modulate the activity of an IRAP-BP polypeptide, or biologically active portion thereof, is accomplished by assaying for the activity of IRAP (e.g., by assaying for aminopeptidase activity) In a preferred embodiment, the cell which expresses the IRAP- BP polypeptide, or biologically active portion thereof, further expressed an IRAP-BP target molecule, or biologically active portion thereof. In another preferred embodiment, the cell expresses IRAP and/or GLUT4, or biologically active portion thereof. In yet another preferred example, the cell is contacted with a compound which stimulates the activity of an IRAP-BP-associated activity (e.g., insulin) and the ability of a test compound to modulate the IRAP-BP-associated activity is determined. In another embodiment, an assay is a cell-based assay in which a cell which expresses an IRAP-BP polypeptide, or biologically active portion thereof, is contacted with a bioactive peptide derived from an IRAP-BP target molecule and a test compound and the ability of the test compound to modulate the activity of the IRAP-BP polypeptide, or biologically active portion thereof, determined. In one embodiment, the bioactive peptide is derived from the amino acid sequence of GLUT4. In another embodiment, the bioactive peptide is derived from the amino acid sequence of IRAP. In another embodiment, the bioactive peptide corresponds to the cytoplasmic domain of either GLUT4 or IRAP. In yet another embodiment, the bioactive peptide corresponds to a trafficking motif of GLUT4 or IRAP. According to the cell-based assays of the present invention, determining the ability of the test compound to modulate the activity of the IRAP-BP polypeptide or biologically active portion thereof, can be determined by assaying for any of the native activities of an IRAP-BP polypeptide described herein. For example, assaying for GLUT4 translocation, IRAP translocation, IRAP and/or GLUT4 sorting, retention of IRAP and/or GLUT4, or intracellular trafficking of IRAP and/or GLUT4-comtaining vesicles. Moreover, the activity of the IRAP-BP polypeptide or biologically active portion thereof, can be determined by assaying for an indirect activity which is coincident the the activity of an IRAP-BP polypeptide. For example, the effect of the test compound on the ability of an IRAP-BP-expressing cell to uptake glucose in an insulin-dependent manner can be assayed in the presence and assay of the test compound. Furthermore, determining the ability of the test compound to modulate the activity of the IRAP-BP polypeptide or biologically active portion thereof, can be determined by assaying for an activity which is not native to the IRAP-BP polypeptide, but for which the cell has been recombinantly engineered. For example, the cell can be engineered to express an IRAP-BP target molecule which is a recombinant protein comprising a bioactive portion of an IRAP-BP target molecule operatively linked to a non-IRAP-BP target molecule polypeptide. In an exemplary embodiment, the cytoplasmic domain of the IRAP-BP target molecule GLUT4 or IRAP is operatively linked to the transmembrane and extracellular domains of, for example, the transferrin receptor, and the effect of the test compound on the ability of the chimeric protein to traffick intracellularly, determined. (Jonhson et al. (1998) J. Biol. Chem. 273:17968- 17977 provide an example of the making of such a chimera.) It is also intended that in preferred embodiments, the cell-based assays of the present invention comprise a final step of identifying the compound as a modulator of IRAP-BP activity. In yet another embodiment, an assay of the present invention is a cell-free assay in which an IRAP-BP polypeptide, or biologically active portion thereof, is contacted with a test compound and the ability of the test compound to bind to the IRAP-BP polypeptide or biologically active portion thereof is determined. Binding of the test compound to the IRAP-BP polypeptide can be accomplished, for example, by coupling the test compound or the IRAP-BP polypeptide with a radioisotope or enzymatic label such that binding of the test compound to the IRAP-BP polypeptide can be determined by detecting the labeled compound or polypeptide in a complex. For example, test compounds or polypeptids can be labeled with 125 I, 3 5 S, 14 C, or 3 H, either directly or indirectly, and the radioisotope detected by direct counting of radioemmission or by scintillation counting. Alternatively, test compounds or polypeptides can be enzymatically labeled with, for example, horseradish peroxidase, alkaline phosphatase, or luciferase, and the enzymatic label detected by determination of conversion of an appropriate substrate to product. Binding of the test compound to the IRAP-BP polypeptide can also be accomplished using a technology such as real-time Biomolecular Interaction Analysis (BIA). Sjolander, S. and Urbaniczky, C. (1991) Anal. Chem. 63:2338-2345 and Szabo et al. (1995) Curr. Opin. Struct. Biol. 5:699-705. As used herein, "BIA" is a technology for studying biospecific interactions in real time, without labeling any of the interactants (e.g., BIAcore™). Changes in the optical phenomenon of surface plasmon resonance (SPR) can be used as an indication of real-time reactions between biological molecules. In a preferred embodiment, the assay includes contacting the IRAP-BP polypeptide or biologically active portion thereof with an IRAP-BP target molecule which binds IRAP-BP to form an assay mixture, contacting the assay mixture with a test compound, and determining the ability of the test compound to interact with an IRAP- BP polypeptide, wherein determining the ability of the test compound to interact with an IRAP-BP polypeptide comprises determining the ability of the test compound to preferentially bind to IRAP-BP or biologically active portion thereof as compared to the IRAP-BP. In one embodiment, the IRAP-BP target molecule is a GLUT4 protein or biologically active portion thereof (e.g., the N-terminal cytoplasmic domain or a trafficking motif). In another embodiment, the IRAP-BP target molecule is an IRAP protein or biologically active portion thereof (e.g., the N-terminal cytoplasmic domain or a trafficking motif). In another embodiment, the assay is a cell-free assay in which an IRAP-BP polypeptide or biologically active portion thereof is contacted with a test compound and the ability of the test compound to modulate (e.g., stimulate or inhibit) the activity of the IRAP-BP polypeptide or biologically active portion thereof is determined. Determining the ability of the test compound to modulate the activity of an IRAP-BP polypeptide can be accomplished, for example, by determining the ability of the IRAP-BP polypeptide to modulate the activity of a downstream IRAP-BP binding partner or target molecule by one of the methods described above for cell-based assays. For example, the catalytic/enzymatic activity of the target molecule on an appropriate substrate can be determined