BINDING OF OSTEOGENIC PROTEIN-I (OP-1) AND ANALOGS THEREOF TO THE CELL SURFACE RECEPTOR ALK-1 AND ANALOGS THEREOF

Abstract: Disclosed are (1) nucleic acid sequences, amino acid sequences, homologies, structural features, and various other data characterizing 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 for identifying compounds capable of modulating endogenous morphogen receptor levels; and (5) methods and compositions for identifying and producing morphogen analogs useful in the design of morphogen agonists and antagonists for therapeutic, diagnostic, and experimental uses.

Description: BINDING OF CISTECJGENIC PROTEIN-1 (OP-1) AND ANALOGS THEREOF TO THE CELL SURFACE RECEPTOR AL -1 AND ANALOGS THEREOF Field of the Invention This invention relates generally to the field of tissue morphogenesis and more particularly to morphogenic protein-specific cell surface receptors. Related Applications The present invention claims priority to Provisional Application U.S.S.N. 60/002,313 filed August 14, 1995, the entire disclosure of which is incorporated herein by reference. 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 ofthe 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 ofthe "superfamily" of structurally related proteins referred to in the art as the "transforming growth factor-β (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 morphogenically 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 morphogenically 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-terminal "seven cysteine" domain, and the related proteins OP-2 (also referred to in related applications as "OP2") and OP-3, both of which share approximately 70-75% identity with OP-1 in the C-terminal seven cysteine domain, as well as BMP-5, BMP-6 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 BMP-6 Xenopus homolog, Vgl, which shares approximately 57% identity with OP-1 in the C-terminal seven cysteine domain. Other bone inductive proteins include the CBMP-2 proteins (also referred to in the art as BMP-2 and BMP-4) 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. Other proteins identified in the art as bone morphogenic proteins include BMP-9 (See PCT/US92/05374; WO 93/00432); BMP-10 (PCT/US94/05288; WO 94/26893); BMP-11 (PCT/US94/05290; WO 94/26892); and BMP-12 and BMP-13 (PCT/US94/14030; WO 95/16035). As stated above, these true tissue morphogenic proteins are recognized in the art as a distinct subfamily of proteins different from other members ofthe 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 ofthe family of morphogenic proteins are capable of all ofthe following in a morphogenically permissive environment: stimulating cell proliferation and cell differentiation, and supporting the growth and maintenance of differentiated 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, BMP-2/4 and OP-1, respectively, and induce endochondral bone formation at a non-bony site in a standard rat bone formation assay. Similarly, BMP-2 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 OP-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 ofthe TGF-β protein superfamily suggests that the Ugands 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 intraceUular domain that consists essentially ofthe kinase, a short, extended hydrophobic sequence sufficient to span the membrane one time, and an extracellular domain characterized by a high concentration of conserved cysteines. 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 68: 775-785); and numerous activin-binding receptors. See, for example, Mathews et al. (1991) Cell 65: 973-982 and international patent application WO 92/20793, pubUshed November 26, 1992, disclosing the " ActR II" sequence; Attisano et al. , ( 1992) Cell 68 : 97- 108, disclosing the " ActR-IIB " sequence; and Legerski et al. (.1992) Biochem Biophys. Res. Commu. 183: 672-679. A different Type II receptor shown to have affinity for activin is Atr-II (Childs et α/.(1993) PNAS 90: 9475-9479.) Two Type π receptors have been identified in C. elegans, the daf-1 gene, (Georgi et al. (1990) Cell 61 : 635-645), having no known Ugand to date, and daf-4, which has been shown to bind BMP-4, 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 Ubraries ((1993) Oncogene 8: 2879-2887, and (1994) Science 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 ((1994) Science 263: 1756-1759). The authors suggest that the protein binds dpp. To date, a ligand having binding affinity for the ALK-1 Type I receptor has not yet been identified. It is an object of this invention to provide molecules, including OP-1 and OP- 1 -related proteins, which are competent to act as Ugands for ALK-1 receptor binding and/or are capable of mediating an intraceUular effect by interaction with the ALK-1 receptor. Another object is to provide methods for identifying genes and naturally occurring sequences in a variety of species and/or tissues, and in a variety of nucleic acid Ubraries encoding ALK- 1 -binding Ugands and ALK- 1 -related receptors. Still another object is to provide means for designing biosynthetic receptor- binding Ugand analogs, particularly OP-1 and OP- 1 -related 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 OP-1 or an OP- 1 -related protein for ALK-1 receptor binding in vivo. 