MORPHOGENIC PROTEIN-SPECIFIC CELL SURFACE RECEPTORS AND USES THEREFOR

Abstract: Disclosed are: (1) nucleic acid sequences, amino acid sequences, homologies, structural features and various other data characterizing a morphogen cell surface receptors particularly OP-1-binding cell surface receptors; (2) methods for producing receptor proteins, including fragments thereof, using recombinant DNA technology; (3) methods for identifying novel morphogen receptors and their encoding DNAs; (4) methods and compositions for identifying compounds capable of modulating endogenous morphogen receptor levels; and (5) methods and compositions for identifying morphogen receptor binding analogs useful in the design of morphogen agonists and antagonists for therapeutic, diagnostic and experimental uses.

Description: "MORPHOGENIC PROTEIN-SPECIFIC CELL SURFACE RECEPTORS AND USES THEREFOR" Field of the Invention This invention relates generally to the field of tissue morphogenesis and more particularly to morphogenic protein-specific cell surface receptors. Background of the Invention. Cell differentiation is the central characteristic of tissue morphogenesis which initiates during embryogenesis, and continues to various degrees throughout the life of an organism in adult tissue repair and regeneration mechanisms. The degree of morphogenesis in adult tissue varies among different tissues and is related, among other things, to the degree of cell turnover in a given tissue. The cellular and molecular events which govern the stimulus for differentiation of cells is an area of intensive research. In the medical and veterinary fields, it is anticipated that the discovery of the factor or factors which control cell differentiation and tissue morphogenesis will advance significantly medicine's ability to repair and regenerate diseased or damaged mammalian tissues and organs. Particularly useful areas for human and veterinary therapeutics include reconstructive surgery and in the treatment of tissue degenerative diseases including arthritis, emphysema, osteoporosis, cardiomyopathy, cirrhosis, degenerative nerve diseases, inflammatory diseases, and cancer, and in the regeneration of tissues, organs and limbs. (In this and related applications, the terms "morphogenetic" and "morphogenic" are used interchangeably. ) A number of different factors have been isolated in recent years which appear to play a role in cell differentiation. Recently, a distinct subfamily of the "superfamily" of structurally related proteins referred to in the art as the "transforming growth factor-b (TGF-β) superfamily of proteins" have been identified as true tissue morphogens. The members of this distinct "subfamily" of true tissue morphogenic proteins share substantial amino acid sequence homology within their morphogenetically active C-terminal domains (at least 50% identity in the C- terminal 102 amino acid sequence) , including a conserved six or seven cysteine skeleton, and share the in vivo activity of inducing tissue-specific morphogenesis in a variety of organs and tissues. The proteins apparently contact and interact with progenitor cells e.g., by binding suitable cell surface molecules, predisposing or otherwise stimulating the cells to proliferate and differentiate in a morphogenetically permissive environment. These morphogenic proteins are capable of inducing the developmental cascade of cellular and molecular events that culminate in the formation of new organ- specific tissue, including any vascularization, connective tissue formation, and nerve innervation as required by the naturally occurring tissue. The proteins have been shown to induce morphogenesis of both bone cartilage and bone, as well as periodontal tissues, dentin, liver, and neural tissue, including retinal tissue. The true tissue morphogenic proteins identified to date include proteins originally identified as bone inductive proteins. These include OP- 1, (osteogenic protein-1, also referred to in related applications as "OP1"), its Drosophila homolog, 60A, with which it shares 69% identity in the C- terrαinal "seven cysteine" domain, and the related proteins OP-2 (also referred to in related applications as "0P2") and 0P-3, both of which share approximately 70-75% identity with OP-1 in the C-terminal seven cysteine domain, as well as BMP5, BMP6 and its murine homolog, Vgr-1, all of which share greater than 85% identity with OP-1 in the C-terminal seven cysteine domain, and the BMP6 Xenopus homolog, Vgl, which shares approximately 57% identity with OP-1 in the C-terminal seven cysteine domain. Other bone inductive proteins include the CBMP2 proteins (also referred to in the art as BMP2 and B P4) and their Drosophila homolog, DPP. Another tissue morphogenic protein is GDF-1 (from mouse) . See, for example, PCT documents US92/01968 and US92/07358, the disclosures of which are incorporated herein by reference. As stated above, these true tissue morphogenic proteins are recognized in the art as a distinct subfamily of proteins different from other members of the TGF-β superfamily in that they share a high degree of sequence identity in the C-terminal domain and in that the true tissue morphogenic proteins are able to induce, on their own, the full cascade of events that result in formation of functional tissue rather than merely inducing formation of fibrotic (scar) tissue. Specifically, members of the family of morphogenic proteins are capable of all of the following in a morphogenetically permissive environment: stimulating cell proliferation and cell differentiation, and supporting the growth and maintenance of di ferentiated cells. The morphogenic proteins apparently may act as endocrine, paracrine or autocrine factors. The morphogenic proteins are capable of significant species "crosstalk." That is, xenogenic (foreign species) homologs of these proteins can substitute for one another in functional activity. For example, DPP and 60A, two Drosophila proteins, can substitute for their mammalian homologs, BMP2/4 and OP-1, respectively, and induce endochondral bone formation at a non-bony site in a standard rat bone formation assay. Similarly, BMP2 has been shown to rescue a dpp~ mutation in Drosophila. In their native form, however, the proteins appear to be tissue-specific, each protein typically being expressed in or provided to one or only a few tissues or, alternatively, expressed only at particular times during development. For example, GDF-1 appears to be expressed primarily in neural tissue, while 0P-2 appears to be expressed at relatively high levels in early (e.g., 8-day) mouse embryos. The endogenous morphogens may be synthesized by the cells on which they act, by neighboring cells, or by cells of a distant tissue, the secreted protein being transported to the cells to be acted on. A particularly potent tissue morphogenic protein is OP-1. This protein, and its xenogenic homologs, are expressed in a number of tissues, primarily in tissues of urogenital origin, as well as in bone, mammary and salivary gland tissue, reproductive tissues, and gastrointestinal tract tissue. It is also expressed in different tissues during embryogenesis, its presence coincident with the onset of morphogenesis of that tissue. The morphogenic protein signal transduction across a cell membrane appears to occur as a result of specific binding interaction with one or more cell surface receptors. Recent studies on cell surface receptor binding of various members of the TGF-β protein superfamily suggests that the ligands can mediate their activity by interaction with two different receptors, referred to as Type I and Type II receptors to form a hetero-complex. A cell surface bound beta-glycan also may enhance the binding interaction. The Type I and Type II receptors are both serine /threonine kinases, and share similar structures: an intracellular domain that consists essentially of the kinase, a short, extended hydrophobic sequence sufficient to span the membrane one time, and an extracellular domain characterized by a high concentration of conserved cysteineε. A number of Type II receptor sequences recently have been identified. These include "TGF-βR II", a TGF-β Type II receptor (Lin et al. (1992) Cell 8:775-785); and numerous activin-binding receptors. See, for example, Mathews et al. (1991) Cell 5:973-982 and international patent application WO 92/20793, published November 26, 1992, disclosing the "ActR II" sequence; Attisano et al. , (1992) Cell 8:97-108, disclosing the "ActR-IIB" sequence; and Legerski et al. (1992) Biochem Biophys. Res. Commun 183:672-679. A different Type II receptor shown to have affinity for activin is Atr-II (Childs et al. (1993) PNAS 90:9475-9479.) Two Type II receptors have been identified in C. elegans, the daf-1 gene, (Georgi et al. (1990) Cell 61:635- 645) , having no known ligand to date, and daf-4, which has been shown to bind BMP4, but not activin or TGF-β (Estevez, et al. (1993) Nature 365:644-649. ) Ten Dijke et al. disclose the cloning of six different Type I cell surface receptors from murine and human cDNA libraries. ((1993) Oncogene j3:2879-2887, and Science (1994) 264:101-104. These receptors, referenced as ALK-1 to ALK-6 ("activin receptor-like kinases"), share significant sequence identities (60-79%) and several have been identified as TGF-β binding (ALK-5) or activin binding (ALK-2, ALK-4) receptors. Xie et al. also report a Drosophila Type I receptor encoded by the sax gene (Science (1994) 263:1756- 1759) . The authors suggest that the protein binds DPP. .To date, the Type I receptors with which the morphogenic proteins described herein interact on the cell surface have not yet been identified, and no Type II receptor has been described as having binding affinity for OP-1 and its related sequences. Identification of these cell surface molecules, with which the morphogens interact and through which they may mediate their biological effect, is anticipated to enhance elucidation of the molecular mechanism of tissue morphogenesis and to enable development of morphogen receptor binding "analogs", e.g., compounds (which may or may not be amino acid-based macromolecules) capable of imicing the binding affinity of a morphogen for its receptor sufficiently to act either as a receptor binding agonist or antagonist. These "analogs" have particular utility in therapeutic, diagnostic and experimental research applications. It is an object of this invention to provide nucleic acid molecules and amino acid sequences encoding morphogenic protein binding cell surface receptors, particularly OP-1-specific binding receptor sequences. Another object is to provide methods for identifying genes in a variety of species and/or tissues, and in a variety of nucleic acid libraries encoding morphogenic protein binding receptors, particularly receptors that bind OP-1. Still another object is to provide means for designing biosynthetic receptor- binding ligand analogs, particularly OP-1 analogs, and/or for identifying natural-occurring ligand analogs, including agonists and antagonists, using the receptor molecules described herein, and analogs thereof. Another object is to provide antagonists, including soluble receptor constructs comprising the extracellular ligand-binding domain, which can modulate the availability of OPl for receptor binding in vivo. Another obect is to provide means and compositions for competing with activin-receptor and BMP2/4-receptor interactions. Yet another object is to provide means and compositions for ligand affinity purification and for diagnostic detection and quantification of ligands in a body fluid using OPl-specific cell surface receptors and ligand-binding fragments thereof. Still another object is to provides means and compositions for modulating the endogenous expression or concentration of these receptor molecules. Yet another object is to provide ligand-receptor complexes and analog sequences thereof, as well as antibodies capable of identifying and distinguishing the complex from its component proteins. Still another object is to provide means and compositions for modulating a morphogenesis in a mammal. These and other objects and features of the invention will be apparent from the description, drawings and claims which follow. Summary of the Invention Type I and Type II cell surface receptor molecules capable of specific binding affinity with true tissue morphogenic proteins, particularly 0P-1- related proteins, now have been identified. Accordingly, the invention provides ligand-receptor complexes comprising at least the ligand binding domain of these receptors and OP-1 or an OP-1 receptor-binding analog as the ligand; means for identifying and/or designing useful OP-1 receptor-binding analogs and OP-1-binding- receptor analogs; and means for modulating the tissue morphogenesis capability of a cell. The morphogen cell surface receptors useful in this invention are referred to in the art as Type I or Type II serine/threonine kinase receptors. They share a conserved structure, including an extracellular, ligand-binding domain generally composed of about 100-130 amino acids (Type I receptors; up to about 196 amino acids for Type II receptors) , a transmembrane domain sufficient to span a cellular membrane one time, and an intracellular (cytoplasmic) domain having serine/threonine kinase activity. The intact receptor is a single polypeptide chain of about 500-550 amino acids and having an apparent molecular weight of about 50-55 kDa. Of particular utility in the methods and compositions of the invention are the Type I cell surface receptors referenced herein and in the literature as, ALK-2, ALK-3 and ALK-6, whose nucleic acids and encoded amino acid sequences are represented by the sequences in Seq. ID Nos. 3, 5 and 7 respectively, and which, as demonstrated herein below, have specific binding affinity for 0P1 and OPl-related analogs. Accordingly, in one embodiment, the receptor sequences contemplated herein include OP-1 binding analogs of the ALK-2, ALK-3 and ALK-6 proteins described herein. As used herein, ligand-receptor binding specificity is understood to mean a specific, saturable noncovalent interaction between the ligand and the receptor, and which is subject to competitive inhibition by a suitable competitor molecule. Preferred binding affinities (defined as the amount of ligand required to fill one-half (50%) of available receptor binding sites) are described herein by dissociation constant (Kd) . In one embodiment, preferred binding affinities of the ligand-receptor complexes described herein have a Kd of less than 10 ~ 'M, preferably less than 5xlO ~ 'M, more preferably less than 10~°M. In another preferred embodiment, the receptor molecules have little or no substantial binding affinity for TGF-β. As used herein, an "OPl-specific receptor analog" is understood to mean a sequence variant of the ALK-2, ALK-3 or ALK-6 sequences which shares at least 40%, preferably at least 45%, and most preferably at least 50%, amino acid identity in the extracellular ligand binding domain with the sequence defined by residues 23-122 of Seq. ID No. 7 (ALK-6) , and which has substantially the same binding affinity for 0P1 as ALK-2, ALK-3 or ALK-6. ALK-6 and ALK-3 share 46% amino acid sequence identity in their ligand binding domains. Accordingly, in one preferred embodiment, the OP1-specific receptor analogs share at least 46% amino acid sequence identity with the extracellular, ligand binding domains of ALK-6 or ALK-3. As will be appreciated by those having ordinary skill in the art, 0P1- specific receptor analogs also can have binding affinity for other, related morphogenic proteins. As used herein, an OPl-specific receptor analog is understood to have substantially the same binding affinity for OP-1 as ALK-2, ALK-3 or ALK-6 if it can be competed successfully for OP-1 binding in a standard competition assay with a known OP-1 binding receptor, e.g., with ALK- 2, ALK-3 or ALK-6. In one preferred embodiment, OPl-specific receptor analogs have a binding affinity for OP-1 defined by a dissociation constant of less than about 10~ 7 M, preferably less than about 5 x 10 "7 M or 10 -8 M. It is anticipated however, that analogs having lower binding affinities, e.g., on the order of 10 " °M also will be useful. For example, such analogs may be provided to an animal to modulate availability of serum-soluble OP1 for receptor binding in vivo■ Similarly, where tight binding interaction is desired, for example as part of a cancer therapy wherein the analog acts as a ligand-receptor antagonist, preferred binding affinities may be on the order of 5xlO "8 M. In another embodiment, the OP-1 binding receptor analogs contemplated by the invention include proteins encoded by nucleic acids which hybridize with the DNA sequence encoding the extracellular, ligand binding domain of ALK-2, ALK-3 or ALK-6 under stringent hybridization conditions, and which have substantially the same OP-1 binding affinity as ALK-2, ALK-3 or ALK-6. As used herein, stringent hybridization conditions are as defined in the art, (see, for example, Molecular Cloning: A Laboratory Manual, Maniatis et al. , eds. 2d.ed., Cold Spring Harbor Press, Cold Spring Harbor, 1989.) An exemplary set of conditions is defined as: hybridization in 40% formamide, 5 X SSPE, 5 X Denhardt's Solution, and 0.1% SDS at 37°C overnight, and washing in 0.1 X SSPE, 0.1% SDS at 50°C. In still another embodiment, the OP-1 binding receptor analogs contemplated by the invention include part or all of a serine/threonine kinase receptor encoded by a nucleic acid that can be amplified with one or more primers derived from ALK-1 (Seq. ID No. 1), ALK-2, ALK-3 or ALK-6 sequence in a standard PCR (polymeraεe chain reaction) amplification scheme. In particular, a primer or, most preferably, a pair of primers represented by any of the sequences of SEQ ID Nos. 12-15 are envisioned to be particularly useful. Use of primer pairs (e.g., SEQ. ID No. 12/15; 13/15; 14/15) are described in WO94/11502 (PCT/GB93/02367) . Useful OPl-specific receptor analogs include xenogenic (foreign species) homologs of the murine and human ALK sequences described herein, including those obtained from other mammalian species, as well as other, eukaryotic, non-mammalian xenogenic homologs. Also contemplated are biosynthetic constructs and naturally-occurring sequence variants of ALK-2, ALK-3 and ALK-6, provided these molecules, in all cases, share the appropriate identity in the ligand binding domain, and bind OP-1 specifically as defined herein. In one embodiment, sequence variants include receptor analogs which have substantially the same binding affinity for 0P1 as ALK-2, ALK-3 or ALK-6 and which are recognized by an antibody having binding specificity for ALK-2, ALK-3 or ALK-6. In another embodiment the receptors and OP-1 binding receptor analogs contemplated herein provide the means by which a morphogen, e.g., OP-1, can mediate a cellular response. In one embodiment these receptors include ALK-2, ALK-3, or ALK-6, or sequence variants or OP-1 binding analogs thereof. In another embodiment, ALK-1, including sequence variants thereof is contemplated to participate in an OP-1 mediated cellular response. OPl-specific receptor analogs may be used as OP1 antagonists. For example, a soluble form of a receptor, e.g., consisting essentially of only the extracellular ligand-binding domain, may be provided systemically to a mammal to bind to soluble ligand, effectively competing with ligand binding to a cell surface receptor, thereby modulating (reducing) the availability of free ligand in vivo for cell surface binding. The true tissue morphogenic proteins contemplated as useful receptor ligands in the invention include OP-1 and OP-1 receptor-binding analogs. As used herein, an "OP-1 analog" or "OP-1 receptor-binding analog" is understood to include all molecules able to functionally substitute for OP-1 in Type I receptor binding, e.g., are able to successfully compete with OP-1 for receptor binding in a standard competition assay. In one embodiment, useful OP-1 receptor-binding analogs include molecules whose binding affinity is defined by a dissociation constant of less than about 5x 10 ~ °M, preferably less than about lO ' ^M or 5x 10 ~ 'M. As for the OP-specific receptor analogs above, both stronger and weaker binding affinities are contemplated to be useful in particular applications. In one preferred embodiment, these receptor-binding OP-1 analogs also bind OP-1 specific Type II serine/kinase receptors. The OP-1 analogs contemplated herein, all of which mimic the binding activity of OP-1 or an OP-1-related protein sufficiently to act as a substitute for OP-1 in receptor binding, can act as OP-1 agonists, capable of mimicking OP-1 both in receptor binding and in inducing a transmembrane effect e.g., inducing threonine or serine-specific phosphorylation following binding. Alternatively, the OP-1 analog can act as an OP-1 antagonist, capable of mimicking OP-1 in receptor binding only, but unable to induce a transmembrane effect, thereby blocking the natural ligand from interacting with its receptor, for example. Useful applications for antagonists include their use as therapeutics to modulate uncontrolled differentiated tissue growth, such as occurs in malignant transformations such as in osteosarcomas or Paget' s disease. OP-1 analogs contemplated by the invention can be amino acid-based, e.g., sequence variants of OP-1, or antibody-derived sequences capable of functionally mimicking OP-1 binding to an OP-1-specific receptor. Examples of such antibodies may include anti-idiotypic antibodies. In a specific embodiment, the anti-idiotypic antibody mimics OP1 both in receptor binding and in ability to induce a transmembrane effect. Alternatively, the OP-1 analogs can be composed in part or in whole of other chemical structures, e.g., the analogs can be comprised in part or in whole of nonproteinaceouε molecules. In addition, the OP-1 analogs contemplated can be naturally sourced or synthetically produced. As used herein, OP-1 related sequences include sequences sharing at least 60%, preferably greater than 65% or even 70% identity with the C- terminal 102 amino acid sequence of OP-1 as defined in Seq ID NO.7, and which are able to substitute for OP-1 in ligand binding to ALK-2, ALK-3 or ALK-6, (e.g., able to compete successfully with OP-1 for binding to one or more of these receptors.) OP-1 related sequences contemplated by the invention include xenogenic homologs (e.g., the Drosophila homolog 60A) , and the related sequences referenced herein and in the literature as OP-2, 0P-3, BMP5, BMP6 (and its xenogenic homolog Vgr-1.) OP-1 related sequences also include