MUTANT OB RECEPTORS AND NUCLEOTIDES ENCODING THEM

Abstract: Mutant ob receptors have been made which a) lack a functional first CK-F3 domain; b) lack a functional second CK-F3 domain or c) lack a functional intracellular domain. These receptors may be used in various assays, such as a transactivation assay to identify novel ligands.

Description: TITLE OF THE INVENTION MUTANT OB RECEPTORS AND NUCLEOTIDES ENCODING THEM CROSS-REFERENCE TO RELATED APPLICATIONS Not Applicable. STATEMENT REGARDING FEDERALLY-SPONSORED R&D Not Applicable. REFERENCE TO MICROFICHE APPENDIX Not Applicable. FIELD OF THE INVENTION This invention relates to mutant ob receptor proteins, to nucleotides encoding them, and to assays using the mutant receptor proteins. BACKGROUND OF THE INVENTION Recently the identification of mutations in several genes involved in the onset of obesity in rodents have been identified. Of particular interest are mutations discovered in the peptide hormone, leptin, which is a component of a novel signal transduction pathway that regulates body weight (Zhang et al. 1994, Nature 372:425-432; Chen et al. 1996, Cell 84:491-495). Leptin was initially discovered by the positional cloning of the obesity gene, ob, in mice. Two different ob alleles have been identified: one mutation causes the premature termination of the leptin peptide resulting in a truncated protein, and the other mutation changes the transcriptional activity of the obesity (ob) gene, resulting in a reduced amount of circulating leptin. There is a correlation between a decrease in the levels of biologically active leptin and the overt obese phenotype observed in ob/ob mice. Recombinant leptin has been shown to induce weight loss in the ob/ob mouse but not in the diabetic phenotype db/db mouse (Campfield et al. 1995, Science 269: 546-549; Halaas et al. 1995, Science 269: 543-546; Pellymounter et al. 1995, Science 269:540-543; Rentsch et al. 1995, Biochem. Biophys. Res. Comm. 214:131-136; and Weigle et al. 1995, J. Clin. Invest. 96:2065-2070). Although the synthesis of leptin occurs in the adipocyte, its ability to decrease food intake and increase metabolic rate appears to be mediated centrally by the hypothalamus. Injection of recombinant leptin into the third ventricle of the brain elicits a similar response as peripheral administration of leptin. Furthermore, the recent cloning of the human receptor for the leptin, the ob-receptor (OB-R), reveals that it is transcribed in the hypothalamus (Tartaglia et al. 1995, Cell 83:1263- 1271; Stephens et al. 1995, Nature 377: 530-532). In addition, a mutation that results in premature termination of the long-form of the mouse OB- R, which is preferentially expressed in the hypothalamus, appears to be responsible for the obese phenotype of the db/db mouse (Lee et al. 1996, Nature 379:632-635; Chua et al. 1996, Science 271:994-996; and Chen et al. 1996, Cell 84:491-495). It would be desirable to clone and produce mutant ob receptor proteins to use in assays for the identification of ligands which may be useful in understanding obesity and for its prevention and treatment. SUMMARY OF THE INVENTION Not Applicable. BRIEF DESCRIPTION OF THE DRAWINGS Not Applicable. DETAILED DESCRIPTION OF THE INVENTION This invention relates to mutant ob receptors ("OB-Rs"), also referred to as "leptin receptors" which differ from wild type receptors by having a mutation selected from the group consisting of: a) lacking a functional first CK-F3 module; b) lacking a functional second CK-F3 module; and c) lacking a functional intracellular domain. The expression "lack a functional" module or domain means that the receptor no longer has biological function associated with structural portion of the receptor molecule. This may be accomplished by deletion of the module or domain, or by substitutions the amino acids which make up one domain by other amino acids such that function is lost. This invention also relates to mutant ob receptors, which are selected from the group consisting of: a) receptors which contain a deletion of amino acid residues which comprise all or substantially all of the first CK-F3 module; b) receptors which contain a substitution of all or substantially all of a F3 domain for a CK domain in a second CK-F3 module, resulting in a F3-F3 module; and c) receptors which have a deletion of all or substantially all of the