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 an ALK-1 receptor and Ugand-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. StiU another object is to provide means and compositions for modulating a morphogenesis in a mammal. These and other objects and features ofthe invention wiU be apparent from the description, drawings and claims which follow. Summary of the Invention It now has been discovered that the Type I cell surface receptor molecule identified in the literature as ALK-1 is capable of specific binding affinity with true tissue morphogenic proteins, particularly OP-1 and OP- 1 -related proteins. Accordingly, the invention provides ligand-receptor complexes comprising at least the ligand binding domain of an ALK-1 receptor and OP-1 or an OP-1 related protein or analog thereof as the Ugand. The invention also provides means for identifying and/or designing useful receptor sequence analogs and OP-1, OP- 1 -related proteins, and analogs hereof; and means for modulating the tissue morphogenesis capability of a cell. The ALK-1 receptor (Seq. ID No. 1) useful in this invention is part of a family of receptor molecules which share a conserved structure, including an extraceUular, Ugand 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 intraceUular (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. As used herein, ligand-receptor binding specificity is understood to mean a specific, saturable noncovalent interaction between the Ugand 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 fiU one-half (50%) of available receptor binding sites) are described herein by dissociation constant (Kd). In one embodiment, preferred binding affinities ofthe ligand-receptor complexes described herein have a Kd of less than IO "7 M, preferably less than 5x10 '7 M, more preferably less than IO "8 M. In another preferred embodiment, the receptor molecules have Uttle or no substantial binding affinity for TGF-β. As wiU be appreciated by those having ordinary skiU in the art, the ALK-1 receptor can also have binding affinity for other, related morphogenic proteins. It further is contemplated that one or more related morphogenic proteins may have better binding affinity and/or signal transducing ability than OP- 1 has for ALK- 1. As used herein, an OP- 1 -related protein ALK- 1 Ugand is understood to have substantially the same binding affinity for ALK-1 as OP-1 if it can be competed successfuUy for ALK-1 binding in a standard competition assay with OP-1. In one preferred embodiment, the Ugand is OP-1 or an OP-1- related protein or analog and has a binding affinity for ALK-1 defined by a dissociation constant of less than about 10 "7 M, preferably less than about 5x10 '7 M or 10 "8 M. It is anticipated however, that analogs having lower binding affinities, e.g., on the order of IO "6 M also will be useful. For example, such analogs may be provided to a mammal to modulate avaUabϋity of serum-soluble Ugand for ALK-1 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 Ugand-receptor antagonist, preferred binding affinities may be on the order of 5x10 "8 M. In another embodiment, the ALK-1 receptor binding analogs contemplated by the invention include proteins encoded by nucleic acids which hybridize with the DNA sequence encoding OP-1 under stringent hybridization conditions, and which have substantially the same or better binding affinity for a ALK-1 as OP-1. 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, 5xSSPE, SxDenhardt's Solution, and 0.1% SDS at 37°C overnight, and washing in O.lxSSPE, 0.1% SDS at 50°C. Useful receptor analogs include xenogenic (foreign species) homologs ofthe human ALK- 1 sequences described herein, including those obtained from other mammaUan species, as well as other, eukaryotic, non-mammalian xenogenic homologs. Also contemplated are biosynthetic constructs and naturaUy-occurring sequence variants of ALK- 1, provided these molecules bind OP-1 or an OP-1 related protein, or analog specifically as defined herein. In one embodiment, sequence variants include receptor analogs which have at least the same binding affinity for OP-1 as ALK- 1 and which are recognized by an antibody having binding specificity for ALK- 1. The ALK-1 receptor analogs may be used as an OP-1 or an OP- 1 -related protein antagonist. For example, a soluble form of a receptor, e.g., consisting essentially of only the extracellular ligand binding domain, may be provided systemicaUy