sequence variants encoded by a nucleic acid which hybridizes with a DNA sequence comprising the C-terminal 102 amino acids of Seq. ID No. 9 under stringent hybridization conditions and which can substitute for 0P1 in an 0P1- receptor binding assay. In another embodiment, an OP1 sequence variant includes a protein which can substitute for 0P1 in a ligand-receptor binding assay and which is recognized by an antibody having binding specificity for OP1. As used herein, "amino acid sequence homology" is understood to mean amino acid sequence similarity, and homologous sequences sharing identical or similar amino acids, where similar amino acids are conserved amino acids as defined by Dayoff et al. , Atlas of Protein Sequence and Structure; vol.5, Suppl.3, pp.345-362 (M.O. Dayoff, ed. , Nat'1 BioMed. Research Fdn., Washington D.C. 1978.) Thus, a candidate sequence sharing 60% amino acid homology with a reference sequence requires that, following alignment of the candidate sequence with the reference sequence, 60% of the amino acids in the candidate sequence are identical to the corresponding amino acid in the reference sequence, or constitute a conserved amino acid change thereto. "Amino acid sequence identity" is understood to require identical amino acids between two aligned sequences. Thus, a candidate sequence sharing 60% amino acid identity with a reference sequence requires that, following alignment of the candidate sequence with the reference sequence, 60% of the amino acids in the candidate sequence are identical to the corresponding amino acid in the reference sequence. As used herein, all receptor homologies and identities calculated use ALK-6 as the reference sequence, with the extracellular domain reference sequence constituting residues 23-122 of Seq. ID No.7; and the intracellular serine/threonine kinase domain reference sequence constituting residues 206- 495 of Seq. ID No.7. Similarly, all OP-1 related protein homologies and identities use OP-1 as the reference sequence, with the C-terminal 102 amino acids described in Seq. ID No. constituting the seven cysteine domain. Also as used herein, sequences are aligned for homology and identity calculations as follows: Sequences are aligned by eye to maximize sequence identity. Where receptor amino acid extracellular domain sequences are compared, the alignment first maximizes alignment of the cysteines present in the two sequences, then modifies the alignment as necessary to maximize amino acid identity and similarity between the two sequences. Where amino acid intracellular domain sequences are compared, sequences are aligned to maximize alignment of conserved amino acids in the kinase domain, where conserved amino acids are those identified by boxes in Fig.3. The alignment then is modified as necessary to maximize amino acid identity and similarity. In all cases, internal gaps and amino acid insertions in the candidate sequence as aligned are ignored when making the homology/identity calculation. Exemplary alignments are illustrated in Figs. 2 and 3 where the amino acid sequences for the extracellular and intracellular domains, respectively are presented in single letter format. In the figures "gaps" created by sequence alignment are indicated by dashes. In one aspect, the invention contemplates isolated ligand-receptor complexes comprising OP-1 or an OP-1 analog as the ligand in specific binding interaction with an OP-1 binding Type I receptor or receptor analog, as defined herein. In another aspect, the invention contemplates the ligand- receptor complex comprises part or all of an OP-1 binding Type II receptor. Type II receptors contemplated to be useful include Type II receptors defined in the literature (referenced hereinabove) as having binding specificity for activin or a bone morphogenic protein such as BMP-4. Such Type II receptors include daf , ActRII and Atrll. In still another aspect, the ligand-receptor complex comprises both a Type I and a Type II receptor and 0P1, or an OP1 analog as the ligand. In all complexes, the bound receptor can comprise just the extracellular, ligand binding domain, or can also include part or all of the transmembrane sequence, and/or the intracellular kinase domain. Similarly, the OP-1 ligand may comprise just the receptor binding sequence, longer sequences, including the mature dimeric species or any soluble form of the protein or protein analog. The OP-1 and OP-1 analogs described herein can interact specifically with Type I and Type II receptors also known to interact with other morphogenic proteins (e.g., BMP2/BMP4) and activin. Thus invention also contemplates the use of OP-1 and OP-1 receptor-binding analogs as competitors of specific BMP-receptor and activin-receptor interactions. As will be appreciated by those having ordinary skill in the art, these binding competitors may act as either agonists or antagonists (e.g., to inhibit an activin or BMP-mediated cellular response) . In another aspect, the invention contemplates binding partners having specific binding affinity for an epitope on the ligand-receptor complex. In a preferred embodiment, the binding partner can discriminate between the complex and the uncomplexed ligand or receptor. In another embodiment, the binding partner has little or no substantial binding affinity for the uncomplexed ligand or receptor. In another preferred embodiment, the binding partner is a binding protein, more preferably an antibody. These antibodies may be monoclonal or polyclonal, may be intact molecules or fragments thereof (e.g., Fab, Fab', (Fab)^), or may be biosynthetic derivatives, including, but not limited to, for example, monoclonal fragments, such as single chain F v fragments, referred to in the literature as εF v ε, BABs and SCAs, and chimeric monoclonalε, in which portions of the monoclonals are humanized (excluding those portions involved in antigen recognition (e.g., complementarity determining regions, "CDRε".) See, for example, U.S. Pat. Nos. 5,091,513 and 5,132,405, the disclosures of which are incorporated herein by reference. Biosynthetic chimeras, fragments and other antibody derivatives may be synthesized using standard recombinant DNA methodology and/or automated chemical nucleic acid synthesis methodology well described in the art and as described below. In still another aspect, the invention provides molecules useful in the design and/or identification of receptor-binding morphogenic protein analogs as described below, as well as kits and methods, e.g., screening assays, for identifying these analogs. The molecules useful in these asεays can include part or all of the receptor sequence of SEQ ID NO. 3, 5 or 7, including amino acid sequence variants and OP-1 binding analogs and amino acid sequence variants thereof. As described above, sequence variants are contemplated to have substantially the same binding affinity for OP-1 as the receptors represented by the sequences in SEQ. ID Nos. 3-7. OP-1 binding receptor analogs include other, known or novel Type I or Type II serine/threonine kinase receptors having binding affinity and specificity for OP-1 as defined herein and which (1) share at least 40% amino acid identity with residueε 23-122 of Seq. ID No. 7, (2) are encoded by a nucleic acid that hybridizeε under stringent conditions with a nucleic acid comprising the sequence defined by nucleotides 256-552 of Seq ID No. 7; or (3) are encoded by a nucleic acid obtainable by amplification with one or more primer sequences defined by Seq. ID Nos. 12-15. Currently preferred for the assays of the invention are receptor sequences comprising at least the sequence which defines the extracellular, ligand binding domainε of theεe proteinε. The kitε and aεεays may include just Type I receptors or both Type I and Type II receptors. Similarly, the kits and screening asεayε can be uεed in the deεign and/or identification of 0P1- specific receptor analogs. The OP-1 receptor-binding analogs and OP-1-binding receptor analogε thuε identified then can be produced in reasonable quantities using standard recombinant expreεεion or chemical εyntheεis technology well known and characterized in the art. Alternatively, promising candidates can be modified using standard biological or chemical methodologies to, for example, enhance the binding affinity of the candidate analog as described in Example 10, below, and the preferred candidate derivative then produced in quantity. In still another aspect, the receptor and/or OPl-specific receptor analogs can be uεed in standard methodologies for affinity purifying and/or quantifying 0P1 and 0P1 analogs. For example, the receptor's ligand binding domain first may be immobilized on a surface of a well or a chromatographic column; ligand in a sample fluid then may be provided to the receptor under conditions to allow specific binding; non-specific binding molecules then removed, e.g., by washing, and the bound ligand then selectively isolated and/or quantitated. Similarly, OP1 and OP1 analogs can be used for affinity purifying and/or quantifying OPl-specific receptors and receptor analogs. In one embodiment, the method is useful in kits and assays for diagnostic purpoεes which detect the presence and/or concentration of 0P1 protein or related morphogen in a body fluid sample including, without limitation, serum, peritoneal fluid, spinal fluid, and breast exudate. The kits and asεayε alεo can be used for detecting and/or quantitating OP-1-specific receptors in a sample. In still another aspect the invention comprises OPl-specific receptors and OP-1-binding receptor analogs useful in screening assays to identify organs, tissues and cell lines which express OPl-specific receptors. These cells then can be used in screening assays to identify ligands that modulate endogenous morphogen receptor expresεion levels, including the density of receptors expressed on a cell surface. Useful assay methodologies may be modeled on those described in PCT US92/07359, and as described below. The invention thus relates to compositions and methods for the use of morphogen-specific receptor sequences in diagnostic, therapeutic and experimental procedures. Active receptors useful in the compositions and methods of this invention can include truncated or full length forms, as well as forms having varying glycoεylation patterns. Active receptors useful in the invention also include chimeric constructε aε described below. Active OPl-specific receptors/analogs can be expresεed from intact or truncated genomic or cDNA, or from εynthetic DNAε in procaryotic or eucaryotic hoεt cells, and purified, cleaved, refolded and oxidized as necessary to form active molecules. Useful host cells include prokaryotes, including E_ ; _ coli and B. subtilis, and eukaryotic cellε, including mammalian cells, such as fibroblast 3T3 cellε, CHO, COS, melanoma or BSC cells, Hela and other human cells, the insect/baculovirus system, as well as yeast and other microbial host cell systems. Thus, in view of this disclosure, skilled genetic engineers now can, for example, identify and produce OPl-specific cell εurface receptorε or analogs thereof; create and perform assays for screening candidate 0P1 receptor-binding analogε and evaluate promising candidates and their progency in therapeutic regimes and preclinical studies; modulate the availability of endogenous morphogen for cell surface interactions; modulate endogenous morphogen-specific cell surface receptor levels; elucidate the signal transduction pathway induced by morphogen-cell εurface receptor binding; and modulate tiεεue morphogeneεiε in vivo. Brief Description of the Drawings Figure 1 is a schematic representation of the encoded ALK-2, ALK-3, ALK-6 amino acid sequences, showing the signal sequence 10, the transmembrane domain 12, the extracellular ligand binding domain 14, and the intracellular serine/threonine kinase domain 16; Figure 2 is a homology alignment of the extracellular domains of ALK-2, ALK-3, and ALK-6, aligned to maximize amino acid identity, wherein conserved amino acids are identified by boxes and conserved cysteines are indicated by asterisks; and Figure 3 is a homology alignment of the intracellular domain of ALK-2, ALK-3 and ALK-6, aligned to maximize amino acid identity, wherein conserved amino acids are boxed and the serine/threonine kinase domain iε indicated by arrows. Detailed Deεcription Diεcloεed herein are Type I and Type II receptors having binding specificity for true tissue morphogenic proteins, particularly OP1 and OP1- related proteins. It further has been determined that 0P1 binds to a broader range of receptors than other known tissue morphogens or TGF-β family members. The Type I receptors disclosed herein, can be used together with 0P1 and 0P1 analogs for therapeutic, diagnostic and experimental uses as described herein below. Moreover, soluble forms of the OPl-binding receptor proteins, e.g., forms consisting essentially of the extracellular domain or a fragment thereof sufficient to bind 0P1 with specificity, can be used as a soluble therapeutic morphogen antagonist, as described below. Following this disclosure, related OPl-specific receptors are available, as are high and medium flux εcreening assays for identifying OP1 analogs and OPl-specific receptor analogs. These analogs can be naturally occurring molecules, or they can be designed and biosynthetically created using a rational drug design and an establiεhed structure/function analysis. The analogs can be amino acid-based or can be composed in part or whole of non-proteinaceous synthetic organic molecules. Useful analogs also can include antibodies, preferably monoclonal antibodies (including fragments thereof, e.g., Fab, Fab', and (Fab)' ), or synthetic derivatives thereof, such as monoclonal single chain F v fragments known in the art as sF v s, BABs, and SCAε (see below), and bispecific antibodies or derivativeε thereof. When these antibodies mimic the binding activity of OP-1 to a cell surface receptor without inducing the biological response OP-1 does upon binding, the antibody can compete for OP-1 binding and act as an antagonist. These antibodies or derivatives thereof also can mimic OP-1 both in receptor binding and signal tranεduction, in which case the antibody acts as an OP-1 agonist. The antibodies and derivatives also can be used for inducing the morphogenic cellular responεe by crosslinking receptors to morphogenic proteins, particularly 0P1 and OPl-related proteins to form either homo- or hetero- complexes of the Type I and Type II receptors. The OPl-binding receptor sequences described herein (ALK2, ALK3, ALK6) also can be used to create chimeric sequences, wherein, for example, part or all of either the extracellular domain or the intracellular domain is a non- ALK sequence or is created from two or more ALK sequences. These chimeric receptors can be synthesized using standard recombinant DNA methodology and/or automated chemical nucleic acid synthesis methodology well described in the art and as disclosed below. Chimerics can be used, for example, in OPl analog assays, wherein the OPl-binding extracellular domain is coupled to a non-ALK intracellular domain that is well characterized and/or readily detectable as a second messenger response system, as described below. Chimerics also can be used, for example, in high flux OPl analogs screens and as part of purification protocols, wherein a soluble ligand binding domain of an OPl-specific receptor is immobilized onto a support e.g., by covalent or non-covalent interactions, with a chromatographic matrix or the well surface of a 96-well plate. When immobilized onto a chromatographic matrix surface. the receptor fragment can be used in a protocol to isolate OPl or OPl analogs. When immobilized on a well surface the receptor fragment is particularly useful in a screening assay to identify receptor-binding OPl analogs in a standard competition assay. The true tissue morphogenic proteins contemplated to be useful in the methods and compositions of the invention include forms having varying glycosylation patterns and varying N-termini. The proteins can be naturally occurring or bioεynthetically derived, and can be produced by expreεεion of recombinant DNA in prokaryotic or eukaryotic hoεt cells. The proteins are active as a single species (e.g., as homodimers) , or combined as a mixed specieε. Useful sequences and eucaryotic and procaryotic expreεεion systems are well described in the art. See, for example, US Patent Nos. 5,061,911 and 5,266,683 for useful expression εyεtemε. Particularly contemplated herein are OPl and OPl-related sequences. Useful OPl sequences are recited in US Pat Nos. 5,011,691; 5,018,753 and 5,266,683; in Ozkaynak et al. (1990) EMBO J 9_:2085-2093; and Sampath et al. (1993) PNAS 90: 6004-6008. OP-1 related sequences include xenogenic homologs, e.g.; 60A, from Drosophila, Wharton et al. (1991) PNAS 8:9214-9218; and proteins sharing greater than 60% identity with OPl in the C-terminal seven cysteine domain, preferably at least 65% identity. Examples of OP-1 related sequences include BMP5, BMP6 (and its species homolog Vgr-1, Lyons et al. (1989) PNAS 86:4554-4558), Celeεte, et al. (1990) PNAS £7:9843-9847 and PCT international application WO93/00432; OP-2 (Ozkaynak et al. (1992) J.Biol. Chem. 2£7:13198-13205) and 0P-3 (PCT international application WO94/06447) . Aε will be appreciated by those having ordinary skill in the art, chimeric constructs readily can be created using standard molecular biology and mutagenesiε techniqueε combining various portions of different morphogenic protein sequences to create a novel sequence, and these forms of the protein also are contemplated herein. A particularly preferred embodiment of the proteins contemplated by the invention includes proteins whose amino acid sequence in the cyεteine-rich C- terminal domain haε greater than 60% identity, and preferably greater than 65% identity with the amino acid sequence of OPS (OP-1 sequence defining the C- terminal conserved six cysteines, e.g., residues 335-431 of Seq. ID No. 9) . In another preferred aspect, the invention contemplates osteogenic proteins comprising species of polypeptide chains having the generic amino acid sequence herein referred to as "OPX" which accommodates the homologies between the various identified species of the osteogenic OPl and 0P2 proteins, and which is described by the amino acid sequence presented below and in Sequence ID No. 11. Cys Xaa Xaa His Glu Leu Tyr Val Ser Phe 1 5 10 Xaa Asp Leu Gly Trp Xaa Asp Trp Xaa lie 15 20 Ala Pro Xaa Gly Tyr Xaa Ala Tyr Tyr Cys 25 30 Glu Gly Glu Cys Xaa Phe Pro Leu Xaa Ser 35 40 Xaa Met Asn Ala Thr Asn His Ala lie Xaa 45 50 Gin Xaa Leu Val His Xaa Xaa Xaa Pro Xaa 55 60 Xaa Val Pro Lys Xaa Cys Cys Ala Pro Thr 65 70 Xaa Leu Xaa Ala Xaa Ser Val Leu Tyr Xaa 75 80 Asp Xaa Ser Xaa Asn Val lie Leu Xaa Lys 85 90 Xaa Arg Asn Met Val Val Xaa Ala Cys Gly 95 100 Cys His, and wherein Xaa at res. 2 = (Lys or Arg); Xaa at res. 3 = (Lys or Arg); Xaa at res. 11 = (Arg or Gin); Xaa at res. 16 = (Gin or Leu); Xaa at res. 19 = (lie or Val); Xaa at res. 23 = (Glu or Gin); Xaa at res. 26 = (Ala or Ser); Xaa at res. 35 = (Ala or Ser); Xaa at res. 39 = (Asn or Asp); Xaa at res. 41 = (Tyr or Cys); Xaa at res. 50 = (Val or Leu); Xaa at res. 52 = (Ser or Thr); Xaa at res. 56 = (Phe or Leu); Xaa at res. 57 = (lie or Met); Xaa at res. 58 = (Asn or Lys); Xaa at res. 60 = (Glu, Asp or Asn); Xaa at res. 61 = (Thr, Ala or Val); Xaa at res. 65 = (Pro or Ala); Xaa at res. 71 = (Gin or Lys); Xaa at res. 73 = (Asn or Ser); Xaa at res. 75 = (lie or Thr); Xaa at res. 80 = (Phe or Tyr); Xaa at res. 82 = (Asp or Ser); Xaa at res. 84 = (Ser or Asn); Xaa at res. 89 = (Lys or Arg) ; Xaa at res. 91 = (Tyr or His); and Xaa at res. 97 = (Arg or Lys) . In still another preferred aspect, the invention contemplates osteogenic proteins encoded by nucleic acids which hybridize to DNA or RNA sequences encoding the C-terminal seven cysteine domain of OPl or OP2 under stringent hybridization conditions. A brief description of the various terms of OP-1 useful in the invention is described below: OPl - Refers generically to the family of osteogenically active proteins produced by expression of part or all of the hOPl gene. Also referred to in related applications as "OPl" and "OP-1". OPl-PP - Amino acid sequence of human OPl protein (prepro form) , Seq. ID No. 9, residues 1-431. Also referred to in related applications as "OPl-PP" and "OPP" . OPl-18Ser - Amino acid sequence of mature human OPl protein, Seq. ID No. 9, residues 293-431. N-terminal amino acid is serine. Originally identified as migrating at 18 kDa on SDS-PAGE in COS cells. Depending on protein glycosylation pattern in different host cells, also migrates at 23kDa, 19kDa and 17kDa on SDS-PAGE. Also referred to in related applications as "OP1-18." 0Pl-16Ser; OPl-16Ala; 0P1-16 Met; 0P1-16 leu; OPl-16Val - N-terminally truncated mature human OPl protein species defined, respectively, by residues 300-431; 316-431; 315-431; 313-431 and 318-431. OPS - Amino acid sequence defining the C-terminal six cysteine domain, residues 335-431 of Seq. ID No. 9. 