TROPHININ AND TROPHININ-ASSISTING PROTEINS

Abstract: The present invention provides substantially purified mammalian trophinin which can mediate cell adhesion. The invention also provides substantially purified trophinin-assisting proteins, which interact with trophinin to mediate cell adhesion. The invention further provides antibodies that are specifically reactive with trophinin or a trophinin-assisting protein. In addition, the invention provides active fragments of trophinin or trophinin-assisting proteins. The invention further provides a nucleic acid molecule encoding trophinin or a trophinin-assisting protein, vectors containing the nucleic acid molecules and host cells containing the vectors. The invention also provides a nucleotide sequence that can hybridize to a nucleic acid molecule encoding trophinin or a trophinin-assisting protein. The invention further provides methods to detect trophinin or a trophinin-assisting protein or a nucleic acid molecule encoding trophinin or a trophinin-assisting protein in a sample. The invention also provides methods of effecting cell adhesion by modifying cells to express trophinin or a trophinin-assisting protein. The invention further provides trophinin antagonists and methods to reduce or inhibit cell adhesion. The invention also provides methods to treat cells with trophinin agonists to increase cell adhesion.

Description: TROPHININ AND TROPHININ-ASSISTING PROTEINS This work was supported by grant number DK37016 awarded by the National Institutes of Health. The United States government has certain rights in this invention. BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates generally to the fields of biochemistry and molecular biology and more specifically to cell adhesion molecules. BACKGROUND INFORMATION The early stages of pregnancy involve fertilization of an egg by a sperm, followed by cell division and implantation of the embryo into the uterine cell wall. The inability of the embryo to properly implant in the uterus is a significant cause of pregnancy failure following in vivo or in vitro fertilization. The early events of implantation are characterized by an initial attachment of the embryo's external cell lining (trophoblast layer) to the cells lining the uterus (endometrial epithelium) followed by or in parallel with adhesion of these two cell types. The molecular events involved in the early steps in implantation are not well understood. Embryo attachment and adhesion to the uterine endometriuirt is unusual in that cells from these two sources adhere at their apical surfaces. In contrast, most other epithelial cell interactions adhere at their basal and lateral cell surfaces. The unique ability of trophoblast and endometrial cells to adhere may result from apical display of adhesion molecules normally located at basal and lateral surfaces. Alternatively, adhesion of these cell types in implantation may be mediated by unique cell surface molecules. Recent experiments suggest that certain endometrial tumor cell lines express characteristics associated with implantation-receptive endometrial tissue. In these experiments, trophoblast cells derived from germ cell tumors adhered to monolayers of endometrial adenocarcinoma cells via their apical cell surfaces. Morphological analysis of the adhering cell surfaces showed characteristics in common with early stage implantation. However, the molecules involved in the critical early adhesion step of embryo implantation were not identified. Thus, a need exists to identify the molecules responsible for adhesion of the embryo to the uterine lining. The present invention satisfies this need and provides related advantages as well. SUMMARY OF THE INVENTION The present invention provides substantially purified mammalian trophinin, which mediates adhesion of cells at their apical surfaces. In addition, the invention provides a family of substantially purified mammalian trophinin-assisting proteins, including tastin, bystin and lastin, which can be involved in trophinin- mediated cell adhesion. The invention also provides antibodies that specifically bind trophinin or a trophinin-assisting protein. Such antibodies can be useful, for example, to detect trophinin or a trophinin- assisting protein in a sample. In addition, the invention provides active fragments of trophinin and trophinin-assisting proteins. The invention also provides nucleic acid molecules encoding trophinin or a trophinin-assisting protein, vectors containing the nucleic acid molecules and host cells containing the vectors. These nucleic acid molecules can be used to express trophinin or a trophinin-assisting protein in a cell that otherwise does not express trophinin or a trophinin-assisting protein or expresses an aberrant trophinin or trophinin-assisting protein. The invention further provides methods for adhering cells together. In addition, the invention provides methods to inhibit trophinin-mediated adhesion of cells by contacting cells with a trophinin antagonist. The invention also provides a method to increase or decrease the likelihood of embryo implantation in a subject. In addition, the invention provides methods to diagnose infertility and determine the optimal time when the uterus of the mammal is most receptive for implanting a fertilized embryo. BRIEF DESCRIPTION OF FIGURES Figures IA, IB, 1C and ID collectively show the results of in vitro adhesion cell assays evaluating the ability of cell lines to undergo trophinin-mediated cell adhesion. Figures IA and IB show binding of embryonic trophoblastic cells HT-H (1), endometrial adenocarcinoma cells SNG-M (2) and monkey kidney cells COS-1 (3) to a monolayer of SNG-M cells (Figure IA) or HT-H cells (Figure IB) . After 20 minutes ( in) at room temperature (RT), nonadherent cells were removed by washing with (+) or without (-) 1 mM EDTA. The x axis indicates the percentage of cells that bound to the monolayer. Figure 1C shows the binding of COS-1 cells transfected with vector alone (1), vector containing tastin cDNA (2) , vector containing trophinin cDNA (3) and a mixture of vectors containing tastin cDNA and trophinin cDNA (4) to a monolayer of SNG-M cells. Non adherent cells were removed by washing with 1 mM EDTA. Figure ID presents the effects of anti- trophinin antibodies on cell adhesion. HT-H cells (1) or SNG-M cells (2) were added to a monolayer of SNG-M cells previously treated with pre-immune serum (-) or with anti-trophinin antiserum anti-GST-553 (+). Non adherent cells were removed by washing with 1 mM EDTA. Figures 2A, 2B, 2C and 2D are electron micrographs showing the interface between adherent HT-H and SNG-M cells. HT-H cells were added to a monolayer of SNG-M cells and electron micrographs were taken after 10 min, 6 hr or 4 days of culture. Figure 2A, 10 min post co-culture revealing microvilli at the lower side of an HT-H cell (H) facing the upper surface of the SNG-M cell (S). The basal surface of the HT-H cell is indicated by short arrows. Scale bar = 5μm. Figure 2B is a 4.4 X higher magnification of the area indicated by the parentheses in figure 2A. Contact of the two cell types via microvilli is evident. Figure 2C, 6 hr post-culture shows a HT-H cell (H) adhered to an SNG-M cell (S). Contact between the two cell types is closer than observed at 10 min culture. The microvilli are flattened in both cells and extend directly from each cell to the plasma membrane of the other cell. The SNG-M cells at this stage of contact often show invagination activity (arrow). Scale bar = 1 μM. Figure 2D, 4 days post co-culture shows a HT-H cell (H) adhered to a SNG-M cell (S). Microvilli are absent from the surfaces of both cells and contact primarily is focal, with occasional development of an adherent junction (arrow). Scale bar - 0.5 μM. Figure 3 presents the complete nucleotide sequence (SEQ ID NO: 1) and deduced amino acid sequence (SEQ ID NO: 2) of trophinin. Single letter amino acid symbols are used. Areas of the protein are indicated as follows: transmembrane domains (underlined), cytoplasmic domains (italics) and cell surface domains (bolded) . Potential sites for N-linked and O-linked glycosylation are underlined; potential sites for protein kinase phosphorylation are indicated by shadowed letters. Figure 4 is an autoradiograph of an SDS- polyacrylamide gel following electrophoresis of 35 S- labeled proteins obtained by in vitro translation of trophinin (lane 1) and tastin (lane 2) cDNA. Numbers on the right indicate the migration of molecular weight markers. Figures 5A and 5B show a schematic representation of the trophinin molecule in the cell membrane and identify a repeating decapeptide sequence in the molecule. Figure 5A shows the topology of a trophinin molecule within the cell membrane. Eight potential transmembrane domains are represented and the portion of trophinin containing the tandem decapeptide repeating sequence is filled-in. The amino terminus (N) , the carboxy terminus (C), potential sites for protein kinase phosphorylation (P) and potential sites for N-linked glycosylation (circles) are indicated. Figure 5B shows the amino acid sequence of trophinin from position 69 to 743 (SEQ ID NO: 3) in a form that identifies the individual tandem decapeptide units. Figure 6 presents the complete nucleotide sequence (SEQ ID NO: 4) and deduced amino acid sequence (SEQ ID NO: 5) of the tastin cDNA clone. Single letter amino acid symbols are used. Potential sites for phosphorylation by protein kinase C (underlined bold) , cAMP/cGMP dependent protein kinase (underlined), casein kinase II (bold) and MAP kinase (shadowed letters) are indicated. The location of 4 tandem repeat sequences that contain the majority of cysteines in the molecule are indicated by italics between residues 516 and 650. Figure 7 presents the complete nucleotide sequence (SEQ ID NO: 6) and deduced amino acid sequence (SEQ ID NO: 7) of bystin. Single letter amino acid symbols are used. Threonine and serine residues within potential sites for phosphorylation by protein kinase C (underlined) and casein kinase II (bolded) are indicated. Potential sites for phosphorylation of tyrosine residues by tyrosine kinase and potential sites for myristoylation of glycine residues are indicated in bold. Figure 8 presents a partial nucleotide sequence (SEQ ID NO: 8) and deduced amino acid sequence (SEQ ID NO: 9) of a portion of the lastin gene. The cDNA obtained for the lastin gene was missing the 3' end of the coding sequence and the poly-A tail. Single letter amino acid symbols are used. Potential threonine and serine within sites for phosphorylation by protein kinase C (underlined) and casein kinase II (bolded) are indicated. Potential sites for myristoylation of glycine residues are indicated in bold. Amino acid residues indicated by an X and nucleotides indicated by an N are unknown. Figures 9A, 9B, 9C and 9D are immunofluorescence micrographs detailing expression of trophinin or tastin in HT-H cells and SNG-M cells as detected by antibodies to the N-terminal region of trophinin (residue 23-31) and the N-terminal region of tastin (residue 41-49). Figure 9A (HT-H) and Figure 9B (SNG-M) show staining for trophinin while Figure 9C (HT- H) and Figure 9D (SNG-M) show staining for tastin. Scale bars = 10 μM. Figures 10A, 10B, IOC and 10D are immunofluorescence micrographs showing staining of trophinin and tastin in various human tissues as detected by antibodies to the N-terminal region of trophinin (residue 23-31) and the N-terminal region of tastin (residue 41-49). Figures 10A and 10B present immunofluorescence micrographs of placenta tissues from early pregnancy stained via anti-trophinin antibodies. Figure 10A shows a region of trophinin staining of the chorionic villus of a placenta obtained at seven weeks pregnancy. Fewer than half the villi in this tissue were stained for trophinin. Staining of trophinin in the villus in figure 10A. is observed at the apical plasma membranes of the syncytiotrophoblasts. Figure 10B is a chorionic villus of placenta obtained at nine weeks pregnancy. Lysosomal vesicles of the syncytiotrophoblasts in some villi show staining for trophinin. Scale bars ■ 10 μM. Figures IOC and 10D display immunofluorescence micrographs of endometrial epithelium stained via anti- trophinin antibodies. Figure IOC shows staining for trophinin at the apical membrane (arrowheads) of the surface epithelium from early secretory phase (approximately day 16/17 of the menstrual cycle). Figure 10D shows staining for trophinin in mucinous materials (arrow; in glandular lumen) associated with endometrial tubular epithelium from middle secretory phase (approximately day 22 of the menstrual cycle). Scale bars - 10 μM. Figures 11A, 11B, 11C and 11D display immunofluorescence micrographs of a monkey embryo and the implantation site from a monkey stained via antibodies to the N-terminal region of trophinin (residue 23-31). Figures 11A and 11B show an expanded blastocyst ( zona pellucida removed) from a rhesus monkey under phase microscopy (11A) and after immunofluorescence staining with an anti-trophinin antibody (11B). The long arrows indicate cell mass in Figure 11A while arrowheads indicate the embryonic pole in Figure 11B. Strong staining for trophinin is associated with cells of the trophectoderm (11B). Staining of cells located at the embryonic pole (arrowheads) is stronger than staining of cells located at the mural pole (small arrows). Scale bars = 25 μM. Figure 11C shows a tissue section taken from the site of implantation of a 15 day macaque monkey blastocyst. A light micrograph shows endometrium (E), trophoblast (T), cytotrophoblasts of blastocyst (short arrow) , anchoring villi of trophoblasts penetrating the endothelial epithelium (long arrows) and plaque cells in hypertrophic endometrial epithelium (asterisks) . The border between the embryo and the uterine epithelium is at the upper left of the figure, between the long arrows and the asterisks. Scale bar = 200 μM. Figure 11D is an immunofluorescence micrograph of a higher magnification of the same tissue section described in Figure 11C (site located in brackets) stained with anti-trophinin antibodies to the N-terminal region of trophinin (residue 23-31). Trophoblast layer (T) and endometrial epithelium (E) show strong staining of trophoblast cells (triangles) and endometrial cells (arrows) located at the interface between the two tissues. Scale bar = 10 μM. DETAILED DESCRIPTION OF THE INVENTION The present invention provides novel proteins involved in embryo adhesion to the uterus during implantation. The invention provides trophinin, which is present in the cell membrane of trophoblast cells and uterine epithelial cells. The invention also provides a family of cytoplasmic trophinin- assisting proteins, including tastin, bystin and lastin, which can interact with trophinin to effect cell adhesion. Although the precise morphological events of implantation vary from species to species, an essential feature is the formation of allogenic and heterotypic cell-to-cell contact between embryonic and maternal cells. The early events of implantation include an initial apposition of the trophoblast to the uterus and subsequent adhesion of the trophoblast to the endometrial epithelium (Enders, et al. In Cellular and Molecular Aspects of Implantation (Plenum Press, New York, 1981); Kaufman, In Bioloσγ of the Trophoblast (Elsevier Scientific 1985); Aplin, J. Reprod. Fert. 91:525-541 (1991); Ringler and Strauss, Current. Qpin. Cell Biol. 2:703-708 (1990)). The initial attachment of the trophoblast to the endometrial epithelium is unusual in that this cell-to-cell contact occurs via their respective apical cell membranes. In general, the basal and lateral surfaces of epithelial cells rather than their apical surfaces provide sites for adhesion between cells. The unique ability of trophoblast and endometrial cells to adhere at their apical surfaces can be due to apical display of adhesion molecules normally located at the basal and lateral surfaces of the cells. For example, atypical expression of heparan sulfate and integrins on the surface of the mouse blastocyst at peri-implantation stage has been observed (Farach et al., Devel. Biol. 123:401-410 (1987); Sutherland et al., J. Cell Biol. 106: 1331-1348 (1988) ; Leivo et al., Devel. Biol. 76:100-114 (1980); Armant et al., Devel. Biol. 116: 519-523 (1986)). Alternatively, unique apical adhesion of trophoblast with endometrial epithelium can be mediated by unique cell surface molecules (Kliman et al.. In Blastocyst Implantation. (Adams Publishing 1989)). Attempts to identify molecules involved in embryo implantation have been conducted both in vivo and in vitro (Lindenberg et al.. Hum. Reprod. 