intracellular domain. Another aspect of this invention is to nucleic acids which encode a mutant OB-R of this invention. Preferably, the nucleic acid is DNA. This invention also includes vectors containing a mutant OB-R gene, host cells containing the vectors, and methods of making mutant OB-R protein comprising the steps of introducing a vector comprising a mutant OB-R gene into a host cell, and cultivating the host cell under appropriate conditions such that mutant OB-R is produced. The mutant OB-R so produced may be harvested from the host cells in conventional ways. Yet another aspect of this invention are assays which employ a mutant OB-R. In these assays, various molecules, suspected of being wild-type OB-R ligands are contacted with a mutant OB-R, and their binding is detected. A particularly preferred type of assay is a transactivation assay, wherein the binding of the ligand to a mutant OB- R initiates a cascade of intracellular events, resulting in transcription and/or translation of a reporter gene which can be detected. Using the assays of this invention, agonists, antagonists, and ligand mimetics may be identified. A further aspect of this invention are the ligands so indentified. Still another aspect of this invention is the use of the mutant receptors of this invention in gene therapy. Genes encoding the mutant receptors may be transferred into an animal in need of treatment, and the regulation of weight gain may be influenced. BRIEF DESCRIPTION OF THE FIGURES FIGURE 1 is the amino acid sequence of a deletion mutant receptor D(41-322) (SEQ.ID.NO:l). FIGURE 2 is the amino acid sequence of a replacement mutant receptor wherein amino acids 420-496 have been deleted and substituted with amino acids 500-632 (SEQ.ID.NO:2). FIGURE 3 is a graph of a binding assay using the deletion mutant of FIGURE 1. FIGURE 4 is a graph of a transactivation assay using the deletion mutant receptor of FIGURE 1. FIGURE 5 is the amino acid sequence of the soluble leptin binding domain containing amino acids 1-22 of the leptin receptor as the signal sequence and residues 420-632 of the leptin receptor (SEQ.ID.NO:3). As used througout the specification and claims, the following definitions apply: "Wild type" means that the gene or protein is substantially the same as that found in the reference species, and which is not considered to have a mutation for that gene or protein. "Substantially the same" when referreing to a nucleic acid or amino acid sequence means either it is the same as the reference sequence, or if not exactly the same, contains changes which do not affect its biological activity or function. "Substantially all" when referring to a deletion means that the deletion is extensive enough so that the particular domain or module no longer retains its biological function. The OB-R is a member of the cytokine receptor family. The cytokine receptor family is a relatively large family and includes: Growth Hormone Receptor (GH-R), Erythropoietin Receptor (EPO-R) and Granulocyte Colony Stimulating Fator (GCSF-R). GH-R is one of two receptors for which high resolution structure is currently available. The extracellular region of the GH-R is composed of two domains, a CK domain and a F3 domain, both of which are characterized by unique consensus residues. The combined CK-F3 module forms the growth hormone binding site. Sequence alignment between the OB-R and the GH-R reveals that the extracellular region of the OB-R contains two homologous units of the CK-F3 module. The first CK-F3 module extends from amino acids 41-322. Its CK domain extends from amino acid 41 to 222, and its F3 domain is from amino acid 223 to 326. The second CK-F3 module extends from amino acids 431-632. Its CK domain is from amino acids 431 to 508 and its F3 domain is from amino acids 509 to 632. The repeating CK-F3 module motif which is present in the OB-R is not commonly found in cytokine receptors, and the localization of the ligand binding site for those cytokine receptors with repeating CK-F3 modules is not known. This specification refers to particular amino acids and domains by reference to the human OB-R. However, it is understood that OB-R from other species may be similarly modified to obtain equivalent mutants of those described specifically herein, and the invention is not limited to mutations of the human OB-R. In accordance with this