0P7 - Amino acid sequence defining the C-terminal seven cysteine domain, residues 330-431 of Seq. ID No. 9. Soluble form OPl - mature dimeric OPl species having one or, preferably two copies of pro domain, e.g., at least residues 158-292 of Seq. ID No. 9, preferably residues 48-292 or 30-292, non-covalently complexed with the dimer. The cloning procedure for obtaining OPl-binding ALK nucleic acid sequences, means for expressing receptor sequences, as well as other material aspects concerning the nature and utility of these sequences, including how to make and how to use the subject matter claimed, will be further understood from the following, which constitutes the best mode currently contemplated for practicing the invention. Example 1. IDENTIFICATION OF ALK-1, ALK-2, ALK-3 and ALK-6 The cloning and characterization of ALK-1, -2, -3, and -6 receptors are described in detail in ten Dijke et al. (1993) Oncoqene 8:2879-2887; and (1994) Science 264:101-104. The general structures of these proteins is described in Fig. 1, and the sequence alignments between the ALK genes are shown in Figs 2 and 3. These molecules have similar domain structures: an N- terminal predicted hydrophobic signal sequence (von Heijne (1986) Nucl. Acids Res. 1 : 4683-4690) is followed by a relatively small extracellular cysteine- rich ligand binding domain, a single hydrophobic transmembrane region (Kyte & Doolittle (1982) J. Mol. Biol. 57, 105-132) and a C-terminal intracellular portion, which consists almost entirely of a kinase domain (Figure 3) . The extracellular domains of these receptors, defined essentially by residues 22-118 (SEQ. ID No. 1 ) for ALK-1; residues 16-123 (SEQ ID No. 3) for ALK-2; residues 24-152 (SEQ. ID No. 5) for ALK-3; and residues 23-122 (SEQ ID No. 7) for ALK-6, have cysteine-rich regions, but sequence similarity varies among the proteins. For example, ALK-3 and ALK-6 share a high degree of sequence similarity in their extracellular domains (46% identity) whereas ALK- 2 shows less similarity with ALK 3 or ALK6 (see Fig. 2.) The positions of many of the cysteine residues in these receptors can be aligned, indicating that the extracellular domains likely adopt a similar structural configuration. The intracellular domains of these receptors are characterized by a serine/threonine kinase, defined essentially by residues 204-494 (SEQ. ID. No. 1) for ALK-1; residues 210-510 (SEQ ID No. 3) for ALK-2; residues 236-527 (SEQ ID No. 5) for ALK-3; and residues 206-497 (SEQ ID No. 7) for ALK-6. The catalytic domains of kinases can be divided into 12 subdomains with stretches of conserved amino-acid residues. The key motifs are found in serine/threonine kinase receptors indicating that they are functional kinases. The consensus sequence for the binding of ATP (Gly-X-Gly-X-X-Gly in subdomain I followed by a Lys residue further downstream in subdomain II) is found in all the ALKS. Moreover, ALK-1, ALK-2, ALK-3 and ALK-6 have the sequence motifs or similar motifs HRDLKSKN (Subdomain VIB) and GTKRYMAPE (Subdomain VIII) , that are found in most of the serine/threonine kinase receptors and can be used to distinguish them from tyrosine kinase receptors. Two short inserts in the kinase domain (between subdomain VIA and VIB and between X and XI are unique to members of this serine/threonine kinase receptor family. In the intracellular domain, these regions, together with the juxtamembrane part and C-terminal tail, are the most divergent between family members. Type II serine/threonine kinase receptors known in the art are described and referenced herein above.Example 2. RECEPTOR EXPRESSION A. General Considerations Receptor DNA, or a synthetic form thereof, can be inserted, using conventional techniques well described in the art (see, for example, Maniatis (1989) Molecular Cloning A Laboratory Manual) , into any of a variety of expression vectors and transfected into an appropriate host cell to produce recombinant protein polypeptide chains, including both full length and truncated forms thereof. Shortened sequences, for example, can be used for the production of soluble receptor fragments. Useful host cells include E. coli, Saccharomyces cerevisiae, Pichia pastoris, the insect/baculovirus cell system, myeloma cells, and various other mammalian cells. The full length forms of the proteins of this invention preferably are expressed in mammalian cells, as disclosed herein. Soluble forms may be expressed from both mammalian or bacterial cell systems. The vector additionally may include various sequences to promote correct expression of the recombinant protein, including transcription promoter and termination sequences, enhancer sequences, preferred ribosome binding site sequences, preferred mRNA leader sequences, preferred protein processing sequences, preferred signal sequences for protein secretion, and the like. The DNA sequence encoding the gene of interest also may be manipulated to remove potentially inhibiting sequences or to minimize unwanted secondary structure formation. The recombinant morphogen receptor also may be expressed as a fusion protein. After translation, the protein may be purified from the cells themselves or recovered from the culture medium. The DNA also may include sequences which aid in expression and/or purification of the recombinant protein. One useful sequence for example, is a hexa-His (Hisg) sequence, which adds a histidine tail to allow affinity purification of the protein on an IMAC Cu2+ column (see below.) For example, the DNA encoding the extracellular domain may be inserted into a suitable expression vector for transformation into a prokaryote host such as E. coli or B. subtilis, to produce a soluble, morphogen binding fragment. The DNA may expressed directly or may be expressed as part of a fusion protein having a readily cleavable fusion junction. An exemplary protocol for prokaryote expression using MR-1 DNA is provided below. Recombinant protein is expressed in inclusion bodies and may be purified therefrom using the technology disclosed in U.S. Patent No. 5,013,653, for example. The DNA also may be expressed in a suitable mammalian host. Useful hosts include fibroblast 3T3 cells, (e.g., NIH 3T3, from CRL 1658) COS (simian kidney ATCC, CRL-1650) or CH0 (Chinese hamster ovary) cells (e.g., CHO-DXB11, from Lawrence Chasin, Proc. Nat'1. Acad. Sci. (1980) 2(7) :4216-4222) , mink- lung epithelial cells (MVlLu) , human foreskin fibroblast cells, human glioblastoma cells, and teratocarcinoma cells. Other useful eukaryotic cell systems include yeast cells, the insect/baculovirus system or myeloma cells. To express an OPl-specific cell surface receptor, the DNA is subcloned into an insertion site of a suitable, commercially available vector along with suitable promoter/enhancer sequences and 3' termination sequences. Useful promoter/enhancer sequence combinations include the CMV promoter (human cytomegalovirus (MIE) promoter) present, for example, on pCDM8, as well as the mammary tumor virus promoter (MMTV) boosted by the Rous sarcoma virus LTR enhancer sequence (e.g., from Clontech, Inc., Palo Alto). A useful induceable promoter includes, for example, A Zn * induceable promoter, such as the Zn 2* metallothionein promoter (Wrana et al. (1992) Cell 71:1003-1014.) Other induceable promoters are well known in the art and can be used with similar success. Expression also may be further enhanced using transactivating enhancer sequences. The plasmid also preferably contains an amplifiable marker, such as DHFR under suitable promoter control, e.g., SV40 early promoter (ATCC #37148) . Transfection, cell culturing, gene amplification and protein expression conditions are standard conditions, well known in the art, such as are described, for example in Ausubel et al. , ed.. Current Protocols in Molecular Biology, John Wiley & Sons, NY (1989) . Briefly, transfected cells are cultured in medium containing 5-10% dialyzed fetal calf serum (FCS) , and stably transfected high expression cell lines obtained by amplification and subcloning and evaluated by standard Western and Northern blot analysis. Southern blots also can be used to assess the state of integrated receptor sequences and the extent of their copy number amplification. The expressed protein then is purified using standard procedures. A currently preferred methodology uses an affinity column, such as a ligand affinity column or an antibody affinity column, the bound material then washed, and receptor molecules selectively eluted in a gradient of increasing ionic strength, changes in pH or addition of mild denaturants. Alternatively, where a useful anchor sequence has been added to the DNA, such as a (His)g sequence, the column may be a standard affinity column such as Cu 2+ IMAC column. Here, for example, the cell culture media containing the recombinant protein is passed over a Cu 2+ IMAC column (for example, prepared with 25 mM imidazole) . The bound protein then is washed with a compatible solution and eluted with EDTA. The anchor sequence can be removed by a standard chemical or enzymatic procedure. Mammalian cell expression is preferred where morphogen receptor expression on a cell surface is desired. For example, cell surface expression may be desired to test morphogen or morphogen analog binding specificity for a cell surface receptor under vivo conditions. Cell surface expression also may be most efficacious for medium flux cellular screen assays as described below. B.1 Exemplary Mammalian Cell Culture The receptors tested in Examples 8 and 9 described below were expressed in (1) COS-1 cells; (2) mink lung epithelial cells (MvlLu) ; (3) AG1518 human foreskin fibroblasts; (4) MG-63 human osteosarcoma cells; (5) PC12 rat pheochromocytoma cells (all obtained from American Type Culture Collection, Rockville, MD) ; (6) U-1240 MG human glioblastoma cells (Bengt Westermark, et al. (1988) Cancer Research 48:3910-3918); (7) Tera-2 teratocarcinoma cells (clone 13, Thompson et al. (1984) J Cell Sci 72:37-64); (8) MC3T3-E1 cells (Sudo et al. (1983) J. Cell Biol. jJ6:191-198, and (9) ROS 17/2.8 rat osteosarcoma cells (Majeska et al. (1985) Endocrinology 116:170-179. The ROS cells were cultured in Ham's F12 medium containing 14mM HEPES buffer, 2.5 mH L-glutamine, 1.1 mM CaCl , 5% fetal bovine serum and antibiotics; MC3T3-E1 cells were cultured in a-MEM with 10% fetal bovine serum and antibiotics, and Tera-2 cells were cultured in 5% C0 2 atmosphere at 37oC in a-MEM containing 10% fetal bovine serum, 100 units/ml of penicillin and 50 mg/ml of streptomycin, using tissue culture dishes pretreated with 0.1% swine skin gelatin (Sigma) in phosphate-buffered saline. Unless otherwise specified, cells were cultured in DMEM containing 10% fetal bovine serum and antibiotics. B.2 Transfection of cDNA The receptors tested in Example 8.1 (ALK 1-6, daf-4) were transfected as follows. Transient expression plasmids of ALK-1 to -6 and daf-4 were generated by subcloning into an expression vector (pSV7d, Truett et al. (1985) DNA :333-349) or into the pcDNA I expression vector (Invitrogen, San Diego) . For transient transfection, COS-1 cells were transfected with 10 mg each of plasmids by a calcium phosphate precipitation method using a mammalian transfection kit (Stratagene, La Jolla) , following the manufacturer's protocol. One day after transfection, the cells were used for the affinity labeling and cross-linking experiments. Example 3 ■ ANTIBODY PRODUCTION A. General Considerations Antibodies capable of specifically binding the receptor molecules, ligand molecules, or the ligand-receptor complex itself, useful as analogs and useful in immunoassays and in the immunopurification of morphogen receptors described may be obtained as described below. Where antibodies specific to the OPl-specific receptors are desired, but which do not interfere with ligand binding, the antigenic sequence preferably comprises the juxtamembrane sequence. Where antibodies capable of competing for ligand binding are desired, the ligand binding domain may be used as the antigen source. Where antibodies to the complex are desired, the complex itself preferably is used as the antigenic sequence and candidate antibodies then tested for cross reactivity with uncomplexed ligand and receptors versus the ligand-receptor complex. Finally, bispecific antibodies may used to complex ligand to a cell surface receptor (Type I or Type II) and/or to target an agent or ligand to cells or tissue expressing a Type I or Type II morphogen-specific receptor. Preferred bispecific antibody derived molecules are single chain binding sites described in US Pat No. 5,091,513 and 5,132,405, the disclosures of which are incorporated hereinabove by reference. Antibodies useful as OPl receptor-binding analogs may be obtained using the receptor ligand binding domain as the immunogen source and testing receptor-binding analogs for their ability to compete with OPl in a competition binding assay. Similarly, where antibodies useful as OPl-specific receptor analogs are desired, OPl is the immunogen source and the antibody candidate tested in a competition assay with receptor protein. Polyclonal antibodies specific for a morphogen receptor of interest may be prepared generally as described below. Each rabbit is given a primary immunization (e.g., 500mg) of antigen in 0.1% SDS mixed with 500 ml Complete Freund's Adjuvant. The antigen is injected intradermally at multiple sites on the back and flanks of the animal. The rabbit is boosted after a month with 500mg of antigen in the same manner using incomplete Freund's Adjuvant. Test bleeds are taken from the ear vein seven days later. Two additional boosts and test bleeds are performed at monthly intervals until antibody against the antigenic sequence is detected in the serum using a standard Western blot. Then, the rabbit is boosted monthly with 100 mg/ml of antigen and bled (15 ml per bleed) at days seven and ten after boosting. Similarly, monoclonal antibodies specific for a given morphogen receptor molecule of interest may be prepared as described below: A mouse is given two injections of the antigenic sequence. The protein preferably is recombinantly produced. Where it is desired that the antibody recognize an epitope on the morphogen binding surface of a receptor an antigenic fragment derived from the extracellular domain preferably is provided. The first injection contains lOOmg of antigen in complete Freund's adjuvant and is given subcutaneously. The second injection contains 50 mg of antigen in incomplete adjuvant and is given intraperitoneally. The mouse then receives a total of 230 mg of antigen in four intraperitoneal injections at various times over an eight month period. One week prior to fusion, the mouse is boosted intraperitoneally with antigen (e.g., 100 mg) and may be additionally boosted with an antigen-specific peptide conjugated to bovine serum albumin with a suitable crosslinking agent. This boost can be repeated five days (IP), four days (IP) , three days (IP) and one day (IV) prior to fusion. The mouse spleen cells then are fused to commercially available myeloma cells at a ratio of 1:1 using PEG 1500 (Boehringer Mannheim, Germany) , and the fused cells plated and screened for ALK-specific antibodies, e.g., using ALK-2, ALK-3 or ALK-6 as antigen. The cell fusion and monoclonal screening steps readily are performed according to standard procedures well described in standard texts widely available in the art. (See, for example, Guide to Protein Purification Murray P. Deutscher, ed.. Academic Press, San Diego, 1990. B. Exemplary ALK-Specific Antisera Antibodies used in the assay of Example 8 were obtained as follows. Rabbit antisera against ALK-1 to -6 were made against synthetic peptides corresponding to the divergent, intracellular juxtamembrane parts. (ALK-1: residues 119-141; ALK-2: residues 151-172; ALK-3 residues 181-202; ALK-6: residues 151-168.) Peptides were synthesized with an Applied Biosystems 430 A Peptide Synthesizer using t-butoxycarbonyl chemistry, and purified by reverse phase HPLC. The synthetic peptides were coupled to keyhole limpet hemocyanin (Calbiochem-Behring) using glutaraldehyde, as decribed by Gullick et al. (1985) EMBO J 4: 2869-2877. The coupled peptides then were mixed with Freund's adjuvant and used to immunize rabbits using standard methodologies. Example 4. 0P1-RECEPT0R BINDING ASSAYS Ligand binding specificity is determined by evaluating the ability of a receptor molecule to bind a specific ligand, and the ability of that ligand to compete against itself and other molecules which bind the receptor. Useful ligands will have a binding affinity for a soluble morphogen receptor extracellular domain such that dissociation constant (Kd) is less than about 10 " °M, preferably less than 5x 10~'M. Where stronger binding interaction is desired, preferred affinities are defined by a Kd of 10 _8 -10~ 9 M. OPl-related proteins are expected to be able to bind with specificity to multiple different receptor molecules, although likely with differing affinities. Ligand binding specificity can be assayed as follows, essentially following standard protocols well described in the art and disclosed, for example, in Legerski et al. (1992) Biochem. Biophys. Res. Comm 183:672-679 and Frakar et al. , (1978) Biochem. Biophys. Res.Comm. 80:849-857. In the ligand binding assays, a ligand having a known, quantifiable affinity for a morphogen receptor molecule of interest is labelled, typically by radioiodination ) , e.g., by chromogenic or fluorogenic labeling, or by metabolic labelling, e.g., 35 S, and aliquots of cells expressing the receptor on their surface are incubated with the labelled ligand, in the presence of various concentrations of unlabelled potential competitor ligand. In the assays described in Examples 8 and 9, below, this competitor typically is the candidate morphogen analog or an aliquot from a broth or extract anticipated to contain a candidate morphogen analog. Alternatively, a crosslinking agent may be used to covalently link the ligand to the bound receptor, and the crosslinked complex then immunoprecipitated with an antibody specific to the ligand, receptor, or complex. (See, Example 8.) A standard, exemplary protocol for determining binding affinity is provided below. Briefly, cells expressing a receptor on their cell surface are plated into 35 mM dishes and incubated for 48 hours in DMEM (Dulbecco's modified Eagle medium) plus 10% fetal calf serum. Purified morphogen, here, e.g., OP-1, or an OPl-analog is iodinated with Na-*- 2 5j by chloramine T oxidation, preferably having a specific activity of about 50-100 mCi/mg, essentially following the protocol of Frolik et al. (1984) J. Biol. Chem. 595:10995-11000. Labelled morphogen then is purified using standard procedures, e.g., chromatographically. Plated cells then are washed twice with physiologically buffered saline in the presence of 0.1% BSA, and incubated at 22oC in the presence of BSA, buffer and labelled morphogen (1 ng) and various concentrations (e.g., 0-lOmg/ml) of unlabelled competitor, e.g., unlabelled morphogen or candidate ligand analogs. Following binding, cells are washed three times with cold buffer, solubilized in 0.5 ml of 0.5 N NaOH, removed from the dish, and radioactivity determined by gamma or scintillation counter. Data then are expressed as percent inhibition, where 100% inhibition of specific binding is the difference between binding in the absence of competitor and binding in the presence of a 100-fold molar excess of unlabelled morphogen. Binding parameters preferably are determined using a computer program such as LIGAND (Munsun et al. (1980) Anal. Biochem. 107:220- 259.) Where the receptor cell surface binding domain is to be provided as a soluble protein, the assay can be performed in solution, most readily as an immunoprecipitation assay. In currently preferred assays the morphogen molecule is labelled and incubated with unlabelled receptor and candidate morphogen analogs. Receptor-specific antibody then is provided to the solution to precipitate the receptor-morphogen complex and the amount of labelled morphogen in the precipitated complex determined using standard detection means. Where the receptor or ligand is to be used in an affinity isolation protocol, the molecule preferably is immobilized on a surface, preferably a matrix surface over which sample fluid will flow, allowing the ligand of interest to bind, at letting nonbinding components pass through as effluent. The complex then can be removed intact or the ligand selectively removed with a desired eluant. 4.1 Screening Assay Considerations In the analog screening assays described in Example 9 below, the preferred protocol for assaying ligand-receptor binding is a standard competition or radioimmunoassay (RIA) . Here the OPl is labelled and the relative binding affinity of a candidate OPl analog ligand in a sample is measured by quantitating the ability of the candidate (unlabelled ligand analog) to inhibit binding of the labelled ligand (competitor morphogen) by the receptor. In performing the assay, fixed concentrations of receptor and labelled morphogen are incubated in the absence and presence of unknown samples containing candidate ligands. Sensitivity can be increased by preincubating the receptor with candidate ligand before adding the labelled morphogen. After the labelled competitor has been added, sufficient time is allowed for adequate competitor binding, and then free and bound labelled morphogen are separated, and one or the other is measured. Useful morphogen labels include radioactive labels, chromogenic or fluorogenic labels, and conjugated enzymes having high turnover numbers, such as horseradish peroxidase, alkaline phosphatase, or b-galactosidase, used in combination with chemiluminescent or fluorogenic substrates. In the same manner, OPl-specific receptor analogs can be assayed for their affinity for OPl in competition assays with labelled OPl specific receptors. Assays for evaluating a candidate OPl receptor-binding analog's ability to mimic OP-1 in signal-transduction across a membrane are exemplified in detail in Example 9.2, below. Briefly, the assay involves use of a cell (1) known to express an OP-1-εpecific receptor; or (2) which can be modified to express an OP-1-specific receptor, and/or (3) which can induce an 0P-1- mediated cellular reεponse. In the assay, the ability of a candidate analog to induce an OP-1-specific cellular response is monitored. Numerous OP-1 responsive cells and OP-1-mediated inducible cellular and biochemical markers are known and described in the art. Alternatively, and as exemplified below, an OP-1 inducible reporter gene system can be constructed and uεed to advantage in the aεεay. 