to a mammal to bind to soluble Ugand, 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, OP-1-related proteins, and analogs thereof. As used herein, an "analog" is understood to include all molecules able to functionaUy substitute for OP-1 or an OP- 1 -related protein in Type I receptor binding, e.g., which are able to compete successfully with OP-1 for receptor binding in a standard competition assay. In one embodiment, useful analogs include molecules whose binding affinity is defined by a dissociation constant of less than about 5x10 "6 M, preferably less than about IO "7 M or 5x10 "7 M. As described above, both stronger and weaker binding affinities are contemplated to be useful in particular applications. The OP-1 analogs contemplated herein, aU 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 foUowing 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 Ugand 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 OP-1 both in receptor binding and in abiUty 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 nonproteinaceous molecules. In addition, the OP-1 analogs contemplated can be naturaUy- sourced or synthetically-produced. As used herein, an OP-1 related protein contemplates a protein having a sequence sharing at least 60%, preferably greater than 65% or even 70% amino acid sequence homology or "similarity" with the C-terminal 96 amino acid sequence of OP-1 (OPS) as defined in Seq. ID Nos. 3-4, and which is able to substitute for OP-1 in ligand binding to ALK-1, (e.g., able to compete successfully with OP-1 for binding to an ALK-1 receptor.) Proteins sharing at least 60% homology with OP- 1 include BMP-2, BMP-4, and GDF- 1. In another embodiment an OP- 1 - related protein is contemplated to include a protein having a sequence sharing at least 60% amino acid sequence homology with the C-terminal 102 amino acid sequence of OP-1, (OP-1 "seven cysteine domain"). In one preferred embodiment OP-1 related protein sequences are contemplated to 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. OP-1 related sequences contemplated by the mvention include xenogenic homologs (e.g., the Drosophila homolog 60A), and the related sequences referenced herein and in the Uterature as OP-2, OP-3, BMP-5, BMP-6 (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. 3 under stringent hybridization conditions and which can substitute for OP-1 in an OP- 1 -receptor binding assay. In another embodiment, OP-1 sequence variants include proteins which can substitute for OP-1 in a Ugand-receptor binding assay and which is recognized by an antibody having binding specificity for OP-1. As used herein, "amino acid sequence homology" is understood to mean amino acid sequence similarity, and homologous sequences sharing identical or simUar amino acids, where siπύlar 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'l BioMed. Research Fdn., Washington D.C. 1978.) Thus, a candidate sequence sharing 60% amino acid homology with a reference sequence requires that, following alignment ofthe candidate sequence with the reference sequence, 60% ofthe 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 aUgnment ofthe candidate sequence with the reference sequence, 60% ofthe amino acids in the candidate sequence are identical to the corresponding amino acid in the reference sequence. As used herein, 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 Nos. 3-4 constituting the seven cysteine domain. Also as used herein, sequences are aUgned for homology and identity calculations using the method of Needleman et al. ((1970) J. Mol. Biol. 48: 443- 453), and identities are calculated by the Align program (DNAstar, Inc.). In all cases, internal gaps and amino acid insertions in the candidate sequence as aligned are ignored when making the homology/identity calculation. In one aspect, the invention contemplates isolated ligand-receptor complexes comprising OP-1 or an OP-1 related protein or analog as the ligand in specific binding interaction with an ALK-1 receptor or receptor analog, as defined herein. In another aspect, the invention contemplates that the ligand-receptor complex also 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 Uterature as having binding specificity for activin or a bone morphogenic protein, including daf-4, ActRII and Atrll. In still another aspect, the ligand-receptor complex comprises both a Type I and a Type II receptor and OP-1, or an OP-1 related protein or analog as the ligand. In all complexes, the bound receptor may comprise just the extracellular, ligand binding domain, or may also include part or all ofthe transmembrane sequence, and/or the intracellular kinase domain. Similarly, the ligand may comprise just the receptor binding sequence, longer sequences, including the mature dimeric species or any soluble form ofthe protein or protein analog. 