as previously described (e.g., the aminopeptidase activity of IRAP). In yet another embodiment, the cell-free assay involves contacting an IRAP-BP polypeptide or biologically active portion thereof with an IRAP-BP target molecule which binds the IRAP-BP polypeptide to form an assay mixture, contacting the assay mixture with a test compound, and determining the ability of the test compound to preferentially modulate the activity of an IRAP-BP binding partner or target molecule, as compared to the IRAP-BP. In more than one embodiment of the above assay methods of the present invention, it may be desirable to immobilize either IRAP-BP or its binding partner/target molecule to facilitate separation of complexed from uncomplexed forms of one or both of the proteins, as well as to accommodate automation of the assay. Binding of a test compound to an IRAP-BP polypeptide, or interaction of an IRAP-BP polypeptide with a target molecule in the presence and absence of a candidate compound, can be accomplished in any vessel suitable for containing the reactants. Examples of such vessels include microtitre plates, test tubes, and micro-centrifuge tubes. In one embodiment, a fusion protein can be provided which adds a domain that allows one or both of the proteins to be bound to a matrix. For example, glutathione- S -transferase/ IRAP-BP fusion proteins or glutathione-S-transferase/target fusion proteins can be adsorbed onto glutathione sepharose beads (Sigma Chemical, St. Louis, MO) or glutathione derivatized microtitre plates, which are then combined with the test compound or the test compound and either the non-adsorbed target protein or IRAP-BP polypeptide, and the mixture incubated under conditions conducive to complex formation (e.g., at physiological conditions for salt and pH). Following incubation, the beads or microtitre plate wells are washed to remove any unbound components, the matrix immobilized in the case of beads, complex determined either directly or indirectly, for example, as described above. Alternatively, the complexes can be dissociated from the matrix, and the level of IRAP-BP binding or activity determined using standard techniques. Other techniques for immobilizing proteins on matrices can also be used in the screening assays of the invention. For example, either an IRAP-BP polypeptide or an IRAP-BP target molecule can be immobilized utilizing conjugation of biotin and streptavidin. Biotinylated IRAP-BP polypeptide or target molecules can be prepared from biotin-NHS (N-hydroxy-succinimide) using techniques well known in the art (e.g., biotinylation kit, Pierce Chemicals, Rockford, IL), and immobilized in the wells of streptavidin-coated 96 well plates (Pierce Chemical). Alternatively, antibodies reactive with IRAP-BP polypeptide or target molecules but which do not interfere with binding of the IRAP-BP polypeptide to its target molecule can be derivatized to the wells of the plate, and unbound target or IRAP-BP polypeptide trapped in the wells by antibody conjugation. Methods for detecting such complexes, in addition to those described above for the GST-immobilized complexes, include immunodetection of complexes using antibodies reactive with the IRAP-BP polypeptide or target molecule, as well as enzyme-linked assays which rely on detecting an enzymatic activity associated with the IRAP-BP polypeptide or target molecule. In another embodiment, modulators of IRAP-BP expression are identified in a method wherein a cell is contacted with a candidate compound and the expression of IRAP-BP mRNA or protein in the cell is determined. The level of expression of IRAP- BP mRNA or protein in the presence of the candidate compound is compared to the level of expression of IRAP-BP mRNA or protein in the absence of the candidate compound. The candidate compound can then be identified as a modulator of IRAP-BP expression based on this comparison. For example, when expression of IRAP-BP mRNA or protein is greater (statistically significantly greater) in the presence of the candidate compound than in its absence, the candidate compound is identified as a stimulator of IRAP-BP mRNA or protein expression. Alternatively, when expression of IRAP-BP mRNA or protein is less (statistically significantly less) in the presence of the candidate compound than in its absence, the candidate compound is identified as an inhibitor of IRAP-BP mRNA or protein expression. The level of IRAP-BP mRNA or protein expression in the cells can be determined by methods described herein for detecting IRAP-BP mRNA or protein. In yet another aspect of the invention, the IRAP-BP polypeptides can be used as "bait proteins" in a two-hybrid assay or three-hybrid assay (see, e.g., U.S. Patent No. 5,283,317; Zervos et al. (1993) Cell 72:223-232; Madura et al. (1993) J. Biol. Chem. 268:12046-12054; Bartel et al. (1993) Biotechniques 14:920-924; Iwabuchi et al. (1993) Oncogene 8:1693-1696; and Brent WO94/10300), to identify other proteins, which bind to or interact with IRAP-BP ("IRAP-BP-binding proteins" or "IRAP-BP- target molecules") and are involved in IRAP-BP activity. Such IRAP-BP-target molecules are also likely to be involved in the regualtion of cellular activities modulated by the IRAP-BP polypeptides. The two-hybrid system is based on the modular nature of most transcription factors, which consist of separable DNA-binding and activation domains. Briefly, the assay utilizes two different DNA constructs. In one construct, the gene that codes for an IRAP-BP polypeptide is fused to a gene encoding the DNA binding domain of a known transcription factor (e.g., GAL-4). In the other construct, a DNA sequence, from a library of DNA sequences, that encodes an unidentified protein ("prey" or "sample") is fused to a gene that codes for the activation domain of the known transcription factor. If the "bait" and the "prey" proteins are able to interact, in vivo, forming an IRAP-BP- dependent complex, the DNA-binding and activation domains of the transcription factor are brought into close proximity. This proximity allows transcription of a reporter gene (e.g., LacZ) which is operably linked to a transcriptional regulatory site responsive to the transcription factor. Expression of the reporter gene can be detected and cell colonies containing the functional transcription factor can be isolated and used to obtain the cloned gene which encodes the protein which interacts with the IRAP-BP polypeptide. This invention further pertains to novel agents identified by the above-described screening assays. Accordingly, it is within the scope of this invention to further use an agent identified as described herein in an appropriate animal model. For example, an agent identified as described herein (e.g., an IRAP-BP modulating agent, an antisense IRAP-BP nucleic acid molecule, an IRAP-BP-specific antibody, or an IRAP-BP-target molecule) can be used in an animal model to determine the efficacy, toxicity, or side effects of treatment with such an