4.2 Diagnostic Asεay Considerations The ability to detect morphogens in solution provides a valuable tool for diagnostic assays, allowing one to monitor the level of morphogen free in the body, e.g., in serum and other body fluidε. For example, OP-1 haε been detected in a number of different body fluidε, including serum and spinal fluid, including cerebro-spinal fluid, saliva, milk and other breast exudateε. (See, for example, PCT US92/07432, PCT US93/07231, WO94/06449) . Aε one example, OP-1 is an intimate participant in normal bone growth and resorption. Thus, εoluble OP-1 iε expected to be detected at higher concentrationε in individualε experiencing high bone formation, εuch as children, and at subεtantially lower levels in individuals with abnormally low rates of bone formation, such as patients with osteoporosis, aplastic bone disease, or osteopenia. Monitoring the level of OP-1 in serum thuε provides a means for evaluating the status of bone tissue and bone homeostasis in an individual, aε well as a means for monitoring the efficacy of a treatment to regenerate damaged or lost bone tissue. Alternatively, the level of OP-1 in bone tissue can be assessed in a bone tissue biopsy. Similarly, OP-1 and other morphogens have been identified in brain tissue. In particular, OP-1 is expresεed and/or localized in developing and adult rat brain and εpinal cord tiεsue, in the hippocampus, substantia nigra and the adendema glial cells, as well as associated with astrocyteε and the extracellular matrix εurrounding neuronal cell bodieε. (See, PCTUS93/07331) . Thuε, monitoring the level of OP-1 in spinal fluid or associated with a nerve tisεue biopsy can provide means for evaluating the status of nerve tissue in an individual, as well aε means for monitoring the efficacy of a nerve regeration or repair therapy. For serum assays, the serum preferably first iε partially purified to remove some of the excess, contaminating serum proteins, such as serum albumin. Preferably the serum iε extracted by precipitation in ammonium sulfate (e.g., 45%) such that the complex is precipitated. Further purification can be achieved using purification strategies that take advantage of the differential solubility of soluble morphogen complex or mature morphogens relative to that of the other proteinε present in serum. Further purification also can be achieved by chromatographic techniqueε well known in the art. The sample fluid then can be asεayed for OPl uεing the OPl-specific receptor(ε) and binding asεayε aε deεcribed herein. For a tissue biopsy, cells can be collected and stained with a labelled OP-1-specific antibody or receptor molecule. Alternatively, the OP-1 protein selectively can be extracted and quantitated as described above. Morphogens useful in the binding/screening assays contemplated herein include the soluble forms of the protein, e.g., the mature dimeric species complexed with one or two copies of the pro domain, the mature dimeric species alone, and truncated forms comprising eεεentially just the C-terminal active domain. Example 5. TRANSMEMBRANE SIGNAL INDUCTION ASSAYS/0P1 MIMETICS The kinase activity of the intracellular domains of the OPl-specific receptors can be tested in an autophophorylation asεay as described by Mathews et al. (PCT/US92/03825, published November 26, 1992) . Briefly, the DNA fragment encoding at least the intracellular kinase domain of an OPl-specific receptor is subcloned into pGEX-2T (Smith et al.(1988) Gene 67:31-40) to create a fusion protein between the putative kinase domain and glutathione S- tranεferaεe (GST) . The plasmid is introduced into E. coli and the expresεed fusion protein purified using glutathione affinity chromatography. About 100- 200 ng of fusion protein or purified GST then are incubated with 25 mCi (gp 32 P) ATP in 50mM tris, lOmM MgCl 2 buffer for 30 minutes at 37° C. Products then are analyzed by gel electrophoresis and autoradiography. The fusion protein, but not GST alone, becomes phophorylated, indicating that the kinaεe domain iε functional. Phosphoamino acid analysis then can be performed to determine the predominant amino acid being phosphorylated. Similar assays can be performed using similar fusion constructs expressed in mammalian cells. Various εignaling tranεduction aεεayε are provided in Example 9, below. An aεεay alεo can be developed for testing kinase activity transduction upon ligand binding uεing a ligand-induced kinase activity assay known in the art. Here, the ability of OP-1 analog to induce phosphorylation upon binding to the receptor is tested. See, for example, various assays for measuring ligand-induced kinase activity deεcribed by Accili et al. (1991) J. Biol.Chem. 266:434-439 and Nakamura et al. (1992) J.Biol. Chem. 267: 18924-18928,. For example, ligand- induced kinase activity (e.g., receptor autophosphorylation) can be measured in vitro by incubating purified receptor in the presence and absence of ligand (here, OPl or OPl analog, e.g., 10~'M) under conditionε sufficient to allow binding of the ligand to the receptor, followed by exposure to 32 P-ATP (e.g., lOOmCi in the presence of lOmM Tris-HCl (pH 7.6), lOmM MgCl 2 , lOmM MnCl 2 , lmM dithiothreitol, 0.15M NaCl 2 , 0.1% Triton X-100 and 3% glycerol) and the amount of phosphorylation measured, e.g., by SDS polyacrylamide gel electrophoresiε and autoradiography following immmunoprecipitation with antiphosphoεerine, antiphoεphothreonine or antiphosphotyrosine antibody (e.g., commercially available or made using standard antibody methodologies. ) While a low level of autophosphorylation may be detected in the absence of ligand, incubation with ligand iε anticipated to significantly increase (e.g., 5-20 fold increase) the amount of phosphorylation detected. In another asεay for detecting ligand-induced receptor autophosphorylation, involving intact cells, receptor DNA is transfected into a suitable host cell, e.g., a fibroblast, which then is grown under standard conditions to create a confluent monolayer of cells expresεing the receptor on their cell surface. On the day of the experiment, cells are incubated with or without ligand (e.g., OPl or OPl analog, e.g., 10~ 7 M) at 37°C, and then quickly washed with a "stopping solution" containing ATP (e.g., 0.1M NaF, 4 mM EDTA, lOmM ATP, 10 mM sodium orthovanadate, 4mM sodium pyrophosphate) . The cells then are frozen in a dry ice/ethanol bath, solubilized and the receptors immunoprecipitated, e.g., with an antireceptor antibody, as described herein. The immune complexes then are segregated, washed, separated by gel electrophoresiε using standard procedures and transferred to a membrane for Weεtern blot analyεiε using standard procedureε. Phoεphorylation of the receptor then can be visualized by immunodetection with a suitable antibody (e.g., antiphosphoserine, antiphosphothreonine or antiphosphotyrosine), as described above. The bound antibody (e.g., bound antiphosphoserine, antiphosphothreonine or antiphosphotyrosine) then can be detected with ι25j labelled protein A, followed by autoradiography. The amount of phosphorylated receptor detected iε anticipated to be εignificantly greater (5-20 fold increase) in receptors incubated with ligand than receptorε exposed to ATP in the absence of ligand. Ligand-induced receptor phosphorylation of exogenous subεtrates εimilarly can be aεεayed eεsentially using the methodology described herein. Here, a suitable εubεtrate (e.g., a εynthetic polypeptide containing εerine, threonine or tyroεine amino acidε) iε provided to the receptor following ligand expoεure and prior to incubation with ATP. The substrate subsequently can be segregated by immunoprecipitation with an antibody specific for the εubεtrate, and phosphorylation detected as deεcribed above. Aε for autophoεphorylation, the amount of phoεphorylated εubεtrate detected following ligand incubation iε anticipated to be greater than that detected for εubεtrates exposed to receptors in the absence of ligand. Alternatively, a reporter gene construct can be created to asεay transmembrane signal induction. Here, the expresεion control elementε for an OP-1 inducible protein marker is fused to the open reading frame sequence for any reporter gene, and induction of the reporter gene expression then assayed. Useful reporter geneε include the luciferase gene or GAL4 as well as other, easily characterizable markers. Example 6. CHIMERIC RECEPTOR MOLECULES Chimeric receptor moleculeε, e.g., compriεing an ALK or ALK analog extracellular and tranεmembrane region and, for example, part or all of an intracellular domain from another, different receptor or an intracellular domain from a different cell εurface molecule, may be conεtructed uεing standard recombinant DNA technology and/or an automated DNA synthesizer to construct the desired sequence. As will be appreciated by persons skilled in the art, useful junctions include sequences within the transmembrane region and/or sequences at the junction of either the intracellular or the extracellular domainε. Alεo enviεioned are chimers where the extracellular domain or the intracellular domains themselves are chimeric sequenceε. Chimeric sequences are envisioned to be particularly useful in screening asεays to determine candidate binding ligands (e.g., OPl analogs, see below) , where the non-receptor intracellular domain provides a suitable second messenger response syεtem that iε easy to detect. Potentially useful other second messenger response systems include those which, when activated, induce phosphoinositide hydrolysis, adenylate cyclase, guanylate cyclase or ion channels. Chimeric receptor moleculeε have particular utility in gene therapy protocolε. For example, a population of cells expresεing a chimeric morphogen receptor molecule on their surface and competent for expresεing a desired phenotype can be implanted in a mammal at a particular tisεue locuε. By careful choice of the ligand binding domain used on these receptorε a physician can administer to the individual a morphogen agonist capable of: (1) binding to the chimeric receptor alone and (2) εtimulating the proliferation and/or differentiation of the implanted cellε without affecting endogenouε cell populationε. Example 7. CONSIDERATIONS FOR IDENTIFYING OTHER OPl SPECIFIC RECEPTORS IN NUCLEIC ACID LIBRARIES Identification of ALK Type I receptorε that can bind OP-1 allowε one to identify other morphogen receptor sequences in different specieε aε well aε in different tissues. The OPl-binding ALK sequences themselves can be used as a probe or the sequence may be modified to account for other potential codon usage (e.g., human codon bias.) Currently preferred probe sequences are those which encode the receptor's extracellular domain. Probes based on the nucleic acid sequence of Seq. ID Noε.l, 3, 5 or 7 can be εyntheεized on commercially available DNA synthesizers, e.g. Applied Biosystems model 381A, using standard techniques, e.g. Gait, Oligonucleotide Syntheεis: A Practical Approach, (IRL Preεs, Washington D.C, 1984). It is preferable that the probes are at least 8-50 bases long, more preferably 18-30 bases long. Probes can be labeled in a variety of wayε εtandard in the art, e.g. using radioactive, enzymatic or colormetric labels as described, for example, by Berent et al, (May/June 1985) Biotechniques: 208-220; and Jablonski et al, (1986) Nucleic Acids Research 14: 6115-6128. Preferably, low stringency conditions are employed when screening a library for morphogen receptor sequenceε uεing a probe derived from OPl- binding receptor. Preferred ALK-specific probes are thoεe corresponding to bases encoding the extracellular domain ("ECD"), or encoding a unique (nonhomologous) sequence within the cytoplasmic domain. Useful probes may be designed from bases encoding the juxtamembrane region, for example. The probe may be further modified to use a preferred species codon bias. Alternatively, probes derived from the serine/threonine kinase domain can be used to identify new members of the receptor kinase family which can be screened for OPl binding using the methods deεcribed in Example 8. For example, for a probe of about 20-40 baεeε a typical prehybridization, hybridization, and wash protocol is as follows: (1) prehybridization: incubate nitrocellulose filters containing the denatured target DNA for 3-4 hours at 55oC in 5X Denhardt'ε εolution, 6X SSC (20X SSC consists of 175g NaCl, 88.2 g sodium citrate in 800 ml H 2 0 adjusted to pH. 7.0 with ION NaOH), 0.1% SDS, and 100 mg/ml denatured salmon εperm DNA, (2) hybridization: incubate filterε in prehybridization εolution pluε probe at 42oC for 14-48 hourε, (3) wash; three 15 minutes waεheε in 6X SSC and 0.1% SDS at room temperature, followed by a final 1-1.5 minute waεh in 6x SSC and 0.1% SDS at 55oC. Other equivalent procedureε, e.g. employing organic solvents such aε formamide, are well known in the art. Alternatively, morphogen receptor-specific DNA can be amplified using a εtandard PCR (polymerase chain reaction) methodology such aε the one disclosed herein, to amplify approximately 500 base pair fragments. As for the hybridization screening probes deεcribed above, the primer sequences preferably are derived from conserved sequences in the serine/threonine kinase domain. The primers discloεed herein, in Seq. ID Nos. 12-15 are envisioned to be particularly useful, particularly in combination.. Examples of useful PCR amplifications, including the use of the primers recited herein, are disclosed in ten Dijke, et al. (1993) Oncogene 8:2879-2887 and (1994) Science 264:101-104, , and which also describe the isolation protocols for ALK-1, ALK-2, ALK-3 and ALK-6. 7.1 TISSUE DISTRIBUTION OF MORPHOGEN RECEPTORS Determining the tissue distribution of OPl-specific receptors can be used to identify tissue and cell sourceε which expreεε theεe receptors, to identify new, related OPl-specific receptor molecules, aε well aε to identify target tiεεueε for OPl-receptor interactions under naturally occurring conditions. The OP-1 εpecific receptor molecules (or their mRNA transcriptε) readily are identified in different tiεεues using standard methodologies and minor modifications thereof in tissueε where expreεεion may be low. For example, protein diεtribution can be determined uεing εtandard Weεtern blot analysis or immunohistological detection techniques, and antibodies specific to the morphogen receptor molecules of interest. Similarly, the diεtribution of morphogen receptor tranεcriptε can be determined uεing standard Northern hybridization protocols and transcript-specific probes or by m situ hybridization. Any probe capable of hybridizing specifically to a transcript, and distinguiεhing the tranεcript of intereεt from other related tranεcripts can be used. Because the morphogen receptorε deεcribed herein likely εhare high sequence homology in their intracellular domains, the tiεεue diεtribution of a specific morphogen receptor transcript may best be determined using a probe specific for the extracellular domain of the molecule. For example, a particularly useful ALK-specific probe sequence is one derived from a unique portion of the 5' coding sequence, the sequence corresponding to the juxtamembrane region, or the 5' or 3 ' noncoding sequenceε. The choεen fragment then iε labelled using standard means well known and described in the art and herein. Using these receptor-specific probes, which can be synthetically engineered or obtained from cloned sequences, receptor transcripts can be identified and localized in various tiεεues of various organisms, using standard methodologies well known to those having ordinary skill in the art. A detailed description of a εuitable hybridization protocol iε deεcribed in Ozkaynak, et al. , (1991) Biochem. Biophys. Res. Commn. 179:116-123, and Ozkaynak, et al. (1992) J Biol. Chem. 267:25220-25227. Briefly, total RNA is prepared from various tissues (e.g., murine embryo and developing and adult liver, kidney, testis, heart, brain, thymus, stomach) by a εtandard methodologieε such aε by the method of Chomczynεki et al. ((1987) Anal. Biochem 162:156-159) and deεcribed below. Poly (A)+ RNA is prepared by using oligo (dT) -cellulose chromatography (e.g., Type 7, from Pharmacia LKB Biotechnology, Inc.) . Poly (A)+ RNA (generally 15 mg) from each tisεue iε fractionated on a 1% agarose/formaldehyde gel and transferred onto a Nytran membrane (Schleicher & Schuell) . Following the tranεfer, the membrane iε baked at 80oC and the RNA is croεs-linked under UV light (generally 30 seconds at 1 mW/cm 2 ) . Prior to hybridization, the appropriate probe is denatured by heating. The hybridization is carried out in a lucite cylinder rotating in a roller bottle apparatus at approximately 1 rev/min for approximately 15 hours at 37oC uεing a hybridization mix of 40% formamide, 5 x Denhardts, 5 x SSPE, and 0.1% SDS. Following hybridization, the non-εpecific counts are washed off the filters in 0.1 x SSPE, 0.1% SDS at 50oC. Example 8. Demonεtration That ALK-2, ALK-3 and ALK-6 are OPl-Binding Receptorε The tiεεue morphogenic proteinε OPl and BMP4 were tested for specific binding interaction with the ALK receptors in receptor-tranεfected cells (where the receptor iε over-expreεεed) , and in nontransfected cells. It previously was known that ALK-5 interacted specifically with TGFβl and ALK-2 and ALK-4 interacted specifically with activin. In the experiment, complexes were croεεlinked and immuno-precipitated with an ALK-specific antibody as described below. To date, no binding with ALK-1 under the conditions of this protocol have been detected. Binding and affinity croεε-linking using disuccinimidyl suberate (Pierce Chemical Co.) were performed using standard methods (e.g., Franzen et al. (1993) Cell 75:681-692 and Ichijo et al. (1990) Exp. Cell Res. 187:10995- 11000.) A typical protocol is described below. Modifications from this protocol for individual experimentε were standard changes anticipated to produce the same result as for the recited procedure. Briefly, cells in multi-well plates were waεhed with binding buffer (e.g., phosphate buffered saline containing 0.9 mM CaCl , 0.49mM MgCl and 1 mg/ml bovine serum albumin (BSA) ) , incubated on ice in the same buffer with labelled ligand, in the presence and absence of excesε unlabelled ligand for εufficient time for the reaction to equilibrate (e.g., 3 hourε. ) Cellε were waεhed and the crosslinking was done in the binding buffer without BSA together with 0.28 mM disuccinimidyl suberate for 15 min on ice. Cells were harvested by addition of 1 ml of detachment buffer (10 mM Tris-HCl, pH 7.4, 1 mM EDTA, 10% glycerol, 0.3 mM PMSF.) Cells then were pelleted by centrifugation, then resuεpended in 50 ml of solubilization buffer (125 mM NaCl, lOmM Tris-HCl, pH 7.4, lmM EDTA, 1% Triton X-100, 0.3 mM PMSF, 1% Traεylol) and incubated for 40 minutes on ice. Cells were centrifuged again and supernatants subjected to analysis by standard SDS-gel electrophoresis using 4%-15% polyacrylamide gels, followed by autoradiography. Cell lysateε obtained following affinity croεε-linking via the general protocol deεcribed above were immunoprecipitated uεing antisera against ALKs (e.g., raised against the ALK juxtamembrane region), or directly analyzed by SDS-gel electrophoresis using gradient gels consisting of 5-12% or 5-10% polyacrylamide. The gels were fixed and dried, and then subjected to autoradiography or analysiε using phoεphorlmager (Molecular Dynamicε) . 8.1 Binding of OP-1 and/or BMP-4 to ALKs in Transfected Cellε. COS-1 cellε tranεfected with ALK cDNA were teεted for the binding of ^ ■ ^I-OP-l and ^ 2 ^I-BMP-4, in the presence or abεence of co-tranεfected Type II receptor DNA: daf-4 cDNA or ActRII (Eεtevez et al. (1993) Nature 365:644-649 and Attisano et al. (1992) Cell £8:97-108, disclosing the DNA sequence for these Type II receptors and the discloεure of which is incorporated herein by reference.) Since the cross-linked complexeε were εometimeε difficult to visualize because of high background, εampleε were immunoprecipitated by antiεera againεt each ALK. The reεultε are preεented in Table I below. In the Table, "N/T" meanε "not teεted" . Binding waε specific as determined by standard competition assays. The values represented by "+/-", "+", "++", "+++", and "-" are all qualitative descriptors of the relative amount of radioactivity measured when the croεεlinked molecules were gel electrophoresed and subjected to autoradiography. More radioactivity measured indicateε a stronger binding interaction detected. In the Table the εtrength of binding interaction iε aε follows: +++ > ++ > +> +/- > -. TABLE I 125 I OPl 125 I BMP4 +daf_ -daf4 ActRII +daf4 -da£4 ÷ActRII ALK1 _ ALK1 N/T ALK2 ++ +/- ++ ALK2 N/T ALK3 ++ - +/- ALK3 N/T ALK4 - - - ALK4 N/T ALK5 - - - ALK5 N/T ALK6 +++ ++ + ALK6 N/T In the absence of daf-4, OP-1 bound to ALK-6, whereas BMP-4 bound to ALK-3 and ALK-6. Weaker binding of OP-1 to ALK-2 was also observed. Other ALKs did not bind OP-1 or BMP-4 in the absence of Daf-4. When ALK cDNAs were co-transfected with the daf-4 cDNA, increaεed binding of OP-1 to ALK-2 and ALK-6 waε εeen. In addition, ALK-3 alεo waε found to bind OP-1 in the co- tranεfected cellε. Similarly, increased binding of BMP-4 to ALK-3 and ALK-6 could be observed. Co-tranεfection of two different typeε of ALKε did not further increase the binding of OP-1 or BMP-4. In cells co-tranεfected with the DNA for ActRII and ALK-2, ALK-3 or ALK-6, OPl-receptor binding waε enhanced. The εizes of the cross-linked complexes were slightly higher for ALK-3 than for ALK-2 and ALK-6, consistent with its slightly larger size. Complexes of about 95 kDa aε well aε multiple componentε of 140-250 kDa were also co- immunoprecipitated with certain of the ALKs. In standard competition asεays performed with the Type I receptors in the presence and absence of the Type II receptors, the binding of OP-1 and BMP-4 could be competed with excess amounts of unlabeled OP-1, verifying the binding specificity of these interactions where they occurred. These results demonstrate that ALK-2, ALK-3 and ALK-6 can serve as Type I receptors for OP-1. Notably, ligand binding apparently can be enhanced in the presence of Type II receptors. Moreover, OP-1 is able to interact with both a "bone morphogen" Type II receptor (daf 4) and an "activin" Type II receptor (ActRII) , whereas, for example, activin only interacts with the ActRII Type II receptor. The data indicate that OPl has a broader spectrum of receptor (Type I and Type II) binding affinities than do other tiεsue morphogenic proteinε, or other memberε of the TGF-β family. It is anticipated that OPl will have specific binding interactionε with other "activin-binding" or "bone morphogen-binding" Type II receptors. The ability of OP-1 to bind to Type I, Type II receptors having binding specificity for activin or BMP4 but not TGF-B, indicates that OP-1 and OP-1 analogε will be uεeful as competitors of activin or BMP4 binding to cell surface receptors. In particular, OP-1 and OP-1 analogs will be useful for competing with activin-ALK-2 binding and/or activin-ALK-2/ActRII (or other Type II) receptor binding; and for competing with BMP4 (or BMP2)-ALK-6 binding, and/or BMP2/4-ALK-6/daf 4 (or other Type II) binding. The OP-1 competitorε may act as antagonists (e.g., binding competitors unable to induce the εignal tranεduction cascade upon binding) or as agonistε (e.g., able both to bind and induce the signal transduction cascade) . 