invention, three classes of leptin receptor mutants were constructed. The first class lacks a functional first CK-F3 module. One embodiment of this class is a deletion mutant where all or substantially all of the first CK-F3 module (which extends from amino acids 41 to 322) has been deleted. In one preferred embodiment of this invention, the entire first CK-F3 module has been deleted. This particular mutant was designated D(41-322). Surprisingly, D(41-322) binds recombinant leptin with the same affinity as the wild type receptor (IC50 = 0.5 nM), and in the transactivation assay of this invention (described more fully below) it is functionally active in inducing the synthesis of luciferase (EC50 = 0.2 nM). This finding is unexpected since point mutations and deletions in the N-terminal domain of the GCSF receptor (which is in the same class of cytokine receptors as OB-R) was shown to negatively affect activity (see Fukunaga et al., 1991 EMBO J. 10:2855-2865). In contrast, the OB-R mutant carrying a deletion in this domain retained activity. Analyses of the mutants of this invention indicates that one third of the extracellular region of the leptin receptor is not required for binding or activation. The leptin binding site lies within residues 420- 632. Using the D(41-322) deletion mutant for compound screening can be advantageous over the wild type because a small molecule compound may bind to both of the CK-F3 modules in the wild type receptor and therefore may complicate data interpretation in the process of establishing a structure-activity relationship for synthetic compounds. A second class of mutants according to this invention lacks a functional second CK-F3 domain. One embodiment of this class are termed substitution mutants. In a substitution mutant, all or substantially all of the CK domain in the second CK-F3 module (which extends from amino acids 420-496) are removed and replaced with amino acids comprising all or substantially all of the F3 domain in the second CK-F3 module (amino acids 500-632). This results in a new module, F3-F3 which replaces the second CK-F3 module. In a preferred embodiment, the entire CK domain is replaces with an entire F3 domain in the second module. This mutant was designated (420-496)TO(500-632). This mutant exhibits no detectable level of radiolabeled leptin binding at 0.2 nM, and it is incapable of inducing the synthesis of luciferase in a transactivation assay with up to 100 nM of leptin. This mutant behaves as a dominant negative mutant of the wild-type OB-R. When the latter mutant was coexpressed with the wild-type OB-R, leptin binding or activation was undetectable, suggesting a homodimerization interaction between the subunits. In contrast, coexpression of the deletion (41-322 mutant) retained wildtype activity. A third class of mutants according to this invention lacks a functional intracellular domain. One group of these mutants contain a deletion of all or substantially all of the intracellular residues (867-1165). A preferred embodiment of this class of mutants has the entire intracellular region deleted. This mutant, D(867-1165), exhibits the same leptin binding affinity as the wild type receptor and yet it is incapable of inducing the synthesis of luciferase in a transactivation assay with up to 100 nM of leptin. This mutant also behaves as a dominant negative mutant of OB-R. This invention also relates to a soluble secreted form of leptin binding domain comprised of a secretion signal sequence (residues 1-22) and residues 420-632 of the OB-R (Figure 4). This leptin bonding domain can be produced by COS or other cells and subsequently purified. Using such a purified soluble binding domain in the screening of OB-R ligands can substantially increase the specificity of the screening assay compared to a cell based assay. Another aspect of this invention is a transactivation assay which uses the mutant OB-R receptors described above. Transactivation assays which involve membrane-bound receptors have been described in the art. Generally, when a ligand binds to a membrane-bound receptor, a cascade of intracellular events is initiated, ultimately resulting in the binding of a molecule to a portion of a DNA sequence known as the response element. This in turn, initiates transcription of the gene associated with the response element. In a transactivation assay, a reporter