agent. Alternatively, an agent identified as described herein can be used in an animal model to determine the mechanism of action of such an agent. Furthermore, this invention pertains to uses of novel agents identified by the above-described screening assays for treatments as described herein. B. Diagnostic Assays Portions or fragments of the cDNA sequences identified herein (and the corresponding complete gene sequences) as well as antibodies to the polypeptides identified herein, can be used in numerous ways as diagnostic and/or detection reagents. In one embodiment, a polynucleotide reagent (e.g., a portion or fragment of the IRAP- BP nucleotide sequences described herein) is used in a method for detecting an IRAP- BP polynucleotide in a biological sample. In another embodiment, an IRAP-BP antibody is used in a method for detecting an IRAP-BP polypeptide in a biological sample. An exemplary method for detecting the presence or absence of IRAP-BP polypeptide or nucleic acid in a biological sample involves obtaining a biological sample from a test subject and contacting the biological sample with a compound or an agent capable of detecting IRAP-BP polypeptide or nucleic acid (e.g., mRNA, genomic DNA) that encodes IRAP-BP polypeptide such that the presence of IRAP-BP polypeptide or nucleic acid is detected in the biological sample. A preferred agent for detecting IRAP- BP mRNA or genomic DNA is a labeled nucleic acid probe capable of hybridizing to IRAP-BP mRNA or genomic DNA. The nucleic acid probe can be, for example, a full- length IRAP-BP nucleic acid, such as the nucleic acid of SEQ ID NO: 1 or SEQ ID NO:4, or a fragment or portion of an IRAP-BP nucleic acid such as an oligonucleotide of at least 15, 30, 50, 100, 250 or 500 nucleotides in length and sufficient to specifically hybridize under stringent conditions to IRAP-BP mRNA or genomic DNA. Other suitable probes for use in the diagnostic assays of the invention are described herein. A preferred agent for detecting IRAP-BP polypeptide is an antibody capable of binding to IRAP-BP polypeptide, preferably an antibody with a detectable label. Antibodies can be polyclonal, or more preferably, monoclonal. An intact antibody, or a fragment thereof (e.g., Fab or F(ab')2) can be used. The term "labeled", with regard to the probe or antibody, is intended to encompass direct labeling of the probe or antibody by coupling (i.e., physically linking) a detectable substance to the probe or antibody, as well as indirect labeling of the probe or antibody by reactivity with another reagent that is directly labeled. Examples of indirect labeling include detection of a primary antibody using a fluorescently labeled secondary antibody and end-labeling of a DNA probe with biotin such that it can be detected with fluorescently labeled streptavidin. The term "biological sample" is intended to include tissues, cells and biological fluids isolated from a subject, as well as tissues, cells and fluids present within a subject. That is, the detection method of the invention can be used to detect IRAP-BP mRNA, protein, or genomic DNA in a biological sample in vitro as well as in vivo. For example, in vitro techniques for detection of IRAP-BP mRNA include Northern hybridizations and in situ hybridizations. In vitro techniques for detection of IRAP-BP polypeptide include enzyme linked immunosorbent assays (ELISAs), Western blots, immunoprecipitations and immunofluorescence. In vitro techniques for detection of IRAP-BP genomic DNA include Southern hybridizations. Furthermore, in vivo techniques for detection of IRAP- BP polypeptide include introducing into a subject a labeled anti-IRAP-BP antibody. For example, the antibody can be labeled with a radioactive marker whose presence and location in a subject can be detected by standard imaging techniques. In one embodiment, the biological sample contains protein molecules from the test subject. Alternatively, the biological sample can contain mRNA molecules from the test subject or genomic DNA molecules from the test subject. A preferred biological sample is a tissue sample (e.g., muscle of adipose) isolated by conventional means from a subject (e.g., a muscle biopsy). In another embodiment, the methods further involve obtaining a control biological sample from a control subject, contacting the control sample with a compound or agent capable of detecting IRAP-BP polypeptide, mRNA, or genomic DNA, such that the presence of IRAP-BP polypeptide, mRNA or genomic DNA is detected in the biological sample, and comparing the presence of IRAP-BP polypeptide, mRNA or genomic DNA in the control sample with the presence of IRAP-BP polypeptide, mRNA or genomic DNA in the test sample. The invention also encompasses kits for detecting the presence of IRAP-BP in a biological sample. For example, the kit can comprise a labeled compound or agent capable of detecting IRAP-BP polypeptide or mRNA in a biological sample; means for determining the amount of IRAP-BP in the sample; and means for comparing the amount of IRAP-BP in the sample with a standard. The compound or agent can be packaged in a suitable container. The kit can further comprise instructions for using the kit to detect IRAP-BP polypeptide or nucleic acid. B.l. Prognostic assays The diagnostic methods described herein can furthermore be utilized to identify subjects having or at risk of developing a disease or disorder associated with aberrant IRAP-BP expression or activity. For example, the assays described herein, such as the preceding diagnostic assays or the following assays, can be utilized to identify a subject having or at risk of developing a disorder associated with IRAP-BP polypeptide, nucleic acid expression or activity such as insulin resistance or diabetes. Thus, the present invention provides a method for identifying a disease or disorder associated with aberrant IRAP-BP expression or activity in which a test sample is obtained from a subject and IRAP-BP polypeptide or nucleic acid (e.g, mRNA, genomic DNA) is detected, wherein the presence of IRAP-BP polypeptide or nucleic acid is diagnostic for a subject having or at risk of developing a disease or disorder associated with aberrant IRAP-BP expression or activity. As used herein, a "test sample" refers to a biological sample obtained from a subject of interest. For example, a test sample can be a biological fluid, cell sample, or tissue (e.g., muscle or adipose). Furthermore, the prognostic assays described herein can be used to determine whether a subject can be administered an agent (e.g., an agonist, antagonist, peptidomimetic, protein, peptide, nucleic acid, small molecule, or other drug candidate) to treat a disease or disorder associated with aberrant IRAP-BP expression or activity. For example, such methods can be used to determine whether a subject can be effectively treated with an agent for a disorder, such as insulin resistance or diabetes. Thus, the present invention provides methods for determining whether