8.2 Identification of OPl-Specific Receptorε in Nontranεfected Cells ALK-5 haε been εhown to bind TGF-βl, and ALKε 2, 4 bind activin A with high affinities in nontranεfected cellε (ten Dijke, Oncogene, (1993_; Science, _1994) referenced herein above.) In the present experiment, the binding affinity of OPl and/or BMP4 to receptors in nontransfected cells waε demonstrated as follows. The resultε corroborate the tranεfected cell data, verifying that OPl interacts specifically with ALK-2, ALK-3 and ALK-6, but not ALK-4 or ALK-5. MC3T3-E1 oεteoblaεtε are well characterized cellε known to respond to OP-1 and BMP-4 in the induction of alkaline phosphatase activity (Paralkar (1991) PNAS 8 : 3397-3401.) These cells were affinity labeled using 125 I-OP-l as described herein above, and croεε-linked complexes of about 75 kDa were seen, which were immunoprecipitated only with the ALK-2 antiserum. Tera-2 teratocarcinoma cells and MvlLu cells responded to OP-1 as measured by production of plasminogen activator inhibitor-1 (PAI-1) . Similar to MC3T3-E1 cellε, cross-linked complexes uεing 125 I-OP-l in Tera-2 teratocarcinoma were immunoprecipitated only by the ALK-2 antiserum. On the other hand, cross-linked complexes using ^"I-OP-l to MvlLu cellε were immunoprecipitated by ALK-2 aε well aε ALK-3 and ALK-6 antiεera. MvlLu cells are known to expresε ALK-4 and ALK-5 (Ebner (1993) Science 260:1344-1348) , but croεε-linked complexeε with 125 I-OP-l were not precipitated by antisera against these receptors. Similarly, crosε-linked complexeε in U-1240 MG glioblaεtoma cellε were immunoprecipitated by ALK-2 and ALK-6 antisera, and weakly by ALK-3 antiεerum. In contrast, cross-linking of 5 I-OP-l to AG1518 human foreskin fibroblasts yielded weak immunoprecipitated components only by ALK-3 antiεerum. Type II receptor-like components of about 95 kDa as well aε high molecular weight complexes of 140-250 kDa co- immunoprecipitated with certain ALKs in the Tera-2 cells, MvlLu cells and U- 1240 MG cells. Receptors for BMP-4 have alεo been inveεtigated using nontransfected cells. Cross-linked complexes uεing 125 I-BMP-4 to MC3T3-E1 cellε and AG1518 human foreεkin fibroblaεtε were immunoprecipitated only by ALK-3. On the other hand, cross-linking of 12 ^I-BMP-4 to Tera-2 cellε did not yield any immunoprecipitated componentε by antiεera against ALKs. 1 ->I-0P-1 and/or • ' ■2 ^I-BMP-4 also were demonstrated by affinity croεε- linking to interact specifically with receptors in other BMP-responsive cellε, e.g., MG-63 osteosarcoma cells and PC12 pheochromocytoma cells. A summary of the binding of ALKε to OP-1 or BMP-4 in different cell typeε is shown in Table II, below. In the Table, "N/T" means not tested, and receptorε presented in brackets indicate comparatively lower quantities of radioactive complexeε detected. TABLE II Cell lineε Binding of OP-1 Binding of BMP-4 Mouse osteoblasts ALK-2 ALK-3 (MC3T3-E1 Mink lung epithelial ALK-2, -3, -6 N/T cells (MvlLu) ALK2, [-3], -6 N/T Human glioblastoma ALK2 Human teratocarcinoma (Tera-2) [ALK3] ALK3 Human foreskin fibroblasts (AG1518) ALK2, [-3] N/T. Rat osteosarcoma (ROS17/2.8) Example 9. OPl, OP1-SPECIFIC RECEPTOR ANALOG SCREENING ASSAYS The present invention is useful to determine whether a ligand, such as a known or putative drug, is capable of binding to and/or activating an OPl- specific cell surface receptor aε described herein. Ligands capable of specific binding interaction with a given OPl-specific receptor (e.g., ALK-2, ALK-3, ALK-6) are referred to herein aε OPl analogε and can be uεed for therapeutic and diagnoεtic applications. Some analogs will have the ability to stimulate morphogenetic activity in the cell, mimicking both the receptor binding and signal transducing activity of OPl. These are referred to OPl agonists or mimetics. Others will have strong binding affinity but will not stimulate morphogenesiε, these are OPl antagonists. The analogs can be amino acid-based, or they can be composed of non-proteinaceous chemical structures. The methods and kitε deεcribed below εimilarly can be used to identify OPl-specific receptor analogs, capable of mimicking the binding affinity of ALK-2,ALK-3 or ALK-6 for OPl. The analogs can be provided to a mammal to interact with serum-soluble OPl, effectively sequestering the protein and modulating its availability for cell surface interaction. Transfection of an isolated clone encoding a morphogen receptor into the cell systems described above provides an assay syεtem for the ability of ligandε to bind to and/or to activate the receptor encoded by the isolated DNA molecule. Transfection systemε, εuch aε thoεe described above, are useful as living cell cultures for competitive binding aεεays between known or candidate drugs and ligands which bind to the receptor and compete with the binding of known morphogens, which are labeled by radioactive, enzymatic, spectroscopic or other reagents. Membrane preparations containing the receptor and isolated from transfected cellε are alεo uεeful in these competitive binding asεayε. Alternatively, and currently preferred, purified receptor molecules or their ligand binding extracellular domains can be plated onto a microtiter well surface, in a modification of a sandwich asεay, e.g., aε a competition aεεay, such as an RIA, described above. Finally, as deεcribed above, solution assays, and using only the receptor extracellular domain, also may be used to advantage in these assays. Functional assays of second mesεenger systems or their sequelae in transfection εyεtemε act aε aεεays for binding affinity and efficacy in the activation of receptor function or efficacy in the antagonism of receptor function. Such a transfection system constitutes a "drug discovery system", useful for the identification of natural or synthetic compounds with potential for drug development that can be further modified or used directly as therapeutic compounds to activate or inhibit the natural functions of the receptor encoded by the isolated DNA molecule. Once such candidate drugs (e.g., OP-1 or receptor-binding analogs thereof) are identified, they can be produced in reasonable, useful quantitieε uεing standard methodologieε known in the art. Amino acid-based molecules can be encoded by εynthetic nucleic acid moleculeε, and expreεεed in a recombinant expression syεtem as deεcribed herein above or in the art. Alternatively, such molecules can be chemically syntheεized, e.g., by means of an automated peptide synthesizer, for example. Non-amino acid-based molecules can be produced by εtandard organic chemical synthesis procedures. Where the candidate molecule is of undetermined structure, or compoεition, itε compoεition readily can be determined by, for example, mass εpectroεcopy. Two approaches to identifying analogs typically are practiced in the art: high flux screenε and rational design of ligand mimetics. High flux screenε typically screen naturally sourced materials or chemical banks for their ability to bind a protein of intereεt, here, e.g., the receptor. Typically, compoundε are obtained from a range of εourceε, e.g., chemical bankε, microbial broths, plant and animal extracts, and the like. In a high flux screen typically, purified receptor, preferably the soluble, ligand binding extracellular domain, is plated onto a microtiter well surface and a standard volume of a sample solution to be teεted then is added. Also added is a standard volume having a known quantity of a purified ligand known to bind the receptor with specificity. Preferably the ligand is labelled with a subεtance that is readily detectable by automated means (e.g., radiolabel, chromophoric, fluorometric, enzymatic or spectroεcopic label.) The wellε then are washed and the amount of label remaining after washing or the amount of label remaining associated with the receptor then is detected. Positive scoreε are identified by the ability of the teεt substance to prevent interaction of the labelled ligand with the receptor. The screening assays can be performed without undue experimentation, using standard molecular and cell biology tools in common use in the art. For example, screening assays can be performed in standard 96-well plates. Fifteen such plates reasonably can be set up at a time to perform multiple screening asεayε in parallel. Thuε, with only 10-11 reiterationε of the εcreening aεεay 15,625 (5 6 ) compoundε can be screened for their binding affinity. Even allowing for a maximum incubation time of 2 hourε, all 15,625 compoundε reaεonably can be aεsayed in a matter of days. High flux screenε exploit both the high degree of specificity of the labelled ligand for its receptor, aε well aε high throughput capacity of computer driven robotics and computer handling of data. Candidate analogs identified in this manner, then can be analyzed εtructurally and this information used to design and to synthesize analogs having enhanced potency, increased duration of action, increased selectivity and reduced side effects. Candidateε alεo can be uεed in a rational design program as described below. Finally, candidate analogs also can be teεted to determine morphogenetic effect, if any, aε deεcribed below. The εecond approach to the identification of analogε uses a rational design approach to create molecules capable of mimicking the binding effect of OPl with an OPl-specific receptor. Here the relevant structure for receptor binding is analyzed to identify critical sequences and structures necesεary for binding activity and thiε information can be used to design and syntheεize minimal εize morphogen analogs. As for candidate compounds in the high flux aεεay, design candidates can be tested for receptor binding activity as deεcribed above. Aε deεcribed above, a candidate sequence can be further modified by, for example standard biological or chemical mutagenesiε techniques to create a candidate derivative having, for example, enhanced binding affinity or another preferred characteristic. Antibodies capable of interacting specifically with the receptor and competing with OPl binding also can be used as an analog. Antibodies can be generated as described above. OPl analogε may be evaluated for their ability to mimic OPl or to inhibit OPl binding (e.g., agoniεtε or antagonists) by monitoring the effect of the analogs on cellε bearing an OPl-εpecific receptor (e.g., ALK-2, ALK-3 or ALK-6.) OPl agoniεtε are anticipated to have utility in any application where tiεεue morphogeneεiε is desired, such as in the regeneration of damaged tissue resulting from mechanical or chemical trauma, degenerative diseaεes, tissue destruction resulting from chronic inflammation, cirrhoεiε, inflammatory diεeaεeε, cancer and the like, and in the regeneration of tiεεues, organs and limbs. OPl antagonists are envisioned to have utility in applications where tissue morphogeneεiε iε to be limited aε, for example, in the treatment of malignant tranεformationε including, but not limited to, osteosarcomaε and Paget' ε diεeaεe. Several exemplary εystems for aεεaying the ability of a candidate analog tranεduce an OP-1-εpecific εignal across the cellular membrane are described below. 9.1 Induction of Osteoblast Differentiation Markerε For example, OPl is known to preferentially induce differentiation of progenitor cells, including embryonic mesenchymal cells and primary osteoblasts (see, for example, PCT US92/07432) As one example, OPl analogs can be tested for their ability to induce differentiation of primary osteoblasts, by measuring the ability of these analogs to induce production of alkaline phosphatase, PTH-mediated cAMP and osteocalcin, all of which are induced when primary osteoblasts are exposed to OP-1, 60A or DPP. Briefly, the assays may be performed as follows. In this and all examples involving osteoblast cultures, rat osteoblast-enriched primary cultureε preferably are used. Although these cultures are heterogeneous in that the individual cells are at different stages of differentiation, these cultures are believed to more accurately reflect the metabolism and function of osteoblastε _in vivo than oεteoblaεt cultureε obtained from eεtablished cell lines. Unless otherwise indicated, all chemicalε referenced are εtandard, commercially available reagentε, readily available from a number of εourceε, including Sigma Chemical, Co., St. Louiε; Calbiochem, Corp., San Diego and Aldrich Chemical Co., Milwaukee. Rat oεteoblast-enriched primary cultures are prepared by sequential collagenase digestion of newborn suture-free rat calvaria (e.g., from 1-2 day- old animalε, Long-Evans strain, Charles River Laboratorieε, Wilmington, MA), following standard procedures, such as are described, for example, in Wong et al., (1975) PNAS 72:3167-3171. Rat osteoblast single cell suspensions then are plated onto a multi-well plate (e.g., a 24 well plate) at a concentration of 50,000 osteoblasts per well in alpha MEM (modified Eagle's medium, Gibco, Inc., Long Island) containing 10% FBS (fetal bovine serum), L-glutamine and penicillin/streptomycin. The cells are incubated for 24 hourε at 37°C, at which time the growth medium is replaced with alpha MEM containing 1% FBS and the cells incubated for an additional 24 hourε εo that the cells are in serum- deprived growth medium at the time of the experiment. Alkaline Phoεphatase Induction of Osteoblasts The cultured cells in serum-free medium are incubated with OPl, OPl analog or a negative control, using a range of concentrations. For example, 0.1, 1.0, 10.0, 40.0 or 80.0 ng OP-l/ml medium typically are uεed. 72 hourε after the incubation period the cell layer is extracted with 0.5 ml of 1% Triton X-100. The resultant cell extract then, iε centrifuged, and 100 ml of the extract iε added to 90 ml of paranitroεophenylphospate (PNPP) /glycerine mixture and incubated for 30 minutes in a 37°C water bath and the reaction stopped with 100 ml NaOH. The samples then are run through a plate reader (e.g., Dynatech MR700 plate reader, and absorbance measured at 400 nm, using p-nitrophenol as a standard) to determine the presence and amount of alkaline phosphate activity. Protein concentrations are determined by the Biorad method. Alkaline phosphatase activity is calculated in units/mg protein, where 1 unit=l nmol p-nitrophenol liberated/30 minuteε at 37°C. OP-1 induces a five -fold increase in the specific activity of alkaline phosphate by this method. Agonists are expected to have similar induction effects. Antagonists should inhibit or otherwise interfere with OPl binding, and diminiεhed alkaline phophataεe induction should result when the aεεay is performed with an antagonist in the presence of a limiting amount of OPl. Induction of PTH-Mediated cAMP. The effect of a morphogen analog on parathyroid hormone-mediated cAMP production in rat osteoblastε in vitro may be demonεtrated aε follows. Rat osteoblasts are prepared and cultured in a multiwell plate as described above. The cultured cellε then are divided into three groups: (1) wells which receive, for example, 1.0, 10.0 and 40.0 ng OP-l/ml medium); (2) wells which receive the candidate analog at various concentration ranges; and (3) a control group which receives no additional factors. The plate is then incubated for another 72 hourε. At the end of the 72 hours the cells are treated with medium containing 0.5% bovine serum albumin (BSA) and lmM 3- isobutyl-1-methylxanthine for 20 minutes followed by the addition into half of the wells of human recombinant parathyroid hormone (hPTH, Sigma, St. Louis) at a concentration of 200 ng/ml for 10 minuteε. The cell layer then is extracted from each well with 0.5 ml of 1% Triton X-100. The cAMP levels then are determined using a radioimmunoaεεay kit (e.g., Amerεham, Arlington Heights, Illinois) . OP-1 doubles cAMP production in the presence of PTH. Agonists are expected to have similar induction effects. Antagonistε are expected to inhibit or otherwiεe interfere with OPl binding, and diminiεhed cAMP production should result when the asεay iε performed with an antagoniεt in the preεence of limiting the amount of OPl. Induction of Osteocalcin Production Osteocalcin is a bone-specific protein synthesized by osteoblasts which plays an integral role in the rate of bone mineralization in vivo. Circulating levels of osteocalcin in serum are used as a marker for osteoblast activity and bone formation m vivo. Induction of osteocalcin synthesiε in oεteoblaεt-enriched cultures can be used to demonstrate morphogenic efficacy in vitro. Rat osteoblasts are prepared and cultured in a multi-well plate aε above. In thiε experiment the medium is supplemented with 10% FBS, and on day 2, cells are fed with freεh medium εupplemented with freεh 10 mM b- glycerophosphate (Sigma, Inc.). Beginning on day 5 and twice weekly thereafter, cells are fed with a complete mineralization medium containing all of the above components plus freεh L(+) -aεcorbate, at a final concentration of 50mg/ml medium. OPl or OPl analog then iε added to the wells directly, e.g., in 50% acetonitrile (or 50% ethanol) containing 0.1% trifluoroacetic acid (TFA) , at no more than 5ml OPl/ml medium. Control wells receive solvent vehicle only. The cells then are re-fed and the conditioned medium sample diluted 1:1 in standard radioimmunoassay buffer containing standard protease inhibitors and stored at -20°C until assayed for osteocalcin. Osteocalcin synthesiε is measured by standard radioimmunoasεay uεing a commercially available osteocalcin-specific antibody and can be confirmed by Northern blot analysis to calculate the amount of osteocalcin mRNA produced in the presence and absence of OP-1 or OPl analog. OP-1 induces a dose-dependent increase in osteocalcin production (5-fold increase using 25 ng of OP-1 protein/ml) , and a 20-fold increase in osteocalcin mRNA. Agonistε are expected to have εimilar induction effects; antagonists are expected to inhibit or otherwiεe interfere with OPl binding, thereby subεtantially interfering with oεteocalcin induction in the preεence of a limiting amount of OPl. Mineralization iε determined on long term cultures (13 day) using a modified von Kossa staining technique on fixed cell layers: cells are fixed in fresh 4% paraformaldehyde at 23o C for 10 min, following rinsing cold 0.9% NaCl. Fixed cells then are stained for endogenous alkaline phosphatase at pH 9.5 for 10 min, using a commercially available kit (Sigma, Inc.) Purple stained cells then are dehydrated with methanol and air dried. After 30 min incubation in 3% AgN0 in the dark, H 0-rinsed samples are exposed for 30 sec to 254 nm UV light to develop the black silver-εtained phoεphate nodules. Individual mineralized foci (at least 20 mm in size) are counted under a diεεecting microεcope and expressed as noduleε/culture. OP-l induceε a 20- fold increase in initial mineralization rate. Agonistε are expected to have similar induction effects; antagonists are expected to inhibit or otherwise interfere with OPl binding, thereby inhibiting mineralization induction in the presence of a limiting amount of OPl. 9.2 Induction of a Constructed Reporter Gene Alternatively, a reporter gene construct can be used to determine the ability of candidate molecule to induce signal transduction across a membrane following receptor binding. For example, PAI-1 protein, (Plaεminogen Activator Inhibitor-1) expreεεion can be induced by OP-l in MvlLu- cells (see above). Alεo, as demontrated above, these cells express ALK-2, -3 and -6 surface receptors. In addition, preliminary studieε indicate that ALK-1, when overexpreεεed in a chemically mutagenized derivative of these cellε, alεo apparently mediateε PAI-1 induction in the presence of OPl. Accordingly, PAI-1 promoter elements can be fused to a reporter gene and induction of the reporter gene monitored following incubation of the transfected cell with a candidate analog. As one example, the luciferase reporter gene can be uεed, in, for example, the construct p3TP-Lux described by Wrana et al. (1992) Cell 71: 1003-1014 and Attisano et al. (1993) Cell 74: 671-680. This reporter gene construct includes a region of the human PAI-1 gene promoter in combination with three sets of tetradecanoyl phorbol - acetate responsive elements upstream of the lucifrase open reading frame. In a typical assay, transfected cells starved in DMEM containing 0.1% fetal bovine serum and antibiotics (e.g., 100 units/ml penicillin and 50 μg/ml streptomycin) for 6 hrs., and then exposed to ligand for 24 hr. Luciferase activity in the cell lysate then is measured using a luminometer in the luciferase assay system, according to the manufacturer's protocol(Promega) . In MvlLu mutant cells, "R mutant" cells co-tranεfected with ALK-2 and Act RII, OPl mediated induction of luciferase activity. 