1:533-538 (1988); Armant et al., supra , 1986; Leivo et al., supra, 1980; Sutherland et al., supra, 1988; Farach et al., supra, 1987; Yamagata and Yamazaki, Biochem. Biophγs. Res. Com un. 181:1004-1009 (1991); Romagnano and Babiarz, In vitro. Devel. Biol. 141:254-261 (1990)), however, none of these studies have identified adhesion molecules that are unique to embryo implantation. As disclosed herein, trophinin is involved in apical cell adhesion between cultured trophoblast HT-H cells and endometrial adenocarcinoma SNG-M cells (see Figure IA and IB) . Trophinin also mediates adhesion between HT-H and HT-H cells and between SNG-M and SNG-M cells (See Example I). In contrast, these two cell types do not adhere to other types of epithelial cells such as HeLa, A431, S 480 and HepG-2 cells (Table 1). Thus, adhesion between HT-H and SNG-M cells is cell-type specific. The invention provides a substantially purified mammalian trophinin having substantially the amino acid sequence of human trophinin shown in Figure 3 (SEQ ID NO: 2) . The amino acid sequence of trophinin was derived from the nucleotide sequence shown in Figure 3 (SEQ ID NO: 1). As used herein, the term "substantially the amino acid sequence" means the amino acid sequence of human trophinin as shown in Figure 3 (SEQ ID NO: 2), as well as amino acid sequences that are similar to SEQ ID NO: 2, but have one or more amino acid additions, deletions or substitutions that do not substantially alter the ability of the encoded protein to function like a trophinin and, for example, mediate cell adhesion or elicit trophinin specific antibodies. In general, an amino acid sequence having at least 65% sequence homology with the amino sequence of Figure 3 (SEQ ID NO: 2) is considered substantially the same sequence. Thus, a mammalian trophinin is characterized, in part, by having a greater homology with other mammalian trophinins such as human trophinin as compared with other cell adhesion type molecules. It is well recognized that various amino acids in a polypeptide can be replaced by other naturally- or non-naturally-occurring L- or D-amino acids having equivalent reactive side chains or by other chemical compounds without substantially changing the biological activity of the polypeptide. For example, a hydrophobic amino acid such as leucine can be replaced by another hydrophobic amino acid such as alanine without substantially changing the amino acid sequence or activity of a trophinin polypeptide. In addition, the N- terminus or C-terminus or a reactive side chain of an amino acid can be modified, for example, by acetylation or amidation, without substantially changing the activity of a trophinin polypeptide. Such modified proteins can have advantageous properties including, for example. increased stability in vivo or in vitro, and are considered to be within the meaning of the term "substantially the amino acid sequence." As used herein, the term "substantially purified" means a protein that is in a form that is relatively free from contaminating lipids, proteins, nucleic acids or other material normally associated with a protein in a cell. Substantially purified trophinin can be obtained, for example, using well known biochemical methods of purification or by expressing a recombinant nucleic acid molecule encoding a trophinin such as the nucleic acid molecule shown in SEQ ID NO: 1. In addition, an amino acid sequence consisting of at least a portion of the amino acid sequence of SEQ ID NO: 2, can be chemically synthesized or can be produced by expressing a portion of the nucleotide sequence shown in SEQ ID NO: 1 (see Example V and VI). A substantially purified protein also includes a protein produced in an environment that is separate or distinct from it's native cellular environment. For example, the substantially purified protein includes a protein expressed in a cell that does not normally express the protein or can be expressed in the cell at a level higher than normally expressed by the cell. The ability to manipulate a cell to express a recombinant form of a protein of the present invention provides distinct advantages such as increased cell adhesion. As used herein, the terms "protein" or "polypeptide" are used in their broadest sense to mean a sequence of amino acids that can be encoded by a cellular gene or by a recombinant nucleic acid sequence or can be chemically synthesized. In some cases, the term "polypeptide" is used in referring to a portion of an amino acid sequence encoding a full length protein. An active fragment of trophinin as defined below can be an example of such a polypeptide. A protein can be a complete, full length gene product, which can be a core protein having no amino acid modifications or can be a post-translationally modified form of a protein such as a phosphoprotein, glycoprotein, proteoglycan, lipoprotein or nucleoprotein. Trophinin is a cell membrane protein that is characterized primarily by its ability to effect cell adhesion. It is recognized that the ability of trophinin to effect cell adhesion can be due to a portion of the full length protein. For example, as discussed below, greater than 90% of trophinin is composed of a repeating decapeptide sequence that can be involved in binding to another trophinin molecule. Thus, a polypeptide that contains only a portion of the full length trophinin protein can be useful for mediating cell adhesion. As used herein, the term "trophinin" means the full length trophinin protein or an active fragment thereof. As used herein, the term "active fragment" means a portion of a full length protein, provided the portion retains at least one activity that is characteristic of the full length protein. For example, an active fragment of trophinin can be a portion of the full length trophinin protein that can effect cell adhesion or can elicit specific antibodies to trophinin. An active fragment of trophinin can be identified, for example, by expressing a portion of the trophinin protein in a cell and determining that the cell can adhere to a trophinin expressing cell (see Example I). The complete amino acid sequence of human trophinin was deduced from the nucleotide sequence of a cDNA clone encoding human trophinin. The trophinin cDNA (SEQ ID NO: 1) contains an open reading frame coding for 749 amino acids (Figure 3). Trophinin has no significant homology to sequences contained in protein and nucleic acid databases. In vitro translation of trophinin cDNA and analysis using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) showed that trophinin is synthesized as a major product of 61 kiloDaltons (kDa) (Figure 4). This experimentally determined molecular mass is in agreement with the predicted molecular mass of 69.29 kDa based on the cDNA open reading frame. Hydropathy analysis (Kyte and Doolittle, J. Mol. Biol. 157:105-132 (1982)) of trophinin indicates trophinin is an intrinsic membrane protein having 8 separate transmembrane domains (Figure 5A) . The relative proportion of trophinin localized in the cytoplasm, in the membrane bilayer and on the cell surface is 10%, 56% and 34%, respectively. The amino terminal portion of trophinin is likely located in the cytoplasm because the first putative membrane spanning domain (amino acids 66 to 120) follows an arginine residue at position 54, which can function as a stop transfer signal during translocation into the endoplasmic reticulum, and because antibodies raised to an amino terminal peptide of trophinin (residues 23-31) react only with cells that have had their membranes permeabilized by detergent treatment (see Example VI) . The amino terminal region of trophinin contains many serine and threonine residues that can function as potential phosphorylation sites for enzymes such as casein kinase II (Kemp and Pearson, Trends Biochem. Sci. 15:342-346 (1990)), protein kinase C, and cAMP/cGMP dependent kinases (see Example III). Four potential N- glycosylation sites and thirteen potential 0- glycosylation sites are present within the predicted cell surface domains of trophinin (Figure 3). Greater than 90% of trophinin is composed of a tandemly repeated decapeptide motif. There are 69 such repeat sequences, which exhibit some variation in sequence and length (Figure 5B) . Portions of the decapeptide motifs are contained within three regions of trophinin that are hydrophilic in character and are exposed on the external side of the cell plasma membrane. The external trophinin domains are located from amino acid positions 278 to 364 (SEQ ID NO: 20), 441 to 512 (SEQ ID NO: 21) and 634 to 719 (SEQ ID NO: 22) (see bold lettering in Figure 3) . Protein secondary structure algorithms (Gamier et al., J. Mol. Biol. 120:97-120 (1978); Gascuel and Golmard, Comput. APPI. Biosci. 4:357- 365 (1988)) predict that the decapeptide repeats conform to a repeated β-turn structure, which can be involved in homophilic adhesion (not shown) . In addition to trophinin, a cell can require the expression of a trophinin-assisting protein in order to effect cell adhesion. The present invention provides a family of substantially purified mammalian trophinin- assisting proteins having substantially the amino acid sequences of human tastin (SEQ ID NO: 5), human bystin (SEQ ID NO: 7) and human lastin (SEQ ID NO: 9) as shown in Figures 6, 7 and 8, respectively. A trophinin- assisting protein can enable adhesion of cells that express trophinin. As used herein, the term "substantially the amino acid sequence" means the disclosed amino acid sequence of human tastin (SEQ ID NO: 5), human bystin (SEQ ID NO: 7) or human lastin (SEQ ID NO: 9) as well as amino acid sequences that are similar to SEQ ID NO: 5, SEQ ID NO: 7 or SEQ ID NO: 9, respectively, but have one or more amino acid additions, deletions or substitutions that do not substantially alter the ability of the encoded protein to function like a trophinin-assisting protein and, for example, mediate cell adhesion or elicit a trophinin-assisting protein specific antibody. As used herein, the term "trophinin-assisting protein" is used generally to mean a member of the trophinin-assisting protein family of proteins as defined by their ability to assist trophinin in mediating adhesion of cells. Trophinin-assisting proteins include such family members as tastin, bystin or lastin and can be a full length trophinin-assisting protein or an active fragment of a trophinin-assisting protein. For example, amino acids 1 to 675 of lastin are a portion of the full length protein and can assist trophinin in mediating cell adhesion (see Example II). While not necessarily structurally related, trophinin-assisting protein family members are characterized, in part, by having the property of assisting trophinin mediated cell adhesion. Trophinin and a trophinin-assisting protein can interact directly or indirectly to effect cell adhesion. For example, cell adhesion can be mediated by the direct binding of a trophinin-assisting protein to trophinin. Cell adhesion also can be due to a trophinin-assisting protein binding to another cellular molecule which then directly or indirectly binds to trophinin. Alternatively, a trophinin-assisting protein can interact indirectly with trophinin by binding to and eliminating the function of a negative regulator of trophinin activity in the cell. A substantially purified trophinin-assisting protein can be obtained, for example, using well known biochemical methods of purification or by expressing a recombinant nucleic acid molecule encoding a trophinin- assisting protein such as the nucleic acid molecules shown in SEQ ID NO: 4, SEQ ID NO: 6 or SEQ ID NO: 8. In addition, an amino acid sequence consisting of at least a portion of the amino acid sequences of SEQ ID NO: 5, SEQ ID NO: 7 or SEQ ID NO: 9 can be chemically synthesized or can be produced by expressing a portion of the nucleotide sequence shown in SEQ ID NO: 4, SEQ ID NO: 6 or SEQ ID NO: 8, respectively. The complete amino acid sequence of tastin (SEQ ID NO: 5) was deduced from the nucleotide sequence of the tastin cDNA clone and is shown in Figure 6. The open reading frame of the tastin cDNA encodes a protein having 778 amino acids. Tastin exhibits an apparent molecular mass of about 80 kDa based on SDS-PAGE analysis of in vitro translated tastin cDNA (Figure 4). This mass is consistent with a molecular weight of 83.75 kDa calculated from the tastin cDNA open reading frame. Tastin lacks a consensus signal sequence characteristic of a secreted protein and contains no transmembrane helices as assessed by hydropathy analysis (Kyte and Doolittle, supra, 1982). Thus, tastin has the characteristics of a cytoplasmic protein. Tastin is rich in proline residues, which account for 15.3% of the total amino acids of the protein, and in cysteine residues. The majority of the cysteines are located between position 516 to 650 and occur primarily within four tandemly repeated sequences of 33 amino acids each (region denoted by italics in Figure 6) . Tastin contains many serine and threonine residues that are potential sites for phosphorylation, including two potential sites for cAMP/cGMP dependent kinase, sixteen sites for protein kinase C (Kemp and Pearson, supra , 1990), eleven sites for casein kinase II and two sites for MAP kinase (Gonzalez et al., J. Biol. Chem. 266:22159-22163 (1991)) (see Example IV). Tastin has no overall significant homology to previously reported protein sequences. Nucleotide sequence homology analysis of tastin identified the sequence HFBCL29 (Genbank accession number M85643), which was derived from a human fetal brain cDNA library. HFBCL29 shows DNA base complementarity to a portion of tastin cDNA (positions 2057 to 2340). Thus, the HFBCL29 sequence can be homologous to a portion of the tastin sequence if HFBCL29 was recorded in the data base in the antisense direction. The protein sequence deduced from HFBCL29 is related to Y box binding protein-1 (Adams et al.. Nature 355:632-634 (1992)). However, the entire nucleotide sequence and deduced amino acid sequence of tastin are not homologous overall to the Y-box binding protein-1. The complete amino acid sequence of bystin was deduced from the nucleotide sequence of the bystin cDNA clone and is shown in Figure 7 (SEQ ID NO: 7). The open reading frame of the bystin cDNA codes for a protein of 306 residues. Bystin contains threonine and serine residues within potential sites for phosphorylation by protein kinase C (underlined) and casein kinase II (bolded) . In addition, bystin contains tyrosine residues (bolded) that are potential sites of phosphorylation by tyrosine kinase and glycine residues within potential sites for myristoylation (bolded) . Amino acid residues 1 to 88 of bystin show a significant degree of sequence homology to the bγs gene previously identified in Drosophila (Stuart et al., Mol. Cell. Biol. 13:2524 (1993) ) . A partial amino acid sequence of lastin was deduced from a partial nucleotide sequence of the lastin cDNA clone and is shown in Figure 8 (SEQ ID NO: 9). The lastin cDNA clone does not contain the 3' end of the gene, including the stop codon and the poly-A tail. The open reading frame of the partial cDNA encodes for 675 amino acids. Lastin contains threonine and serine within potential sites for phosphorylation by protein kinase C (underlined) and casein kinase II (bolded) . Lastin also contains potential sites for myristoylation of glycine residues. The present invention also provides antibodies that are specifically reactive with trophinin or with a trophinin-assisting protein. As used herein, the term "antibody" is used in its broadest sense to include polyclonal and monoclonal antibodies, as well as polypeptide fragments of antibodies that retain a specific binding affinity for trophinin or a trophinin- assisting protein of at least about 1 X 10 5 M" 1 . One skilled in the art would know that antibody fragments such as Fab, F(ab') 2 and Fv fragments can retain specific binding activity for their target antigen and, thus, are included within the definition of an antibody to trophinin or to a trophinin-assisting protein. In addition, the term "antibody" as used herein includes naturally occurring antibodies as well as non-naturally occurring antibodies such as domain-deleted antibodies (Morrison and Oi, WO 89/07142, Aug. 10, 1989, which is incorporated herein by reference) or single chain Fv (Ladner and Bird, U.S. Patent NO. 5,250,203, Nov. 9, 1993, which is incorporated herein by reference). Such non-naturally occurring antibodies can be constructed using solid phase peptide synthesis, can be produced recombinantly or can be obtained, for example, by screening combinatorial libraries consisting of variable heavy chains and variable light chains as described by Huse et al.. Science 246:1275-1281 (1989), which is incorporated herein by reference. Particularly useful non-naturally occurring antibodies include chimeric antibodies and humanized antibodies. Methods to produce chimeric antibodies and humanized antibodies by the method of CDR grafting are known in the art (see, for example. Winter, U.S. Patent No. 5,225,539, July 6, 1993, which is incorporated herein by reference) . As used herein, the term "chimeric antibody" means an antibody having a human constant region and a variable region from an organism other than a human. For example, a chimeric antibody useful in the invention can consist of a human IgG constant region and a variable region obtained from a mouse anti-human trophinin antibody. As used herein, the term "humanized antibody" means an antibody having constant and framework regions derived from human and hypervariable regions derived from an organism other than a human. For example, a humanized antibody useful in the invention can consist of the amino acids that form the hypervariable region of a mouse anti- human trophinin antibody and the amino acids that form the framework region and constant regions of a human IgG class antibody. Chimeric antibodies and humanized antibodies are particularly useful for administration to a human subject, since the likelihood of an immune response by the subject against the antibody is minimized. Other non-naturally occurring antibodies within the present invention include bispecific antibodies, in which the antibody contains at least two different binding specificities that can be univalent or multi-valent for each particular binding specificity. Methods for producing bispecific antibodies by chemical crosslinking or by heterohybridoma formation are well known in the art (for trivalent antibodies, see, for example, Ahlem and Huang, U.S. Patent No. 5,273,743, Dec. 28, 1993), which is incorporated herein by reference). An anti-trophinin antibody or an anti- trophinin-assisting protein antibody can be prepared using substantially purified trophinin or a trophinin- assisting protein, respectively, either of which can be obtained from natural sources or produced by recombinant DNA methods or chemical synthesis. For example, recombinant DNA methods can be used to express trophinin alone or as a fusion protein, which can facilitate purification of the antigen and enhance its immunogenicity (see Example II). Similarly, an active fragment of trophinin or of a trophinin-assisting protein also can be obtained as described above and can be used as an immunogen (see Example V). If not sufficiently immunogenic, such fragments or peptides can be made immunogenic by expressing the hapten as a fusion protein or by coupling the hapten to an immunogenic