a subject can be effectively treated with an agent for a disorder associated with aberrant IRAP-BP expression or activity in which a test sample is obtained and IRAP-BP polypeptide or nucleic acid expression or activity is detected (e.g., wherein the abundance of IRAP-BP polypeptide or nucleic acid expression or activity is diagnostic for a subject that can be administered the agent to treat a disorder associated with aberrant IRAP-BP expression or activity.) The methods of the invention can also be used to detect genetic alterations in an IRAP-BP gene, thereby determining if a subject with the altered gene is at risk for a disorder such as insulin resistance or diabetes. In preferred embodiments, the methods include detecting, in a sample of cells or tissue from the subject, the presence or absence of a genetic alteration characterized by at least one of an alteration affecting the integrity of a gene encoding an IRAP-BP-protein, or the mis-expression of the IRAP-BP gene. For example, such genetic alterations can be detected by ascertaining the existence of at least one of 1 ) a deletion of one or more nucleotides from an IRAP-BP gene; 2) an addition of one or more nucleotides to an IRAP-BP gene; 3) a substitution of one or more nucleotides of an IRAP-BP gene, 4) a chromosomal rearrangement of an IRAP-BP gene; 5) an alteration in the level of a messenger RNA transcript of an IRAP-BP gene, 6) aberrant modification of an IRAP-BP gene, such as of the methylation pattern of the genomic DNA, 7) the presence of a non-wild type splicing pattern of a messenger RNA transcript of an IRAP-BP gene, 8) a non-wild type level of an IRAP-BP-protein, 9) allelic loss of an IRAP-BP gene, and 10) inappropriate post-translational modification of an IRAP-BP-protein. As described herein, there are a large number of assay techniques known in the art which can be used for detecting alterations in an IRAP-BP gene. A preferred biological sample is a tissue or cell sample isolated by conventional means from a subject. In certain embodiments, detection of the alteration involves the use of a probe/primer in a polymerase chain reaction (PCR) (see, e.g., U.S. Patent Nos. 4,683,195 and 4,683,202), such as anchor PCR or RACE PCR, or, alternatively, in a ligation chain reaction (LCR) (see, e.g., Landegran et al. (1988) Science 241 :1077-1080; and Nakazawa et al (1994) PNAS 91 :360-364), the latter of which can be particularly useful for detecting point mutations in the IRAP-BP-gene (see Abravaya et al. (1995) Nucleic Acids Res .23:675-682). This method can include the steps of collecting a sample of cells from a patient, isolating nucleic acid (e.g., genomic, mRNA or both) from the cells of the sample, contacting the nucleic acid sample with one or more primers which specifically hybridize to an IRAP-BP gene under conditions such that hybridization and amplification of the IRAP-BP-gene (if present) occurs, and detecting the presence or absence of an amplification product, or detecting the size of the amplification product and comparing the length to a control sample. It is anticipated that PCR and/or LCR may be desirable to use as a preliminary amplification step in conjunction with any of the techniques used for detecting mutations described herein. Alternative amplification methods include: self sustained sequence replication (Guatelli, J.C. et al, 1990, Proc. Natl. Acad. Sci. USA 87:1874-1878), transcriptional amplification system (Kwoh, D.Y. et al, 1989, Proc. Natl. Acad. Sci. USA 86:1173- 1177), Q-Beta Replicase (Lizardi, P.M. et all, 1988, Bio/Technology 6:1197), or any other nucleic acid amplification method, followed by the detection of the amplified molecules using techniques well known to those of skill in the art. These detection schemes are especially useful for the detection of nucleic acid molecules if such molecules are present in very low numbers. In an alternative embodiment, mutations in an IRAP-BP gene from a sample cell can be identified by alterations in restriction enzyme cleavage patterns. For example, sample and control DNA is isolated, amplified (optionally), digested with one or more restriction endonucleases, and fragment length sizes are determined by gel electrophoresis and compared. Differences in fragment length sizes between sample and control DNA indicates mutations in the sample DNA. Moreover, the use of sequence specific ribozymes (see, for example, U.S. Patent No. 5,498,531) can be used to score for the presence of specific mutations by development or loss of a ribozyme cleavage site. In other embodiments, genetic mutations in IRAP-BP can be identified by hybridizing a sample and control nucleic acids, e.g., DNA or RNA, to high density arrays containing hundreds or thousands of oligonucleotides probes (Cronin, M.T. et al. (1996) Human Mutation 1: 244-255; Kozal, M.J. et al. (1996) Nαtwre Medicine 2: 753- 759). For example, genetic mutations in IRAP-BP can be identified in two dimensional arrays containing light-generated DΝA probes as described in Cronin, M.T. et al. supra. Briefly, a first hybridization array of probes can be used to scan through long stretches of DΝA in a sample and control to identify base changes between the sequences by making linear arrays of sequential ovelapping probes. This step allows the identification of point mutations. This step is followed by a second hybridization array that allows the characterization of specific mutations by using smaller, specialized probe arrays complementary to all variants or mutations detected. Each mutation array is composed of parallel probe sets, one complementary to the wild-type gene and the other complementary to the mutant gene. In yet another embodiment, any of a variety of sequencing reactions known in the art can be used to directly sequence the IRAP-BP gene and detect mutations by comparing the sequence of the sample IRAP-BP with the corresponding wild-type (control) sequence. Examples of sequencing reactions include those based on techniques developed by Maxim and Gilbert ((1977) PNAS 74:560) or Sanger ((1977) PNAS 74:5463). It is also contemplated that any of a variety of automated sequencing procedures can be utilized when performing the diagnostic assays ((1995) Biotechniques 19:448), including sequencing by mass spectrometry (see, e.g., PCT International Publication No. WO 94/16101; Cohen et al. (1996) Adv. Chromatogr. 