construct which contains a reporter gene (such as luciferase or CAT) is placed under the transcriptional control of a response element and introduced into a host cell. The host cell also expresses the receptor of interest (either endogenously or through recombinant expression). When a ligand binds to the receptor, various intracellular signals are generated and ultimately, the reporter gene is transcribed and/or translated. The transcription and/or translation may be detected and measured by a variety of methods, and may even be quantified. Co-pending U.S. provisional patent applications Serial Nos. 60/016051, filed April 22, 1996 and , (attorney docket number 19686PV2) filed November 15, 1996, both of which are hereby incorporated by reference describe transactivation assays for various native OB-Rs. In the assay of the present invention, the mutant OB-Rs can be used in the same general transactivation assay protocol to screen for receptor ligands (agonists and/or antagonists) and to generally study receptor structure and function. A further aspect of this invention is a method for determining whether an OB-R ligand is present in a sample comprising. a) contacting a cell with the sample wherein the cell expresses a mutant OB-R selected from the group consisting of receptors which lack a functional first CK-F3 module; and b) determining if transcription or translation of the reporter gene occurs. Another aspect of this invention is a method for determining whether an OB-R ligand is present in a sample comprising: a) contacting a cell with the sample, wherein the cell expresses an OB-R mutant selected from the group consisting of: receptors which contain a deletion of amino acid residues which comprise all or substantially all of a first CK-F3 module; wherein the cell also comprises a reporter gene construct, wherein the reporter gene construct comprises a leptin response element operatively linked to a reporter gene; and b) determining if transcription or translation of the reporter gene occurs. In preferred embodiments of this assay, the cell which expresses the mutant receptor does not express endogenous OB-R, or alternatively only expresses a small amount of endogenous receptor. Thus, the amount of transcription and/or translation of the reporter gene will be directly correlated to the binding and activation of the mutant receptor. The reporter gene construct contains a promoter region which includes a leptin response element, and a reporter gene. A leptin response element which may be used in accordance with this invention is an IRF-1 derived gamma-interferon activation sequence. IRF-1 derived activation sequences have been described in the art (Pine et al, 1994 EMBO J. 13: 158-167). A promoter region is constructed which contains a promoter of choice and one or more leptin response elements. The promoter region may be a hybrid promoter region, i.e. contain a promoter and one or more response elements which do not naturally occur together, or it may be a naturally occurring promoter region with one or more associated response elements. In a preferred embodiment the promoter is a well-characterized promoter, such as the herpes simplex virus thymidine kinase promoter, but any promoter which is known to function in the host cell chosen may be used. It is also preferred that the promoter be a minimal promoter, so that there are no transcriptional control sequences which may influence the activity of the leptin response element(s). The leptin-response element is preferably placed proximal to the promoter. Intervening sequences may be present between the leptin response element and the promoter, provided they do not interfere with the functioning of the leptin response element. In accordance with this invention, a suitable leptin response element is an IRF-1 derived gamma interferon activation sequence 5'-CTGATTTCCC CGAAATGACG-3' (SEQ.ID.NO.4). In a preferred embodiment, the promoter region comprises a plurality of IRF-1 derived gamma interferon activation sequences; and more preferably it comprises at least three such sequences. When a plurality of leptin response elements are present, they are preferably joined in tandem. However, there may be interviewing sequences present provided they do not interfere with the functioning of the leptin response elements. Other cytokine activation sequences, known in the