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 Uttle or no substantial binding affinity for the uncomplexed Ugand 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)' 2 ), 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 sF v s, BABs and SCAs, and chimeric monoclonals, in which portions ofthe monoclonals are humanized (excluding those portions involved in antigen recognition (e.g., complementarity determining regions, "CDRs".) 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 morphogenic protein analogs as described below, as well as kits and methods, e.g., screening assays, for identifying these analogs. The molecules usefiil in these assays can include part or all ofthe ALK-1 receptor sequence of Seq. ID No. 1, including analogs and amino acid sequence variants thereof. Currently preferred for these assays are receptor sequences comprising at least the sequence which defines the extracellular, ligand binding domains. SimUarly, the kits and screening assays can be used in the design and/or identification of ALK- 1 receptor analogs. The OP-1 analogs and OP-1 receptor analogs identified then can be produced in reasonable quantities using standard recombinant expression or chemical synthesis technology. In still another aspect, the receptor and/or OP- 1 -specific receptor analogs can be used in standard methodologies for affinity purifying and/or quantifying OP-1, OP- 1 -related proteins and analogs. For example, the receptor's ligand binding domain first may be immobilized on a surface of a weU or a chromatographic column; Ugand in a sample fluid then may be provided to the receptor under conditions to aUow specific binding; non-specific binding molecules then removed, e.g., by washing, and the bound ligand then selectively isolated and/or quantitated. Similarly, OP- 1, OP- 1 -related proteins and OP-1 analogs can be used for affinity purifying and/or quantifying ALK-1 receptors and receptor analogs. In one embodiment, the method is useful in kits and assays for diagnostic purposes wliich detect the presence and/or concentration of OP-1 protein or related morphogen in a body fluid sample including, without Umitation, serum, peritoneal fluid, spinal fluid, and breast exudate. The kits and assays can also be used for detecting and/or quantitating ALK-1 receptors in a sample. In stiU another aspect the invention comprises ALK-1 receptors and receptor analogs useful in screening assays to identify organs, tissues and cell lines which express ALK-1 receptors. These cells then can be used in screening assays to identify Ugands that modulate endogenous morphogen receptor expression 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 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 glycosylation patterns. Active receptors useful in the invention also include chimeric constructs as described below. Active OP- 1 -specific receptors/analogs can be expressed from intact or truncated genomic or cDNA, or from synthetic DNAs in prokaryotic or eukaryotic host ceUs, 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 ceUs, including mammalian cells, such as fibroblast 3T3 cells, CHO, COS, melanoma or BSC ceUs, Hela and other human cells, the insect aculovirus system, as well as yeast and other microbial host ceU systems. Thus, in view of this disclosure, skiUed genetic engineers now can, for example, identify and produce ALK-1 ceU surface receptors or analogs thereof; create and perform assays for screemng candidate OP-1 or OP- 1 -related protein analogs and evaluate promising candidates in therapeutic regimes and preclinical studies; modulate the availabiUty of endogenous morphogen for cell surface interactions; modulate endogenous ALK-1 cell surface receptor levels; elucidate the signal transduction pathway induced or otherwise mediated by ALK-1 receptor binding; and modulate tissue morphogenesis in vivo. Brief Description of the Drawings Fig 1. is a schematic representation of OP- 1 -induced PAI-1 production in ceUs transfected with ALK-1, ALK-3 or ALK-5; Fig 2A is a graphic representation of 3 H-thymidine incorporation in response to OP-1 in R4-2 ceUs transfected with ALK-1 (closed circles), ALK-3 (closed triangles) and ALK-5 (closed squares) as a function of OP-1 concentration; Fig 2B is a graphic representation of 3 H-thymidine incorporation in response to OP-1 or TGF-β in cells transfected with ALK- 1 , ALK-3 or ALK-5. Detailed Description Disclosed herein is a Type I receptor, ALK-1, having binding specificity for true tissue morphogenic proteins, particularly OP-1 and OP- 1 -related proteins. The ALK-1 Type I receptors disclosed herein, can be used together with OP-1, OP- 1 -related proteins and OP-1 analogs for therapeutic, diagnostic and experimental uses as described herein below. Moreover, soluble forms ofthe receptor proteins, e.g., forms consistmg essentially ofthe extraceUular