36:127-162; and Griffin et al. ( 1993) Appl. Biochem. Biotechnol 38 : 147- 159). Other methods for detecting mutations in the IRAP-BP gene include methods in which protection from cleavage agents is used to detect mismatched bases in RNA/RNA or RNA/DNA heteroduplexes (Myers et al. (1985) Science 230:1242). In general, the art technique of "mismatch cleavage" starts by providing heteroduplexes of formed by hybridizing (labeled) RNA or DNA containing the wild-type IRAP-BP sequence with potentially mutant RNA or DNA obtained from a tissue sample. The double-stranded duplexes are treated with an agent which cleaves single-stranded regions of the duplex such as which will exist due to basepair mismatches between the control and sample strands. For instance, RNA/DNA duplexes can be treated with RNase and DNA/DNA hybrids treated with S 1 nuclease to enzymatically digesting the mismatched regions. In other embodiments, either DNA/DNA or RNA/DNA duplexes can be treated with hydroxylamine or osmium tetroxide and with piperidine in order to digest mismatched regions. After digestion of the mismatched regions, the resulting material is then separated by size on denaturing polyacrylamide gels to determine the site of mutation. See, for example, Cotton et al (1988) Proc. Natl Acad Sci USA 85:4397; Saleeba et al. (1992) Methods Enzymol 217:286-295. In a preferred embodiment, the control DNA or RNA can be labeled for detection. In still another embodiment, the mismatch cleavage reaction employs one or more proteins that recognize mismatched base pairs in double-stranded DNA (so called "DNA mismatch repair" enzymes) in defined systems for detecting and mapping point mutations in IRAP-BP cDNAs obtained from samples of cells. For example, the mutY enzyme of E. coli cleaves A at G/A mismatches and the thymidine DNA glycosylase from HeLa cells cleaves T at G/T mismatches (Hsu et al. (1994) Carcinogenesis 15:1657-1662). According to an exemplary embodiment, a probe based on an IRAP- BP sequence, e.g., a wild-type IRAP-BP sequence, is hybridized to a cDNA or other DNA product from a test cell(s). The duplex is treated with a DNA mismatch repair enzyme, and the cleavage products, if any, can be detected from electrophoresis protocols or the like. See, for example, U.S. Patent No. 5,459,039. In other embodiments, alterations in electrophoretic mobility will be used to identify mutations in IRAP-BP genes. For example, single strand conformation polymoφhism (SSCP) may be used to detect differences in electrophoretic mobility between mutant and wild type nucleic acids (Orita et al. (1989) Proc Natl. Acad. Sci USA: 86:2766, see also Cotton (1993) Mutat Res 285:125-144; and Hayashi (1992) Genet Anal Tech Appl 9:73-79). Single-stranded DNA fragments of sample and control IRAP-BP nucleic acids will be denatured and allowed to renature. The secondary structure of single-stranded nucleic acids varies according to sequence, the resulting alteration in electrophoretic mobility enables the detection of even a single base change. The DNA fragments may be labeled or detected with labeled probes. The sensitivity of the assay may be enhanced by using RNA (rather than DNA), in which the secondary structure is more sensitive to a change in sequence. In a preferred embodiment, the subject method utilizes heteroduplex analysis to separate double stranded heteroduplex molecules on the basis of changes in electrophoretic mobility (Keen et al. (1991) Trends Genet 7:5). In yet another embodiment the movement of mutant or wild-type fragments in polyacrylamide gels containing a gradient of denaturant is assayed using denaturing gradient gel electrophoresis (DGGE) (Myers et al (1985) Nature 313 :495). When DGGE is used as the method of analysis, DNA will be modified to insure that it does not completely denature, for example by adding a GC clamp of approximately 40 bp of high-melting GC-rich DNA by PCR. In a further embodiment, a temperature gradient is used in place of a denaturing gradient to identify differences in the mobility of control and sample DNA (Rosenbaum and Reissner (1987) Biophys Chem 265:12753). Examples of other techniques for detecting point mutations include, but are not limited to, selective oligonucleotide hybridization, selective amplification, or selective primer extension. For example, oligonucleotide primers may be prepared in which the known mutation is placed centrally and then hybridized to target DNA under conditions which permit hybridization only if a perfect match is found (Saiki et al. (1986) Nature 324:163); Saiki et al. (1989) Proc. Natl Acad. Sci USA 86:6230). Such allele specific oligonucleotides are hybridized to PCR amplified target DNA or a number of different mutations when the oligonucleotides are attached to the hybridizing membrane and hybridized with labeled target DNA. Alternatively, allele specific amplification technology which depends on selective PCR amplification may be used in conjunction with the instant invention. Oligonucleotides used as primers for specific amplification may carry the mutation of interest in the center of the molecule (so that amplification depends on differential hybridization) (Gibbs et al. (1989) Nucleic Acids Res. 17:2437-2448) or at the extreme 3' end of one primer where, under appropriate conditions, mismatch can prevent, or reduce polymerase extension (Prossner (1993) Tibtech 11 :238). In addition it may be desirable to introduce a novel restriction site in the region of the mutation to create cleavage-based detection (Gasparini et al (1992) Mol. Cell Probes 6:1). It is anticipated that in certain embodiments amplification may also be performed using Taq ligase for amplification (Barany (1991) Proc. Natl. Acad. Sci USA 88:189). In such cases, ligation will occur only if there is a perfect match at the 3' end of the 5' sequence making it possible to detect the presence of a known mutation at a specific site by looking for the presence or absence of amplification. The methods described herein may be performed, for example, by utilizing prepackaged diagnostic kits comprising at least one probe nucleic acid or antibody reagent described herein, which may be conveniently used, e.g., in clinical settings to diagnose patients exhibiting symptoms or family history of a disease or illness involving an IRAP-BP gene. Furthermore, any cell type or tissue in which IRAP-BP is expressed may be utilized in the prognostic assays described herein. B.2. Monitoring of Effects During Clinical Trials Monitoring the influence of agents (e.g., drugs, compounds) on the expression or activity of an IRAP-BP polypeptide (e.g., modulation insulin resistance) an be applied not only in basic drug screening, but also in clinical trials. For example, the effectiveness of an agent determined by a screening assay as described herein to increase IRAP-BP gene expression, protein levels, or upregulate IRAP-BP activity, can be monitored in clinical trails of subjects exhibiting decreased IRAP-BP gene expression, protein levels, or downregulated IRAP-BP activity. Alternatively, the effectiveness of an agent determined by a screening assay to decrease IRAP-BP gene expression, protein levels, or downregulate IRAP-BP activity, can be monitored in clinical trails of subjects exhibiting increased IRAP-BP gene expression, protein levels, or upregulated IRAP-BP activity. In such clinical trials, the expression or activity of an IRAP-BP gene, and preferably, other genes that have been implicated in, for example, insulin resistance can be used as a "read out" or markers of the phenotype of a particular