9.3 Inhibition of Epithelial cell proliferation OPl is known to inhibit epithelial cells. Thus, the ability of a candidate analog to inhibit cell proliferation, as measured by H-thymidine uptake by an epithelial cell can be used in an asεay to evaluate signal transduction activity of the candidate. Analogs competent to inhibit epithelial cell growth are contemplated to have particular utility in therapeutic applications where limitation of a proliferating cell population is desired. Such applications include chemotherapies and radiation therapies where limiting the growth of a normally proliferating population of cellε can protect theεe cells from the cytotoxic effects of these cancer therapies. (see e.g., W094/06420) . In addition, psoriasiε and other tissue disorders resulting from uncontrolled cell proliferation, including benign and malignant neoplasties, can be modulated by use of an OPl analog. As an example, mink lung epithelial cell growth is inhibited by OP-l. (see, PCT US93/08885; WO94/06420.) As deεcribed above, derivativeε of theεe cells [e.g., "R-4 mutants", clone 4-2, Laiho et al. (1990) J. Biol. Chem. 265: 18518-18524] can be transfected with DNA encoding OPl-specific receptors and induced to expresε these receptors. The transfected cells, then can be asεayed for a candidate analog's ability to block cell growth. As one example, when R-4 cells are transfected with ALK-3 under a Zn 2* -inducible promoter, and induced to expreεε the receptor following induction with ZnCl , cell growth can be inhibited in the presence of OPl in a dose dependent manner. Preliminary experiments with ALK-1 indicates that this receptor alεo can mediate this OP-1-specific effect. In a typical aεεay, cellε are εeeded in 24-well cell culture plateε at a denεity of 10^ cells per well in DMEM with 10 % FBS, and incubated overnight. The medium is replaced with DMEM containing 0.2 % FBS and lOOuM ZnLC2, and the cells are incubated for 5 h, after which the medium iε replaced with fresh DMEM containing 0.2 % FBS, lOOuM ZnCL 2 and variouε concentrations of OP-l or an analog candidate. After 16 h of incubation, 0.25 ci of [ 3 H] thymidine (Amersham) are added and the cells incubated for an additional 2 h. Thereafter, the cells are fixed in 10% trichloroacetic acid for more than 15 min on ice, and εolubilized with 1 M NaOH. The cell extracts are neutralized with 1 M HC1 and 3 H radioactivity determined in a liquid scintillation counter. Example 10. SCREENING ASSAY FOR COMPOUNDS WHICH ALTER ENDOGENOUS OPl RECEPTOR EXPRESSION LEVELS Candidate compound(ε) which can be administered to affect the level of a given endogenous OPl receptor can be found using the following screening assay, in which the level of OPl receptor production by a cell type which produces measurable levelε of the receptor is determined by incubating the cell in culture with and without the candidate compound, in order to asseεε the effects of the compound on the cell. This also can be accompliεhed by detection of the OPl receptor either at the protein level by Western blot or immunolocalization, or at the RNA level by Northern blot or situ hydridization. The protocol iε baεed on a procedure for identifying compounds which alter endogenous levels of OPl expression, a detailed description also may be found in PCT US 92/07359, incorporated herein by reference. Cell cultures of, for example, bone, brain, intestine, lung, heart, eye, breast, gonadε, kidney, adrenalε, urinary bladder, brain, or other organs, may be prepared as described widely in the literature. For example, kidneyε may be explanted from neonatal or new born or young or adult rodentε (mouεe or rat) and uεed in organ culture as whole or sliced (1-4 mm) tiεsues. Primary tiεεue cultureε and eεtablished cell lines, also derived from kidney, adrenals, urinary, bladder, brain, mammary, or other tisεues may be established in multiwell plates (6 well or 24 well) according to conventional cell culture techniques, and are cultured in the absence or presence of serum for a period of time (1-7 days) . Cells can be cultured, for example, in Dulbecco's Modified Eagle medium (Gibco, Long Island, NY) containing serum (e.g., fetal calf serum at 1%-10%, Gibco) or in serum-deprived medium, as desired, or in defined medium (e.g., containing insulin, transferrin, glucoεe, albumin, or other growth factorε) . Cell samples for testing the level of OPl receptor production are collected periodically and evaluated for receptor production by immunoblot analysis (Sambrook et al. , eds., 1989, Molecular Cloning, Cold Spring Harbor Presε, Cold Spring Harbor, NY) , or, alternatively, a portion of the cell culture itεelf can be collected periodically and used to prepare polyA÷ RNA for mRNA analysis by Northern blot analysiε. To monitor de novo receptor synthesis, some cultures are labeled according to conventional procedures with an "s-methionine/ 3 - > S-cysteine mixture for 6-24 hourε and then evaluated to quantitate receptor synthesis by conventional immunoassay methods. Alternatively, anti-receptor antibodies may be labelled and incubated with the cells or cell lysates, and the bound complexes detected and quantitated by conventional means, such as those described hereinabove. Northern blotε may be performed using a portion of the OPl receptor coding sequence to create hybridization probeε, and following the RNA hybridization protocol described herein. Example 11. GENERAL FORMULATION/ ADMINISTRATION CONSIDERATIONS The analogs and constructs described herein can be provided to an individual as part of a therapy to enhance, inhibit, or othewise modulate the in vivo binding interaction between OPl and one or more OPl-specific cell surface receptors. The molecules then comprise part of a pharmaceutical composition as deεcribed herein below and can be adminiεtered by any εuitable meanε, preferably directly or systemically, e.g., parenterally or orally. Where the therapeutic molecule is to be provided directly (e.g., locally, as by injection, to a desired tissue site) , or parenterally, such as by intravenouε, εubcutaneous, intramuscular, intraorbital, ophthalmic, intraventricular, intracranial, intracapsular, intraspinal, intracisternal, intraperitoneal, buccal, rectal, vaginal, intranasal or by aerosol adminiεtration, the therapeutic preferably compriεeε part of an aqueous solution. The solution preferably is physiologically acceptable so that in addition to delivery of the desired morphogen to the patient, the solution does not otherwise adversely affect the patient's electrolyte and volume balance. The aqueous medium for the therapeutic molecule thus may comprise normal physiologic saline (0.9% NaCl, 0.15M), pH 7-7.4 or other pharmaceutically acceptable salts thereof. Useful solutions for oral or parenteral administration can be prepared by any of the methods well known in the pharmaceutical art, described, for example, in Remington'ε Pharmaceutical Sciences, (Gennaro, A., ed. ) , Mack Pub., 1990. Formulations may include, for example, polyalkylene glycolε such as polyethylene glycol, oils of vegetable origin, hydrogenated naphthalenes, and the like. Formulationε for direct adminiεtration, in particular, can include glycerol and other compositions of high viscosity. Biocompatible, preferably bioresorbable polymers, including, for example, hyaluronic acid, collagen, tricalcium phosphate, polybutyrate, polylactide, polyglycolide and lactide/glycolide copolymers, may be useful excipients to control the release of the morphogen in vivo. Other potentially useful parenteral delivery systems for these therapeutic molecules include ethylene-vinyl acetate copolymer particles, osmotic pumps, implantable infusion systemε, and lipoεomeε. Formulationε for inhalation administration may contain as excipients, for example, lactose, or can be aqueous solutions containing, for example, polyoxyethylene-9-lauryl ether, glycocholate and deoxycholate, or oily solutionε for administration in the form of nasal drops, or as a gel to be applied intranasally. Alternatively, the morphogens described herein may be administered orally. The therapeutic molecules also can be associated with means for targeting the therapeutic to a desired tisεue. For example, tetracycline and diphosphonates (bisphosphonates) are known to bind to bone mineral, particularly at zones of bone remodeling, when they are provided systemically in a mammal. Accordingly, theεe molecules may be included as useful agents for targeting therapeutics to bone tissue. Alternatively, an antibody or other binding protein that interactε specifically with a surface molecule on the desired target tissue cells also can be used. Such targeting molecules further can be covalently asεociated to the therapeutic molecule e.g., by chemical croεεlinking, or by using standard genetic engineering means to create, for example, an acid labile bond such as an Asp-Pro linkage. Uεeful targeting molecules can be designed, for example, using the single chain binding site technology disclosed, for example, in U.S. Pat. No. 5,091,513. Finally, therapeutic molecules can be administered alone or in combination with other molecules known to have a beneficial effect on tissue morphogenesis, including molecules capable of tissue repair and regeneration and/or inhibiting inflammation. Examples of useful cofactors for stimulating bone tisεue growth in oεteoporotic individualε, for example, include but are not limited to, vitamin D 3 , calcitonin, prostaglandins, parathyroid hormone, dexamethasone, estrogen and IGF-I or IGF-II. Useful cofactors for nerve tissue repair and regeneration can include nerve growth factors. Other uεeful cofactors include symptom-alleviating cofactors, including antiseptics, antibiotics, antiviral and antifungal agents and analgesics and anestheticε. Therapeutic moleculeε further can be formulated into pharmaceutical compoεitions by admixture with pharmaceutically acceptable nontoxic excipients and carriers. As noted above, such compositions can be prepared for parenteral administration, particularly in the form of liquid solutionε or suspensions; for oral administration, particularly in the form of tablets or capsules; or intranasally, particularly in the form of powderε, naεal dropε or aeroεols. Where adhesion to a tissue surface is desired the composition may include the morphogen dispersed in a fibrinogen-thrombin composition or other bioadhesive such aε iε disclosed, for example in PCT US91/09275, the disclosure of which is incorporated herein by reference. The composition then can be painted, sprayed or otherwise applied to the desired tissue surface. The compoεitionε can be formulated for parenteral or oral administration to humans or other mammals in therapeutically effective amounts, e.g., amounts which provide appropriate concentrations of the analog to target tisεue for a time sufficient to induce the desired effect. Where the analog is to be used as part of a transplant procedure, it can be provided to the living tiεεue or organ to be transplanted prior to removal of tisεue or organ from the donor. The analog may be provided to the donor host directly, as by injection of a formulation comprising the analog into the tissue, or indirectly, e.g., by oral or parenteral adminiεtration, using any of the means described above. Alternatively or, in addition, once removed from the donor, the organ or living tisεue can be placed in a preεervation εolution containing the therapeutic molecule. In addition, the recipient alεo preferably iε provided with the analog just prior to, or concommitant with, transplantation. In all caseε, the analog can be adminiεtered directly to the tiεεue at riεk, as by injection to the tissue, or it may be provided syεtemically, either by oral or parenteral administration, using any of the methods and formulations described herein and/or known in the art. Where the therapeutic molecule compriseε part of a tiεεue or organ preservation solution, any commercially available preservation εolution can be uεed to advantage. For example, uεeful εolutionε known in the art include Collinε solution, Wisconεin εolution, Belzer εolution, Eurocollinε solution and lactated Ringer's solution. Generally, an organ preservation solution usually posseεses one or more of the following properties: (a) an osmotic presεure substantially equal to that of the inside of a mammalian cell, (εolutions typically are hyperosmolar and have K+ and/or Mg++ ions present in an amount εufficient to produce an oεmotic pressure εlightly higher than the inεide of a mammalian cell) ; (b) the solution typically iε capable of maintaining substantially normal ATP levels in the cells; and (c) the solution uεually allowε optimum maintenance of glucose metabolism in the cells. Organ preεervation solutionε also may contain anticoagulants, energy sources such as glucose, fructose and other sugars, metabolites, heavy metal chelators, glycerol and other materialε of high viscosity to enhance survival at low temperatures, free oxygen radical inhibiting and/or scavenging agents and a pH indicator. A detailed deεcription of preεervation solutions and useful components can be found, for example, in US Patent No. 5,002,965, the disclosure of which is incorporated herein by reference. As will be appreciated by thoεe εkilled in the art, the concentration of the compoundε deεcribed in a therapeutic composition will vary depending upon a number of factors, including the dosage of the drug to be administered, the chemical characteristics (e.g., hydrophobicity) of the compounds employed, and the route of administration. The preferred dosage of drug to be administered also is likely to depend on such variables aε the type and extent of tissue losε or defect, the overall health εtatuε of the particular patient, the relative biological efficacy of the compound εelected, the formulation of the compound, the presence and types of excipientε in the formulation, and the route of adminiεtration. In general terms, the therapeutic molecules of this invention may be provided to and individual where typical dose ranges are from about 10 ng/kg to about 1 g/kg of body weight per day; a preferred dose range being from about 0.1 mg/kg to 100 mg/kg of body weight. No obvious morphogen- induced pathological lesions are induced when mature morphogen (e.g., OP-l, 20 mg) is administered daily to normal growing rats for 21 consecutive days. Moreover, 10 mg systemic injections of morphogen (e.g., OP-l) injected daily for 10 days into normal newborn mice does not produce any groεε abnormalities. Other Embodiments The invention may be embodied in other specific formε without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. SEQUENCE LISTING (1) GENERAL INFORMATION: (i) APPLICANT: (B) NAME: CREATIVE BIOMOLCULES, INC (C) STREET: 45 SOUTH STREET (D) STATE: HOPKINTON (E) COUNTRY: USA (F) POSTAL CODE (ZIP) : 01748 (G) TELEPHONE: (508) -435-9001 (H) TELEFAX: (508) -435-0992 (I) TELEX: (i) APPLICANT: (B) NAME: LUDWIG INSTITUTE FOR CANCER RESEARCH (C) STREET: 1345 AVENUE OF THE AMERICAS (D) STATE: NY (E) COUNTRY: USA (F) POSTAL CODE (ZIP) : 10105 (G) TELEPHONE: (212) -765-3000 (H) TELEFAX: (I) TELEX: (ii) TITLE OF INVENTION: MORPHOGENIC PROTEIN-SPECIFIC CELL SURFACE RECEPTORS AND USES THEREFOR (iii) NUMBER OF SEQUENCES: 15 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: PATENT ADMINISTRATOR, TESTA, HURWITZ & THIBEAULT (B) STREET: 53 STATE STREET (C) CITY: BOSTON (D) STATE: MA (E) COUNTRY: USA (F) ZIP: 02109 (V) COMPUTER READABLE FORM: (A) MEDIUM TYPE: Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: Patentin Release #1.0, Version #1.25 (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: (B) FILING DATE: (C) CLASSIFICATION: (viii) ATTORNEY/AGENT INFORMATION: (A) NAME: PITCHER, EDMUND R. (B) REGISTRATION NUMBER: 27,829 (C) REFERENCE/DOCKET NUMBER: CRP-097PC (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: (617) 248-7000 (B) TELEFAX: (617) 248-7100 (2) INFORMATION FOR SEQ ID NO:l: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 1509 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (ix) FEATURE: (A) NAME/KEY: CDS (B) LOCATION: 1..1509 (D) OTHER INFORMATION: /product= "Human ALK1" (xi) SEQUENCE DESCRIPTION: SEQ ID NO:l: ATG ACC TTG GGC TCC CCC AGG AAA GGC CTT CTG ATG CTG CTG ATG GCC 48 Met Thr Leu Gly Ser Pro Arg Lys Gly Leu Leu Met Leu Leu Met Ala 1 5 10 15 TTG GTG ACC CAG GGA GAC CCT GTG AAG CCG TCT CGG GGC CCG CTG GTG 96 Leu Val Thr Gin Gly Asp Pro Val Lys Pro Ser Arg Gly Pro Leu Val 20 25 30 ACC TGC ACG TGT GAG AGC CCA CAT TGC AAG GGG CCT ACC TGC CGG GGG 144 Thr Cys Thr Cys Glu Ser Pro His Cys Lys Gly Pro Thr Cys Arg Gly 35 40 45 GCC TGG TGC ACA GTA GTG CTG GTG CGG GAG GAG GGG AGG CAC CCC CAG 192 Ala Trp Cys Thr Val Val Leu Val Arg Glu Glu Gly Arg His Pro Gin 50 55 60 GAA CAT CGG GGC TGC GGG AAC TTG CAC AGG GAG CTC TGC AGG GGG CGC 240 Glu His Arg Gly Cys Gly Asn Leu His Arg Glu Leu Cys Arg Gly Arg 65 70 75 80 CCC ACC GAG TTC GTC AAC CAC TAC TGC TGC GAC AGC CAC CTC TGC AAC 288 Pro Thr Glu Phe Val Asn His Tyr Cys Cys Asp Ser His Leu Cys Asn 85 90 95 CAC AAC GTG TCC CTG GTG CTG GAG GCC ACC CAA CCT CCT TCG GAG CAG 336 His Asn Val Ser Leu Val Leu Glu Ala Thr Gin Pro Pro Ser Glu Gin 100 105 110 CCG GGA ACA GAT GGC CAG CTG GCC CTG ATC CTG GGC CCC GTG CTG GCC 384 Pro Gly Thr Asp Gly Gin Leu Ala Leu lie Leu Gly Pro Val Leu Ala 115 120 125 TTG CTG GCC CTG GTG GCC CTG GGT GTC CTG GGC CTG TGG CAT GTC CGA 432 Leu Leu Ala Leu Val Ala Leu Gly Val Leu Gly Leu Trp His Val Arg 130 135 140 CGG AGG CAG GAG AAG CAG CGT GGC CTG CAC AGC GAG CTG GGA GAG TCC 480 Arg Arg Gin Glu Lys Gin Arg Gly Leu His Ser Glu Leu Gly Glu Ser 145 150 155 160 AGT CTC ATC CTG AAA GCA TCT GAG CAG GGC GAC ACG ATG TTG GGG GAC 528 Ser Leu lie Leu Lys Ala Ser Glu Gin Gly Asp Thr Met Leu Gly Asp 165 170 175 CTC CTG GAC AGT GAC TGC ACC ACA GGG AGT GGC TCA GGG CTC CCC TTC 576 Leu Leu Asp Ser Asp Cys Thr Thr Gly Ser Gly Ser Gly Leu Pro Phe 180 185 190 CTG GTG CAG AGG ACA GTG GCA CGG CAG GTT GCC TTG GTG GAG TGT GTG 624 Leu Val Gin Arg Thr Val Ala Arg Gin Val Ala Leu Val Glu Cys Val 195 200 205 GGA AAA GGC CGC TAT GGC GAA GTG TGG CGG GGC TTG TGG CAC GGT GAG 672 Gly Lys Gly Arg Tyr Gly Glu Val Trp Arg Gly Leu Trp His Gly Glu 210 215 220 AGT GTG GCC GTC AAG ATC TTC TCC TCG AGG GAT GAA CAG TCC TGG TTC 720 Ser Val Ala Val Lys lie Phe Ser Ser Arg Asp Glu Gin Ser Trp Phe 225 230 235 240 CGG GAG ACT GAG ATC TAT AAC ACA GTA TTG CTC AGA CAC GAC AAC ATC 768 Arg Glu Thr Glu lie Tyr Asn Thr Val Leu Leu Arg His Asp Asn lie 245 250 255 CTA GGC TTC ATC GCC TCA GAC ATG ACC TCC CGC AAC TCG AGC ACG CAG 816 Leu Gly Phe lie Ala Ser Asp Met Thr Ser Arg Asn Ser Ser Thr Gin 260 265 270 CTG TGG CTC ATC ACG CAC TAC CAC GAG CAC GGC TCC CTC TAC GAC TTT 864 Leu Trp Leu lie Thr His Tyr His Glu His Gly Ser Leu yr Asp Phe 275 280 285 CTG CAG AGA CAG ACG CTG GAG CCC CAT CTG GCT CTG AGG CTA GCT GTG 912 Leu Gin Arg Gin Thr Leu Glu Pro His Leu Ala Leu Arg Leu Ala Val 290 295 300 TCC GCG GCA TGC GGC CTG GCG CAC CTG CAC GTG GAG ATC TTC GGT ACA 960 Ser Ala Ala Cys Gly Leu Ala His Leu His Val Glu lie Phe Gly Thr 305 310 315 320 CAG GGC AAA CCA GCC ATT GCC CAC CGC GAC TTC AAG AGC CGC AAT GTG 1008 Gin Gly Lys Pro Ala lie Ala His Arg Asp Phe Lys Ser Arg Asn Val 325 330 335 CTG GTC AAG AGC AAC CTG CAG TGT TGC ATC GCC GAC CTG GGC CTG GCT 1056 Leu Val Lys Ser Asn Leu Gin Cys Cys lie Ala Asp Leu Gly Leu Ala 340 345 350 GTG ATG CAC TCA CAG GGC AGC GAT TAC CTG GAC ATC GGC AAC AAC CCG 1104 Val Met His Ser Gin Gly Ser Asp Tyr Leu Asp lie Gly Asn Asn Pro 355 360 365 AGA GTG GGC ACC AAG CGG TAC ATG GCA CCC GAG GTG CTG GAC GAG CAG 1152 Arg Val Gly Thr Lys Arg Tyr Met Ala Pro Glu Val Leu Asp Glu Gin 370 375 380 ATC CGC ACG GAC TGC TTT GAG TCC TAC AAG TGG ACT GAC ATC TGG GCC 1200 lie Arg Thr Asp Cys Phe Glu Ser Tyr Lys Trp Thr Asp lie Trp Ala 385 390 395 400 TTT GGC CTG GTG CTG TGG GAG ATT GCC CGC CGG ACC ATC GTG AAT GGC 1248 Phe Gly Leu Val Leu Trp Glu lie Ala Arg Arg Thr lie Val Asn Gly 405 410 415 ATC GTG GAG GAC TAT AGA CCA CCC TTC TAT GAT GTG GTG CCC AAT GAC 1296 lie Val Glu Asp Tyr Arg Pro Pro Phe Tyr Asp Val Val Pro Asn Asp 420 425 430 CCC AGC TTT GAG GAC ATG AAG AAG GTG GTG TGT GTG GAT CAG CAG ACC 1344 Pro Ser Phe Glu Asp Met Lys Lys Val Val Cys Val Asp Gin Gin Thr 435 440 445 CCC ACC ATC CCT AAC CGG CTG GCT GCA GAC CCG GTC CTC TCA GGC CTA 1392 Pro Thr lie Pro Asn Arg Leu Ala Ala Asp Pro Val Leu Ser Gly Leu 450 455 460 GCT CAG ATG ATG CGG GAG TGC TGG TAC CCA AAC CCC TCT GCC CGA CTC 1440 Ala Gin Met Met Arg Glu Cys Trp Tyr Pro Asn Pro Ser Ala Arg Leu 465 470 475 480 ACC GCG CTG CGG ATC AAG AAG ACA CTA CAA AAA ATT AGC AAC AGT CCA 1488 Thr Ala Leu Arg lie Lys Lys Thr Leu Gin Lys lie Ser Asn Ser Pro 485 490 495 GAG AAG CCT AAA GTG ATT CAA 1509 Glu Lys Pro Lys Val lie Gin 500 (2) INFORMATION FOR SEQ ID NO:2: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 503 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2: Met Thr Leu Gly Ser Pro Arg Lys Gly Leu Leu Met Leu Leu Met Ala 1 5 10 15 Leu Val Thr Gin Gly Asp Pro Val Lys Pro Ser Arg Gly Pro Leu Val 20 25 30 Thr Cys Thr Cys Glu Ser Pro His Cys Lys Gly Pro Thr Cys Arg Gly 35 40 45 Ala Trp Cys Thr Val Val Leu Val Arg Glu Glu Gly Arg His Pro Gin 50 55 60 Glu His Arg Gly Cys Gly Asn Leu His Arg Glu Leu Cys Arg Gly Arg 65 70 75 80 Pro Thr Glu Phe Val Asn His Tyr Cys Cys Asp Ser His Leu Cys Asn 85 90 95 His Asn Val Ser Leu Val Leu Glu Ala Thr Gin Pro Pro Ser Glu Gin 100 105 110 Pro Gly Thr Asp Gly Gin Leu Ala Leu lie Leu Gly Pro Val Leu Ala 115 120 125 Leu Leu Ala Leu Val Ala Leu Gly Val Leu Gly Leu Trp His Val Arg 130 135 140 Arg Arg Gin Glu Lys Gin Arg Gly Leu His Ser Glu Leu Gly Glu Ser 145 150 155 160 Ser Leu lie Leu Lys Ala Ser Glu Gin Gly Asp Thr Met Leu Gly Asp 165 170 175 Leu Leu Asp Ser Asp Cys Thr Thr Gly Ser Gly Ser Gly Leu Pro Phe 180 185 190 Leu Val Gin Arg Thr Val Ala Arg Gin Val Ala Leu Val Glu Cys Val 195 200 205 Gly Lys Gly Arg Tyr Gly Glu Val Trp Arg Gly Leu Trp His Gly Glu 210 215 220 Ser Val Ala Val Lys lie Phe Ser Ser Arg Asp Glu Gin Ser Trp Phe 225 230 235 240 Arg Glu Thr Glu lie Tyr Asn Thr Val Leu Leu Arg His Asp Asn lie 245 250 255 Leu Gly Phe lie Ala Ser Asp Met Thr Ser Arg Asn Ser Ser Thr Gin 260 265 270 Leu Trp Leu lie Thr His Tyr His Glu His Gly Ser Leu Tyr Asp Phe 275 280 285 Leu Gin Arg Gin Thr Leu Glu Pro His Leu Ala Leu Arg Leu Ala Val 290 295 300 Ser Ala Ala Cys Gly Leu Ala His Leu His Val Glu lie Phe Gly Thr 305 310 315 320 Gin Gly Lys Pro Ala lie Ala His Arg Asp Phe Lys Ser Arg Asn Val 325 330 335 Leu Val Lys Ser Asn Leu Gin Cys Cys lie Ala Asp Leu Gly Leu Ala 340 345 350 Val Met His Ser Gin Gly Ser Asp Tyr Leu Asp lie Gly Asn Asn Pro 355 360 365 Arg Val Gly Thr Lys Arg Tyr Met Ala Pro Glu Val Leu Asp Glu Gin 370 375 380 lie Arg Thr Asp Cys Phe Glu Ser Tyr Lys Trp Thr Asp lie Trp Ala 385 390 395 400 Phe Gly Leu Val Leu Trp Glu lie Ala Arg Arg Thr lie Val Asn Gly 405 410 415 lie Val Glu Asp Tyr Arg Pro Pro Phe Tyr Asp Val Val Pro Asn Asp 420 425 430 Pro Ser Phe Glu Asp Met Lys Lys Val Val Cys Val Asp Gin Gin Thr 435 440 445 Pro Thr lie Pro Asn Arg Leu Ala Ala Asp Pro Val Leu Ser Gly Leu 450 455 460 Ala Gin Met Met Arg Glu Cys Trp Tyr Pro Asn Pro Ser Ala Arg Leu 465 470 475 480 Thr Ala Leu Arg lie Lys Lys Thr Leu Gin Lys lie Ser Asn Ser Pro 485 490 495 Glu Lys Pro Lys Val lie Gin 500 (2) INFORMATION FOR SEQ ID NO:3: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 2724 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (ix) FEATURE: (A) NAME/KEY: CDS (B) LOCATION: 104..1633 (D) OTHER INFORMATION: /product= "Human ALK2" (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3: CTCCGAGTAC CCCAGTGACC AGAGTGAGAG AAGCTCTGAA CGAGGGCACG CGGCTTGAAG 60 GACTGTGGGC AGATGTGACC AAGAGCCTGC ATTAAGTTGT ACA ATG GTA GAT GGA 115 Met Val Asp Gly 1 GTG ATG ATT CTT CCT GTG CTT ATC ATG ATT GCT CTC CCC TCC CCT AGT 163 Val Met lie Leu Pro Val Leu lie Met lie Ala Leu Pro Ser Pro Ser 5 10 15 20 ATG GAA GAT GAG AAG CCC AAG GTC AAC CCC AAA CTC TAC ATG TGT GTG 211 Met Glu Asp Glu Lys Pro Lys Val Asn Pro Lys Leu Tyr Met Cys Val 25 30 35 TGT GAA GGT CTC TCC TGC GGT AAT GAG GAC CAC TGT GAA GGC CAG CAG 259 Cys Glu Gly Leu Ser Cys Gly Asn Glu Asp His Cys Glu Gly Gin Gin 40 45 50 TGC TTT TCC TCA CTG AGC ATC AAC GAT GGC TTC CAC GTC TAC CAG AAA 307 Cys Phe Ser Ser Leu Ser lie Asn Asp Gly Phe His Val Tyr Gin Lys 55 60 65 GGC TGC TTC CAG GTT TAT GAG CAG GGA AAG ATG ACC TGT AAG ACC CCG 355 Gly Cys Phe Gin Val Tyr Glu Gin Gly Lys Met Thr Cys Lys Thr Pro 70 75 80 CCG TCC CCT GGC CAA GCT GTG GAG TGC TGC CAA GGG GAC TGG TGT AAC 403 Pro Ser Pro Gly Gin Ala Val Glu Cys Cys Gin Gly Asp Trp Cys Asn 85 90 95 100 AGG AAC ATC ACG GCC CAG CTG CCC ACT AAA GGA AAA TCC TTC CCT GGA 451 Arg Asn lie Thr Ala Gin Leu Pro Thr Lys Gly Lys Ser Phe Pro Gly 105 110 115 ACA CAG AAT TTC CAC TTG GAG GTT GGC CTC ATT ATT CTC TCT GTA GTG 499 Thr Gin Asn Phe His Leu Glu Val Gly Leu lie lie Leu Ser Val Val 120 125 130 TTC GCA GTA TGT CTT TTA GCC TGC CTG CTG GGA GTT GCT CTC CGA AAA 547 Phe Ala Val Cys Leu Leu Ala Cys Leu Leu Gly Val Ala Leu Arg Lys 135 140 145 TTT AAA AGG CGC AAC CAA GAA CGC CTC AAT CCC CGA GAC GTG GAG TAT 595 Phe Lys Arg Arg Asn Gin Glu Arg Leu Asn Pro Arg Asp Val Glu Tyr 150 155 160 GGC ACT ATC GAA GGG CTC ATC ACC ACC AAT GTT GGA GAC AGC ACT TTA 643 Gly Thr lie Glu Gly Leu lie Thr Thr Asn Val Gly Asp Ser Thr Leu 165 170 175 180 GCA GAT TTA TTG GAT CAT TCG TGT ACA TCA GGA AGT GGC TCT GGT CTT 691 Ala Asp Leu Leu Asp His Ser Cys Thr Ser Gly Ser Gly Ser Gly Leu 185 190 195 CCT TTT CTG GTA CAA AGA ACA GTG GCT CGC CAG ATT ACA CTG