carrier molecule such as bovine serum albumin or keyhole limpet hemocyanin (KLH) . Various other carrier molecules and methods for coupling a non-immunogenic peptide to a carrier molecule are well known in the art (see, for example, Harlow and Lane, Antibodies: A laboratory Manual Cold Spring Harbor Laboratory Press, (1988), which is incorporated herein by reference). Methods for raising an antibody are routine and described, for example, by Harlow and Lane ( supra , 1988). An antiserum containing polyclonal antibodies to trophinin or to a trophinin-assisting protein can be raised in rabbits, goats or other animals. The resulting antiserum can be processed by purification of an IgG antibody fraction using protein A Sepharose chromatography and, if desired, can be further purified by affinity chromatography using, for example, Sepharose conjugated with a peptide antigen (see Example V). The ability of polyclonal antibodies to specifically bind to a given molecule can be manipulated, for example, by dilution or by adsorption to remove crossreacting antibodies to a non-target molecule. Methods to manipulate the specificity of polyclonal antibodies are well known to those in the art (See Harlow and Lane, supra , 1988). A monoclonal anti-trophinin or anti-trophinin- assisting protein antibody can be produced using known methods (Harlow and Lane, supra , 1988). Essentially, spleen cells from a trophinin- or a trophinin-assisting protein-immunized animal can be fused to an appropriate myeloma cell line such as SP2/0 myeloma cells to produce hybridoma cells. Cloned hybridoma cell lines can be screened using a labeled trophinin or trophinin-assisting protein polypeptide to identify clones that secrete an appropriate monoclonal antibody. A trophinin or a trophinin-assisting protein polypeptide can be labeled as described below. A hybridoma that expresses an antibody having a desirable specificity and affinity can be isolated and utilized as a continuous source of monoclonal antibodies. Methods for identifying an anti- trophinin or anti-trophinin-assisting protein antibody having an appropriate specificity and affinity and, therefore, useful in the invention are known in the art and include, for example, enzyme-linked immunoadsorbance assays, radioimmunoassays, precipitin assays and immunohistochemical analyses (see for example, Harlow and Lane, supra , 1988; chap. 14). An anti-trophinin antibody can be characterized by its ability to bind a portion of a mammalian trophinin protein, such as the portion of trophinin that is exposed on the external side of the plasma membrane of a cell (see, for example. Figure ID). An anti-trophinin- assisting protein antibody can be characterized by its ability to bind to an epitope that is unique to one or more members of the trophinin-assisting protein family of proteins. An anti-trophinin antibody or an anti- trophinin-assisting protein antibody of the invention can be useful to purify trophinin or a trophinin-assisting protein, respectively, from a sample. For example, an anti-trophinin antibody can be attached to a solid substrate such as a resin and can be used to affinity purify trophinin. In addition, an anti-trophinin antibody can be used to identify the presence of trophinin in a sample. In this case, the antibody can be labeled with a detectable moiety such as a radioisotope, an enzyme, a fluorochrome or biotin. An anti-trophinin or anti-trophinin-assisting protein antibody can be detectably labeled using methods well known in the art (see, for example, Harlow and Lane, supra , (1988); chap. 9) . Following contact of a labeled antibody with a sample such as a tissue homogenate or a histological section of a tissue, specifically bound labeled antibody can be identified by detecting the labeled moiety. A labeled second antibody also can be used to identify specific binding of an unlabeled anti-trophinin or anti-trophinin-assisting protein antibody. A second antibody generally will be specific for the particular class of the first antibody. For example, if an anti- trophinin antibody is of the IgG class, a second antibody will be an anti-IgG antibody. Such second antibodies are readily available from commercial sources. The second antibody can be labeled using a detectable moiety as described above. When a sample is labeled using a second antibody, the sample is first contacted with a first antibody, then the sample is contacted with the labeled second antibody, which specifically binds to the first antibody and results in a labeled sample. Alternatively, a labeled second antibody can be one that reacts with a chemical moiety, for example biotin or a hapten that has been conjugated to the first antibody (see for example, Harlow and Lane, supra (1988); chapter 9). The present invention also provides nucleic acid molecules encoding trophinin or a trophinin- assisting protein. Nucleic acid molecules encoding the disclosed proteins, which are involved in mediating apical cell adhesion, were obtained by functional selection from an expression cDNA library (see Example II). Essentially, a cDNA library was prepared from HT-H cells and transfected into non-adhering COS-1 cells, which then were selected for adherence to SNG-M cells. Both trophinin and trophinin-assisting protein clones were simultaneously discovered since COS-1 cells only became adherent following co-transfection with a trophinin and a trophinin-assisting protein cDNA sequence (see Figure 1C) . The present invention provides a substantially purified nucleic acid molecule encoding a mammalian trophinin. For example, the invention provides a substantially purified nucleic acid molecule encoding human trophinin having substantially the nucleotide sequence shown in Figure 3 (SEQ ID NO: 1). As used herein, the term "substantially purified" means that the nucleic acid is relatively free from contaminating materials such as lipids, proteins, carbohydrates or cellular material normally associated with a nucleic acid in a cell. For example, a nucleic acid molecule that is chemically synthesized is considered substantially purified. Recombinant DNA methods for producing a substantially purified nucleic acid are well known in the art and include cloning a sequence or polymerase chain reaction (PCR) amplification of a sequence (see Sambrook et al.. Molecular Cloning: A laboratory manual (Cold Spring Harbor Laboratory Press 1989), which is incorporated herein by reference; see, also, Erlich, PCR Technology: Principles and applications for DNA amplification (Stockton Press 1989), which is incorporated herein by reference) . A substantially purified nucleic acid also includes a nucleic acid produced in an environment that is separate or distinct from the nucleic acid in it's native cellular environment. For example, the substantially purified nucleic acid is a nucleic acid expressed by recombinant techniques in a cell that does not normally express the nucleic acid or expressed at levels that are higher than normally expressed by the cell. The ability to manipulate a cell to express the nucleic acids of the present invention provides distinct advantages such as increased cell adhesion. As used herein, the term "substantially the nucleotide sequence" means a sequence that contains, for example, different nucleotides than shown in figure 3 (SEQ ID NO: 1) but that, as a result of the degeneracy of the genetic code, encodes substantially the same amino acid sequence as shown in Figure 3 (SEQ ID NO: 2). Such nucleotide sequences can be either DNA or RNA and can encode either the coding or non-coding nucleotide strand. In addition, nucleic acid molecules encoding substantially the sequence of a protein include a nucleic acid that hybridizes under relatively stringent conditions to the nucleotide sequence or a portion thereof encoding the protein. For example, nucleic acid molecules encoding a mammalian trophinin include nucleic acid molecules that hybridize under relatively stringent conditions to the nucleotide sequence of SEQ ID NO: 2 or its complement or a portion thereof containing at least 25-30 contiguous nucleotides. The cloned nucleic acid molecule encoding trophinin (SEQ ID NO: 1) contains 2524 nucleotides with an open reading frame encoding 749 amino acids (see Figure 3) . The 3' untranslated region of trophinin consists of 250 nucleotides and contains a polyadenylation signal located twelve nucleotides upstream of the poly-A tail. Among the ATG codons in the 5' region, the sequence around the ATG at position 1 (see Figure 3) closely matches a Kozak sequence optimal for translation initiation (Kozak, Nucleic Acid Res. 12, 857- 872, (1984)). No other ATG codon near the 5' end conforms to the consensus sequence for translation initiation. Jn vitro translation of the trophinin cDNA confirms that the ATG beginning at position 1 in Figure 3 encodes the initiation methionine in trophinin. The invention also provides a nucleotide sequence that can hybridize to a portion of the nucleic acid molecule encoding trophinin under relatively stringent hybridization conditions. Relatively stringent hybridization conditions can be determined empirically or can be estimated based, for example, on the relative GC:AT content of the hybridizing nucleotide sequence and the target sequence, the length of the hybridizing nucleotide sequence and the number, if any, of mismatches between the hybridizing nucleotide sequence and the target sequence. The extent of hybridization can be controlled, for example, by the temperature, pH or ionic strength of the hybridization reaction mixture or the subsequent wash solutions (Sambrook et al., supra , 1989). A nucleotide sequence useful for hybridizing to a nucleic acid molecule encoding trophinin should be at least ten nucleotides in length and can be prepared, for example, by restriction endonuclease digestion of a cloned nucleic acid molecule, such as the nucleic acid molecule shown in Figure 3 (SEQ ID NO: 1), by PCR amplification of a portion of a nucleic acid encoding trophinin or by chemical synthesis using well known methods. A nucleotide sequence can be labeled with a detectable moiety and can be used as a probe to detect a nucleic acid molecule or as a primer for PCR. Methods for detectably labeling a nucleic acid are well known in the art (see, for example, Sambrook et al., supra , 1989; see, also, Ausubel et al.. Current Protocols in Molecular Biology (John Wiley & Sons 1987), which is incorporated herein by reference). The invention also provides a substantially purified nucleic acid molecule encoding a trophinin- assisting protein. For example, the invention provides a substantially purified nucleic acid molecule encoding human tastin, bystin or a portion of human lastin having substantially the nucleotide sequence shown in Figure 6 (SEQ ID NO: 4), Figure 7 (SEQ ID NO: 6) and Figure 8 (SEQ ID NO: 8), respectively. The cloned nucleic acid molecule encoding human tastin (SEQ ID NO: 4) contains 2,578 nucleotides having an open reading frame encoding 778 amino acids (see Figure 6). The 3' untranslated region contains 133 nucleotides and has a polyadenylation signal located eleven nucleotides upstream of the poly-A tail. The nucleotide sequence around the ATG at position 1 conforms to the consensus sequence for the translation initiation site (Kozak, supra, 1984). Jn vitro translation of the tastin cDNA confirms that the ATG beginning at position 1 in Figure 6 encodes the initiation methionine in tastin. The cloned nucleic acid molecule encoding human bystin (SEQ ID NO: 6) contains 1,293 nucleotides having an open reading frame encoding 306 amino acids (see Figure 7). The 3' untranslated region contains 306 nucleotides. The cloned nucleic acid molecule encoding a portion of human lastin is based on the sequence of a partial cDNA clone (SEQ ID NO: 8) that contains 2,223 nucleotides having an open reading frame encoding 675 amino acids beginning at the ATG start site (see Figure 7). The 5' untranslated region contains 198 nucleotides. The nucleotide sequence around the ATG at position 1 conforms to the consensus sequence for the translation initiation site (Kozak, supra , 1984). The invention also provides a nucleotide sequence that can hybridize to a portion of a nucleic acid molecule encoding a trophinin-assisting protein under relatively stringent hybridization conditions. A nucleotide sequence useful for hybridizing to a nucleic acid encoding a trophinin-assisting protein should be at least ten nucleotides in length and can be prepared as described above. The invention provides vectors containing a nucleic acid molecule encoding a mammalian trophinin or a mammalian trophinin-assisting protein and host cells containing the vectors. Vectors are well known in the art and include, for example, cloning vectors and expression vectors, as well as plasmids or viral vectors (see, for example, Goedell, Methods in Enzymologv. vol. 185 (Academic Press 1990), which is incorporated herein by reference). For example, an expression vector that contains a nucleic acid molecule encoding trophinin can be particularly useful for expressing large amounts of trophinin protein, which can be purified and used as an immunogen to raise anti-trophinin antibodies. A baculovirus vector is an example of a vector that can be used to express large amounts of trophinin or a trophinin-assisting protein. A vector containing a nucleic acid molecule encoding a trophinin or a trophinin-assisting protein can also contain a promoter or enhancer element, which can be constitutive or inducible and, if desired, can be tissue specific. Host cells also are known in the art and can be selected based on the particular vector. An appropriate host cell can be selected based, on the particular vector used, for example, baculovirus transfer vectors can be used with baculovirus DNA to infect insect cell lines such as SF21 cells. An expression vector can also be used to effect the ability of cells to undergo trophinin-mediated cell adhesion. A variety of nucleic acid molecules can be used to effect cell adhesion under various situations. For example, an expression vector that contains a nucleic acid molecule encoding trophinin can be introduced into a cell that previously expressed an insufficient level of trophinin to mediate cell adhesion. Under the appropriate conditions, cells containing such expression vectors can increase their expression of trophinin, thus enhancing their ability to undergo trophinin mediated cell adhesion. In addition, an expression vector containing a nucleic acid molecule encoding a trophinin- assisting protein can be used to increase trophinin- mediated cell adhesion by introducing the expression vector into cells that fail to exhibit trophinin-mediated cell adhesion due to a deficiency in the expression of a trophinin-assisting protein. An expression vector also can contain an exogenous nucleic acid molecule encoding an antisense nucleotide sequence that is complementary to a nucleotide sequence encoding a portion of trophinin. When introduced into a cell under the appropriate conditions, such an expression vector can produce the antisense nucleic acid molecule, which can selectively hybridize to the trophinin gene or to an RNA molecule encoding trophinin in a cell and, thereby, affect trophinin expression in the cell. For example, the antisense nucleic acid molecule can hybridize to a trophinin gene in the cell and can reduce or inhibit transcription of the trophinin gene. Also, the antisense molecule can hybridize to the an RNA molecule encoding trophinin in the cell and can reduce or inhibit translation, processing and cell stability or half-life of the RNA. Expression vectors also can be used to effect trophinin-mediated cell adhesion by introducing into a cell an exogenous nucleic acid molecule encoding a rxbozyme that can specifically cleave RNA encoding trophinin. Introducing an expression vector into a cell and expressing a ribozyme specific for an RNA encoding trophinin can reduce or inhibit trophinin expression. An antisense nucleic acid molecule or a ribozyme can be chemically synthesized and incorporated into an expression vector using recombinant DNA techniques. An antisense nucleic acid molecule or a ribozyme also can be added directly to a cell without having been incorporated into the expression vector. The above described methods for effecting trophinin-mediated cell adhesion by using an expression vector to obtain expression of an exogenous nucleic acid molecule in a cell also can be accomplished if the exogenous nucleic acid molecule encodes a trophinin- assisting protein or an antisense or ribozyme sequence specific for a trophinin-assisting protein. For example, an increase in trophinin-mediated cell adhesion can be achieved by introducing an expression vector encoding a trophinin-assisting protein into cells that are deficient in trophinin-assisting protein expression or produce a non-functional trophinin-assisting protein. In addition, a decrease in trophinin-mediated cell adhesion can be accomplished by introducing into a cell an expression vector that encodes for an antisense or ribozyme specific for a trophinin-assisting protein. In such cases the expressed antisense or ribozyme can reduce or inhibit trophinin-mediated cell adhesion by decreasing the effective level of trophinin-assisting protein in a cell below that required to effect trophinin-mediated cell adhesion. The ability of cells to undergo trophinin- mediated cell adhesion also can be effected by introducing two or more expression vectors into a cell, each encoding a different exogenous nucleic acid molecule or introducing an expression vector capable of expressing more than one exogenous nucleic acid molecule. To reduce or inhibit the level of expression of trophinin, for example, expression vectors coding for both an antisense and a ribozyme specific for trophinin can be introduced into a cell. The expression of both such exogenous nucleic acid sequences simultaneously in a cell can be more effective at reducing trophinin expression than when either sequence is expressed alone. Alternatively, a