art may also be used in place of or in addition to the IRF-1 derived gamma interferon activation sequence. These include the STAT binding sequences. In order to detect transcription which occurs as a result of the leptin-OB-R binding, it is preferred to operatively link a reporter gene to the promoter region which contains at least one leptin response element. A reporter gene may be any gene which encodes a peptide which is easily detected, or otherwise allows for easy detection of transcription or translation. It generally encodes a protein which does naturally occur in the host cell or only is produced in small amounts by the host cell. Examples of well known reporter genes include: chloramphenicol acetyl transferase (CAT), green fluorescent protein (GFP), luciferase (either bacterial or firefly), and other enzyme-based detection systems such as β-galactosidase, alkaline phosphatase, and the like. In a particularly preferred embodiment, luciferase is a reporter gene. In alternative embodiments, the mRNA transcribed from the reporter gene DNA may be measured rather than the translation product. The reporter gene construct, which comprises a) a promoter region which comprises a promoter and at least one leptin response element and b) the reporter gene (operatively linked to the promoter region) is preferably placed in an appropriate vector and in used to transfect a host cell. The vector may be any known vector, including plasmids, cosmids and viral vectors which can function in a chosen host cell. The host cell may be any cell or cell line which is conveniently cultured. In general, a preferred host cell will be eukaryotic, preferably mammalian, and in particularly preferred embodiments it is a mouse hypothalamic cell (such as GT1-7, NLT or Gnll), a mammalian neuroblastoma cell line, human brain neuron (SK-NMC or SK-N-BE2) mouse fibroblast cell lines such as NIH/3T3 (ATCC CRL 1658) or L (ATCC CCL 1), Chinese hamster ovary CHO-Kl cells (ATCC CCL 61) human embryonic kidney-derived 293 cells (ATCC CRL 1573), or CDS cells. Another aspect of this invention is an assay to determine if a putative OB-R ligand is present in a sample comprising i) contacting a mutant OB-R with the putative ligand wherein the mutant OB-R differ from a wild type receptor by having a mutation selected from the group consisting of: a) lacking a functional first CK-F3 module; b) lacking a functional CK-F3 module and c) lacking a functional intracellular domain; and ii) determining if binding occurs. Determination of binding may be done by any conventional method, including by sue of radiolabeled ligand. One preferred assay compares the amount of binding of radiolabeled leptin which occurs in the presence and absence of the putative ligand. The amount of binding of the putative ligand is related to the inhibition of labeled leptin binding. Another aspect of this invention is a set of vectors suitable for transfecting a host cell so that it can be used in the assays of this invention. The set of vectors comprises a first vector which contains a mutant OB-R construct. The OB-R construct includes nucleic acids encoding a desired mutated form of the receptor. It may be under the control of its native promoter or any other desired heterologous promoter. Optionally it may also contain other expression-control elements, such as enhancers and sequences which assist in expressing the receptor on the membrane. The set of vectors also comprises a second vector comprising the reporter gene construct described previously. In a preferred assay of this invention, compounds which are suspected of being leptin receptor ligands can be assayed. In this embodiment, cells which express a mutant OB-R are transfected with the reporter gene construct described previously. The putative OB-R ligand is placed in contact with the transfected cells, and the presence of the reporter gene transcription or translation product is detected. This may be compared to the amount of transcription or translation measured in a control assay, where an identically transfected cell is placed in contact with leptin. A further advantage of the assay of this invention is that it is dose-dependent; i.e. as more leptin ligand-OB-R binding occurs, transcription and/or translation of reporter gene increases, and therefore allows for a quantitative determination of OB-R binding activity. A