TTG GAG 739 Pro Phe Leu Val Gin Arg Thr Val Ala Arg Gin lie Thr Leu Leu Glu 200 205 210 TGT GTC GGG AAA GGC AGG TAT GGT GAG GTG TGG AGG GGC AGC TGG CAA 787 Cys Val Gly Lys Gly Arg Tyr Gly Glu Val Trp Arg Gly Ser Trp Gin 215 220 225 GGG GAA AAT GTT GCC GTG AAG ATC TTC TCC TCC CGT GAT GAG AAG TCA 835 Gly Glu Asn Val Ala Val Lys lie Phe Ser Ser Arg Asp Glu Lys Ser 230 235 240 TGG TTC AGG GAA ACG GAA TTG TAC AAC ACT GTG ATG CTG AGG CAT GAA 883 Trp Phe Arg Glu Thr Glu Leu Tyr Asn Thr Val Met Leu Arg His Glu 245 250 255 260 AAT ATC TTA GGT TTC ATT GCT TCA GAC ATG ACA TCA AGA CAC TCC AGT 931 Asn lie Leu Gly Phe lie Ala Ser Asp Met Thr Ser Arg His Ser Ser 265 270 275 ACC CAG CTG TGG TTA ATT ACA CAT TAT CAT GAA ATG GGA TCG TTG TAC 979 Thr Gin Leu Trp Leu lie Thr His Tyr His Glu Met Gly Ser Leu Tyr 280 285 290 GAC TAT CTT CAG CTT ACT ACT CTG GAT ACA GTT AGC TGC CTT CGA ATA 1027 Asp Tyr Leu Gin Leu Thr Thr Leu Asp Thr Val Ser Cys Leu Arg lie 295 300 305 GTG CTG TCC ATA GCT AGT GGT CTT GCA CAT TTG CAC ATA GAG ATA TTT 1075 Val Leu Ser lie Ala Ser Gly Leu Ala His Leu His lie Glu lie Phe 310 315 320 GGG ACC CAA GGG AAA CCA GCC ATT GCC CAT CGA GAT TTA AAG AGC AAA 1123 Gly Thr Gin Gly Lys Pro Ala lie Ala His Arg Asp Leu Lys Ser Lys 325 330 335 340 AAT ATT CTG GTT AAG AAG AAT GGA CAG TGT TGC ATA GCA GAT TTG GGC 1171 Asn lie Leu Val Lys Lys Asn Gly Gin Cys Cys lie Ala Asp Leu Gly 345 350 355 CTG GCA GTC ATG CAT TCC CAG AGC ACC AAT CAG CTT GAT GTG GGG AAC 1219 Leu Ala Val Met His Ser Gin Ser Thr Asn Gin Leu Asp Val Gly Asn 360 365 370 AAT CCC CGT GTG GGC ACC AAG CGC TAC ATG GCC CCC GAA GTT CTA GAT 1267 Asn Pro Arg Val Gly Thr Lys Arg Tyr Met Ala Pro Glu Val Leu Asp 375 380 385 GAA ACC ATC CAG GTG GAT TGT TTC GAT TCT TAT AAA AGG GTC GAT ATT 1315 Glu Thr lie Gin Val Asp Cys Phe Asp Ser Tyr Lys Arg Val Asp lie 390 395 400 TGG GCC TTT GGA CTT GTT TTG TGG GAA GTG GCC AGG CGG ATG GTG AGC 1363 Trp Ala Phe Gly Leu Val Leu Trp Glu Val Ala Arg Arg Met Val Ser 405 410 415 420 AAT GGT ATA GTG GAG GAT TAC AAG CCA CCG TTC TAC GAT GTG GTT CCC 1411 Asn Gly lie Val Glu Asp Tyr Lys Pro Pro Phe Tyr Asp Val Val Pro 425 430 435 AAT GAC CCA AGT TTT GAA GAT ATG AGG AAG GTA GTC TGT GTG GAT CAA 1459 Asn Asp Pro Ser Phe Glu Asp Met Arg Lys Val Val Cys Val Asp Gin 440 445 450 CAA AGG CCA AAC ATA CCC AAC AGA TGG TTC TCA GAC CCG ACA TTA ACC 1507 Gin Arg Pro Asn lie Pro Asn Arg Trp Phe Ser Asp Pro Thr Leu Thr 455 460 465 TCT CTG GCC AAG CTA ATG AAA GAA TGC TGG TAT CAA AAT CCA TCC GCA 1555 Ser Leu Ala Lys Leu Met Lys Glu Cys Trp Tyr Gin Asn Pro Ser Ala 470 475 480 AGA CTC ACA GCA CTG CGT ATC AAA AAG ACT TTG ACC AAA ATT GAT AAT 1603 Arg Leu Thr Ala Leu Arg lie Lys Lys Thr Leu Thr Lys lie Asp Asn 485 490 495 500 TCC CTC GAC AAA TTG AAA ACT GAC TGT TGACATTTTC ATAGTGTCAA 1650 Ser Leu Asp Lys Leu Lys Thr Asp Cys 505 510 GAAGGAAGAT TTGACGTTGT TGTCATTGTC CAGCTGGGAC CTAATGCTGG CCTGACTGGT 1710 TGTCAGAATG GAATCCATCT GTCTCCCTCC CCAAATGGCT GCTTTGACAA GGCAGACGTC 1770 GTACCCAGCC ATGTGTTGGG GAGACATCAA AACCACCCTA ACCTCGCTCG ATGACTGTGA 1830 ACTGGGCATT TCACGAACTG TTCACACTGC AGAGACTAAT GTTGGACAGA CACTGTTGCA 1890 AAGGTAGGGA CTGGAGGAAC ACAGAGAAAT CCTAAAAGAG ATCTGGGCAT TAAGTCAGTG 1950 GCTTTGCATA GCTTTCACAA GTCTCCTAGA CACTCCCCAC GGGAAACTCA AGGAGGTGGT 2010 GAATTTTTAA TCAGCAATAT TGCCTGTGCT TCTCTTCTTT ATTGCACTAG GAATTCTTTG 2070 CATTCCTTAC TTGCACTGTT ACTCTTAATT TTAAAGACCC AACTTGCCAA AATGTTGGCT 2130 GCGTACTCCA CTGGTCTGTC TTTGGATAAT AGGAATTCAA TTTGGCAAAA CAAAATGTAA 2190 TGTCAGACTT TGCTGCATTT TACACATGTG CTGATGTTTA CAATGATGCC GAACATTAGG 2250 AATTGTTTAT ACACAACTTT GCAAATTATT TATTACTTGT GCACTTAGTA GTTTTTACAA 2310 AACTGCTTTG TGCATATGTT AAAGCTTATT TTTATGTGGT CTTATGATTT TATTACAGAA 2370 ATGTTTTTAA CACTATACTC TAAAATGGAC ATTTTCTTTT ATTATCAGTT AAAATCACAT 2430 TTTAAGTGCT TCACATTTGT ATGTGTGTAG ACTGTAACTT TTTTTCAGTT CATATGCAGA 2490 ACGTATTTAG CCATTACCCA CGTGACACCA CCGAATATAT TATCGATTTA GAAGCAAAGA 2550 TTTCAGTAGA ATTTTAGTCC TGAACGCTAC GGGGAAAATG CATTTTCTTC AGAATTATCC 2610 ATTACGTGCA TTTAAACTCT GCCAGAAAAA AATAACTATT TTGTTTTAAT CTACTTTTTG 2670 TATTTAGTAG TTATTTGTAT AAATTAAATA AACTGTTTTC AAGTCAAAAA AAAA 2724 (2) INFORMATION FOR SEQ ID NO:4 : (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 509 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4: Met Val Asp Gly Val Met lie Leu Pro Val Leu lie Met lie Ala Leu 1 5 10 15 Pro Ser Pro Ser Met Glu Asp Glu Lys Pro Lys Val Asn Pro Lys Leu 20 25 30 Tyr Met Cys Val Cys Glu Gly Leu Ser Cys Gly Asn Glu Asp His Cys 35 40 45 Glu Gly Gin Gin Cys Phe Ser Ser Leu Ser lie Asn Asp Gly Phe His 50 55 60 Val Tyr Gin Lys Gly Cys Phe Gin Val Tyr Glu Gin Gly Lys Met Thr 65 70 75 80 Cys Lys Thr Pro Pro Ser Pro Gly Gin Ala Val Glu Cys Cys Gin Gly 85 90 95 Asp Trp Cys Asn Arg Asn lie Thr Ala Gin Leu Pro Thr Lys Gly Lys 100 105 110 Ser Phe Pro Gly Thr Gin Asn Phe His Leu Glu Val Gly Leu lie lie 115 120 125 Leu Ser Val Val Phe Ala Val Cys Leu Leu Ala Cys Leu Leu Gly Val 130 135 140 Ala Leu Arg Lys Phe Lys Arg Arg Asn Gin Glu Arg Leu Asn Pro Arg 145 150 155 160 Asp Val Glu Tyr Gly Thr lie Glu Gly Leu lie Thr Thr Asn Val Gly 165 170 175 Asp Ser Thr Leu Ala Asp Leu Leu Asp His Ser Cys Thr Ser Gly Ser 180 185 190 Gly Ser Gly Leu Pro Phe Leu Val Gin Arg Thr Val Ala Arg Gin lie 195 ' 200 205 Thr Leu Leu Glu Cys Val Gly Lys Gly Arg Tyr Gly Glu Val Trp Arg 210 215 220 Gly Ser Trp Gin Gly Glu Asn Val Ala Val Lys lie Phe Ser Ser Arg 225 230 235 240 Asp Glu Lys Ser Trp Phe Arg Glu Thr Glu Leu Tyr Asn Thr Val Met 245 250 255 Leu Arg His Glu Asn lie Leu Gly Phe lie Ala Ser Asp Met Thr Ser 260 265 270 Arg His Ser Ser Thr Gin Leu Trp Leu lie Thr His Tyr His Glu Met 275 280 285 Gly Ser Leu Tyr Asp Tyr Leu Gin Leu Thr Thr Leu Asp Thr Val Ser 290 295 300 Cys Leu Arg lie Val Leu Ser lie Ala Ser Gly Leu Ala His Leu His 305 310 315 320 lie Glu lie Phe Gly Thr Gin Gly Lys Pro Ala lie Ala His Arg Asp 325 330 335 Leu Lys Ser Lys Asn lie Leu Val Lys Lys Asn Gly Gin Cys Cys lie 340 345 350 Ala Asp Leu Gly Leu Ala Val Met His Ser Gin Ser Thr Asn Gin Leu 355 360 365 Asp Val Gly Asn Asn Pro Arg Val Gly Thr Lys Arg Tyr Met Ala Pro 370 375 380 Glu Val Leu Asp Glu Thr lie Gin Val Asp Cys Phe Asp Ser Tyr Lys 385 390 395 400 Arg Val Asp lie Trp Ala Phe Gly Leu Val Leu Trp Glu Val Ala Arg 405 410 415 Arg Met Val Ser Asn Gly lie Val Glu Asp Tyr Lys Pro Pro Phe Tyr 420 425 430 Asp Val Val Pro Asn Asp Pro Ser Phe Glu Asp Met Arg Lys Val Val 435 440 445 Cys Val Asp Gin Gin Arg Pro Asn lie Pro Asn Arg Trp Phe Ser Asp 450 455 460 Pro Thr Leu Thr Ser Leu Ala Lys Leu Met Lys Glu Cys Trp Tyr Gin 465 470 475 480 Asn Pro Ser Ala Arg Leu Thr Ala Leu Arg lie Lys Lys Thr Leu Thr 485 490 495 Lys lie Asp Asn Ser Leu Asp Lys Leu Lys Thr Asp Cys 500 505 (2) INFORMATION FOR SEQ ID NO:5: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 2932 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (ix) FEATURE: (A) NAME/KEY: CDS (B) LOCATION: 310..1908 (D) OTHER INFORMATION: /product= "Human ALK3" (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5: GCTCCGCGCC GAGGGCTGGA GGATGCGTTC CCTGGGGTCC GGACTTATGA AAATATGCAT 60 CAGTTTAATA CTGTCTTGGA ATTCATGAGA TGGAAGCATA GGTCAAAGCT GTTTGGAGAA 120 AATCAGAAGT ACAGTTTTAT CTAGCCACAT CTTGGAGGAG TCGTAAGAAA GCAGTGGGAG 180 TTGAAGTCAT TGTCAAGTGC TTGCGATCTT TTACAAGAAA ATCTCACTGA ATGATAGTCA 240 TTTAAATTGG TGAAGTAGCA AGACCAATTA TTAAAGGTGA CAGTACACAG GAAACATTAC 300 AATTGAACA ATG ACT CAG CTA TAC ATT TAC ATC AGA TTA TTG GGA GCC 348 Met Thr Gin Leu Tyr lie Tyr lie Arg Leu Leu Gly Ala 1 5 10 TAT TTG TTC ATC ATT TCT CGT GTT CAA GGA CAG AAT CTG GAT AGT ATG 396 Tyr Leu Phe lie lie Ser Arg Val Gin Gly Gin Asn Leu Asp Ser Met 15 20 25 CTT CAT GGC ACT GGG ATG AAA TCA GAC TCC GAC CAG AAA AAG TCA GAA 444 Leu His Gly Thr Gly Met Lys Ser Asp Ser Asp Gin Lys Lys Ser Glu 30 35 40 45 AAT GGA GTA ACC TTA GCA CCA GAG GAT ACC TTG CCT TTT TTA AAG TGC 492 Asn Gly Val Thr Leu Ala Pro Glu Asp Thr Leu Pro Phe Leu Lys Cys 50 55 60 TAT TGC TCA GGG CAC TGT CCA GAT GAT GCT ATT AAT AAC ACA TGC ATA 540 Tyr Cys Ser Gly His Cys Pro Asp Asp Ala lie Asn Asn Thr Cys lie 65 70 75 ACT AAT GGA CAT TGC TTT GCC ATC ATA GAA GAA GAT GAC CAG GGA GAA 588 Thr Asn Gly His Cys Phe Ala lie lie Glu Glu Asp Asp Gin Gly Glu 80 85 90 ACC ACA TTA GCT TCA GGG TGT ATG AAA TAT GAA GGA TCT GAT TTT CAG 636 Thr Thr Leu Ala Ser Gly Cys Met Lys Tyr Glu Gly Ser Asp Phe Gin 95 100 105 TGC AAA GAT TCT CCA AAA GCC CAG CTA CGC CGG ACA ATA GAA TGT TGT 684 Cys Lys Asp Ser Pro Lys Ala Gin Leu Arg Arg Thr lie Glu Cys Cys 110 115 120 125 CGG ACC AAT TTA TGT AAC CAG TAT TTG CAA CCC ACA CTG CCC CCT GTT 732 Arg Thr Asn Leu Cys Asn Gin Tyr Leu Gin Pro Thr Leu Pro Pro Val 130 135 140 GTC ATA GGT CCG TTT TTT GAT GGC AGC ATT CGA TGG CTG GTT TTG CTC 780 Val lie Gly Pro Phe Phe Asp Gly Ser lie Arg Trp Leu Val Leu Leu 145 150 155 ATT TCT ATG GCT GTC TGC ATA ATT GCT ATG ATC ATC TTC TCC AGC TGC 828 lie Ser Met Ala Val Cys lie lie Ala Met lie lie Phe Ser Ser Cys 160 165 170 TTT TGT TAC AAA CAT TAT TGC AAG AGC ATC TCA AGC AGA CGT CGT TAC 876 Phe Cys Tyr Lys His Tyr Cys Lys Ser lie Ser Ser Arg Arg Arg Tyr 175 180 185 AAT CGT GAT TTG GAA CAG GAT GAA GCA TTT ATT CCA GTT GGA GAA TCA 924 Asn Arg Asp Leu Glu Gin Asp Glu Ala Phe lie Pro Val Gly Glu Ser 190 195 200 205 CTA AAA GAC CTT ATT GAC CAG TCA CAA AGT TCT GGT AGT GGG TCT GGA 972 Leu Lys Asp Leu lie Asp Gin Ser Gin Ser Ser Gly Ser Gly Ser Gly 210 215 220 CTA CCT TTA TTG GTT CAG CGA ACT ATT GCC AAA CAG ATT CAG ATG GTC 1020 Leu Pro Leu Leu Val Gin Arg Thr lie Ala Lys Gin lie Gin Met Val 225 230 235 CGG CAA GTT GGT AAA GGC CGA TAT GGA GAA GTA TGG ATG GGC AAA TGG 1068 Arg Gin Val Gly Lys Gly Arg Tyr Gly Glu Val Trp Met Gly Lys Trp 240 245 250 CGT GGC GAA AAA GTG GCG GTG AAA GTA TTC TTT ACC ACT GAA GAA GCC 1116 Arg Gly Glu Lys Val Ala Val Lys Val Phe Phe Thr Thr Glu Glu Ala 255 260 265 AGC TGG TTT CGA GAA ACA GAA ATC TAC CAA ACT GTG CTA ATG CGC CAT 1164 Ser Trp Phe Arg Glu Thr Glu lie Tyr Gin Thr Val Leu Met Arg His 270 275 280 285 GAA AAC ATA CTT GGT TTC ATA GCG GCA GAC ATT AAA GGT ACA GGT TCC 1212 Glu Asn lie Leu Gly Phe lie Ala Ala Asp lie Lys Gly Thr Gly Ser 290 295 300 TGG ACT CAG CTC TAT TTG ATT ACT GAT TAC CAT GAA AAT GGA TCT CTC 1260 Trp Thr Gin Leu Tyr Leu lie Thr Asp Tyr His Glu Asn Gly Ser Leu 305 310 315 TAT GAC TTC CTG AAA TGT GCT ACA CTG GAC ACC AGA GCC CTG CTT AAA 1308 Tyr Asp Phe Leu Lys Cys Ala Thr Leu Asp Thr Arg Ala Leu Leu Lys 320 325 330 TTG GCT TAT TCA GCT GCC TGT GGT CTG TGC CAC CTG CAC ACA GAA ATT 1356 Leu Ala Tyr Ser Ala Ala Cys Gly Leu Cys His Leu His Thr Glu lie 335 340 345 TAT GGC ACC CAA GGA AAG CCC GCA ATT GCT CAT CGA GAC CTA AAG AGC 1404 Tyr Gly Thr Gin Gly Lys Pro Ala lie Ala His Arg Asp Leu Lys Ser 350 355 360 365 AAA AAC ATC CTC ATC AAG AAA AAT GGG AGT TGC TGC ATT GCT GAC CTG 1452 Lys Asn lie Leu lie Lys Lys Asn Gly Ser Cys Cys lie Ala Asp Leu 370 375 380 GGC CTT GCT GTT AAA TTC AAC AGT GAC ACA AAT GAA GTT GAT GTG CCC 1500 Gly Leu Ala Val Lys Phe Asn Ser Asp Thr Asn Glu Val Asp Val Pro 385 390 395 TTG AAT ACC AGG GTG GGC ACC AAA CGC TAC ATG GCT CCC GAA GTG CTG 1548 Leu Asn Thr Arg Val Gly Thr Lys Arg Tyr Met Ala Pro Glu Val Leu 400 405 410 GAC GAA AGC CTG AAC AAA AAC CAC TTC CAG CCC TAC ATC ATG GCT GAC 1596 Asp Glu Ser Leu Asn Lys Asn His Phe Gin Pro Tyr lie Met Ala Asp 415 420 425 ATC TAC AGC TTC GGC CTA ATC ATT TGG GAG ATG GCT CGT CGT TGT ATC 1644 He Tyr Ser Phe Gly Leu He He Trp Glu Met Ala Arg Arg Cys He 430 435 440 445 ACA GGA GGG ATC GTG GAA GAA TAC CAA TTG CCA TAT TAC AAC ATG GTA 1692 Thr Gly Gly He Val Glu Glu Tyr Gin Leu Pro Tyr Tyr Asn Met Val 450 455 460 CCG AGT GAT CCG TCA TAC GAA GAT ATG CGT GAG GTT GTG TGT GTC AAA 1740 Pro Ser Asp Pro Ser Tyr Glu Asp Met Arg Glu Val Val Cys Val Lys 465 470 475 CGT TTG CGG CCA ATT GTG TCT AAT CGG TGG AAC AGT GAT GAA TGT CTA 1788 Arg Leu Arg Pro He Val Ser Asn Arg Trp Asn Ser Asp Glu Cys Leu 480 485 490 CGA GCA GTT TTG AAG CTA ATG TCA GAA TGC TGG GCC CAC AAT CCA GCC 1836 Arg Ala Val Leu Lys Leu Met Ser Glu Cys Trp Ala His Asn Pro Ala 495 500 505 TCC AGA CTC ACA GCA TTG AGA ATT AAG AAG ACG CTT GCC AAG ATG GTT 1884 Ser Arg Leu Thr Ala Leu Arg He Lys Lys Thr Leu Ala Lys Met Val 510 515 520 525 GAA TCC CAA GAT GTA AAA ATC TGATGGTTAA ACCATCGGAG GAGAAACTCT 1935 Glu Ser Gin Asp Val Lys He 530 AGACTGCAAG AACTGTTTTT ACCCATGGCA TGGGTGGAAT TAGAGTGGAA TAAGGATGTT 1995 AACTTGGTTC TCAGACTCTT TCTTCACTAC GTGTTCACAG GCTGCTAATA TTAAACCTTT 2055 CAGTACTCTT ATTAGGATAC AAGCTGGGAA CTTCTAAACA CTTCATTCTT TATATATGGA 2115 CAGCTTTATT TTAAATGTGG TTTTTGATGC CTTTTTTTAA GTGGGTTTTT ATGAACTGCA 2175 TCAAGACTTC AATCCTGATT AGTGTCTCCA GTCAAGCTCT GGGTACTGAA TTGCCTGTTC 2235 ATAAAACGGT GCTTTCTGTG AAAGCCTTAA GAAGATAAAT GAGCGCAGCA GAGATGGAGA 2295 AATAGACTTT GCCTTTTACC TGAGACATTC AGTTCGTTTG TATTCTACCT TTGTAAAACA 2355 GCCTATAGAT GATGATGTGT TTGGGATACT GCTTATTTTA TGATAGTTTG TCCTGTGTCC 2415 TTAGTGATGT GTGTGTGTCT CCATGCACAT GCACGCCGGG ATTCCTCTGC TGCCATTTGA 2475 ATTAGAAGAA AATAATTTAT ATGCATGCAC AGGAAGATAT TGGTGGCCGG TGGTTTTGTG 2535 CTTTAAAAAT GCAATATCTG ACCAAGATTC GCCAATCTCA TACAAGCCAT TTACTTTGCA 2595 AGTGAGATAG CTTCCCCACC AGCTTTATTT TTTAACATGA AAGCTGATGC CAAGGCCAAA 2655 AGAAGTTTAA AGCATCTGTA AATTTGGACT GTTTTCCTTC AACCACCATT TTTTTTGTGG 2715 TTATTATTTT TGTCACGGAA AGCATCCTCT CCAAAGTTGG AGCTTCTATT GCCATGAACC 2775 ATGCTTACAA AGAAAGCACT TCTTATTGAA GTGAATTCCT GCATTTGATA GCAATGTAAG 2835 TGCCTATAAC CATGTTCTAT ATTCTTTATT CTCAGTAACT TTTAAAAGGG AAGTTATTTA 2895 TATTTTGTGT ATAATGTGCT TTATTTGCAA ATCACCC 2932 (2) INFORMATION FOR SEQ ID NO: 6: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 532 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 6: Met Thr Gin Leu Tyr He Tyr He Arg Leu Leu Gly Ala Tyr Leu Phe 1 5 10 15 He He Ser Arg Val Gin Gly Gin Asn Leu Asp Ser Met Leu His Gly 20 25 30 Thr Gly Met Lys Ser Asp Ser Asp Gin Lys Lys Ser Glu Asn Gly Val 35 40 45 Thr Leu Ala Pro Glu Asp Thr Leu Pro Phe Leu Lys Cys Tyr Cys Ser 50 55 60 Gly His Cys Pro Asp Asp Ala He Asn Asn Thr Cys He Thr Asn Gly 65 70 75 80 His Cys Phe Ala He He Glu Glu Asp Asp Gin Gly Glu Thr Thr Leu 85 90 95 Ala Ser Gly Cys Met Lys Tyr Glu Gly Ser Asp Phe Gin Cys Lys Asp 100 105 110 Ser Pro Lys Ala Gin Leu Arg Arg Thr He Glu Cys Cys Arg Thr Asn 115 120 125 Leu Cys Asn Gin Tyr Leu Gin Pro Thr Leu Pro Pro Val Val He Gly 130 135 140 Pro Phe Phe Asp Gly Ser He Arg Trp Leu Val Leu Leu He Ser Met 145 150 155 160 Ala Val Cys He He Ala Met He He Phe Ser Ser Cys Phe Cys Tyr 165 170 175 Lys His Tyr Cys Lys Ser He Ser Ser Arg Arg Arg Tyr Asn Arg Asp 180 185 190 Leu Glu Gin Asp Glu Ala Phe He Pro Val Gly Glu Ser Leu Lys Asp 195 200 205 Leu He Asp Gin Ser Gin Ser Ser Gly Ser Gly Ser Gly Leu Pro Leu 210 215 220 Leu Val Gin Arg Thr He Ala Lys Gin He Gin Met Val Arg Gin Val 225 230 235 240 Gly Lys Gly Arg Tyr Gly Glu Val Trp Met Gly Lys Trp Arg Gly Glu 245 250 255 Lys Val Ala Val Lys Val Phe Phe Thr Thr Glu Glu Ala Ser Trp Phe 260 265 270 Arg Glu Thr Glu He Tyr Gin Thr Val Leu Met Arg His Glu Asn He 275 280 285 Leu Gly Phe He Ala Ala Asp He Lys Gly Thr Gly Ser Trp Thr Gin 290 295 300 Leu Tyr Leu He Thr Asp Tyr His Glu Asn Gly Ser Leu Tyr Asp Phe 305 310 315 320 Leu Lys Cys Ala Thr Leu Asp Thr Arg Ala Leu Leu Lys Leu Ala Tyr 325 330 335 Ser Ala Ala Cys Gly Leu Cys His Leu His Thr Glu He Tyr Gly Thr 340 345 350 Gin Gly Lys Pro Ala He Ala His Arg Asp Leu Lys Ser Lys Asn He 355 360 365 Leu He Lys Lys Asn Gly Ser Cys Cys He Ala Asp Leu Gly Leu Ala 370 375 380 Val Lys Phe Asn Ser Asp Thr Asn Glu Val Asp Val Pro Leu Asn Thr 385 390 395 400 Arg Val Gly Thr Lys Arg Tyr Met Ala Pro Glu Val Leu Asp Glu Ser 405 410 415 Leu Asn Lys Asn His Phe Gin Pro Tyr He Met Ala Asp He Tyr Ser 420 425 430 Phe Gly Leu He He Trp Glu Met Ala Arg Arg Cys He Thr Gly Gly 435 440 445 He Val Glu Glu Tyr Gin Leu Pro Tyr Tyr Asn Met Val Pro Ser Asp 450 455 460 Pro Ser Tyr Glu Asp Met Arg Glu Val Val Cys Val Lys Arg Leu Arg 465 470 475 480 Pro He Val Ser Asn Arg Trp Asn Ser Asp Glu Cys Leu Arg Ala Val 485 490 495 Leu Lys Leu Met Ser Glu Cys Trp Ala His Asn Pro Ala Ser Arg Leu 500 505 510 Thr Ala Leu Arg He Lys Lys Thr Leu Ala Lys Met Val Glu Ser Gin 515 520 525 Asp Val Lys He 530 (2) INFORMATION FOR SEQ ID NO:7: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 1952 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (ix) FEATURE: (A) NAME/KEY: CDS (B) LOCATION: 187..1695 (D) OTHER INFORMATION: /product= "Murine ALK6" (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7: AAGCGGCGGC AGAAGTTGCC GGCGTGGTGC TCGTAGTGAG GGCGCGGAGG ACCCGGGACC 60 TGGGAAGCGG CGGCGGGTTA ACTTCGGCTG AATCACAACC ATTTGGCGCT GAGCTATGAC 120 AAGAGAGCAA ACAAAAAGTT AAAGGAGCAA CCCGGCCATA AGTGAAGAGA GAAGTTTATT 180 GATAAC ATG CTC TTA CGA AGC TCT GGA AAA TTA AAT GTG GGC ACC AAG 228 Met Leu Leu Arg Ser Ser Gly Lys Leu Asn Val Gly Thr Lys 1 5 10 AAG GAG GAT GGA GAG AGT ACA GCC CCC ACC CCT CGG CCC AAG ATC CTA 276 Lys Glu Asp Gly Glu Ser Thr Ala Pro Thr Pro Arg Pro Lys He Leu 15 20 25 30 CGT TGT AAA TGC CAC CAC CAC TGT CCG GAA GAC TCA GTC AAC AAT ATC 324 Arg Cys Lys Cys His His His Cys Pro Glu Asp Ser Val Asn Asn He 35 40 45 TGC AGC ACA GAT GGG TAC TGC TTC ACG ATG ATA GAA GAA GAT GAC TCT 372 Cys Ser Thr Asp Gly Tyr Cys Phe Thr Met He Glu Glu Asp Asp Ser 50 55 60 GGA ATG CCT GTT GTC ACC TCT GGA TGT CTA GGA CTA GAA GGG TCA GAT 420 Gly Met Pro Val Val Thr Ser Gly Cys Leu Gly Leu Glu Gly Ser Asp 65 70 75 TTT CAA TGT CGT GAC ACT CCC ATT CCT CAT CAA AGA AGA TCA ATT GAA 468 Phe Gin Cys Arg Asp Thr Pro He Pro His Gin Arg Arg Ser He Glu 80 85 90 TGC TGC ACA GAA AGG AAT GAG TGT AAT AAA GAC CTC CAC CCC ACT CTG 516 Cys Cys Thr Glu Arg Asn Glu Cys Asn Lys Asp Leu His Pro Thr Leu 95 100 105 110 CCT CCT CTC AAG GAC AGA GAT TTT GTT GAT GGG CCC ATA CAC CAC AAG 564 Pro Pro Leu Lys Asp Arg Asp Phe Val Asp Gly Pro He His His Lys 115 120 125 GCC TTG CTT ATC TCT GTG ACT GTC TGT AGT TTA CTC TTG GTC CTC ATT 612 Ala Leu Leu He Ser Val Thr Val Cys Ser Leu Leu Leu Val Leu He 130 135 140 ATT TTA TTC TGT TAC TTC AGG TAT AAA AGA CAA GAA GCC CGA CCT CGG 660 He Leu Phe Cys Tyr Phe Arg Tyr Lys Arg Gin Glu Ala Arg Pro Arg 145 150 155 TAC AGC ATT GGG CTG GAG CAG GAC GAG ACA TAC ATT CCT CCT GGA GAG 708 Tyr Ser He Gly Leu Glu Gin Asp Glu Thr Tyr He Pro Pro Gly Glu 160 165 170 TCC CTG AGA GAC TTG ATC GAG CAG TCT CAG AGC TCG GGA AGT GGA TCA 756 Ser Leu Arg Asp Leu He Glu Gin Ser Gin Ser Ser Gly Ser Gly Ser 175 180 185 190 GGC CTC CCT CTG CTG GTC CAA AGG ACA ATA GCT AAG CAA ATT CAG ATG 804 Gly Leu Pro Leu Leu Val Gin Arg Thr He Ala Lys Gin He Gin Met 195 200 205 GTG AAG CAG ATT GGA AAA GGC CGC TAT GGC GAG GTG TGG ATG GGA AAG 852 Val Lys Gin He Gly Lys Gly Arg Tyr Gly Glu Val Trp Met Gly Lys 210 215 220 TGG CGT GGA GAA AAG GTG GCT GTG AAA GTG TTC TTC ACC ACG GAG GAA 900 Trp Arg Gly Glu Lys Val Ala Val Lys Val Phe Phe Thr Thr Glu Glu 225 230 235 GCC AGC TGG TTC CGA GAG ACT GAG ATA TAT CAG ACG GTC CTG ATG CGG 948 Ala Ser Trp Phe Arg Glu Thr Glu He Tyr Gin Thr Val Leu Met Arg 240 245 250 CAT GAG AAT ATT CTG GGG TTC ATT GCT GCA GAT ATC AAA GGG ACT GGG 996 His Glu Asn He Leu Gly Phe He Ala Ala Asp He Lys Gly Thr Gly 255 260 265 270 TCC TGG ACT CAG TTG TAC CTC ATC ACA GAC TAT CAT GAA AAC GGC TCC 1044 Ser Trp Thr Gin Leu Tyr Leu He Thr Asp Tyr His Glu Asn Gly Ser 275 280 285 CTT TAT GAC TAT CTG AAA TCC ACC ACC TTA GAC GCA AAG TCC ATG CTG 1092 Leu Tyr Asp Tyr Leu Lys Ser Thr Thr Leu Asp Ala Lys Ser Met Leu 290 295 300 AAG CTA GCC TAC TCC TCT GTC AGC GGC CTA TGC CAT TTA CAC ACG GAA 1140 Lys Leu Ala Tyr Ser Ser Val Ser Gly Leu Cys His Leu His Thr Glu 305 310 315 ATC TTT AGC ACT CAA GGC AAG CCA GCA ATC GCC CAT CGA GAC TTG AAA 1188 He Phe Ser Thr Gin Gly Lys Pro Ala He Ala His Arg Asp Leu Lys 320 325 330 AGT AAA AAC ATC CTG GTG AAG AAA AAT GGA ACT TGC TGC ATA GCA GAC 1236 Ser Lys Asn He Leu Val Lys Lys Asn Gly Thr Cys Cys He Ala Asp 335 340 345 350 CTG GGC TTG GCT GTC AAG TTC ATT AGT GAC ACA AAT GAG GTT GAC ATC 1284 Leu Gly Leu Ala Val Lys Phe He Ser Asp Thr Asn Glu Val Asp He 355 360 365 CCA CCC AAC ACC CGG GTT GGC ACC AAG CGC TAT ATG CCT CCA GAA GTG 1332 Pro Pro Asn Thr Arg Val Gly Thr Lys Arg Tyr Met Pro Pro Glu Val 370 375 380 CTG GAC GAG AGC TTG AAT AGA AAC CAT TTC CAG TCC TAC ATT ATG GCT 1380 Leu Asp Glu Ser Leu Asn Arg Asn His Phe Gin Ser Tyr He Met Ala 385 390 395 GAC ATG TAC AGC TTT GGA CTC ATC CTC TGG GAG ATT GCA AGG AGA TGT 1428 Asp Met Tyr Ser Phe Gly Leu He Leu Trp Glu He Ala Arg Arg Cys 400 405 410 GTT TCT GGA GGT ATA GTG GAA GAA TAC CAG CTT CCC TAT CAC GAC CTG 1476 Val Ser Gly Gly He Val Glu Glu Tyr Gin Leu Pro Tyr His Asp Leu 415 420 425 430 GTG CCC AGT GAC CCT TCT TAT GAG GAC ATG AGA GAA ATT GTG TGC ATG 1524 Val Pro Ser Asp Pro Ser Tyr Glu Asp Met Arg Glu He Val Cys Met 435 440 445 AAG AAG TTA CGG CCT TCA TTC CCC AAT CGA TGG AGC AGT GAT GAG TGT 1572 Lys Lys Leu Arg Pro Ser Phe Pro Asn Arg Trp Ser Ser Asp Glu Cys 450 455 460 CTC AGG CAG ATG GGG AAG CTT ATG ACA GAG TGC TGG GCG CAG AAT CCT 1620 Leu Arg Gin Met Gly Lys Leu Met Thr Glu Cys Trp Ala Gin Asn Pro 465 470 475 GCC TCC AGG CTG ACG GCC CTG AGA GTT AAG AAA ACC CTT GCC AAA ATG 1668 Ala Ser Arg Leu Thr Ala Leu Arg Val Lys Lys Thr Leu Ala Lys Met 480 485 490 TCA GAG TCC CAG GAC ATT AAA CTC TGACGTCAGA TACTTGTGGA CAGAGCAAGA 1722 Ser Glu Ser Gin Asp He Lys Leu 495 500 ATTTCACAGA AGCATCGTTA GCCCAAGCCT TGAACGTTAG CCTACTGCCC AGTGAGTTCA 1782 GACTTTCCTG GAAGAGAGCA CGGTGGGCAG ACACAGAGGA ACCCAGAAAC ACGGATTCAT 1842 CATGGCTTTC TGAGGAGGAG AAACTGTTTG GGTAACTTGT TCAAGATATG ATGCATGTTG 1902 CTTTCTAAGA AAGCCCTGTA TTTTGAATTA CCATTTTTTT ATAAAAAAAA 1952 (2) INFORMATION FOR SEQ ID NO:8: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 502 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8: Met Leu Leu Arg Ser Ser Gly Lys Leu Asn Val Gly Thr Lys Lys Glu 1 5 10 15 Asp Gly Glu Ser Thr Ala Pro Thr Pro Arg Pro Lys He Leu Arg Cys 20 25 30 Lys Cys His His His Cys Pro Glu Asp Ser Val Asn Asn He Cys Ser 35 40 45 Thr Asp Gly Tyr Cys Phe Thr Met He Glu Glu Asp Asp Ser Gly Met 50 55 60 Pro Val Val Thr Ser Gly Cys Leu Gly Leu Glu Gly Ser Asp Phe Gin 65 70 75 80 Cys Arg Asp Thr Pro He Pro His Gin Arg Arg Ser He Glu Cys Cys 85 90 95 Thr Glu Arg Asn Glu Cys Asn Lys Asp Leu His Pro Thr Leu Pro Pro 100 105 110 Leu Lys Asp Arg Asp Phe Val Asp Gly Pro He His His Lys Ala Leu 115 120 125 Leu He Ser Val Thr Val Cys Ser Leu Leu Leu Val Leu He He Leu 130 135 140 Phe Cys Tyr Phe Arg Tyr Lys Arg Gin Glu Ala Arg Pro Arg Tyr Ser 145 150 155 160 He Gly Leu Glu Gin Asp Glu Thr Tyr He Pro Pro Gly Glu Ser Leu 165 170 175 Arg Asp Leu He Glu Gin Ser Gin Ser Ser Gly Ser Gly Ser Gly Leu 180 185 190 Pro Leu Leu Val Gin Arg Thr He Ala Lys Gin He Gin Met Val Lys 195 200 205 Gin He Gly Lys Gly Arg Tyr Gly Glu Val Trp Met Gly Lys Trp Arg 210 215 220 Gly Glu Lys Val Ala Val Lys Val Phe Phe Thr Thr Glu Glu Ala Ser 225 230 235 240 Trp Phe Arg Glu Thr Glu He Tyr Gin Thr Val Leu Met Arg His Glu 245 250 255 Asn He Leu Gly Phe He Ala Ala Asp He Lys Gly Thr Gly Ser Trp 260 265 270 Thr Gin Leu Tyr Leu He Thr Asp Tyr His Glu Asn Gly Ser Leu Tyr 275 280 285 Asp Tyr Leu Lys Ser Thr Thr