combination of expression vectors where one encodes for trophinin and the other for a trophinin- assisting protein can be used to increase the expression of trophinin in a cell and thereby increase trophinin- mediated cell adhesion. Methods for introducing expression vectors into cells are well known in the art. Such methods are described in Sambrook et al supra (1989) and in Kriegler M. Gene Transfer and Expression: A Laboratory Manual (W. H. Freeman and Co. New York NY (1990), which is incorporated herein by reference) and, include, for example, transfection methods such as calcium phosphate, electroporation or lipofection, or viral infection. Recombinant viral vectors are available for introducing exogenous nucleic acid molecules into mammalian cells and include, for example, adenovirus, herpesvirus and retrovirus-derived vectors. For example, a viral vector encoding trophinin or a trophinin- assisting protein can be packaged into a virus to enable delivery of the genetic information and expression of these proteins in endometrial cells following infection by the virus. Also, a recombinant virus which contains an antisense sequence or a ribozyme specific for a nucleotide sequence encoding trophinin or a trophinin- assisting protein can be used to reduce or inhibit the ability of trophinin to mediate cell adhesion in cells infected by the virus. Recombinant viral infection can be more selective than direct DNA delivery due to the natural ability of viruses to infect specific cell types. This natural ability for selective viral infection can be exploited to limit infection to specific cell types within a mixed cell population. For example, adenoviruses can be used to restrict viral infection principally to cells of epithelial origin. In addition, a retrovirus can be modified by recombinant DNA techniques to enable expression of a unique receptor or ligand that provides further specificity to viral gene delivery. Retroviral delivery systems can also provide high infection rates, stable genetic integration, and high levels of exogenous gene expression. As described above, recombinant viral delivery systems exist that provide the means to deliver genetic information into a selected type of cell. The choice of viral system will depend on the desired cell type to be targeted, while the choice of vector will depend on the intended application. Recombinant viral vectors are readily available to those in the art and can be easily modified by one skilled in the art using standard recombinant DNA methods. The invention also provides methods to detect trophinin or a nucleic acid molecule encoding trophinin in a sample using an agent that specifically binds to trophinin or to a nucleic acid molecule encoding trophinin. As used herein the term "agent" means a chemical or biological molecule that can specifically bind to trophinin or to a trophinin-assisting protein or to a nucleic acid molecule encoding trophinin or a trophinin-assisting protein. For example, an agent specific for trophinin can be another trophinin molecule or can be an anti-trophinin antibody. In addition, an agent can be a nucleotide sequence that binds to a nucleic acid molecule encoding trophinin or a trophinin- assisting protein. As used herein, "sample" means a specimen such as a cell, tissue or an organ, which can be obtained, for example, by biopsy from a subject or can be a serum, urine or mucin specimen obtained from a subject. A sample containing trophinin can be used directly or can be processed prior to testing. For example, a biopsy tissue sample can be cut into tissue sections for histologic examination or can be further processed to release trophinin from cells within the tissue. Methods to process a sample such as a tissue, cells or a biological fluid for detecting a protein are known in the art (see, for example, Harlow and Lane, supra , (1988)). The presence of trophinin in a sample can be determined by contacting the sample with an agent that can bind to trophinin under suitable conditions, which allow the agent to specifically bind to trophinin. Suitable conditions can be achieved using well known methods and can be optimized, for example, by varying the concentration of reactants or the temperature of the reaction. After the agent specifically binds to trophinin in a sample, the presence of trophinin can be determined by detecting specific binding of the agent. An agent that can be detectably labeled can be used as a probe. For example, a probe for detecting the presence of trophinin in a sample can be an anti- trophinin antibody that is detectably labeled or that can be bound by a second antibody that is detectably labeled. In addition, a probe for detecting a nucleic acid molecule encoding trophinin or a trophinin-assisting protein can be an agent such as a nucleotide sequence that can hybridize to the nucleic acid molecule and that can be detected directly, for example, by a radioactive moiety incorporated into the nucleotide sequence, or indirectly, for example, by PCR analysis. As used herein, "detectable label" means a molecule whose presence can be detected due to a physical, chemical or biological characteristic of the molecule. Detectable labels include, for example, radioisotopes, fluorescent molecules, enzyme/substrate systems, or visually detectable molecules. Methods to produce a probe for detecting a protein are well known in the art (see, for example, Harlow and Lane, supra , (1988)) and include, for example labeling the agent with a radioisotope, fluorescence molecule or histochemically useful enzyme or visible particle or colloid. Methods to produce a probe for detecting a nucleic acid molecule are also well known in the art (see, for example, Sambrook et al, supra , 1989; Hames and Higgins, Nucleic acid Hybridization: a practical approach. IRL press, New York, (1985), which are incorporated herein by reference). An agent often can bind to a limited but detectable level of non-target substances such as the assay container and can result in background binding. Thus, to properly conclude that the presence of an agent binding in a sample represents the presence of trophinin, it is necessary to determine that the specific binding observed in a sample is greater than the background binding of the agent. The level of background binding of an agent can be determined using a control sample, which is similar in composition to the sample being tested but which contains a defined amount of trophinin or no trophinin. The invention also provides methods to detect a trophinin-assisting protein in a sample using an agent that specifically binds to a trophinin-assisting protein. Such an agent can be an anti-trophinin-assisting protein antibody that specifically binds to a particular trophinin-assisting protein such as tastin, bystin and lastin. The presence of a trophinin-assisting protein in a sample can be determined using the methods described above. A nucleic acid molecule encoding trophinin can be detected in a sample using an agent such an antisense nucleotide sequence that is specific for trophinin as described above. The target nucleic acid molecule can be extracted from a sample by methods well known in the art (See Sambrook et al., supra , 1989). Methods to detect the presence of a particular nucleic acid molecule within a population of nucleic acid molecules are well known to those in the art and include, for example, include Southern blotting, northern blotting, slot blotting and PCR amplification (see, for example, Sambrook et al., supra , 1989). In situ hybridization also can be particularly useful for identifying nucleic acids in a sample (see for example, Pardue, in Nucleic Acid Hybridisation: a practical approach (IRL Press, 1991), which is incorporated herein by reference) . To detect a nucleic acid molecule encoding trophinin in a sample, the sample is contacted with a nucleotide sequence probe that can hybridize to a nucleic acid molecule encoding trophinin under relatively stringent conditions. The presence of a nucleic acid molecule encoding trophinin in the sample can be determined, for example, by detecting the presence of a specifically bound nucleotide sequence probe. The degree of background binding of the probe also can be determined in a control sample to confirm that binding seen in the sample is due to the presence of the target nucleic acid molecule. A nucleic acid molecule encoding a trophinin- assisting protein also can be detected in a sample using methods as described above. For this purpose, the agent can be a nucleotide sequence specific for a nucleic acid molecule encoding a single trophinin-assisting protein such as tastin. The target nucleic acid molecule can be extracted from the sample or can be directly detected by in situ hybridization. A combination of both protein detecting and nucleic acid detecting methods, when used together, can provide more information than either method used alone. For example, when the expression of RNA encoding trophinin and tastin was evaluated in samples of human tissues by northern blotting, low levels of trophinin mRNA and tastin mRNA were observed in placenta, lung and liver. However, immunofluorescence analysis of these tissues using anti-trophinin antibodies and anti-tastin antibodies was negative for these tissues except for macrophages present in the tissues (not shown) . Thus, the combination of nucleic acid hybridization and immunofluorescence techniques together demonstrated that trophinin and tastin are not expressed by the majority of cell types within the body but are expressed by macrophages which are resident in certain tissues. The expression of trophinin in vivo indicates that trophinin has a role in human embryo implantation. For example, immunofluorescence studies using anti- trophinin antibodies demonstrated that trophinin was absent in term placental tissues. Although trophinin was absent from the majority of placental tissues from early (7-10 week) pregnancy (except for macrophages), trophinin was readily detected in focal regions in the apical plasma membranes of syncytiotrophoblasts of chorionic villi at 7 weeks pregnancy (Figure 10A) . Trophinins also were found in cytoplasmic vesicles of syncytiotrophoblasts in the chorionic villi from 7-10 week pregnancy (Figure 10B) . Double immunostaining with the lamp-1 lysosome marker (Fukuda, J. Biol. Chem. 266:21327-21330 (1991), which is incorporated herein by reference) showed co-localization of trophinin and lamp-1 in these vesicles, indicating that trophinins are present in lysosomes or endosomes. These results indicate that trophinin expression is strictly regulated in vivo and is present on the surface of syncytiotrophoblasts at early stages of pregnancy but not at later stages of pregnancy. Trophinins that are present in lysosomes of syncytiotrophoblasts at later stages of pregnancy can be undergoing degradation following removal from the cell surface. Tastin was not detected in most of the chorionic villi from 7-10 week pregnancy, except that a weak signal was observed in the lysosomes of the syncytiotrophoblasts. In addition to expression by the embryo, trophinin also is expressed in the uterus at the apical plasma membrane of the surface epithelium on day 16/17 endometrium (Figure 10C), but not in endometrium during the proliferation stage (day 6-13) or ovulation stage (day 14). Endometrial biopsy samples taken from the late secretory phase (day 20-28) showed staining for trophinin in the mucin. Tastin could not be detected in any of the above endometrial samples except for mucin. These results, like those for the embryo, demonstrate that trophinin expression is strictly regulated in endometrial tissue and is present for only a short time on the cell surface. The expression of trophinin is consistent with the concept of an implantation window for embryo implantation (Yoshinaga, Biochem. Biophys. Res. Comm. 181:1004-1009 (1988); Harper, Ballieres Clin. Obstet. Gvnaecol. 6:351-371 (1992)). The level of trophinin or of a trophinin- assisting protein in a sample of endometrial tissue can be diagnostic of infertility due to failure of implantation. For example, insufficient expression of trophinin in endometrial epithelial cells or in trophoblast cells of the embryo can result in a failure of implantation. As described above, agents to detect trophinin or a trophinin-assisting protein can be used to detect the level of these proteins or can be used to detect the level of nucleic acid molecules encoding these proteins at various times during the menstrual cycle. For example, immunofluorescence staining with anti- trophinin antibodies showed that trophinin was present in mucin shed from endometrial epithelium of late secretory phases (day 20-28; see Figure 10D) . With implantation of the embryo, mucin shedding from the endometrial epithelium does not occur. Thus, the disclosed methods to detect trophinin are useful for testing for the absence of pregnancy since detection of trophinin shed into body fluids, for example, in cervical mucus or in serum, can provide an early indication that implantation had not occurred and therefore, that the individual was not pregnant. The level of trophinin or a trophinin-assisting protein can also be used to diagnose the peak of fertility of a mammal. For example, the diagnosis of fertility is useful to determine the optimal time when the uterus of the mammal is most receptive for implanting a fertilized embryo. A source of body fluids or biopsy tissues is tested to determine the amount of trophinin or trophinin-assisting proteins or nucleic acids encoding such proteins that is produced by the uterus. The time point when trophinin expression or trophinin-assisting protein expression in the uterus is maxxmal or approaches the maximum identifies the optimal time for introduction of a fertilized embryo to maximize the chances for adherence and subsequent implantation of the embryo in the uterus. The ability to adhere cells at their apical surfaces using the methods described in the present invention can have a significant effect on cell morphology and function as exemplified by adhesion of HT- H cells to SNG-M cells. Initial cell attachment of HT-H to SNG-M cells is associated with the extension of the microvilli from one cell to another (Figures 2A and 2B) . Within 6 hr after co-culture, each microvillus becomes flattened into the plasma membrane (Figure 2C) and adherent junctions appear after 20 hr of co-culture (not shown) . Desmosomes are formed between HT-H and SNG-M cells at sites in the plasma membrane that were originally the upper (apical) surface of these cells (Figure 2D) . This finding contrasts to the situation in typical epithelial cells where desmosomes normally form in plasma membranes located at the lateral or basal sides of the cell. The ability to form desmosomes at a new membrane surface can result from a sequential reorganization of the proteins that control the structure and polarity of epithelial cells. Trophinin is expressed on the surfaces of HT-H and SNG-M cells in a unique lace-like pattern (Figures 9A and 9B) . This expression indicates that trophinin proteins cluster to form patches in the plasma membrane. Trophinin contains decapeptide repeats that form multiple β-turn structures (see Figure 5A and 5B) . This unique structure can be responsible for self-aggregation of trophinin in the cell membrane and for mediating cell adhesion. The subcellular localization of tastin in HT-H and SNG-M cells (Figures 9C and 9D) indicates that tastin can associate with cytoskeletal elements such as cytokeratins present in these cells. Thus, trophinin- assisting proteins can function to segregate trophinin molecules into clusters on the apical plasma membrane by interacting with trophinin in cells. Evidence from recent studies on cell adhesion molecules indicates that their function is regulated by association with cytoplasmic proteins and cytoskeletal structures (Gumbiner, Neuron 11:551-564 (1993); Stappert and Kemler, Cur . Opin. Neurol. 