domain or a fragment thereof sufficient to bind OP-1 with specificity, can be used as a soluble therapeutic morphogen antagonist, as described below. Following this disclosure, related ALK-1 receptors are available, as are high and medium flux screening assays for identifying Ugand analogs and ALK-1 receptor analogs. These analogs can be naturally occurring molecules, or they can be designed and biosynthetically created using a rational drug design and an established 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)' 2 ), or synthetic derivatives thereof, such as monoclonal single chain F v fragments known in the art as sF v s, BABs, and SCAs (see below), and bispecific antibodies or derivatives 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 can also mimic OP-1 both in receptor binding and signal transduction, in which case the antibody acts as an OP-1 agonist. The antibodies and derivatives can also be used for inducing the morphogenic ceUular response by crosslinking receptors to morphogenic proteins, particularly OP-1 and OP- 1 -related proteins to form either homo- or hetero-complexes ofthe Type I and Type II receptors. The ALK-1 receptor sequences described herein also can be used to create chimeric sequences, wherein, for example, part or all of either the extracellular domain or the intraceUular 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 OP-1 analog assays, wherein the OP- 1 -binding extraceUular 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 can also be used, for example, in high flux OP-1 analogs screens and as part of purification protocols, wherem a soluble ligand binding domain of an OP- 1 -specific receptor is immobiUzed on to 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 OP-1 or OP-1 analogs. When immobilized on a well surface the receptor fragment is particularly useful in a screening assay to identify receptor-binding OP-1 analogs in a standard competition assay. The true tissue morphogenic proteins contemplated to be useful in the methods and compositions ofthe invention include forms having varying glycosylation patterns and varying N- termini. The proteins can be naturally occurring or biosyntheticaUy derived, and can be produced by expression of recombinant DNA in prokaryotic or eukaryotic host ceUs. The proteins are active as a single species (e.g., as homodimers), or combined as a mixed species. Useful sequences and eukaryotic and prokaryotic expression systems are well described in the art. See, for example, US Patent Nos. 5,061,911 and 5,266,683 for useful expression systems. Contemplated herein are OP-1 and OP- 1 -related sequences. Particularly useful OP-1 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 88: 9214-9218; and proteins sharing at least 60% amino acid sequence homology or "similarity" with OP-1 in the C-terminal seven cysteine domain. In a preferred embodiment, the related proteins share greater than 60% identity in the C-terminal seven cysteine domain, preferably at least 65% identity. Examples of OP-1 related sequences include BMP-5, BMP-6 (and its species homolog Vgr-1, Lyons et al. (1989) PNAS 86: 4554-4558), Celeste, et al. (1990) PNAS 87: 9843-9847 and PCT intemational appUcation WO 93/00432; OP-2 (Ozkaynak et al. (1992) J. Biol. Chem. 267: 13198-13205) and OP-3 (PCT international appUcation WO 94/06447). As will be appreciated by those having ordinary skill in the art, chimeric constructs readily can be created using standard molecular biology and mutagenesis techniques combining various portions of different morphogenic protein sequences to create a novel sequence, and these forms ofthe protein also are contemplated herein. A particularly preferred embodiment ofthe proteins contemplated by the invention includes proteins whose amino acid sequence in the cysteine-rich C-terminal domain has 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 Nos. 3-4). 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 ofthe osteogenic OP-1 and OP-2 proteins, and which is described by the amino acid sequence presented below and in Seq. ID No. 5. Cys Xaa Xaa His Glu Leu Tyr Val Ser Phe 1 5 10 Xaa Asp Leu Gly Trp Xaa Asp Trp Xaa Ile 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 Ile 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 Ile Leu Xaa Lys 85 90 Xaa Arg Asn Met Val Val Xaa Ala Cys Gly 95 100 Cys His,r 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 = (Ile 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 = (Ile 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 = (Ile 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 OP-1 or OP-2 under stringent hybridization conditions. A brief description ofthe various terms of OP-1 useful in the invention is described below: OP-1 - Refers generically to the family of osteogenically active proteins produced by expression of part or all