counterscreen may also be employed as a part of this assay. In the counterscreen, a second cell of the same cell type is transfected with the reporter gene construct described previously, but is not transfected with the OB-R gene construct. The putative OB-R ligand is placed in contact with the transfected cells and the presence of of the reporter gene transcription or translation is detected. Those putative OB-R ligands which activate the reporter gene construct only in the presence of leptin receptor are determined to be specific leptin agonists. Using this embodiment of the assay, leptin agonists and antagonists may be identified. A leptin agonist is a compound which binds to the OB-R, such as a leptin mimetic, and produces a cellular response which is at least about equivalent to that of leptin, and which may be greater than that of leptin. Such compounds would be useful in situations where leptin insufficiency causes obesity, diabetes or infertility. Also using this embodiment of the assay, leptin antagonists may be identified. A leptin antagonist is a compound which can bind to the OB-R, but produces a lesser response than that of native leptin. Such compounds would be useful in the treatment of anorexia and cachexia. The mutant leptin receptors of this invention are also useful in gene therapy for weight control. For example, the first class of receptor mutants described (containing a deletion of all or substantially all of the first CK3 module) can be transferred to an animal which suffers from obesity due to an insufficient amount of OB-R receptors or which has OB-R receptors which do not function as efficiently as wild- type receptors. The dominant negative mutants of this invention are useful for gene therapy in animals which need to increase their weight, i.e. who suffer from anorexia or other conditions which are manifested by having too many OB-Rs or those which are functioning above the wild-type efficiency. The following non-limiting Examples are presented to better illustrate the invention. EXAMPLE 1 Receptor Mutant Construction The deletion mutant of the human OB-R, designated as D(41-322), was constructed as follows. The full length cDNA encoding the wild type human leptin receptor was inserted into the plasmid pcDNA3 (Invitrogen, San Diego, CA). The plasmid was cleaved by restriction enzymes SphI at residue 38 and Seal at residue 324, and the nucleic acids encoding residues 39-323 were removed by gel purification. An adapter was generated by annealing two oligonucleotides (5'-CCACCAAGT (SEQ.ID.NO:5) and δ'-ACTTGGTGGCATG (SEQ.ID.NO:6), and the adapter was ligated to the SphI and Seal cut human OB-R cDNA to regenerate residues 39, 40 and 323. The substitution mutant (420-496)TO(500-632) was constructed as follows. A PCR product was generated using the human leptin receptor cDNA as template and two oligonucleotide primers (5 * GAACATGAATGCCATTTCCAGCCAATCTTCCTATTATC (SEQ.ID.NO:7) and 5' GGAAAATGCATTCAACTGTGTAGGC- TGGATTGCTC (SEQ.ID.NO:8) which amplified the region encoding residues 500-632. This PCR fragment was cleaved by restriction enzyme Bsml. In a parallel experiment, the full length cDNA encoding the wild type human OB-R was inserted into the expression vector pcDNA3 (Invitrogen). This plasmid was cleaved by Bsml at residues 419 and 499, followed by ligation with the PCR fragment encoding residues 500-632. The deletion mutant D(867-1165) was constructed as follows. A PCR fragment encoding residues 500-866 was generated using the human OB-R cDNA as template and two oligonucleotides (5' TTCCAGCCAATCTTCCTATTATC (SEQ. ID NO:9) and 5' GACATCTCTAGACATTCTTTGGTGTGATATTAATAATG (SEQ. ID NO:10). This PCR fragment was cleaved with restriction enzymes EcoNI and Xbal. In parallel, the plasmid pcDNA3 with the human leptin receptor cDNA insert was cleaved with EoNI at residue 640 and with Xbal downstream from the stop codon. The DNA encoding residues 640-1165 was removed by gel purification, and the remaining receptor DNA in the pcDNA3 plasmid was ligated with the PCR fragment encoding residues 640-866. EXAMPLE 2 Cell Culture COS cells, a monkey kidney cell line, were maintained in Dulbecco-modified Eagle Medium (DMEM) (Gibco/BRL, Gaithersburg, MD), 10 % bovine calf serum, 50 μg/ml streptomycin, and 50 U/ml penicillin (growth medium) at 37°C in an atmosphere of 95% air, 5% C02- Cells were seeded at