3:60-66 (1993); Garrod, Curr. Qpin. Cell Biol. 5:30-40 (1993; Hynes, Cell 69:11- 25 (1992)). Such molecular organization is important for cell-to-cell adhesion and cell movement. Cytoplasmic proteins involved in regulating cell adhesion molecules are associated with kinases that play a role in signal transduction, which occurs upon binding of cell adhesion molecules at the cell surface. Both trophinin and tastin contain serine and threonine residues that can serve as potential phosphorylation sites for protein kinases. For example, the amino terminal region of trophinin contains three serine and threonine residues that are potential phosphorylation sites (Figure 3). The presence of phosphorylation sites in trophinin and trophinin- assisting proteins indicates that the adhesion of trophinins expressed on one cell to those on another cell can be involved in triggering phosphorylation of trophinin and trophinin-assisting proteins as a signal to initiate the morphological changes occurring subsequent to trophinin-mediated cell adhesion. The invention provides methods to modify the ability of cells to adhere to each other. Cell adhesion can allow the cells to undergo subsequent physiological changes associated with cell adhesion. Such physiological changes can result from an increase in the adherence between cells due to increasing the level of trophinin expressed on the cell surface. An increase in adherence can be achieved by introducing an exogenous nucleic acid molecule encoding trophinin into cells and allowing the cells to adhere under appropriate conditions (see Example VII). This method of increasing adherence between cells can be used with any cell that can express functional trophinin proteins. Such cells include, for example, cells obtained from human or non-human primates or other mammalian cells, such as bovine, ovine, porcine or murine cells. A nucleic acid molecule encoding trophinin can be introduced into a population of first cell types, which can be allowed to adhere to each other. In addition, a cell from the population of first cell types, which contain a nucleic acid molecule encoding trophinin, can be combined with a second cell type, wherein a DNA molecule encoding a trophinin binding protein has been introduced into the second cell type. In this case, adhesion between the first cell type and the second cell type can occur due to binding of trophinin on one cell to the trophinin binding protein of the other cell. Similarly, a third or additional cell types expressing trophinin or a trophinin binding protein can be included so as to provide adhesion among three or more cell types. As used herein, the term "trophinin binding protein" means a molecule that can bind to trophinin with an affinity of about 1 x 10" 5 M or greater as measured, for example, by ELISA. A trophinin binding protein can include, for example, trophinin itself, an anti-trophinin antibody or a trophinin-assisting protein. Cell types that naturally express trophinin can adhere to a cell type that has been modified to express trophinin (see Example VII). In some cases, the expression of trophinin alone in cells may not enable cell adhesion. In such cases, adhesion may require the expression of a trophinin-assisting protein in addition to trophinin. The present invention also provides nucleic acid molecules encoding members of the trophinin- assisting protein family of proteins as well as methods for introducing such exogenous nucleic acid molecules into cells to obtain expression of a trophinin-assisting protein. This method of increasing adherence between cells by introducing an exogenous nucleic acid molecule can be used with any cell that can express functional trophinin-assisting proteins. Such cells include, for example, human and non-human primates or other mammalian cells, as described above. The level of expression of trophinin in a cell can be increased on the cell surface by contacting the cell with a trophinin agonist. As used herein, "trophinin agonist" means a chemical or biological molecule such as a simple or complex organic molecule, a peptide, peptido-mimetic, protein, carbohydrate or nucleotide sequence that can increase the expression level of functional trophinin in a cell and, thereby, increase the capacity of the cell for trophinin-mediated cell adhesion. A nucleic acid encoding trophinin is an example of a trophinin agonist. An expression vector that contains an exogenous nucleic acid molecule encoding trophinin can also be used as a trophinin agonist. For example, the introduction of an expression vector encoding trophinin into a cell can result in increased expression of trophinin and increased ability of the cell to undergo trophinin-mediated cell adhesion. Another example of a trophinin agonist can be a trophinin- assisting protein or an expression vector that contains an exogenous nucleic acid molecule encoding a trophinin- assisting protein. For example, a cell that can express trophinin but cannot efficiently mediate cell adhesion can be due to the inability of the cell to express a level of trophinin-assisting protein sufficient to interact with trophinin or a trophinin binding protein. In such cells, a trophinin agonist can, for example, be a trophinin-assisting protein or an expression vector encoding a trophinin-assisting protein. Particular types of trophinin agonists also can include hormones, cytokines or other types of molecules that interact directly or indirectly, for example, with genetic regulatory elements that control the expression level of trophinin or a trophinin-assisting protein. Genetic regulatory elements include, for example, promoters, enhancers, or intronic sequences that can regulate protein expression at the transcriptional or translational level. For example, a trophinin agonist can increase the expression of trophinin in a cell by binding to the promoter region of a trophinin gene and increase the efficiency of transcription. A trophinin agonist also can increase the expression of trophinin indirectly by binding to a regulatory protein, which, in turn, can activate an enhancer sequence to increase transcription of the trophinin gene. Trophinin mediated cell adhesion also can be increased by directly contacting a cell with purified trophinin. The ability of cells to adsorb a protein such as trophinin by an active or a passive process can result in a greater level of trophinin available on the cell surface for contact with another cell, thus, increasing the likelihood of trophinin-mediated cell adhesion. Trophinin agonists, which are useful for increasing trophinin-mediated cell adherence, are useful. for example, for preventing or minimizing the likelihood of implantation failure. Humans or other mammals that exhibit implantation failure can be tested for the level of trophinin or a trophinin-assisting protein expressed by endometrial cells using the methods described herein. Subjects having cells that fail to express sufficient levels of trophinin or trophinin-assisting proteins to achieve trophinin-mediated adhesion or express an aberrant or non-functional form of trophinin or a trophinin-assisting protein can be identified and a trophinin agonist can be used to achieve cell adhesion. The invention also provides methods to reduce or inhibit trophinin-mediated cell adhesion by contacting a cell with a trophinin antagonist, which can reduce or inhibit trophinin binding. Such methods can be used with human or other mammalian cells that express trophinin. For example, methods to reduce or inhibit trophinin- mediated cell adhesion can be used to block or terminate embryo implantation in humans or other mammals. As used herein, "trophinin antagonist" means a chemical or biological molecule such as a simple or complex organic molecule, a peptide, peptido-mimetic, protein, carbohydrate, antibody or nucleotide sequence that can reduce or inhibit the ability of trophinin to mediate cell adhesion. A trophinin antagonist can act by binding to a trophinin molecule of a first cell and, as a result of such binding, inhibit binding to a trophinin molecule on a second cell. Thus, the binding between two trophinin molecules is reduced or inhibited by the trophinin antagonist to a level below that required for a biological activity. An antibody molecule that binds to a portion of trophinin exposed on the external side of the cell membrane is an example of a trophinin antagonist. The present invention provides methods to produce such antibodies (see Example V) and to evaluate such antibodies for their ability to act as trophinin antagonists in an in vitro cell binding assay (see Figure ID and Example VII). An active fragment trophinin antagonist is another example of a trophinin antagonist that can bind to trophinin on a cell and prevent the cell from binding to a second cell that expresses a trophinin binding protein. As used herein, an "active fragment trophinin antagonist" means a portion of trophinin or a trophinin binding protein that is ineffective in promoting cell adhesion but can bind to a trophinin molecule. Such active fragment trophinin antagonists can be peptides as small as about five amino acids and can be identified, for example, by screening a peptide library (see for example, Ladner et. al., U.S. Patent No: 5,223,409, June 29, 1993, which is incorporated herein by reference) to identify peptides that bind to trophinin but do not mediate cell adhesion. A trophinin antagonist also can interfere with the interaction of a trophinin-assisting protein with trophinin. Thus, a chemical or biological molecule such as a simple or complex organic molecule, a peptide, peptido-mimetic, protein, carbohydrate or nucleotide can be a trophinin antagonist by binding to the site on a trophinin-assisting protein or on a trophinin molecule that is involved in the interaction between a trophinin- assisting protein and trophinin. A trophinin antagonist need not bind directly to the site in trophinin that binds to another trophinin molecule or the site in trophinin that binds to a trophinin-assisting protein, in order to inhibit cell adhesion. Thus, for example, a trophinin antagonist of sufficient size, when bound to a region in trophinin that is near the trophinin binding site can physically block another trophinin molecule from binding to the site. Also, a trophinin antagonist can bind to trophinin and change the structure of the trophinin binding site rendering it unsuitable for adhesion to another trophinin molecule. Thus, a trophinin antagonist can act like an allosteric inhibitor of an enzyme. A trophinin antagonist can also function to inhibit trophinin- mediated cell adhesion by binding to a trophinin- assisting protein in a cell, thereby inhibiting the ability of the trophinin-assisting protein to assist trophinin in mediating cell adhesion. A trophinin antagonist also can function by reducing the level of expression of trophinin or a trophinin-assisting protein, thereby reducing or inhibiting cell adhesion. For example, nucleic acid molecules encoding an antisense nucleotide sequence or encoding a ribozyme for a trophinin or a trophinin- assisting protein can be incorporated into vectors and introduced into cells by methods well known to those in the art as described above. The level of trophinin or trophinin-assisting protein expression also can be reduced by treating cells with hormones, cytokines or other type molecules that interact directly or indirectly with genetic regulatory elements controlling the expression level of trophinin or a trophinin-assisting protein in a cell. A trophinin antagonist can effect trophinin-mediated cell adhesion by reducing the level of expression of trophinin in the cell by blocking regulatory elements involved in maintaining expression of trophinin. A trophinin antagonist can also reduce the level of trophinin expression by acting directly or indirectly as a negative regulator. Reducing or inhibiting adhesion of cells by trophinin-mediated cell adhesion can be useful in vitro or in vivo. In vitro, trophinin antagonists can be identified and compared to each other to determine potency, which can be derived from concentration versus activity curves and can be represented as the concentration of antagonist that achieves 50% inhibition of activity. In vitro potency can be one criterion for selecting trophinin antagonists that can be useful in vivo. The in vitro method for measuring potency is based on the adhesion assay used to discover trophinin and trophinin-assisting protein molecules (see Figure IC and Example I). In this method, a radiolabeled cell line expressing trophinin and a trophinin-assisting protein (e.g. HT-H cells) is contacted with the antagonist to be tested, then the mixture is added to a paraformaldehyde fixed-monolayer of trophinin and trophinin-assisting protein expressing cells (e.g. SNG-M cells). After a period of time, the unbound cells are removed by washing and the percentage of attached cells determined by counting the bound radioactivity. A potent trophinin antagonist can be identified by its ability to significantly reduce or to inhibit trophinin-mediated cell adhesion. The ability of trophinin to mediate cell adhesion can have other in vitro uses besides that of a trophinin antagonist. For example, trophinin can be used to bind trophinin- expressing cells to a solid support, which is useful, for example, to purify a population of trophinin expressing cells from a mixed population containing trophinin expressing and non-trophinin expressing cells or to purify a trophinin expressing embryo. Also, trophinin attached to a prosthetic device can be used to bind a layer of trophinin expressing cells to the device to render the device more suitable for introduction in vivo . Trophinin can be bound to a solid support using methods known in the art (for example see Harlow and Lane, supra , (1988)). For example, purified trophinin in PBS (phosphate buffered saline, 10 mM phosphate buffer, pH 7.4 ) can be directly adsorbed to a plastic tissue culture surface, a polyvinyl chloride surface or a nitrocellulose surface. Trophinin also can be covalently coupled to beads such as, for example, agarose or polyacrylamide that had been previously activated by a coupling agent such as glutaraldehyde or cyanogen bromide. In addition, trophinin can be attached indirectly to a solid support, for example, by first coating or coupling an agent that can specifically bind to trophinin. A population of trophinin-expressing cells can be enriched from a mixed population of trophinin- expressing and cells that do not express trophinin by applying the mixed cell population to a solid support or surface containing trophinin. After a period of time sufficient to allow the trophinin- expressing cells to adhere to the solid support, cells that do not express trophinin can be washed from the support. The enriched population of trophinin expressing cells can be used directly on the solid support or can be removed from the solid support by vigorous washing or by treating the cells with a trophinin antagonist. A trophinin antagonist or agonist can be used to prepare a medicament for the treatment of a condition such as infertility, for treatment of a disease or for intervening in a potential pregnancy. For example, a trophinin antagonist can be administered to a subject to block embryo implantation following fertilization by inhibiting binding of the embryo trophoblast cell layer to the uterine epithelial cell layer. A trophinin antagonist also can be used to terminate implantation after it has already occurred by administering a trophinin antagonist to effect detachment of the embryo from the uterine cell lining. In contrast, a trophinin agonist can be administered to a subject to alleviate implantation failure by enhancing the binding between the trophoblast cell layer of the embryo and the endothelial cell layer of the uterus. Trophinin antagonists and agonists of the invention are particularly useful when administered as a pharmaceutical composition containing the trophinin antagonist or agonist and a pharmaceutically acceptable carrier. Pharmaceutically acceptable carriers are well known in the art and include, for example, aqueous solutions such as physiologically buffered saline or other solvents or vehicles such as glycols, glycerol, oils such as olive oil or injectable organic esters. A pharmaceutically acceptable carrier can contain physiologically acceptable compounds that act, for example, to stabilize or to increase the absorption of a trophinin antagonist or agonist. Such physiologically acceptable compounds include, for example, carbohydrates, such as glucose, sucrose or dextrans, antioxidants, such as ascorbic acid or glutathione, chelating agents, low molecular weight proteins or other stabilizers or excipients. One skilled in the art would know that the choice of a pharmaceutically acceptable carrier, including a physiologically acceptable compound, depends, for example, on the route of administration of the composition. One skilled in the art would know that a pharmaceutical composition containing a trophinin antagonist or agonist can be administered to a subject by various routes including, for example, by intra-uterine instillation, orally or parenterally, such as intravenously, intramuscularly, subcutaneously or intraperitoneally. The composition can be administered by injection or by intubation. The pharmaceutical composition also can be incorporated, if desired, into liposomes or microspheres or can be microencapsulated in other polymer matrices (Gregoriadis, Liposome Technology, Vol. 1 (CRC Press, Boca Raton, FL 1984), which is incorporated herein by reference) . Liposomes, for example, which consist of phospholipids or other lipids, are nontoxic, physiologically acceptable and metabolizable carriers that are relatively easy to make and administer. In order to inhibit embryo implantation, the trophinin antagonist is administered in an effective amount, which is about 0.01 to 100 mg/kg body weight. As used herein, the term "effective amount" means the amount of trophinin antagonist that can effectively block a cell adhesion event. For example in the case of implantation, an effective amount is that which blocks embryo implantation. In the case of a trophinin agonist, the "effective amount" means the amount of agonist that can effectively increase the level of trophinin-mediated cell adhesion. For example, in implantation failure, an effective amount of a trophinin agonist is the amount that allows for successful implantation. An effective amount of a trophinin antagonist or agonist in a subject can be determined using methods known to those in the art. The total effective amount can be administered to a subject as a single dose, either as a bolus or by infusion over a relatively short period of time, or can be administered using a fractionated treatment protocol, in which the multiple doses are administered over a more prolonged period of time. One skilled in the art would know that the concentration of trophinin antagonist or agonist required to obtain an effective dose in a subject depends on many factors including the age and general health of the subject as well as the route of administration and the number of treatments to be administered and the chemical form of the antagonist or agonist. In view of these factors, the skilled artisan would adjust the particular amount so as to obtain an effective amount for the subject being treated. The cadherin and integrin families of adhesion molecules, which are involved in cell-cell and cell- matrix adhesion, are implicated in epithelial differentiation, carcinogenesis and metastasis. A further understanding of how such adhesion receptors exert their biological effects on the cell was accomplished through the discovery of a cell adhesion regulator gene (Pullman and Bodmer, Nature 356:529-533 (1992)). The cell adhesion regulator gene codes for a protein that is located in the cytoplasm and functions as a signal transduction molecule for integrin adhesion receptors. The cell adhesion receptor gene has the characteristics of a tumor suppressor gene because inactivation of the gene can