Leu Asp Ala Lys Ser Met Leu Lys Leu 290 295 300 Ala Tyr Ser Ser Val Ser Gly Leu Cys His Leu His Thr Glu He Phe 305 310 315 320 Ser Thr Gin Gly Lys Pro Ala He Ala His Arg Asp Leu Lys Ser Lys 325 330 335 Asn He Leu Val Lys Lys Asn Gly Thr Cys Cys He Ala Asp Leu Gly 340 345 350 Leu Ala Val Lys Phe He Ser Asp Thr Asn Glu Val Asp He Pro Pro 355 360 365 Asn Thr Arg Val Gly Thr Lys Arg Tyr Met Pro Pro Glu Val Leu Asp 370 375 380 Glu Ser Leu Asn Arg Asn His Phe Gin Ser Tyr He Met Ala Asp Met 385 390 395 400 Tyr Ser Phe Gly Leu He Leu Trp Glu He Ala Arg Arg Cys Val Ser 405 410 415 Gly Gly He Val Glu Glu Tyr Gin Leu Pro Tyr His Asp Leu Val Pro 420 425 430 Ser Asp Pro Ser Tyr Glu Asp Met Arg Glu He Val Cys Met Lys Lys 435 440 445 Leu Arg Pro Ser Phe Pro Asn Arg Trp Ser Ser Asp Glu Cys Leu Arg 450 455 460 Gin Met Gly Lys Leu Met Thr Glu Cys Trp Ala Gin Asn Pro Ala Ser 465 470 475 480 Arg Leu Thr Ala Leu Arg Val Lys Lys Thr Leu Ala Lys Met Ser Glu 485 490 495 Ser Gin Asp He Lys Leu 500 (2) INFORMATION FOR SEQ ID NO:9: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 1822 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (ix) FEATURE: (A) NAME/KEY: CDS (B) LOCATION: 49..1341 (D) OTHER INFORMATION: /function= "OSTEOGENIC PROTEIN" /product= "OPl" /evidence= EXPERIMENTAL /standard_name= "OPl" (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9: GGTGCGGGCC CGGAGCCCGG AGCCCGGGTA GCGCGTAGAG CCGGCGCG ATG CAC GTG 57 Met His Val 1 CGC TCA CTG CGA GCT GCG GCG CCG CAC AGC TTC GTG GCG CTC TGG GCA 105 Arg Ser Leu Arg Ala Ala Ala Pro His Ser Phe Val Ala Leu Trp Ala 5 10 15 CCC CTG TTC CTG CTG CGC TCC GCC CTG GCC GAC TTC AGC CTG GAC AAC 153 Pro Leu Phe Leu Leu Arg Ser Ala Leu Ala Asp Phe Ser Leu Asp Asn 20 25 30 35 GAG GTG CAC TCG AGC TTC ATC CAC CGG CGC CTC CGC AGC CAG GAG CGG 201 Glu Val His Ser Ser Phe He His Arg Arg Leu Arg Ser Gin Glu Arg 40 45 50 CGG GAG ATG CAG CGC GAG ATC CTC TCC ATT TTG GGC TTG CCC CAC CGC 249 Arg Glu Met Gin Arg Glu He Leu Ser He Leu Gly Leu Pro His Arg 55 60 65 CCG CGC CCG CAC CTC CAG GGC AAG CAC AAC TCG GCA CCC ATG TTC ATG 297 Pro Arg Pro His Leu Gin Gly Lys His Asn Ser Ala Pro Met Phe Met 70 75 80 CTG GAC CTG TAC AAC GCC ATG GCG GTG GAG GAG GGC GGC GGG CCC GGC 345 Leu Asp Leu Tyr Asn Ala Met Ala Val Glu Glu Gly Gly Gly Pro Gly 85 90 95 GGC CAG GGC TTC TCC TAC CCC TAC AAG GCC GTC TTC AGT ACC CAG GGC 393 Gly Gin Gly Phe Ser Tyr Pro Tyr Lys Ala Val Phe Ser Thr Gin Gly 100 105 110 115 CCC CCT CTG GCC AGC CTG CAA GAT AGC CAT TTC CTC ACC GAC GCC GAC 441 Pro Pro Leu Ala Ser Leu Gin Asp Ser His Phe Leu Thr Asp Ala Asp 120 125 130 ATG GTC ATG AGC TTC GTC AAC CTC GTG GAA CAT GAC AAG GAA TTC TTC 489 Met Val Met Ser Phe Val Asn Leu Val Glu His Asp Lys Glu Phe Phe 135 140 145 CAC CCA CGC TAC CAC CAT CGA GAG TTC CGG TTT GAT CTT TCC AAG ATC 537 His Pro Arg Tyr His His Arg Glu Phe Arg Phe Asp Leu Ser Lys He 150 155 160 CCA GAA GGG GAA GCT GTC ACG GCA GCC GAA TTC CGG ATC TAC AAG GAC 585 Pro Glu Gly Glu Ala Val Thr Ala Ala Glu Phe Arg He Tyr Lys Asp 165 170 175 TAC ATC CGG GAA CGC TTC GAC AAT GAG ACG TTC CGG ATC AGC GTT TAT 633 Tyr He Arg Glu Arg Phe Asp Asn Glu Thr Phe Arg He Ser Val Tyr 180 185 190 195 CAG GTG CTC CAG GAG CAC TTG GGC AGG GAA TCG GAT CTC TTC CTG CTC 681 Gin Val Leu Gin Glu His Leu Gly Arg Glu Ser Asp Leu Phe Leu Leu 200 205 210 GAC AGC CGT ACC CTC TGG GCC TCG GAG GAG GGC TGG CTG GTG TTT GAC 729 Asp Ser Arg Thr Leu Trp Ala Ser Glu Glu Gly Trp Leu Val Phe Asp 215 220 225 ATC ACA GCC ACC AGC AAC CAC TGG GTG GTC AAT CCG CGG CAC AAC CTG 777 He Thr Ala Thr Ser Asn His Trp Val Val Asn Pro Arg His Asn Leu 230 235 240 GGC CTG CAG CTC TCG GTG GAG ACG CTG GAT GGG CAG AGC ATC AAC CCC 825 Gly Leu Gin Leu Ser Val Glu Thr Leu Asp Gly Gin Ser He Asn Pro 245 250 255 AAG TTG GCG GGC CTG ATT GGG CGG CAC GGG CCC CAG AAC AAG CAG CCC 873 Lys Leu Ala Gly Leu He Gly Arg His Gly Pro Gin Asn Lys Gin Pro 260 265 270 275 TTC ATG GTG GCT TTC TTC AAG GCC ACG GAG GTC CAC TTC CGC AGC ATC 921 Phe Met Val Ala Phe Phe Lys Ala Thr Glu Val His Phe Arg Ser He 280 285 290 CGG TCC ACG GGG AGC AAA CAG CGC AGC CAG AAC CGC TCC AAG ACG CCC 969 Arg Ser Thr Gly Ser Lys Gin Arg Ser Gin Asn Arg Ser Lys Thr Pro 295 300 305 AAG AAC CAG GAA GCC CTG CGG ATG GCC AAC GTG GCA GAG AAC AGC AGC 1017 Lys Asn Gin Glu Ala Leu Arg Met Ala Asn Val Ala Glu Asn Ser Ser 310 315 320 AGC GAC CAG AGG CAG GCC TGT AAG AAG CAC GAG CTG TAT GTC AGC TTC 1065 Ser Asp Gin Arg Gin Ala Cys Lys Lys His Glu Leu Tyr Val Ser Phe 325 330 335 CGA GAC CTG GGC TGG CAG GAC TGG ATC ATC GCG CCT GAA GGC TAC GCC 1113 Arg Asp Leu Gly Trp Gin Asp Trp He He Ala Pro Glu Gly Tyr Ala 340 345 350 355 GCC TAC TAC TGT GAG GGG GAG TGT GCC TTC CCT CTG AAC TCC TAC ATG 1161 Ala Tyr Tyr Cys Glu Gly Glu Cys Ala Phe Pro Leu Asn Ser Tyr Met 360 365 370 AAC GCC ACC AAC CAC GCC ATC GTG CAG ACG CTG GTC CAC TTC ATC AAC 1209 Asn Ala Thr Asn His Ala He Val Gin Thr Leu Val His Phe He Asn 375 380 385 CCG GAA ACG GTG CCC AAG CCC TGC TGT GCG CCC ACG CAG CTC AAT GCC 1257 Pro Glu Thr Val Pro Lys Pro Cys Cys Ala Pro Thr Gin Leu Asn Ala 390 395 400 ATC TCC GTC CTC TAC TTC GAT GAC AGC TCC AAC GTC ATC CTG AAG AAA 1305 He Ser Val Leu Tyr Phe Asp Asp Ser Ser Asn Val He Leu Lys Lys 405 410 415 TAC AGA AAC ATG GTG GTC CGG GCC TGT GGC TGC CAC TAGCTCCTCC 1351 Tyr Arg Asn Met Val Val Arg Ala Cys Gly Cys His 420 425 430 GAGAATTCAG ACCCTTTGGG GCCAAGTTTT TCTGGATCCT CCATTGCTCG CCTTGGCCAG 1411 GAACCAGCAG ACCAACTGCC TTTTGTGAGA CCTTCCCCTC CCTATCCCCA ACTTTAAAGG 1471 TGTGAGAGTA TTAGGAAACA TGAGCAGCAT ATGGCTTTTG ATCAGTTTTT CAGTGGCAGC 1531 ATCCAATGAA CAAGATCCTA CAAGCTGTGC AGGCAAAACC TAGCAGGAAA AAAAAACAAC 1591 GCATAAAGAA AAATGGCCGG GCCAGGTCAT TGGCTGGGAA GTCTCAGCCA TGCACGGACT 1651 CGTTTCCAGA GGTAATTATG AGCGCCTACC AGCCAGGCCA CCCAGCCGTG GGAGGAAGGG 1711 GGCGTGGCAA GGGGTGGGC CATTGGTGTC TGTGCGAAAG GAAAATTGAC CCGGAAGTTC 1771 CTGTAATAAA TGTCACAATA AAACGAATGA ATGAAAAAAA AAAAAAAAAA A 1822 (2) INFORMATION FOR SEQ ID NO:10: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 431 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:10: Met His Val Arg Ser Leu Arg Ala Ala Ala Pro His Ser Phe Val Ala 1 5 10 15 Leu Trp Ala Pro Leu Phe Leu Leu Arg Ser Ala Leu Ala Asp Phe Ser 20 25 30 Leu Asp Asn Glu Val His Ser Ser Phe He His Arg Arg Leu Arg Ser 35 40 45 Gin Glu Arg Arg Glu Met Gin Arg Glu He Leu Ser He Leu Gly Leu 50 55 60 Pro His Arg Pro Arg Pro His Leu Gin Gly Lys His Asn Ser Ala Pro 65 70 75 80 Met Phe Met Leu Asp Leu Tyr Asn Ala Met Ala Val Glu Glu Gly Gly 85 90 95 Gly Pro Gly Gly Gin Gly Phe Ser Tyr Pro Tyr Lys Ala Val Phe Ser 100 105 110 Thr Gin Gly Pro Pro Leu Ala Ser Leu Gin Asp Ser His Phe Leu Thr 115 120 125 Asp Ala Asp Met Val Met Ser Phe Val Asn Leu Val Glu His Asp Lys 130 135 140 Glu Phe Phe His Pro Arg Tyr His His Arg Glu Phe Arg Phe Asp Leu 145 150 155 160 Ser Lys He Pro Glu Gly Glu Ala Val Thr Ala Ala Glu Phe Arg He 165 170 175 Tyr Lys Asp Tyr He Arg Glu Arg Phe Asp Asn Glu Thr Phe Arg He 180 185 190 Ser Val Tyr Gin Val Leu Gin Glu His Leu Gly Arg Glu Ser Asp Leu 195 200 205 Phe Leu Leu Asp Ser Arg Thr Leu Trp Ala Ser Glu Glu Gly Trp Leu 210 215 220 Val Phe Asp He Thr Ala Thr Ser Asn His Trp Val Val Asn Pro Arg 225 230 235 240 His Asn Leu Gly Leu Gin Leu Ser Val Glu Thr Leu Asp Gly Gin Ser 245 250 255 He Asn Pro Lys Leu Ala Gly Leu He Gly Arg His Gly Pro Gin Asn 260 265 270 Lys Gin Pro Phe Met Val Ala Phe Phe Lys Ala Thr Glu Val His Phe 275 280 285 Arg Ser He Arg Ser Thr Gly Ser Lys Gin Arg Ser Gin Asn Arg Ser 290 295 300 Lys Thr Pro Lys Asn Gin Glu Ala Leu Arg Met Ala Asn Val Ala Glu 305 310 315 320 Asn Ser Ser Ser Asp Gin Arg Gin Ala Cys Lys Lys His Glu Leu Tyr 325 330 335 Val Ser Phe Arg Asp Leu Gly Trp Gin Asp Trp He He Ala Pro Glu 340 345 350 Gly Tyr Ala Ala Tyr Tyr Cys Glu Gly Glu Cys Ala Phe Pro Leu Asn 355 360 365 Ser Tyr Met Asn Ala Thr Asn His Ala He Val Gin Thr Leu Val His 370 375 380 Phe He Asn Pro Glu Thr Val Pro Lys Pro Cys Cys Ala Pro Thr Gin 385 390 395 400 Leu Asn Ala He Ser Val Leu Tyr Phe Asp Asp Ser Ser Asn Val He 405 410 415 Leu Lys Lys Tyr Arg Asn Met Val Val Arg Ala Cys Gly Cys His 420 425 430 (2) INFORMATION FOR SEQ ID NO:11: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 102 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (ix) FEATURE: (A) NAME/KEY: Protein (B) LOCATION: 1..102 (D) OTHER INFORMATION: /label= OPX /note= "Each Xaa is independently selected from a group of one or more specified amino acids as defined in the specification" (Xi) SEQUENCE DESCRIPTION: SEQ ID NO:11: Cys Xaa Xaa His Glu Leu Tyr Val Xaa Phe Xaa Asp Leu Gly Trp Xaa 1 5 10 15 Asp Trp Xaa He Ala Pro Xaa Gly Tyr Xaa Ala Tyr Tyr Cys Glu Gly 20 25 30 Glu Cys Xaa Phe Pro Leu Xaa Ser Xaa Met Asn Ala Thr Asn His Ala 35 40 45 He Xaa Gin Xaa Leu Val His Xaa Xaa Xaa Pro Xaa Xaa Val Pro Lys 50 55 60 Xaa Cys Cys Ala Pro Thr Xaa Leu Xaa Ala Xaa Ser Val Leu Tyr Xaa 65 70 75 80 Asp Xaa Ser Xaa Asn Val Xaa Leu Xaa Lys Xaa Arg Asn Met Val Val 85 90 95 Xaa Ala Cys Gly Cys His 100 (2) INFORMATION FOR SEQ ID NO:12: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 28 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: CDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:12 GCGGATCCTG TTGTGAAGGN AATATGTG 28 (2) INFORMATION FOR SEQ ID NO:13: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 24 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:13: GCGATCCGTC GCAGTCAAAA TTTT 24 (2) INFORMATION FOR SEQ ID NO:14: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 26 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:14: GCGGATCCGC GATATATTAA AAGCAA 26 (2) INFORMATION FOR SEQ ID NO:15: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (iii) HYPOTHETICAL: NO (iv) ANTI-SENSE: NO (xi) SEQUENCE DESCRIPTION: SEQ ID NO:15: CGGAATTCTG GTGCCATATA 20

Claims: What is claimed iε: 1. A method for identifying an OP-l receptor-binding analog, εaid analog being characterized as having substantially the same binding affinity for a cell surface receptor aε OP-l, the method comprising the steps of: (a) providing a sample containing a protein selected from the group consisting of: (i) a polypeptide chain compriεing an amino acid sequence defined by residues 16-123 of Seq. ID No. 3 (ALK-2), or an OPl-binding analog thereof; (ii) a polypeptide chain comprising an amino acid sequence defined by residueε 24-152 of Seq. ID No. 5 (ALK-3),, or an OPl-binding analog thereof; (iii) a polypeptide chain compriεing an amino acid sequence defined by residues 23-122 of Seq. ID No. 7 (ALK-6) , , or an OPl binding analog thereof; (iv) a polypeptide chain having binding affinity for OP-l and sharing at least 40% amino acid identity with residues 23- 122 of Seq. ID No. 7 (ALK-6) , ; (v) a polypeptide chain having binding affinity for OP-l and encoded by a nucleic acid obtainable by amplification with one or more primer sequences defined by Seq. ID Nos. 12-15; or (vi) a polypeptide chain having binding affinity for OP-l and encoded by a nucleic acid that hybridizes under stringent conditions with a nucleic acid comprising the sequence defined by nucleotides 256-552 of Seq. ID No. 7 (ALK-6) , ; (b) contacting εaid εample with a candidate OPl receptor- binding analog; and (c) detecting specific binding between εaid candidate OPl receptor- binding analog and εaid protein. 2. A method for identifying an OP-l receptor-binding analog, said analog being characterized as having subεtantially the same binding affinity for a cell surface receptor as OPl, the method comprising the stepε of: (a) providing a cell that expresses a surface receptor protein having binding specificity for OP-l selected from the group consisting of: (i) a polypeptide chain comprising an amino acid sequence defined by residues 16-123 of Seq. ID No. 3 (ALK-2), or an OPl-binding analog thereof; (ii) a polypeptide chain comprising an amino acid sequence defined by residues 24-152 of Seq. ID No. 5 (ALK-3),, or an OPl-binding analog thereof; (iii) a polypeptide chain comprising an amino acid sequence defined by residues 23-122 of Seq. ID No. 7 (ALK-6),, or an OPl binding analog thereof; (iv) a polypeptide chain having binding affinity for OP-l and sharing at least 40% amino acid identity with residues 23- 122 of Seq. ID No. 7 (ALK-6) , ; (v) a polypeptide chain having binding affinity for OP-l and encoded by a nucleic acid obtainable by amplification with one or more primer sequences defined by Seq. ID Nos. 12-15; or (vi) a polypeptide chain having binding affinity for OP-l and encoded by a nucleic acid that hybridizeε under stringent conditions with a nucleic acid compriεing the εequence defined by nucleotides 256-552 of Seq. ID No. 7 (ALK-6),; (b) contacting said cell with a candidate OPl receptor-binding analog; and (c) detecting induction of an OP-1-mediated cellular response. 3. The method of claim 2 wherein said OP-l mediated cellular reεponεe detected in step (c) is induction of a kinase activity, inhibition of epithelial cell growth, or induction of a cell differentiation marker. 4. The method of claim 2 or 3 wherein said cell compriseε a transfected nucleic acid comprising a reporter gene in operative association with a control element derived from an OP-l inducible protein. 5. The method of any of claims 1-4 wherein εaid εample further comprises part or all of a Type II serine/threonine kinase receptor protein having binding affinity for OP-l, activin or BMP-4. 6. A method for producing an OP-l receptor binding analog, the method comprising the steps of: (a) obtaining, by the method of any of claims 1-5, a candidate OP-l binding analog, and (b) producing either said candidate analog or a second OP-l binding analog derived from said candidate and having substantially the same OP-l receptor-binding domain as said candidate. 7. The method of producing an OP-l receptor-binding analog of claim 6 wherein said analog produced in step (b) is by recombinant DNA techniques, or by nonbiological peptide synthesis. 8. A kit for identifying OP-l or a candidate OP-l receptor binding analog in a sample, the kit comprising: (a) a receptacle adapted to receive a sample and containing a protein selected from the group consiεting of: (i) a polypeptide chain compriεing an amino acid sequence defined by residueε 16-123 of Seq. ID No. 3 (ALK-2), or an OPl-binding analog thereof; (ii) a polypeptide chain comprising an amino acid sequence defined by residues 24-152 of Seq. ID No. 5 (ALK-3),, or an OPl-binding analog thereof; (iii) a polypeptide chain comprising an amino acid sequence defined by residueε 23-122 of Seq. ID No. 7 (ALK-6) , , or an OPl binding analog thereof; (iv) a polypeptide chain having binding affinity for OP-l and sharing at least 40% amino acid identity with residueε 23- 122 of Seq. ID No. 7 (ALK-6),; (v) a polypeptide chain having binding affinity for OP-l and encoded by a nucleic acid obtainable by amplification with one or more primer sequences defined by Seq. ID Nos. 12-15; or (vi) a polypeptide chain having binding affinity for OP-l and encoded by a nucleic acid that hybridizeε under stringent conditions with a nucleic acid comprising the sequence defined by nucleotides 256-552 of Seq. ID No. 7 (ALK-6),; and (b) means for detecting interaction of OP-l or a candidate OP-l receptor-binding analog with said protein of part (a) , εaid OP-l or candidate analog comprising part of said εample provided to said receptacle. 9. The kit of claim 8 wherein said means in part (b) compriseε either (i) means for detecting specific binding interaction of OP-l or said candidate analog with said protein; or (ii) meanε for detecting induction of an OP-l mediated cellular response. 10. The kit of claim 8 or 9 further comprising a εerine/threonine Type II receptor having binding specificity for OP-l, activin or BMP-4. 11. An OP-l receptor-binding analog produced by the method of any of claims 1-7 or use of the kit of claims 8-10. 12. The analog produced by the method of any of claims 1-8, said analog (i)comprising an amino acid sequence sharing greater than 60% identity with the C-terminal 96 amino acids of the sequence represented by Seq. ID No. 9 (OP-l, residues 335-431), and (ii) being substantially incapable of inducing an OP-l mediated cellular response. 13. The analog of claim 11 or 12 further having binding affinity for a Type II serine/threonine kinase cell surface receptor. 14. The analog of claim 13 wherein said Type II receptor also haε binding affinity for activin or BMP-4. 15. An iεolated ligand-receptor complex compriεing two moleculeε interacting as specific binding partners, the first said molecule defining said ligand and comprising at least the C-terminal 96 amino acids of OPl (residues 335-431 of Seq ID No. 9) or a receptor-binding analog thereof, and the second said molecule defining said receptor and being selected from the group consisting of: (i) a polypeptide chain comprising an amino acid sequence defined by residues 16-123 of Seq. ID No. 3 (ALK-2), or an OPl-binding analog thereof; (ii) a polypeptide chain comprising an amino acid sequence defined by residues 24-152 of Seq. ID No. 5 (ALK-3),, or an OPl-binding analog thereof; (iii) a polypeptide chain comprising an amino acid sequence defined by residues 23-122 of Seq. ID No. 7 (ALK-6), , or an OPl binding analog thereof; (iv) a polypeptide chain having binding affinity for OP-l and sharing at least 40% amino acid identity with reεidueε 23- 122 of Seq. ID No. 7 (ALK-6),; (v) a polypeptide chain having binding affinity for OP-l and encoded by a nucleic acid obtainable by amplification with one or more primer sequences defined by Seq. ID Nos. 12-15; or (vi) a polypeptide chain having binding affinity for OP-l and encoded by a nucleic acid that hybridizes under stringent conditions with a nucleic acid comprising the sequence defined by nucleotides 256-552 of Seq. ID No. 7 (ALK-6) , . 16. The complex of claim 15 further compriεing part or all of a Type II εerine/threonine kinase receptor . 17. The complex of claim 16 wherein said Type II receptor also has binding affinity for activin or BMP-4. 18. The complex of any of claims 15-17 wherein said firεt molecule defining said ligand is an OP-l receptor-binding analog compriseε part or all of the proteins selected from the group consisting of 60A, BMP-5, BMP- 6, Vgr-1, OP2, 0P3 and receptor-binding amino acid sequence variants or xenogenic homologs thereof. 19. An isolated binding partner having εpecific binding affinity for an epitope on a ligand-receptor complex, said complex being characterized as comprising an OP-l protein or an analog thereof in specific binding interaction with the ligand binding domain of a cell surface receptor defined by Seq. ID No. 3 (ALK-2) , 5, or 7, or an OPl-binding analog thereof; said binding partner having εubstantially no binding affinity for the uncomplexed form of said OP-l protein or OP-l protein analog. 20. The iεolated binding partner of claim 19 wherein said binding partner is further characterized as having substantially no binding affinity for the uncomplexed form of εaid cell surface receptor protein or said analog thereof. 21. The binding partner of claim 19 wherein said binding partner iε a monoclonal or polyclonal antibody. 22. Use of the OP-l receptor-binding analog of any claims 11-14 in a method for (i) antagonizing OP-l binding to a cell surface receptor; or (ii) antagonizing induction of an OP-l mediated cellular response. 23. The use according to claim 22 wherein said OP-l receptor-binding analog compriseε an antibody having binding specificity for (i) the ligand binding domain of a cell surface receptor defined by Seq. ID Nos. 3, 5, or 7 or an OP-l binding analog thereof; or (ii) the receptor binding domain of OP-l, represented by Seq. ID No. 9, or a receptor-binding analog thereof. 24. Use of a protein selected from the group consisting of: (i) a polypeptide chain comprising an amino acid sequence defined by residues 16-123 of Seq. ID No. 3 (ALK-2), or an OPl-binding analog thereof; (ii) a polypeptide chain comprising an amino acid sequence defined by residueε 24-152 of Seq. ID No. 5 (ALK-3),, or an OPl-binding analog thereof; (iii) a polypeptide chain comprising an amino acid sequence defined by residues 23-122 of Seq. ID No. 7 (ALK-6) , , or an OPl binding analog thereof; (iv) a polypeptide chain having binding affinity for OP-l and sharing at least 40% amino acid identity with residueε 23- 122 of Seq. ID No. 7 (ALK-6) , ; (v) a polypeptide chain having binding affinity for OP-l and encoded by a nucleic acid obtainable by amplification with one or more primer sequences defined by Seq. ID Nos. 12-15; or (vi) a polypeptide chain having binding affinity for OP-l and encoded by a nucleic acid that hybridizes under stringent conditions with a nucleic acid comprising the sequence defined by nucleotides 256-552 of Seq. ID No. 7 (ALK-6) , ; in a method for antagonizing (i) OP-l binding to a cell surface receptor; or (ii) induction of an OP-l mediated cellular response. 25. A method for antagonizing activin binding to a cell surface receptor, the method comprising the step of: providing a cell expresεing a said receptor with a protein having binding specificity for the amino acid sequence defined by residues 16-123 of Seq ID No. 3 or an OP-l binding sequence variant thereof, said protein εharing at leaεt 60% amino acid sequence identity with residue 335-431 of the sequence defined by Seq ID No. 9, such that said protein, when provided to said cell, is competent fcfo interact specifically with said receptor, thereby substantially inhibiting activin binding to said receptor. 26. A method for antagonizing BMP-4 binding to a cell surface receptor, the method comprising the step of: providing a cell expressing a said receptor with a protein having binding specificity for the ligand binding domain defined by residueε 24-152 of Seq ID No. 5 (ALK-3), or reεidueε 23-122 of Seq ID No. 7 (ALK-6), or an OP-l binding sequence variant thereof, said protein sharing at least 60% amino acid sequence identity with residues 335-431 of the sequence defined by Seq ID No. 9, εuch that εaid protein, when provided to εaid cell, iε competent to interact εpecifically with εaid receptor, thereby substantially inhibiting BMP-4 binding to said receptor. 27. Use of the OP-l receptor binding analog of claim 12-14 in the method of claim 25 or 26.

Inventors: DIJKE PETER T, HELDIN CARL-HENRIK, MIYAZANO KOHEI, SAMPATH KUBER T, DIJKE, PETER, T., HELDIN, CARL-HENRIK, MIYAZANO, KOHEI, SAMPATH, KUBER, T.
Applicants: CREATIVE BIOMOLECULES INC, DIJKE PETER T, HELDIN CARL HENRIK, LUDWIG INST CANCER RES, MIYAZANO KOHEI, SAMPATH KUBER T, CREATIVE BIOMOLECULES, INC., DIJKE, PETER, T., HELDIN, CARL-HENRIK, MIYAZANO, KOHEI, SAMPATH, KUBER, T., THE LUDWIG INSTITUTE FOR CANCER RESEARCH
Application year: 1995
Main classification: C12N15
IPC codes: C07K14, C12N15, C12P21, C12Q1, C12R1, G01N33