ofthe hOP-1 gene. Also referred to in related applications as "OPI" and "OP- 1". OP-l-PP - Amino acid sequence of human OP-1 protein (prepro form), Seq. ID Nos. 3-4, residues 1-431. Also referred to in related applications as "OP-l-PP" and "OPP". OP-l-18Ser - Amino acid sequence of mature human OP-1 protein, Seq. ID Nos. 3-4, residues 293-431. N-terminal amino acid is serine. Originally identified as migrating at 18 kDa on SDS-PAGE in COS ceUs. Depending on protein glycosylation pattem in different host cells, also migrates at 23 kDa, 19 kDa and 17 kDa on SDS-PAGE. Also referred to in related appUcations as "OP-1-18." OP-l-16Ser; OP-l-16Ala; OP-1-16 Met; OP-l-16Leu; OP-l-16Val - N-terminaUy tmncated mature human OP-1 protein species defined, respectively, by residues 300-431; 316-431; 315-431; 313-431 and 318-431 of Seq. ID Nos. 3-4. OPS - Amino acid sequence defining the C-terminal six cysteine domain, residues 335-431 of Seq. ID Nos. 3-4. OP7 - Amino acid sequence defining the C-terminal seven cysteine domain, residues 330-431 of Seq. ID Nos. 3-4. Soluble form OP-1 - mature dimeric OP-1 species having one or, preferably two copies of pro domain, e.g., at least residues 158-292 of Seq. ID Nos. 3-4 , preferably residues 48-292 or 30-292, non-covalently complexed with the dimer. The cloning procedure for obtaining OP- 1 -binding ALK nucleic acid sequences, means for expressing receptor sequences, as well as other material aspects concerning the nature and utiUty 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 The cloning and characterization of ALK- 1, -2, -3, and -6 receptors are described in detail in ten Dijke et al. (1993) Oncogene 8: 2879-2887; and (1994) Science 264: 101-104. These molecules have similar domain stmctures: an N-terminal predicted hydrophobic signal sequence (von Heijne (1986) Nucl. Acids Res. 14: 4683-4690) is followed by a relatively small extraceUular cysteine-rich ligand binding domain, a single hydrophobic transmembrane region (Kyte & DooUttle (1982) J. Mol. Biol. 157: 105-132) and a C-terminal intracellular portion, which consists almost entirely of a kinase domain. The extraceUular domains ofthe ALK-1 receptor is defined essentiaUy by residues 22-118 (Seq. ED Nos. 1-2). The positions of many ofthe cysteine residues in these receptors can be aUgned, indicating that the extraceUular domains likely adopt a similar stmctural configuration. The intracellular domains of these receptors are characterized by a serine/threonine kinase, defined essentially by residues 204-494 (Seq. ID Nos. 1-2) for ALK-1. 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 aU the ALK. 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. 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 tmncated 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 ceU system, myeloma cells, and various other mammaUan cells. The full length forms ofthe proteins of this invention preferably are expressed in mammalian ceUs, as disclosed herein. Soluble forms may be expressed from both mammaUan or bacterial cell systems. The vector additionally may include various sequences to promote correct expression ofthe 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 Uke. The DNA sequence encoding the gene of interest also may be manipulated to remove potentially inhibiting sequences or to minimize unwanted secondary stmcture 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 ofthe recombinant protein. One useful sequence for example, is a hexa-His (Hisβ) sequence, which adds a histidine tail to allow affinity purification ofthe protein on an Cu 2+ HV AC 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 mammaUan host. Useful hosts include fibroblast 3T3 ceUs, (e.g., NIH 3T3, from CRL 1658) COS (simian kidney ATCC, CRL-1650) or CHO (Chinese hamster ovary) cells (e.g. , CHO-DXB 11 , from Lawrence Chasin, (1980) PNAS 77: 4216-4222), mink-lung epithelial ceUs (MVlLu), human foreskin fibroblast ceUs, human gUoblastoma ceUs, and teratocarcinoma ceUs. Other useful eukaryotic ceU systems include yeast cells, the insect/baculovims system or myeloma ceUs. To express an ALK-1 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 vims promoter (MMTV) boosted by the Rous sarcoma vims LTR enhancer sequence (e.g., from Clontech, Inc., Palo Alto). 