result in loss of differentiation induction of a cell and subsequent acquisition of invasive and metastatic character. The genes encoding the trophinin-assisting proteins of the present invention also can function as tumor suppressor genes. For example, the structural features of the trophinin-assisting proteins, as derived from the deduced amino acid sequences (see SEQ ID NO: 5, SEQ ID NO: 7 and SEQ ID NO: 9), are consistent with a cytoplasmic regulatory protein that can mediate intracellular signaling of trophinin or other cell adhesion molecules. The present invention provides methods to increase the level of expression of trophinin-assisting proteins, thus increasing the tumor suppressor activity of a cell. Such methods can, for example, be useful for the treatment of cancer. As used herein, a trophinin- assisting protein agonist means a chemical or biological molecule such as a sxmple or complex organic molecule, a peptide, peptido-mimetic, protein, carbohydrate or nucleotide sequence that can increase the expression level of a trophinin-assisting protein in a cell. Particular types of trophinin-assisting protein agonists can include hormones, cytokines or other types of molecules that interact either directly or indirectly with genetic regulatory elements controlling the expression level of a trophinin-assisting protein. The following examples are intended to illustrate but not limit the present invention. EXAMPLE I CELL CULTURE ADHESION METHOD This example describes methods for performing cell adhesion assays and for evaluating their specificity and impact on cell morphology. A. Cell Lines The human teratocarcinoma cell line HT-H was used as a source of embryonic trophoblast cells for the adhesion assay. HT-H cells (Izhar et al., Biol. 116:510- 518 (1986), which is incorporated herein by reference) were maintained in RPM 1640 medium containing 10% fetal bovine serum, 2 mM glutamine, 100 units/ml penicillin and lOOμg/ml streptomycin. Trophoblastic HT-H cells were separated from undifferentiated HT-H cells as described (Izhar et al., supra , 1986) and subcloned for use in the experiments described. The endometrial adenocarcinoma cell line SNG-M (Ishiwata et al.. Cancer Res. 37:1777- 1785 (1977), which is incorporated herein by reference) was maintained in RPMI medium as described above. Endometrial adenocarcinoma cell lines HeclA, RL95-2 AN3CA and KLE were obtained from American Type Culture Collection (Rockville MD) and were cultured in Dulbecco's modified Eagle's (DME) medium containing 10% fetal bovine serum, 2 mM glutamine, 100 units/ml penicillin and lOOμg/ml streptomycin. Additional epithelial cells which were tested included COS-1 cells (monkey kidney) , HeLa (uterine cervical carcinoma), HepG2 (hepatocellular carcinoma) , SW480 (colonic adenocarcinoma) , and A431 (epidermoid carcinoma). These cells were obtained from the American Type Culture Collection and were cultured as described above. B. Cell Adhesion Assay The adhesion of HT-H cells to several different human endometrial epithelial cells was examined. HT-H cells were metabolxcally labeled with 35 S-methionine using Trans-label (DuPont/NEN, Boston MA) in methionine- and cysteine-free RPMI medium supplemented with 10% dialyzed fetal bovine serum, 2 mM glutamine, lOOμg/ml pyruvate, 100 units/ml penicillin and lOOμg/ml streptomycin. Cells were labeled at 37°C in a humidified C0 2 incubator. After 20 minutes (min) , the medium was replaced with complete medium and cells were incubated an additional 2 hr. The 35 S-labeled HT-H cells were detached from the tissue culture dish using cell dissociation solution (Specialty Media, Lavalette, NJ) supplemented with 1 mM EDTA. The cells were pelleted by centrifugation and resuspended in Hank's balanced salt solution (HBS). Cell suspensions (0.2 ml, 5 x 10* cells) were added to a monolayer of endometrial epithelial cells grown in a 24 well tissue culture plate. HT-H cells that did not adhere to the monolayer were removed by washing 3x with HBS with or without 1 mM EDTA. The cells remaining in each well were solubilized with 0.5 ml of 0.5 N NaOH and 1% SDS. The lysate was transferred to a scintillation vial and radioactivity was counted. The data were expressed as % radioactivity remaining on the monolayer relative to the total radioactivity of 35 S-labeled HT-H cells added. The results were obtained from triplicate cultures. The results of the cell adhesion assays indicate that HT-H cells attached to the SNG-M, HeclA, KLE and RL95-2 cells, but attached minimally to the AN3CA cells (Table 1). The addition of 1 mM EDTA did not change significantly the percentage of HT-H cells which bound to SNG-M, HeclA and KLE cells (Table 1), whereas, the attachment of HT-H cells to RF95-2 cells was reduced significantly in the presence of EDTA. These results indicate that adhesion of HT-H cells with SNG-M, HeclA or KLE cells is divalent cation independent. In contrast, HT-H adhesion to RL95-2 cells is largely divalent cation dependent since a relatively large number of cells failed to adhere after washing with EDTA (Table 1). Since a relatively high proportion of HT-H cells adhered to SNG-M cells and that adherence was not cation dependent, these cells were chosen for further study. TABLE I ADHESION OF HT-H CELL TO ENDOMETRIAL ADENOCARCINOMA CELLS Cell Line* Percentage HT-H cells attached + EDTA - EDTA SNG-M 56.9 ± 8.2 49.7 ± 7.3 HeclA 29.6 ± 10.2 24.2 ± 8.3 KLE 32.5 ± 8.5 27.2 ± 4.8 RL95 83.5 ± 9.7 20.4 ± 12.1 AN3CA 4.2 ± 0.8 2.1 ± 0.6 ' - Used as a monolayer without fixation. Cell adhesion assays also were conducted using fixed cell monolayers. In this case, cells grown in 24 well tissue culture plates were treated with 1% paraformaldehyde in PBS for 15 min at RT. The fixed cells were washed in PBS and were used for cell adhesion assays as described above. The results showed that HT-H cells adhered efficiently to the surface of paraformaldehyde-fixed monolayer of SNG-M cells in a divalent cation independent manner (Figure IA) . Furthermore, when SNG-M cells were added to a fixed monolayer of SNG-M cells, they adhered efficiently in a divalent cation independent manner (Figure IA) . COS-1 cells adhered minimally to SNG-M cells (Figure IA) , whereas HeLa, HepG2, SW480 and A431 did not detectably adhere (not shown). Essentially the same results were obtained when fixed HT-H cell monolayers were used in place of SNG-M cell monolayers (Figure IB) . In summary, adhesion between HT-H and SNG-M cells is cell type specific and divalent cation independent. C. Morphological Evaluation of Adherent Cells Electron microscopy was used to characterize the adherent interface formed during the co-culture of HT-H and SNG-M cells. SNG-M cells were grown in a Falcon 3001 (25 x 10 mm) tissue culture dish until reaching 50% confluency. HT-H cells were detached from the tissue culture dish by trypsin/EDTA treatment and were added to monolayers of SNG-M cells. The combined cells were cultured for up to 4 days. At various times, individual cultures were processed for transmission electron microscopy. Cells were fixed in freshly prepared fixative (10 mM NaI04, 75 mM lysine, 37.5 mM sodium phosphate buffer, 2% paraformaldehyde, pH 6.2) for 15 min at RT. Cells then were washed with phosphate buffer, treated with glutaraldehyde and processed for electron microscopy as described previously (Klier et al., Devel. Biol. 57:440-449 (1977), which is incorporated herein by reference) . Electron microscopy was performed using a Hitachi K-600 electron microscope. Following 10 min co-culture, the apical surface of HT-H cells faced the apical surface of SNG-M cells (Figure 2A) and many microvilli were present between the cells (Figure 2B) . After 6 hr, there were closer adhesive interactions between the cells (Figure 2C), with the edges of the microvilli extending from one cell type and attaching to the cell surface of the other cell type (Figure 2C) . Occasional invagination in the plasma membrane of the SNG-M cells was observed (shown by an arrow in Figure 2C) . After 4 days, the microvilli had disappeared completely from the surfaces of both cell types and desmosome-like adherent junctions were present between the cell (Figure 2D). The results described using the in vitro cell adhesion assay are similar to the morphological studies of human implantation in vivo and in vitro (Lindenberg et al.. Hum. Reprod. 1:533-538 (1986); Knoth and Larsen, Acta Obstet. Gvnecol. Scand. 51:385-393 (1972)). For example, during implantation, the trophoblast endometrial epithelial cells show characteristics which include: 1) reduction of microvilli in areas of attachment, 2) an invagination response of endometrial cells at the contact site, 3) formation of a junction complex or sign of focal adhesions between the trophoblast and endometrial cells in a later stage of implantation and 4) intrusion of the trophoblast between the endometrial epithelia. Thus, the HT-H and SNG-M cells provided are a useful in vitro model of embryo implantation. EXAMPLE II EXPRESSION CLONING OF MOLECULES MEDIATING ADHESION OF HT- H CELLS TO SNG-M CELLS This example describes a method to clone cDNA molecules that are involved in mediating cell adhesion. A. Expression of a cDNA Library in COS-1 Cells A functional cDNA expression cloning strategy was used to obtain the nucleic acid molecules encoding the proteins responsible for the initial, EDTA independent cell adhesion between HT-H and SNG-M cells (Aruffo and Seed, Proc. Natl. Acad. Sci. (USA) 84:8573- 8577 (1987), which is incorporated herein by reference). COS-1 cells, which did not adhere efficiently to monolayers of SNG-M cells (see Figure IA) , were chosen for transfection with a cDNA library derived from HT-H cells (obtained from Invitrogen Corp. Ltd; San Diego, CA) . Poly-A mRNA prepared from freshly harvested HT-H cells was used to construct a unidirectional cDNA expression library in the mammalian expression vector pcDNAI. The cDNA library consisted of 2 x 10 6 independent clones with an insert size ranging from 0.5 to 3.0 kb. COS-1 cells (1 X 10 8 cells) were transfected with the HT-H cDNA library using electroporation. This method of transfection was used because the diethylaminoethyl-dextran and lipofection reagents used for other transfection methods increased the nonspecific adhesion of COS-1 cells to the SNG-M cells. COS-1 cells were grown in Falcon culture dishes until the cells reached about 75% confluency, were detached using 0.1% trypsin and 1 mM EDTA and suspended in DME medium containing 10% fetal bovine serum. The cells were pelleted by centrifugation, washed 2x with cold PBS and 1-0.5 x 10 7 cells/ml were suspended in PBS containing 100 μg/ml plasmid DNA. Electroporation was performed using a Gene Pulser (Biorad, Hercules, CA) at 0.4 kvolt with a capacitance of 125μF. The transfected cells were cultured for 48 hr in DME medium containing 10% fetal bovine serum, 2 mM glutamine, 100 units/ml penicillin and 100 μg/ml streptomycin. B. Screening the cDNA Expression Library by Cell Adhesion The transfected COS-1 cells were selected for binding to an SNG-M cell monolayer. The SNG-M cells were cultured to confluency in a Falcon 3001 or 3005 tissue culture dish, then fixed with 1% paraformaldehyde in PBS at RT for 15 min. The fixed SNG-M cells were washed 3x with PBS and lx with HBS. Two days following electroporation, transfected COS-1 cells were detached from the tissue culture plate by incubating for 5-10 min with cell dissociation solution (Specialty Media, Lavalette, NJ) supplemented with 1 mM EDTA. The cells were suspended in HBS containing 1 mM EDTA and were added to a fixed SNG-M cell monolayer and allowed to attach at RT for 20 min. The plate was washed 3x with HBS containing 1 mM EDTA and transfected COS-1 cells that remained on the SNG-M cell monolayer were mechanically detached by flushing HBS with a Pasteur pipet. Approximately 1 x 10 4 COS-1 cells were recovered and added to a second SNG-M monolayer to adhere as described above. After 20 min, the nonadherent COS-1 cells were discarded by washing as above and the adherent cells (representing about 1 x 10 3 COS-1 cells) were solubilized with 1% SDS. Plasmid DNA was recovered from the SDS soluble extract and amplified in E. coli MC1061/P3 cells. The plasmid DNA obtained from the transfected COS-1 cells was subjected to a second round of electroporation in COS-1 cells followed by selection by adhesion as described above, except that the number of cells used for transfection was reduced to 1 X 10 8 to 1 X 10 7 cells. After the first adhesion selection step, about 2 X 10 3 transfected COS-1 cells were attached to the SNG-M monolayer. After the second cell adhesion step, plasmid DNA was obtained by extraction with SDS as described above. E. coli MC1061/P3 cells were transformed using plasmid DNA following the second transfection. Plasmid DNA from two hundred E. coli clones was divided into ten groups containing 20 plasmids each. Fresh COS-1 cells were transfected with each group containing the mixture of 20 clones and allowed to adhere to a monolayer of the SNG-M cells. The individual clones derived from a group that was positive for adhesion were each transfected into COS-1 cells and tested for adhesion. However, the transfected COS-1 cells derived from individual clones failed to adhere to the SNG-M cells. The 20 clones were screened again using a series of mixtures containing a decreasing number of clones. A mixture of two specific cDNA sequences were the minimum required to enable transfected COS-1 cells to adhere to SNG-M cells. The initial pair of cDNA clones identified were trophinin and tastin. The results in Figure IC demonstrate that COS-1 cells transfected with a mixture of trophinin and tastin cDNA adhered to a monolayer of SNG-M cells, while COS-1 cells transfected only with trophinin cDNA or tastin cDNA failed to adhere. Further screening of the remaining 200 clones for co-transfection with the trophinin clone identified two other clones which were required for adhesion. The additional two clones were named bystin and lastin. The trophinin cDNA clone encodes an intrinsic membrane protein, while the tastin, bystin and lastin clones likely encode a cytoplasmic protein. The cDNA clones were sequenced by the dideoxy nucleotide chain termination method of Sanger et al. fProc. Natl. Acad. Sci. ( USA) . 74:5463-5467 (1977), which is incorporated herein by reference) using a modified T7 DNA polynuclease ( "SEQUENASE", United States Biochemicals, Cleveland,OH) . The nucleotide sequence of the trophinin cDNA was determined from restriction fragments subcloned into Bluescript from nested deletion mutants generated by exonuclease III (Boehringer Mannheim, Indianapolis, IN). The nucleotide sequence of the tastin, bystin and lastin cDNA were determined using oligonucleotide primers. Editing and analysis of the sequence was done using DNASIS (Hitachi, Tokyo, Japan) and PCgene software (Intelligenetics, Mountain View, CA) . Sequence comparisons with the databases were performed using the "blast" network program (National Center for Biotechnology Information, NIH) . The complete nucleotide sequence for trophinin cDNA is shown in Figure 3 (SEQ ID NO: 1), while the complete nucleotide sequence of tastin and bystin and a partial nucleotide sequence of lastin are shown in Figure 6 (SEQ ID NO: 4), Figure 7 (SEQ ID NO: 6) and Figure 8 (SEQ ID NO: 8), respectively. The clone which contained the lastin sequence was missing the 3' end of the gene including the poly-A tail of the mRNA. EXAMPLE III CHARACTERIZATION OF TROPHININ The complete cDNA sequence and the deduced amino acid sequence of trophinin are shown in Figure 3. The trophinin cDNA clone covers 2524 nucleotides with an open reading frame encoding 749 amino acids. The 3' untranslated region consists of 250 nucleotides and contains a polyadenylation signal 12 bp upstream of the poly-A tail. An optimal translation initiation sequence (Kozak, supra , 1984) is associated with only one of the ATG codons in the near 5' region. Use of this ATG for translation initiation would result in a predicted molecular mass of 69.29 kDa for trophinin. The plasmid cDNA clones were subjected to in vitro translation using T7 oligonucleotide primer, rabbit reticulocyte lysate (Promega, Madison, WI), RNA polymerase and 35 S-methionine. The products were processed by SDS-PAGE and visualized by autoradiography. Jn vitro translation of trophinin cDNA showed a major product at 61 kDa (Figure 4), which is in agreement with the predicted molecular mass of 69.29 kDa. Hydropathy analysis (Kyte and Doolittle, supra , 1982) of trophinin defines this molecule as an intrinsic membrane protein having 8 separate transmembrane domains (figure 5A) . No cleavable signal sequence was found in the cDNA clone coding for trophinin, however, the first putative membrane spanning domain (amino acids 66 to 120) follows an arginine residue at position 54 that can function as a stop transfer signal during translocation into the endoplasmic reticulum. Employment of the stop transfer sequence during translocation can result in the amino terminal segment of trophinin from the methionine at position 1 to the serine at position 65 being located in the cytoplasm. The location of the amino terminal portion of trophinin was examined using antibodies raised against a peptide within the predicted cytoplasmic tail of the trophinin (residues 23 to 31). In these experiments, the antibodies only reacted with HT-H cells that had their cell membranes removed by detergent treatment, indicating that the amino terminal portion of trophinin is located in the cytoplasm. The amino terminal region of trophinin contains three serine and/or threonine residues that can function as potential phosphorylation sites (see Figure 3). The threonine at position 7 is contained within a consensus site for phosphorylation by casein kinase II (Kemp and Pearson, supra , 1990). The serine residues at position 46 and 52 are located within consensus sequence sites for protein kinase C phosphorylation. The serine residue at position 46 also is contained within a consensus