cell. For example, and not by way of limitation, genes, including IRAP-BP, that are modulated in cells by treatment with an agent (e.g., compound, drug or small molecule) which modulates IRAP-BP activity (e.g., identified in a screening assay as described herein) can be identified. Thus, to study the effect of agents on insulin resistance, for example, in a clinical trial, cells can be isolated and RNA prepared and analyzed for the levels of expression of IRAP-BP and other genes implicated in insulin resistance, respectively. The levels of gene expression (i.e., a gene expression pattern) can be quantified by Northern blot analysis or RT-PCR, as described herein, or alternatively by measuring the amount of protein produced, by one of the methods as described herein, or by measuring the levels of activity of IRAP-BP or other genes. In this way, the gene expression pattern can serve as a marker, indicative of the physiological response of the cells to the agent. Accordingly, this response state may be determined before, and at various points during treatment of the individual with the agent. In a preferred embodiment, the present invention provides a method for monitoring the effectiveness of treatment of a subject with an agent (e.g., an agonist, antagonist, peptidomimetic, protein, peptide, nucleic acid, small molecule, or other drug candidate identified by the screening assays described herein) comprising the steps of (i) obtaining a pre-administration sample from a subject prior to administration of the agent; (ii) detecting the level of expression of an IRAP-BP polypeptide, mRNA, or genomic DNA in the preadministration sample; (iii) obtaining one or more post- administration samples from the subject; (iv) detecting the level of expression or activity of the IRAP-BP polypeptide, mRNA, or genomic DNA in the post-administration samples; (v) comparing the level of expression or activity of the IRAP-BP polypeptide, mRNA, or genomic DNA in the pre-administration sample with the IRAP-BP polypeptide, mRNA, or genomic DNA in the post administration sample or samples; and (vi) altering the administration of the agent to the subject accordingly. For example, increased administration of the agent may be desirable to increase the expression or activity of IRAP-BP to higher levels than detected, i.e., to increase the effectiveness of the agent. Alternatively, decreased administration of the agent may be desirable to decrease expression or activity of IRAP-BP to lower levels than detected, i.e. to decrease the effectiveness of the agent. According to such an embodiment, IRAP-BP expression or activity may be used as an indicator of the effectiveness of an agent, even in the absence of an observable phenotypic response. C. Methods of Treatment: The present invention provides for both prophylactic and therapeutic methods of treating a subject at risk of (or susceptible to) a disorder or having a disorder associated with aberrant IRAP-BP expression or activity. C.l. Prophylactic Methods In one aspect, the invention provides a method for preventing in a subject, a disease or condition associated with an aberrant IRAP-BP expression or activity, by administering to the subject an IRAP-BP or an agent which modulates IRAP-BP expression or at least one IRAP-BP activity. Subjects at risk for a disease which is caused or contributed to by aberrant IRAP-BP expression or activity can be identified by, for example, any or a combination of diagnostic or prognostic assays as described herein. Administration of a prophylactic agent can occur prior to the manifestation of symptoms characteristic of the IRAP-BP aberrancy, such that a disease or disorder is prevented or, alternatively, delayed in its progression. Depending on the type of IRAP- BP aberrancy, for example, an IRAP-BP, IRAP-BP agonist or IRAP-BP antagonist agent can be used for treating the subject. The appropriate agent can be determined based on screening assays described herein. The prophylactic methods of the present invention are further discussed in the following subsections. C.2. Therapeutic Methods Another aspect of the invention pertains to methods of modulating IRAP-BP expression or activity for therapeutic puφoses. Accordingly, in an exemplary embodiment, the modulatory method of the invention involves contacting a cell with a IRAP-BP molecule of the present invention such that the activity of an IRAP-BP is modulated. Alternatively, the modulatory method of the invention involves contacting a cell with an agent that modulates one or more of the activities of IRAP-BP polypeptide activity associated with the cell. An agent that modulates IRAP-BP polypeptide activity can be an agent as described herein, such as a nucleic acid or a protein, a naturally- occurring target molecule of an IRAP-BP polypeptide (e.g., a carbohydrate), an IRAP- BP antibody, an IRAP-BP agonist or antagonist, a peptidomimetic of an IRAP-BP agonist or antagonist, or other small molecule. In one embodiment, the agent stimulates one or more IRAP-BP activites. Examples of such stimulatory agents include active IRAP-BP polypeptide and a nucleic acid molecule encoding IRAP-BP that has been introduced into the cell. In another embodiment, the agent inhibits one or more IRAP- BP activites. Examples of such inhibitory agents include antisense IRAP-BP nucleic acid molecules and anti-IRAP-BP antibodies. These modulatory methods can be performed in vitro (e.g., by culturing the cell with the agent) or, alternatively, in vivo (e.g, by administering the agent to a subject). As such, the present invention provides methods of treating an individual afflicted with a disease or disorder characterized by aberrant expression or activity of an IRAP-BP polypeptide or nucleic acid molecule. In one embodiment, the method involves administering an agent (e.g., an agent identified by a screening assay described herein), or combination of agents that modulates (e.g., upregulates or downregulates) IRAP-BP expression or activity. In another embodiment, the method involves administering an IRAP-BP polypeptide or nucleic acid molecule as therapy to compensate for reduced or aberrant IRAP-BP expression or activity. Stimulation of IRAP-BP activity is desirable in situations in which IRAP-BP is abnormally downregulated and/or in which increased IRAP-BP activity is likely to have a beneficial effect (e.g., insulin resistance or diabetes). Likewise, inhibition of IRAP-BP activity is desirable in situations in which IRAP-BP is abnormally upregulated and/or in which decreased IRAP-BP activity is likely to have a beneficial effect. This invention is further illustrated by the following examples which should not be construed as limiting. The contents of all references, patents and published patent applications cited throughout this application are hereby incoφorated by reference. References throughout the instant specification to websites maintained as part of the World Wide Web are referred to herein by the prefix