approximately 4x lO^/cm^ and passaged before confluence. EXAMPLE 3 Transfection of COS Cells Eighteen hours before transfection, cells were plated at 3.5 x 10^ cells per 35 mm tissue culture dish. For binding assay, the plasmid encoding the OB-R mutant (500 ng) was transfected. For luciferase assay, the plasmids AH32 (described below), OB-R mutants and pCHHO (300 ng each) were introduced into cells using liposome-mediated transfection. The plasmid pCHHO was obtained from Promega (Madison, WI). Specifically, plasmid or plasmid combination was added to 100 μl of Optimem™ (Gibco/BRL, Gaithersburg, MD). Ten μg of Lipofectamine™ (Gibco/BRL, Gaithersburg, MD) was added to a second 100 μl volume of Optimem™. The two solutions were mixed, incubated at room temperature for 30 minutes and total volume adjusted to 1000 μl with Optimem™. Cells were washed twice with phosphate buffered saline. The DNA-liposome complex was applied to cells. Cells were incubated for 6 hours at which time 1 ml of growth medium was added to cells. Sixteen hours later, the transfection mixture was removed and 2 ml fresh growth medium was added. AH32, an expression vector transactivated by leptin, was constructed as follows. Two complimentary oligonucleotides containing the Stat Binding Element (SBE) from the IRF-1 gene (Sims et al., 1993 Mol Cell Biol. 13:690-702; and Pine et al., 1994 EMBO J. 13:158-167; both of which are hereby incorporated by reference) were synthesized, kinased and annealed using standard molecular biology techniques (Sambrook et al., 1989 Molecular Cloning: A Laboratory Manual, 2nd Ed, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY). The sequences of the oligonucleotides used is 5'-CTGATTTCCC CGAATGACG-3' and 5'-CGTCATTTCG GGGAAATCAT-3'. The double stranded oligonucleotides were cloned into a Sma I site of pTKLuc. Plasmid pTKLuc contains a promoter sequence from -35 to +10 of the herpes simplex virus thymidine kinase gene (McKnight et al., 1982 Science, 217:316-324) upstream of the firefly luciferase gene found in pZLuc (Schadlow, et al, 1992 Mol. Biol Cell 3:941-951). The ligated DNA was used to transform E. coli using DH5α competent cells (Gibco/BRL, Gaithersburg, MD). DNA obtained from the transformed colonies was analyzed by restriction digest analysis and the orientation and number of copies of SBE oligonucleotides found in each of the transformants was confirmed by sequencing through the insert using a sequencing kit (US Biochemical, Cleveland, OH). Clone AH32 was found to contain 3 copies of the SBE inserted in the same orientation and was used for further studies with a human fibroblast cell line (WI-38 VA13 subline 2RA; ATCC No. CCL 75.1). Cells transfected with the AH32 showed a 6-10 fold increase in the luciferase activity after 6h of treatment with IFN-α or IFN-γ. EXAMPLE 4 Luciferase Assay Approximately 24 hours after transfection, various amounts of recombinant human leptin were added to cells. Cells were then incubated for 24 hours. Cell culture medium was removed and cells were washed promptly with 2 ml of ice-cold calcium and magnesium-free phosphate buffered saline. Cells were then prepared for assay with 150 μl Promega reporter lysis reagent (Promega, Madison, WI) according to the manufacturer's protocol. 20 μl of cell lysates was then assayed for luciferase activity using a Dynatech ML 3000 luminometer (Dynatech, Chantilly, VA) using cycle mode after initiating the reaction with 100 μl of Promega luciferase assay reagent. The β-galactosidase activity in 50 μl of each cell lysate was determined using a β-galactosidase assay (Promega, Madison, WI). Results were normalized by dividing the relative light units obtained for each sample by the β-galactosidase activity. Results are shown in Figure 4. EXAMPLE 5 Binding Assay Approximately 36 hours after transfection, the culture dish was washed with binding buffer (Hank's balanced salt solution supplemented with 0.5 % BSA-25 mM HEPES-0.5 % NaN3). 1 5l-leptin (NEN, Boston, MA) was diluted to 0.2 nM in the binding buffer, and 0.5 ml was added to each well. Unlabeled leptin was added to the final concentrations ranging from 10 pM to 10 nM. The cells were incubated at room temperature for 3 hours, after which the cells were washed with the binding buffer 4 times and lysed with 0.1% SDS. The cell lysate was counted in gamma counter. Results are shown in Figure 3.