sequence site for cAMP/cGMP dependent phosphorylation. The presence of phosphorylation sites in a transmembrane protein such as trophinin indicates a likely mechanism for signaling the morphological changes in cells that are known to occur subsequent to trophinin-mediated adhesion (see Figure 2A, 2B, 2C and 2D). Trophinin contains eight potential membrane spanning regions (Figure 5A) . The relative proportion of trophinin localized in the cytoplasm, in the membrane bilayer and on the cell surface is 10%, 56% and 34%, respectively. Four potential N-glycosylation sites, and thirteen potential O-glycosylation sites, are found within the predicted cell surface domains of trophinin (Figure 3) . Greater than 90% of trophinin contains a tandemly repeated decapeptide motif (Figure 5B) . There are 69 such repeat sequences and they exhibit some variation in sequence and length. The predicted exposed cell surface domains of trophinin contain regions of decapeptide repeats that are hydrophilic in character. Three such exposed domains can be identified in trophinin, at amino acid positions 278 to 364 (SEQ ID NO: 20), 441 to 512 (SEQ ID NO: 21) and 634 to 719 (SEQ ID NO: 22) (see Figure 3; bold lettering). Protein secondary structure algorithms (Gamier et al., supra , 1978; Gascuel and Golmard, supra , 1988), predict that the decapeptide repeats conform to a repeated β-turn structure that can be a key structural element for efficient homophilic adhesion (not shown) . Four potential N-glycosylation sites, N-X-S(T), and thirteen potential O-glycosylation sites, P-S(T) or S(T)- P, are found within the predicted cell surface domains of trophinin (Figure 3). Trophinin has no significant homology to sequences in protein and nucleic acid databases. EXAMPLE IV CHARACTERIZATION OF TROPHININ-ASSISTING PROTEINS The complete nucleotide sequence of the tastin cDNA clone (SEQ ID NO: 4) and the deduced amino acid sequence (SEQ ID NO: 5) are shown in Figure 6. The tastin cDNA clone contains 2,577 nucleotides with an open reading frame encoding 778 amino acids. The 3' untranslated region contains 133 nucleotides and has a polyadenylation signal 11 bp upstream of the poly-A tail. The nucleotide sequence around the ATG at position 11 is contained within a consensus sequence for a translation initiation site (Kozak, supra , 1984). In vitro translation of the tastin cDNA produce a predominant product of 80 kDa (Figure 4), consistent with the predicted molecular weight 83.75 kDa based on the cDNA open reading frame. Tastin is likely a cytoplasmic protein since it lacks an obvious secretory signal sequence and transmembrane helices as defined by hydropathy analysis (Kyte and Doolittle, supra , 1982), and shows a pattern of cell staining similar to other cytoplasmic proteins (see Figure 9C and 9D) . Tastin is rich in prolines, which account for 15.3% of the total amino acids of the protein. In addition, the region from residues 516 to 650 is cysteine rich (see italics in Figure 6), with the majority of the cysteines located within four tandem repeat sequences of 33 amino acids each (not shown). Tastin also contains many serine and threonine residues and a tyrosine residue that, when considered with their adjacent amino acid residues, provide sequence motifs for protein kinase phosphorylation (Figure 6). Specifically, tastin contains two cAMP/cGMP- dependent phosphorylation sites located at position 234 and 350 and sixteen protein kinase C phosphorylation sites, among which the threonine at position 179 most closely matches the consensus sequence (Kemp and Pearson, supra , 1990). Tastin also contains eleven serine and threonine residues that are potential casein kinase II phosphorylation sites and two threonines at positions 177 and 363 that are within a consensus MAP kinase phosphorylation site (Gonzalez et al., supra , 1991). Tastin has no overall significant homology to previously reported protein sequences. Nucleotide sequence homology analysis of tastin identified the sequence HFBCL29 (Genbank accession number M85643), which was derived from a human fetal brain cDNA library. HFBCL29 shows homology to a portion of tastin cDNA (positions 2340 to 2057) provided the HFBCL29 sequence represents the non-coding strand of the DNA (ie. the homology is due to nucleotide base complementarity) . Thus, HFBCL29 sequence would be homologous to a portion of the tastin if the former sequence had been recorded in the data base in the antisense direction. The protein sequence deduced from HFBCL29 is believed to be related to Y box binding protein-1 (Adams et al., supra , 1992). However, the entire nucleotide sequence and deduced amino acid sequence of tastin overall are not homologous to the Y-box binding protein-1. EXAMPLE V PRODUCTION OF ANTIBODIES TO TROPHININ AND A TROPHININ ASSISTING PROTEIN (TASTIN. Peptide sequences of trophinin and tastin were analyzed to predict useful antigenic sites using the method of Hopp and Wood, Mol. Immunol. 20:483-489 (1983), which is incorporated herein by reference. A short sequence from the amino terminal end of trophinin and from tastin were selected as antigens. The sequences FEIEARAQE (SEQ ID NO: 10), representing residues 23 to 31 of trophinin, and DQENQDPRR (SEQ ID NO: 11), representing residues 41 to 49 of tastin, were chemically synthesized with a cysteine residue added to the amino terminus to facilitate protein conjugation. The peptides were conjugated to KLH using meta-maleimidobenzoyl N- hydroxysuccinimide ester (Sigma Chemical Co., St. Louis, MO) as described by Kitagawa and Aikawa (J. Biochem. 79:342-346 (1976)), which is incorporated herein by reference). New Zealand white rabbits were immunized the peptide-KLH conjugates according to the following procedure. On day 1, animals were injected subcutaneously with peptide conjugate emulsified in Freund's complete adjuvant. On day 14, the animals were boosted by subcutaneous injection of peptide conjugate emulsified in Freund's incomplete adjuvant. Animals were bled (30 ml) on days 24, 31 and 38 to obtain a source of antisera. Anti-peptide antibodies were purified from rabbit antisera by protein A affinity chromatography and peptide affinity chromatography as described by Richardson (J. Virol. 54:186-193 (1985), which is incorporated herein by reference). Rabbit antibodies to trophinin and tastin were used to detect these molecules in samples of cells and tissues (see Example VI) . To raise antibodies specific for portions of the trophinin molecule that are expressed on the external surface of the cell membrane, the three hydrophilic domains containing decapeptide repeats were separately expressed in bacteria as a fusion to glutathionine S- transferase (GST) . The trophinin cDNA from the aspartic acid residue at position 278 to the serine residue at position 364 was amplified by PCR using the oligonucleotide prxmers GGAATTCATGGATGGCTCTCCCAGCACTGGTG (SEQ ID NO: 14) and GCAGCTGAGTGCTGGTGCTTAGTGTACCACC (SEQ ID NO: 15) to produce the fusion protein GST551. The trophinin cDNA from the proline residue at position 441 to the serine residue at position 512 was amplified by PCR using the oligonucleotide primers GGAATTCATGCCCAGCAACAGCATTGGC (SEQ ID NO: 16) and GCAGCTGAGTACTGGTGCTGGGTCCATCACAAAAAC (SEQ ID NO: 17) to produce the fusion protein GST552. The trophinin cDNA from amino acid residues serine at position 634 to asparagine at position 719 was amplified by PCR using oligonucleotide prxmers GGAATTCATGAGCGATGGCTTTGGCAGTAG (SEQ ID NO: 12) and CGTCGACTCAGTTTGGTCCACCGCCGAAGCCAG (SEQ ID NO: 13) to produce the fusion protein GST553. The trophinin cDNA from the methionine residue at position 1 to the serine residue at position 66 was amplified by PCR using the oligonucleotide primers GGAATTCATGGATATCGACTGCCTA (SEQ ID NO: 18) and GCAGCTGAGTCTGGAGCTGGGTGCACCAT (SEQ ID NO: 19) to produce the fusion protein GST-N-terminal trophinin. The amplified DNA fragments of the fusion proteins were ligated into pGEX-4T-l vector (Pharmacia, Piscataway NJ) at the EcoRI and Xhol sites. E. coli HB101 was transformed with the plasmid vectors and the GST fusion proteins were produced as described by the manufacturer. The fusion proteins were initially purified by affinity chromatography on Glutathionine- agarose beads (Pharmacia) . For immunization to produce antibodies to the external domains of trophinin, GST551, GST552 and GST553 fusion proteins were electrophoresed in SDS-PAGE, the gel was stained with Coomassie blue, and the band containing the fusion protein excised from the gel. The polyacrylamide gel containing the purified fusion proteins were injected into rabbits to produce antibodies according to the procedure described previously for the synthetic peptides except that antibodies were not purified from the antiserum. EXAMPLE VI DETECTION OF TROPHININ AND A TROPHININ ASSISTING PROTEIN (TASTIN, IN CELLS AND TISSUES This example provides methods to identify and localize trophinin and tastin in various types of samples. A. Localization of Trophinin and Tastin in Cultured Cells HT-H and SNG-M cells were grown on glass coverslips in Falcon 3005 tissue culture dishes for 2-3 days. The cells were fixed at RT for 15 min with 1% paraformaldehyde in PBS, then washed 4x with PBS. Fixed cells were incubated in PBS containing 5% bovine serum albumin (IIF buffer) plus 0.1% saponin at RT for 30 min, then incubated 45 min at RT with anti-trophinin or anti- tastin antibody diluted in IIF buffer plus 0.1% saponin to permeabilize the cells. After further washing with IIF buffer plus 0.1% saponin, cells were incubated for 30 min at RT with fluorescein isothiocyanate (FITC)- conjugated goat anti rabbit IgG F(ab') 2 (Cappel, Durham, NC) diluted in IIF buffer. Coverslips containing the cells were washed 3x with IIF buffer and lx with PBS, then placed upside down on a slide glass in an aliquot of 95% glycerol and 5% PBS. Micrographs were obtained with a Zeiss Axioplan fluorescence microscope or a Zeiss LSM410 confocal laser scanning microscope. Antibodies to an N-terminal peptide of trophinin (residue 23-31) showed staining of permeabilized HT-H and SNG-M cells that appears as a lace-like pattern due to clustering of the fluorescence over the cell surface (Figures 9A and 9B) . A tangential view by confocal microscopy (not shown) showed that the majority of trophinin is detected in the upper plasma membranes of these cells. A small amount of trophinin staining is detected inside the cells and in their basal plasma membranes. Antibody to an N-terminal peptide of tastin exhibited a diffuse staining consistent with detection of fibers in the cytoplasm of permeabilized HT-H and SNG-M (Figure 9C and 9D) . The fibers spread from the perinuclear region toward the edge of the cells indicating that tastin likely associates with the cytoskeleton in HT-H and SNG-M cells. Thus, tastin containing fibers that associate with the cytoskeleton can be involved in organizing trophinin as patches in the plasma membranes to effect efficient cell adhesion. Antisera to GST551, GST552 and GST553, reactive with the hydrophilic domains of trophinin, were tested for staining the cell surfaces of unpermeabilized HT-H cells. For these experiments, cells were processed as for permeabilized cells except that saponin was not used. The staining pattern observed for all three antibodies was similar to that obtained when permeabilized cells were stained by antibodies to the N-terminal domain of trophinin (residue 23-31, see Figure 9A) . Similar results were obtained using SNG-M cells as the cell targets (see Figure 9B) . These results demonstrate that all three hydrophilic domains of trophinin are exposed on the cell surface of HT-H and SNG-M cells. COS-1 cells transfected with trophinin cDNA were also tested for staining with the antisera to the trophinin hydrophilic external domains. The cells showed a weak and diffuse staining on the surface with all three antisera. In contrast, COS-1 cells transfected with a mixture of trophinin and tastin cDNA showed stronger and more clustered staining with the antisera. These data indicate that tastin functions to create multivalent patches of trophinin on the cell surface. Such clustering of trophinin provides a basis for the observed requirement of COS-1 cells to be transfected with cDNA encoding for trophinin and a trophinin-assisting protein in order to undergo trophinin-mediated cell adhesion. The detection of trophinin exposed on the cell surface of cultured cells was also determined using cell surface labeling and immunoprecipitation techniques. Proteins exposed on the external side of the plasma cell membrane were labeled at their cysteine residues with biotin-maleimide (Sigma). HT-H and SNG-M cells were removed from culture flasks by scraping with a rubber policeman and washed with ice-cold PBS. 1 x 10 6 cells were suspended in 1 ml PBS and mixed with 20 μl of dimethylformamide containing 10 μg biotin-maleimide. After 1 hr on ice, the cells were washed and resuspended in 1% NP-40 nonionic detergent to produce a soluble lysate and an insoluble material. After centrifugation to remove the NP-40 soluble lysate, the insoluble material was solubilized in 0.1% SDS essentially as described previously (Oshxma et al. Dev. Biol. 99:447-455 (1983), which is incorporated herein by reference). The NP-40 soluble lysate was mixed with avidin-agarose beads (Sigma) for two hr at RT and the beads were centrifuged to yield an avidin-unbound fraction of the NP-40 soluble lysate. The beads were then washed and bound proteins eluted by boiling the beads for 2 minutes in SDS sample buffer (see Harlow and Lane, supra , 1988). Both bound and non-bound avidin fractions and the SDS soluble fraction were evaluated for their content of trophinin by immunoblotting with the antiserum to GST553 and with the antibodies to the N-terminal domain of trophinin (residue 23-31). Immunoblotting was performed by SDS-PAGE and nitrocellulose transfer essentially as described by Towbin et al. (Proc. Natl. Acad. Sci. (USA.. 76:4350-4354 (1976)) except that enhanced chemiluminescence was used for detection of immunoreactive bands (ECL kit, Amersham, Buckinghamshire, UK) . Immunoblotting of surface labeled cells showed three bands with apparent molecular masses of 90 kDa, 120 kDa and 140 kDa in HT-H cells and SNG-M cells. The majority of trophinin was detected in the avidin-bound fraction as compared to the non-bound fraction, indicating that trophinin is exposed on the external side of the cell surface. A significant amount of trophinin was insoluble in NP-40 detergent, indicating that some trophinin molecules are associated with the cytoskeleton and, therefore, that trophinin is an intrinsic plasma membrane protein. Trophinin was evaluated by indirect immunofluorescence to determine if the hydrophilic extracellular domains of trophinin could be used to detect trophinin expressing cells. The trophinin extracellular domain fusion proteins GST551, GST552 and GST553, the GST-N-terminal domain (residue 1-66) and GST were labeled with biotin succinamide (Sigma) . SNG-M, HT- H and COS-1 cells transfected with a mixture of trophinin and tastin cDNA were grown on coverslips and processed for cell staining with the biotinylated proteins essentially as was described for the antibodies to the external domains of trophinin, except that avidin-FITC (Cappel) was used in place of a FITC-secondary antibody. All three biotinylated trophinin extracellular domain fusion proteins stained unpermeabilized HT-H, SNG- M and the COS-1 cells transfected with trophinin and tastin cDNA. In contrast, no staining was seen when the cells were reacted with biotinylated GST or the biotinylated GST-N-terminal domain of trophinin. These results indicate that the soluble trophinin domains can bind trophinin exposed on the surface of the cells and, therefore, that the trophinin cell surface hydrophilic domains can be used to detect trophinin expressing cells. B. Detection of Trophinin and Tastin by Northern Blotting Total RNA was isolated from HT-H cells, SNG-M cells and COS-1 cells by the acid- guanidine:phenol:chloroform method (Chirgwin et al., Biochem. 18:5294-5299 (1979), which is incorporated herein by reference). Poly-A mRNA was prepared using oligo dT cellulose affinity chromatography (poly A Quick, Strategene). Five μg of poly-A RNA was electrophoresed in a 1% agarose formaldehyde gel and the RNA was transferred by blotting to nitrocellulose filter paper. The filter paper was heated at 80° C for 2 hr to fix the RNA and was prehybridized and hybridized as described by Thomas (Thomas, Proc. Natl. Acad. Sci. (USA. 77:5201- 5205, (1980), which is incorporated herein by reference). cDNA clones were labeled with 32 P-α-dCTP (DuPont/NEN, Boston MA) using a random oligo labeling kit (Boehringer- Mannheim, Indianapolis IN) . Northern blotting was also performed using MTN-1 filters containing human tissue RNA (Clontech, Palo Alto, CA) prepared as described above. A 3 P-cDNA probe for trophinin detected a 3.5, 7.5 and 10 kb mRNA species from both HT-H and SNG-M cells which were not detectable from COS-1 cells (not shown). A 32 P-cDNA probe for tastin detected a 3.2 and 3.3 kb mRNA species from both HT-H and SNG-M cells (not shown) . The probes also detected the appropriate sized mRNA species in the poly-A mRNA from placenta, lung and liver, but at lower levels than that seen in the cell lines. Poly-A mRNA from heart, brain, muscle, kidney and pancreas failed to react with either the trophinin or tastin probes. C. Expression and Localization of Trophinin and Tastin in Human Placental and Endometrial Tissues. Tissues embedded in paraffin were obtained from the University of California at San Diego Tissue Bank. These tissues included placenta, endometrial, liver, lung, kidney, ovary, spleen, colon, testes, brain, and spinal cord. Paraffin embedded tissue sections (0.5 or 3 μM thick) were deparaffinized and microwaved in 10 mM citrate buffer, pH 6.0, in order to recover antigenic activities (Shi et al., J. Histochem. Cγtochem. 