http://. The information contained in such websites is publically-avialable and can be accessed elctronically by contacting the cited address. EXAMPLES Example 1 : Identification And Characterization of murine IRAP-BP-1 cDNA In this example, the identification and characterization of the gene encoding murine IRAP-BP-1 is described. Isolation of the murine IRAP-BP-1 cDNA In order to identify novel proteins which bound to the insulin-responsive aminopeptidase, IRAP, the yeast two-hybrid screening method was utilized to analyse the sequences of several cDNAs of a library derived differentaited 3T3-L1 adipocytes. Briefly, the assay utilized the cDNA encoding amino acid residues 55-82 of IRAP polypeptide as the "bait" construct, fused to the gene encoding the DNA binding domain of GAL-4. In the "prey" construct, cDNA sequences from the differentaited 3T3-L1 library that encode unidentified proteins were fused to the gene that codes for the activation domain of GAL-4. Interaction of the "bait" and "prey" proteins in vivo, results in the formation of IRAP-dependent complexes, bringing the the DNA-binding and activation domains of GAL4 into close proximity. This proximity allows transcription of a reporter gene (e.g., LacZ) which is operably linked to a transcriptional regulatory site responsive to GAL4. In a first series of experiments, Y190 cells were made competent by treatment with LiOAc, followed by co-transformation with the bait and prey constructs. Approximately 10 6 transformants were plated on Tφ-, Leu-, His- plates. Approximately 10 2 His+ colonies were replated on Tφ-, Leu- plates and assayed for β- galactosidase activity. Plasmid DNA was isolated from 9 strong β-galactosidase positive clones and transformed into Y190/IRAP(55-82) cells. One positive clone, A89, was sequenced resulting in nucleotides 1-2947 of SEQ ID NO:l. In a second series of experiments, Y190/IRAP(55-82) cells were made competent by treatment with LiOAc, followed by transformation with the prey constructs. Approximately 10 7 transformants were plated on Tφ-, Leu-, His- plates. Approximately 10 3 His+ colonies were replated on Tφ-, Leu- plates and assayed for β- galactosidase activity. Plasmid DNA was isolated from 6 strong β-galactosidase positive clones and transformed into Y190/IRAP(55-82) cells. One positive clone, E89, was sequenced resulting in nucleotides 451 -2947 of SEQ ID NO: 1. The nucleotide sequence encoding the murine IRAP-BP-1 protein is shown in Figure 1 and is set forth as SEQ ID NO: 1. The protein encoded by this nucleic acid is comprised of about 499 amino acids and has the amino acid sequence shown in Figure 1 and set forth as SEQ ID NO:2. The coding portion (open reading frame) of SEQ ID NO: 1 is set forth as SEQ ID NO:3. Alternatively, the nucleotide sequence of SEQ ID NO:l encodes a protein of at least 728 amino acid residues and has the amino acid sequence of SEQ ID NO: 7. The corresponding coding portion (open reading frame) is set forth as SEQ ID NO:8. Tissue Distribution of IRAP-BP-1 mRNA This Example describes the tissue distribution of IRAP-BP mRNA, as determined by Northern blot hybridization. Northern blot hybridizations with various RNA samples were performed under standard conditions and washed under stringent conditions. A 3.5 Kb mRNA transcript was detected in heart, brain, lung, liver, skeletal muscle, kidney, and testis with the expression being the highest in testis. Northern blot hybridizations were also performed with mRNA isolated from 3T3L1 preadipocytes, adipocytes which had been differentiated for 3 days, and adipocytes which had been differentiated for 6 days. IRAP-BP mRNA levels increased significantly with differentiation. Example 2 Identification And Characterization of human IRAP-BP-1 cDNA In this example, the identification and characterization of the gene encoding human IRAP-BP-1 is described. Two PCR primers were designed based on the nucleotide sequence of murine IRAP-BP. A forward primer was designed based on nucleotides 508-527 of murine IRAP-BP (SEQ ID NO: 1). Forward Primer 5'-GGAGCCCGAAGGACAAGTTC-3' 508-527 (SEQ ID NO:9) A reverse primer was designed based on nucleotides 2312-2293 of murine IRAP-BP (SEQ ID NO: 1). Reverse Primer 5'-AGCGACAAAAACCTGTCCCC-3' 2312-2293 (SEQ ID NO:10) RT-PCR was performed utilizing human skeletal muscle total RNA as a template. A fragment of approximately 1805 bp was purified and subcloned into PCR vector 2.1 (InVitrogen). Sequencing of the insert resulted in nucleotides comprising the coding region of human IRAP-BP (SEQ ID NO:4). A TBLASTN search of the dBEST database of Expressed Sequence Tags (ESTs) revealed an EST having significant homology to a portion of the 3' untranslated region of human IRAP-BP, Accession No. W84417. The entire 912 bp insert of this clone was sequenced and corresponds to the 3' untranslated region of human IRAP-BP (SEQ ID NO:4). The nucleotide sequence encoding the human IRAP-BP-1 protein is shown in Figure 2 and is set forth as SEQ ID NO:4. The protein encoded by this nucleic acid is comprised of about 500 amino acids and has the amino acid sequence shown in Figure 2 and set forth as SEQ ID NO:5. The coding portion (open reading frame) of SEQ ID NO:4 is set forth as SEQ ID NO:6. Alternatively, the nucleotide sequence of SEQ ID NO: 1 encodes a protein of at least 561 amino acid residues and has the amino acid sequence of SEQ ID NO: 12. The corresponding coding portion (open reading frame) is set forth as SEQ ID NO:l l. An alignment of murine IRAP-BP and human IRAP-BP demonstrates that the proteins are very highly conserved at the amino acid level. Figure 4 shown such an alignment generated using the ALIGN algorithm, PAM 120 weight residue table, gap length penalty = 12, gap penalty = 4 indicating that the proteins are greater than 91% identical (97.4% identical.) An alignment of the human and murine nucleic acid sequences is depicted in Figure 3A-C. The alignment was generated using the BESTFIT algorithm of the GCG software analysis package and the following parameters: Symbol comparison table: /GCG/gcg/gcgcore/data/rundata/swgapdna. cmp CompCheck: 2335 Gap Weight : 50 Average Match: 10.000 Length Weight 3 Average Mismatch: -9.000 Quality: 18180 Length : 2067 Ratio: 8.830 Gaps : 4 Percent Similarity: 94.504 Percent Identity: 94.504 Match display thresholds for the alignment (s) I = IDENTITY : = 5 . = 1 An alignment of the human and murine hypothetical amino acid sequences is depicted in Figure 3D. The alignment was generated using the BESTFIT algorithm of the GCG software analysis package and the following parameters: Symbol comparison table: /GCG/gcg/gcgcore/data/rundata/blosum62. cmp CompCheck: 6430 Gap Weight : 12 Average Match: 2.912 Length Weight 4 Average Mismatch: -2.003 Quality 3058 Length: 591 Ratio 5.183 Gaps : 1 Percent Similarity 98.136 Percent Identity: 97.458 Match display thresholds for the alignment (s) I = IDENTITY : = 2 . = 1 Equivalents Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.