Claims: WHAT IS CLAIMED IS: 1. A mutant ob receptor ("OB-R") which differs from a wild-type receptor by having a mutation selected from the group consisting of: a) lacking a functional first CK-F3 module; b) lacking a functional second CK-F3 module; and c) lacking a functional intracellular domain. 2. A receptor according to Claim 1 which is selected from the group consisting of a) receptors which contain a deletion of amino acid residues which comprise all or substantially all of a first CK-F3 module; b) receptors which contain a substitution of all or substantially all of a F3 domain for a CK domain in a second CK-F3 module, resulting in a F3-F3 module; and c) receptors which contain a deletion of all or substantially all of an intracellular domain. 3. A receptor according to Claim 2 which contains a deletion of amino acid residues which comprise all of a first CK-F3 module. 4. A receptor according to Claim 3 wherein amino acids 41-322 are deleted. 5. A receptor according to Claim 2 wherein all of the second CK domain is substituted by an F3 domain. 6. A receptor according to Claim 5 wherein amino acids 420-496 are substituted with amino acids 500-632. 7. A receptor according to Claim 6 which contains a deletion of the intracellular domain. 8. A receptor according to Claim 7 wherein amino acids 867 to 1165 are deleted. 9. A method for determining if an OB-R ligand is present in a sample comprising: contacting a cell with the sample, wherein the cell expresses an OB-R of Claim 1; and determining if binding occurs. 10. A method for determining if an OB-R ligand is present in a sample comprising: a) contacting a cell with the sample wherein the cell expresses a mutant OB-R selected from the group consisting of: receptors which lack a functional first CK-F3 module; and wherein the cell also comprises a reporter gene construct comprising a leptin response element operatively linked to a reporter gene; and b) determining if transcription or translation of the reporter gene occurs. 11. A method according to Claim 10 wherein the cell expresses an OB-R mutant selected from the group consisting of i) receptors which contain a deletion of amino acid residues which comprise all or substantially all of the first CK-F3 module; ii) receptors which contain a substitution of all or substantially all of an F3 domain for a CK domain in the second CK-F3 module, resulting in an F3-F3 module; and iii) receptors which contain a deletion of all or substantially all of the intracellular domain. 12. A method according to Claim 11 wherein the leptin response element is an IRF-1 derived gamma-interferon activation sequence. 13. A method according to Claim 12 wherein translation of a luciferase reporter gene is determined. 14. A nucleic acid which encodes a receptor of Claim 1. 15. A vector comprising a nucleic acid of Claim 14. 16. A host cell comprising a receptor of Claim 1. 17. A host cell according to Claim 16 further comprising a reporter gene construct comprising a leptin response element operatively linked to a reporter gene. 18. A method for determining if an OB-R ligand is present in a sample comprising: a) contacting a cell with the sample wherein the cell expresses a mutant OB-R selected from the group consisting of: i) receptors which lack a functional first CK-F3 module; and ii) receptors which lack a functional intracellular domain; and b) determining the amount of binding which occurs. 19. A method according to Claim 18 further comprising the step of contacting the cell with labeled leptin and determining the amount of binding of the putative ligand;

Inventors: FONG TUNG M, HUANG RUEY-RUEY C, VAN DER PLOEG LEONARDUS, FONG, TUNG, M., HUANG, RUEY-RUEY, C., VAN DER PLOEG, LEONARDUS
Applicants: FONG TUNG M, HUANG RUEY RUEY C, MERCK & CO INC, PLOEG LEONARDUS V D, FONG, TUNG, M., HUANG, RUEY-RUEY, C., MERCK & CO., INC., VAN DER PLOEG, LEONARDUS
Application year: 1997
Main classification: C12N1
IPC codes: C07K14, C12N1, C12N5, C12N15, C12Q1, G01N33