39:741- 748 (1991), which is incorporated herein by reference). The sections were stained with anti-trophinin or anti- trophinin-assisting protein antibodies and FITC goat anti-rabbit antibodies according to methods described above (see Example VI, subsection A). Mouse anti-lamp-1 antibodies were used to detect endosomes and lysosomes (Fukuda, supra , 1991). Double immunostaining for lamp-1 and trophinin was performed in de-paraffinized and microwaved sections using the following sequence of reagents (Williams and Fukuda, J. Cell Biol. 111:955-966 (1990) which is incorporated herein by reference): 1) anti-lamp-1 antibody, 2) rhodamine conjugated goat anti- mouse IgG antibody (Sigma, St Louis, MO), 3) anti- trophinin antibody and 4) FITC goat anti-rabbit IgG antibody (Cappel, Durham, NC). Anti-trophinin and anti-tastin antibodies failed to stain cells in placenta, liver, lung, kidney, ovary, spleen, colon, testes, brain, and spinal cord, except for macrophages present in the tissues (not shown) . Trophinin was not detected in term placental tissues or in placental tissues from early (7-10 week) pregnancy (except for macrophages) , whereas trophinin was readily detected in focal regions in the apical plasma membranes of syncytiotrophoblasts of chorionic villi at 7 weeks pregnancy (Figure 10A) . Trophinin also was present in cytoplasmic vesicles of syncytiotrophoblasts in the chorionic villi from 5-10 week pregnancy (Figure 10B) . Double immunostaining with the lamp-1 lysosome marker showed co-localization of trophinin and lamp-1 in these vesicles, indicating that trophinins are present in lysosomes and/or endosomes (not shown) . These observations indicate that trophinin expression is strictly regulated and appears on the surface membranes of syncytiotrophoblasts at early stages of pregnancy but not at later stages of pregnancy. Trophinins seen in lysosomes of syncytiotrophoblasts at later stages of pregnancy can be undergoing degradation after being removed from the cell surface. Tastin was not detected in most of the chorionic villi from 7-10 week pregnancy, except for a weak signal in the lysosomes in syncytiotrophoblasts (not shown) . In addition to expression by the embryo, trophinin also is expressed in the uterus at the apical plasma membrane of the surface epithelium on day 16/17 endometrium (early secretory phase; Figure IOC), but not in endometrium during the proliferation stage (day 6-13) or ovulation stage (day 14). Endometrial biopsy samples taken from middle and late secretory phases (day 20-28) showed staining for trophinin only in the mucus. Similarly, tastin was detected in the trophinin- expressing endometrial epithelial cells on day 16/17 and in mucus of middle and late secretory phase endometrium but was not detected in proliferation and ovulation stage endometrial tissues. These results, like those for the embryo, demonstrate that trophinin and tastin expression is strictly regulated in endometrial tissue, with trophinin and tastin appearing for only a short time on the cell surface. Thus, these both trophinin and tastin are involved in embryo implantation as their pattern of expression is consistent with the concept of an implantation window. Further evidence that trophinin is involved in implantation comes for immunofluorescence analysis of a blastocyst taken from a Rhesus monkey. After removal of the exterior Zona pellucida , the expanded blastocyst showed strong staining at the apical plasma membranes of the trophectoderm cells. More intense staining for trophinin was observed on trophoblast cells located at the embryonic pole as opposed to the mural pole (see Figure 11A and 11B) . Such polarized staining is consistent with the observation that the embryonic pole of both primate and human blastocysts is the site of attachment to the endometrial epithelium (Enders et al, supra (1981); Knoth and Larson, supra (1972) and Lindenberg et al, supra (1986)). Trophinin was also detected both in trophoblasts and endometrial epithelial cells at the implantation site of a Macaque monkey (see Figure 11C and 11D) . Trophinin positive cells were seen among those anchoring villi and cytotrophoblasts of the blastocyst and in plaque cells or hypertrophic endometrial epithelium (not shown). As shown in Figure 11D, the most intense staining for trophinin was observed among trophoblast and endometrial epithelial cells located at the site of adhesion between these two tissues. These results with non-human prxmate embryos together with the studies on human endometrial and implantation site tissues provide strong support for the conservation of trophinin as a mediator of implantation among all primates. EXAMPLE VII USE OF TROPHININ TO MEDIATE ADHESION BETWEEN CELLS This example provides methods to adhere cells together using trophinin. COS-1 cells were transfected with a mixture of trophinin and tastin cDNA and evaluated for cell adhesion capability. Transfected cells that were suspended in HBS with 1 mM EDTA and maintained at RT formed distinct cell aggregates after about 10-20 min, while untransfected COS-1 cells formed little if any aggregates under the same conditions. Thus, the expression of both trophinin and tastin in COS-1 cells provided these cells with the ability to aggregate together in suspension. The ability of various cells to adhere to a monolayer of COS-1 cells transfected with trophinin and tastin cDNA was evaluated in the adhesion cell assay in the presence of 1 mM EDTA. COS-1 cells transfected with trophinin and tastin cDNA bound the monolayer while COS-1 cells transfected with the control pcDNAI vector failed to show significant binding. When the monolayer was pretreated for 1 hr at RT with antisera to GST551, GST552 or GST552 trophinin external domain fusion proteins, the ability of the monolayer to adhere to COS-1 cells transfected with trophinin and tastin cDNA was greatly diminished. These results indicate that transfection with both trophinin and tastin, a trophinin-assisting protein, can confer the property of undergoing adhesion mediated by trophinin. The inhibition of cell adhesion by antibodies to the hydrophilic external domains of trophinin confirms the role of such domains in trophinin- mediated cell adhesion. Trophinin-mediated cell adhesion between HT-H suspension cells and a monolayer of SNG-M cells and between SNG-M cells and a monolayer of SNG-M cells was also tested for inhibition by antibodies to trophinin. In both cases, pretreatment of the monolayer with antiserum to GST553 (Figure ID) or with Fab' fragments of antibodies to GST553 significantly inhibited the amount of cell adhesion. Similar results were obtained when SNG-M cells were added to an SNG-M cell monolayer. In contrast to these results, pretreatment of the SNG-M cell monolayer with preimmune rabbit sera or with antibodies to a synthetic peptide of the amino terminal region of trophinin (residues 23-31) failed to inhibit adhesion of SNG-M or HT-H cells. These experiments provide further evidence for the role of the external hydrophilic domains of trophinin in trophinin-mediated cell adhesion. Although the invention has been described with reference to the examples provided above, it should be understood that various modifications can be made without departing from the spirit of the invention. Accordingly, the invention is limited only by the claims.

Claims: I Claim: 1. Substantially purified mammalian trophinin or an active fragment thereof. 2. The mammalian trophinin of claim 1, wherein said trophinin is human trophinin. 3. The human trophinin of claim 2, having the amino acid sequence shown as SEQ ID NO: 2 in Figure 3. 4. The active fragment of claim 2, having the amino acid sequence Phe-Glu-Ile-Glu-Ala-Arg-Ala-Gln-Glu (SEQ ID NO: 10) . 5. The active fragment of claim 2, having an amino acid sequence selected from the group consisting of: residues 278 to 364 (SEQ ID NO: 20; Figure 3); residues 441 to 512 (SEQ ID NO: 21; Figure 3); and residues 634 to 719 (SEQ ID NO: 22; Figure 3). 6. A trophinin antagonist, which can reduce or inhibit trophinin-mediated cell adhesion. 7. The antagonist of claim 6, wherein said antagonist can specifically bind to trophinin. 8. An agent that can specifically bind to the trophinin of claim 1. 9. The agent of claim 8, wherein said agent is trophinin or an active fragment thereof. 10. The agent of claim 8, wherein said agent is an anti-trophinin antibody. 11. A probe, comprising the agent of claim 8 and a detectable label. 12. A trophinin agonist, which can increase trophinin-mediated cell adhesion. 13. A substantially purified nucleic acid molecule encoding a mammalian trophinin or an active fragment thereof. 14. The nucleic acid molecule of claim 13, wherein said mammalian trophinin is a human trophinin. 15. The nucleic acid molecule of claim 14, encoding the amino acid sequence shown as SEQ ID NO: 2 in Figure 3. 16. The nucleic acid molecule of claim 14, having the nucleotide sequence sequence shown as SEQ ID NO: 1 in Figure 3. 17. A vector, comprising the nucleic acid molecule of claim 13. 18. A host cell, containing the vector of claim 17. 19. A nucleotide sequence, comprising at least 10 nucleotides that hybridize under relatively stringent conditions to a portion of the nucleic acid molecule of claim 13 or a complementary molecule thereof. 20. A probe for detecting a nucleic acid molecule, comprising the nucleotide sequence of claim 19 and a detectable label. 21. The nucleic acid molecule of claim 13, encoding an active fragment of trophinin having an amino acid sequence selected from the group consisting of: residues 278 to 364 (SEQ ID NO: 20; Figure 3); residues 441 to 512 (SEQ ID NO: 21; Figure 3); residues 634 to 719 (SEQ ID NO: 22; Figure 3); and Phe-Glu-Ile-Glu-Ala-Arg-Ala-Gln-Glu (SEQ ID NO: 10) 22. Substantially purified mammalian trophinin-assisting protein or an active fragment thereof. 23. The mammalian trophinin-assisting protein of claim 22, wherein said trophinin-assisting protein is a human trophinin-assisting protein. 24. The human trophinin-assisting protein of claim 23, which is tastin, having the amino acid sequence shown as SEQ ID NO: 5 in Figure 6. 25. The active fragment of claim 23, having the amino acid sequence Asp-Gln-Glu-Asn-Gln-Asp-Pro-Arg- Arg (SEQ ID NO: 11) . 26. The human trophinin-assisting protein of claim 23, which is bystin, having an amino acid sequence shown as SEQ ID NO: 7 in Figure 7. 27. The human trophinin-assisting protein of claim 23, which is lastin, having an amino acid sequence, a portion of which, is the amino acid sequence shown as SEQ ID NO: 9 in Figure 8. 28. An agent that can specifically bind to the trophinin-assisting protein of claim 23. 29. The agent of claim 28, wherein said agent is trophinin or an active fragment thereof. 30. A probe, comprising the agent of claim 28 and a detectable label. 31. The agent of claim 28, wherein said agent is an anti-trophinin-assisting protein antibody. 32. The antibody of claim 31, wherein said trophinin-assisting protein is tastin. 33. The antibody of claim 31, wherein said trophinin-assisting protein is bystin. 34. The antibody of claim 31, wherein said trophinin-assisting protein is lastin. 35. A substantially purified nucleic acid molecule encoding a mammalian trophinin-assisting protein or an active fragment thereof. 36. The nucleic acid molecule of claim 35, wherein said mammalian trophinin-assisting protein is a human trophinin-assisting protein. 37. The nucleic acid molecule of claim 36, which encodes the amino acid sequence shown as SEQ ID NO: 5 in Figure 6. 38. The nucleic acid molecule of claim 36, having the nucleotide sequence shown as SEQ ID NO: 4 in Figure 6. 39. The nucleic acid molecule of claim 36, which encodes the amino acid sequence shown as SEQ ID NO: 7 in Figure 7. 40. The nucleic acid molecule of claim 36, having the nucleotide sequence shown as SEQ ID NO: 6 in Figure 7. 41. The nucleic acid molecule of claim 36, which encodes an amino acid sequence that includes the amino acid sequence shown as SEQ ID NO: 9 in Figure 8. 42. The nucleic acid molecule of claim 36, having a nucleotide sequence that includes the nucleotide sequence shown as SEQ ID NO: 8 in Figure 8. 43. A vector, comprising the nucleic acid molecule of claim 35. 44. A host cell, containing the vector of claim 43 45. A nucleotide sequence, comprising at least 10 nucleotides which hybridize under relatively stringent conditions to a portion of the nucleic acid molecule of claim 35or a complementary molecule thereof. 46. The nucleotide sequence of claim 45 wherein said trophinin-assisting protein is selected from the group consisting of tastin, bystin and lastin. 47. A probe, comprising the nucleotide sequence of claim 45 and a detectable label. 48. A method to detect the presence of trophinin in a sample comprising the steps of: a. obtaining the sample; b. contacting said sample with the agent of claim 8 under suitable conditions, which allow specific binding of said agent to trophinin; and c. detecting specific binding of said agent, which indicates the presence of trophinin. 49. A method to detect the presence of a nucleic acid molecule encoding trophinin in a sample, comprising the steps of: a. obtaining the sample; b. contacting said sample with the nucleotide sequence of claim 19, wherein said contact is under suitable conditions, which allow specific binding of said nucleotide sequence to said nucleic acid molecule encoding trophinin; and c. detecting said specifically bound nucleotide sequence, which indicates the presence of a nucleic acid molecule encoding trophinin. 50. The method of claim 49, wherein said nucleic acid molecule encoding trophinin is RNA. 51. The method of claim 49, wherein said nucleic acid molecule encoding trophinin is DNA. 52. A method to detect the presence of a trophinin-assisting protein in a sample, comprising the steps of: a. obtaining the sample; b. contacting said sample with the agent of claim 28, wherein said contacting is under suitable conditions, which allow specific binding of said agent to the trophinin-assisting protein; and c. detecting specific binding of said agent, which indicates the presence of trophinin- assisting protein. 53. A method to detect the presence of a nucleic acid molecule encoding a trophinin-assisting protein in a sample, comprising the steps of: a. obtaining the sample; b. contacting said sample with the nucleotide sequence of claim 45, under suitable conditions, which allow specific binding of said nucleotide sequence with the nucleic acid molecule; and c. detecting said specifically bound nucleotide sequence, which indicates the presence of a nucleic acid molecule encoding a trophinin-assisting protein. 54. The method of claim 53, wherein said nucleic acid molecule encoding a trophinin-assisting protein is RNA. 55. The method of claim 53, wherein said nucleic acid molecule encoding a trophinin-assisting protein is DNA. 56. A method for modifying a first cell so as to allow the first cell to adhere to a second cell, comprising the steps of: a. introducing the nucleic acid molecule of claim 13 into the first cell to produce a modified first cell; and b. contacting said modified first cell with the second cell, wherein said contact results in adhesion of said first cell to said second cell. 57. The method of claim 56, wherein said second cell is modified by the introduction of the nucleic acid molecule of claim 13. 58. The method of claim 56, wherein said first cell or said second cell or both is further modified by introducing the nucleic acid molecule of claim 35 into said first cell or said second cell or both. 59. The method of claim 56, wherein said cells are the same cell type. 60. The method of claim 56, wherein at least one of said cells express a trophinin-assisting protein. 61. A method for modifying a first cell so as to allow the first cell to adhere to a second cell, comprising the steps of: a. introducing the nucleic acid molecule of claim 35 into the first cell to produce a modified first cell; and b. contacting said modified first cell with the second cell, wherein said contact results in adhesion of said first cell to said second cell. 62. A method for reducing or inhibiting trophinin-mediated cell adhesion, comprising contacting a trophinin expressing cell with an effective amount of the trophinin antagonist of claim 6. 63. The method of claim 62, wherein said trophinin expressing cell is an endometrial cell. 64. The method of claim 62, wherein said trophinin expressing cell is a trophoblast cell. 65. The method of claim 62, wherein said antagonist is an antibody. 66. A method for reducing or inhibiting trophinin-mediated cell adhesion, comprising introducing into a trophinin-expressing cell, a nucleic acid molecule having the nucleotide sequence of claim 19. 67. A method for increasing trophinin-mediated cell adhesion, comprising contacting a cell with an effective amount of the trophinin agonist of claim 12. 68. The method of claim 67, wherein said cell is an endometrial cell. 69. The method of claim 67, wherein said cell is a trophoblast cell. 70. The method of claim 67, wherein said cell expresses a trophinin-assisting protein. 71. A method of inhibiting or terminating embryo implantation in a subject, comprising administering an effective amount of the trophinin antagonist of claim 6. 72. A method of increasing embryo implantation in a subject, comprising administering an effective amount of the trophinin agonist of claim 12 to the subject. 73. A method for determining the optimal time for introducing an in vitro-fertilized embryo into a uterus of a mammal to achieve implantation, comprising detecting the level of trophinin expressed in a sample taken from the mammal. 74. A method for diagnosing infertility in a mammal due to a lack of adhesion between the embryo and the uterus, comprising detecting the level of trophinin expressed in a sample taken from the mammal.

Inventors: FUKUDA MICHIKO N, FUKUDA, MICHIKO, N.
Applicants: JOLLA CANCER RES FOUND, LA JOLLA CANCER RESEARCH FOUNDATION
Application year: 1995
Main classification: A61K38
IPC codes: A61K38, A61K39, A61K45, A61P13, A61P15, C07K7, C07K14, C07K16, C12N5, C12N15, C12P21, C12Q1, C12R1, G01N33