Claims: What is claimed: 1. An isolated nucleic acid molecule comprising a nucleotide sequence which encodes an insulin responsive aminopeptidase-binding protein (IRAP-BP) polypeptide, wherein said polypeptide binds to an insulin responsive aminopeptidase (IRAP) polypeptide. 2. The isolated nucleic acid molecule of claim 1, comprising a nucleotide sequence at least about 60% identical to a nucleotide sequence of SEQ ID NO:l or SEQ ID NO:4, or complement thereof. 3. The isolated nucleic acid molecule of claim 1, comprising the nucleotide sequence of SEQ ID NO:l or SEQ ID NO:4. 4. An isolated nucleic acid molecule of claim 1, which specifically detects an IRAP-BP nucleic acid molecule relative to a nucleic acid molecule encoding a non- IRAP-BP polypeptide. 5. An isolated nucleic acid molecule comprising a nucleotide sequence which encodes an insulin responsive aminopeptidase-binding protein (IRAP-BP) polypeptide, wherein said polypeptide binds to an insulin responsive aminopeptidase (IRAP) polypeptide. 6 The isolated nucleic acid molecule of claim 5, comprising a nucleotide sequence which encodes a polypeptide which comprises an amino acid sequence at least about 60 % identical to the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO:5. 7. The isolated nucleic acid molecule of claim 6, which encodes a polypeptide having the amino acid sequence of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. 8. An isolated nucleic acid molecule comprising a nucleotide sequence which hybridizes under stringent hybridization conditions to a nucleic acid molecule comprising a nucleotide sequence of SEQ ID NO:l or SEQ ID NO:4, wherein said nucleic acid molecule encodes an insulin responsive aminopeptidase-binding protein (IRAP-BP) polypeptide, or biologically active portion thereof. 9. An isolated nucleic acid molecule at least 550 nucleotides in length which hybridizes under stringent hybridization conditions to a nucleic acid molecule comprising the nucleotide sequence of SEQ ID NO:l or SEQ ID NO:4. 10. An isolated nucleic acid molecule which encodes a fusion protein comprising an insulin responsive aminopeptidase-binding protein (IRAP-BP) polypeptide operatively linked to a non-IRAP-BP polypeptide 11. An isolated nucleic acid molecule which is antisense to the nucleic acid molecule of claim 1. 12. A vector comprising the nucleic acid molecule of claim 1. 13. A host cell containing the vector of claim 12. 14. An isolated insulin responsive aminopeptidase-binding protein (IRAP- BP) polypeptide, wherein said polypeptide binds to an insulin responsive aminopeptidase (IRAP) polypeptide. 15. The isolated polypeptide of claim 14, comprising an amino acid sequence at least 60% identical to the amino acid sequence of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. 16. The isolated protein of claim 14, wherein the protein is encoded by a nucleic acid molecule comprising a nucleic acid sequence 60% homologous to the nucleic acid sequence of SEQ ID NO:l or SEQ ID NO:3. 17. The isolated polypeptide of claim 14, wherein said polypeptide comprises the amino acid sequence of SEQ ID NO:2, SEQ ID NO:5, or SEQ ID NO:8. 18. A fusion protein comprising an insulin responsive aminopeptidase- binding protein (IRAP-BP) polypeptide operatively linked to a non-IRAP-BP polypeptide. 19. An antibody that specifically binds an insulin responsive aminopeptidase- binding protein (IRAP-BP) polypeptide. 20. A pharmaceutical composition comprising the protein of claim 14 and a pharmaceutically acceptable carrier. 21. A method for modulating a cell-associated activity comprising contacting the cell with an agent which modulates the activity or expression of an insulin responsive aminopeptidase-binding protein (IRAP-BP) such that the cell associated activity is altered relative to the cell-associated activity of the cell in the absence of the agent. 22. The method of claim 21 , wherein the cell is present within a subject and the agent is administered to the subject. 23. A method for treating a subject having a disorder characterized by aberrant activity expression of an insulin responsive aminopeptidase-binding protein (IRAP-BP), comprising administering to the subject an IRAP-BP modulator such that treatment of the subject occurs. 24. A method for detecting the activity or expression of an insulin responsive aminopeptidase-binding protein (IRAP-BP) in a biological sample comprising contacting the biological sample with an agent capable of detecting activity or expression of IRAP-BP such that activity or expression of IRAP-BP is detected in the biological sample. 25. A kit for detecting the expression or activity of an insulin responsive aminopeptidase-binding protein (IRAP-BP) in a biological sample comprising an agent capable of detecting activity or expression of IRAP-BP in a biological sample. 26. A diagnostic assay for identifying a genetic lesion in a cell sample, the presence or absence of the genetic lesion characterized by at least one of (i) aberrant modification or mutation of a gene encoding an insulin responsive aminopeptidase- binding protein (IRAP-BP) polypeptide, and (ii) mis-regulation of said gene or (iii) aberrant post-translational modification of an IRAP-BP polypeptide. 27. A method for identifying a compound that modulates the activity of an insulin responsive aminopeptidase-binding protein (IRAP-BP) polypeptide, comprising: a. providing a indicator composition comprising an IRAP-BP polypeptide or biologically active portion thereof; b. contacting the indicator composition with a test compound; and c. determining the effect of the test compound on IRAP-BP activity in the indicator composition to thereby identify a compound that modulates the activity of an IRAP-BP polypeptide.

Inventors: LIN BAOZHEN, PILCH PAUL F, TOJO HIDEAKI, LIN, BAOZHEN, PILCH, PAUL, F., TOJO, HIDEAKI
Applicants: LIN BAOZHEN, PILCH PAUL F, TOJO HIDEAKI, UNIV BOSTON, LIN, BAOZHEN, PILCH, PAUL, F., TOJO, HIDEAKI, TRUSTEES OF BOSTON UNIVERSITY
Application year: 1999
Main classification: C07K14
IPC codes: A61K38, A61K45, A61P3, C07K14, C07K16, C07K19, C12N1, C12N5, C12N15, C12Q1, G01N33