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 semm (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 mtegrated 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 Ugand 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. Altematively, where a useful anchor sequence has been added to the DNA, such as a (His) 6 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 in vivo conditions. Cell surface expression also may be most efficacious for medium flux cellular screen assays as described below. B.l Exemplary Mammalian Cell Culture The receptors described herein and tested in Example 8 described below, were expressed in (1) COS-l cells; (2) mink lung epithelial ceUs (MvlLu); and (3) ATDC5, a chondrogenic cell hne, (Atnami (1990) Cell Diff. Dev. 30: 109-116). They also can be tested in a variety of other cells including, without limitation, AG1518 human foreskin fibroblasts; MG-63 human osteosarcoma ceUs; PC 12 rat pheochromocytoma ceUs (all obtainable from American Type Culture Collection, RockviUe, MD); human ghoblastoma cells (U-1240 MG, Bengt Westermark, et al. (1988) Cancer Research 48: 3910-3918); Tera-2 teratocarcinoma cells (clone 13, Thompson et al. (1984) J Cell Sci 72: 37-64); MC3T3-E1 cells (Sudo et al. (1983) J. Cell Biol. 96: 191-198, and ROS 17/2.8 rat osteosarcoma ceUs (Majeska et al. (1985) Endocrinology 116: 170-179. Exemplary culturly conditions for the ROS cells include culture in Ham's F12 medium containing 14 mM HEPES buffer, 2.5 mM L-glutamine, 1.1 mM CaCl 2 , 5% fetal bovine semm and antibiotics; for MC3T3-E1 ceUs, culture in a-MEM with 10% fetal bovine semm and antibiotics, and for Tera-2 cells culture in 5% CO 2 atmosphere at 37°C 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, in the examples provided herein, ceUs were cultured in DMEM containing 10% fetal bovine serum and antibiotics. 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 ALK-1 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. FinaUy, 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 OP-1 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 OP-1 in a competition binding assay. SimUarly, where antibodies useful as OP- 1 -specific receptor analogs are desired, OP-1 is the immunogen source and the antibody 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., 500 mg) 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 ofthe animal. The rabbit is boosted after a month with 500 mg 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 semm 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. SimUarly, monoclonal antibodies specific for a given morphogen receptor molecule of interest may be prepared as described below: A mouse is given two injections ofthe 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 extraceUular domain preferably is provided. The first injection contains 100 mg 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 intraperitoneaUy with antigen (e.g., 100 mg) and may be additionally boosted with an antigen-specific peptide conjugated to bovine semm 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 ceUs 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 using ALK-1, 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 assays described herein were obtained as reported in Franzen et al. (1993) Ce// 75: 885-892, and ten Dijla et al. (1994) Science 264: 101-104. Briefly, rabbit antisera against ALK-1 were made against synthetic peptides corresponding to the divergent, intraceUular juxtamembrane parts: residues 119-141. Peptides were synthesized with an AppUed 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 described 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. OP-1 -RECEPTOR BINDING ASSAYS Ligand binding specificity is determined by evaluating the abuity of a receptor molecule to bind a specific Ugand, and the abUity of that Ugand to compete against itself and other molecules which bind the receptor. Useful ligands will have a binding affinity for a soluble morphogen receptor extraceUular domain such that dissociation constant (Kd) is less than about IO "6 M, preferably less than 5x10 "7 M. Where stronger binding interaction is desired, preferred affinities are defined by a Kd of 10 "8 -10 "9 M. OP- 1 -related proteins are expected to be able to bind with specificity to multiple different receptor molecules, although Ukely with differing affinities. Ligand binding specificity can be assayed as foUows, 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 /., (1978) Biochem. Biophys. Res. Comm. 80: 849-857. In the Ugand binding assays, a ligand having a known, quantifiable affinity for a morphogen receptor molecule of interest is labeled, typically by radioiodination ( 125 I), e.g. , by chromogenic or fluorogenic labeling, or by metabolic labeling, e.g., 35 S, and aUquots of cells expressing the receptor on their surface are incubated with the labeled Ugand, in the presence of various concentrations of unlabeUed potential competitor ligand. In the assays described in Examples 8 and 9, below, this competitor typically is the candidate morphogen analog or an aUquot from a broth or extract anticipated to contain a candidate morphogen analog. Altematively, a crosslinking agent may be used to covalently Unk the ligand to the bound receptor, and the crossUnked complex then immunoprecipitated with an antibody specific to the Ugand, receptor, or complex. A standard, exemplary protocol for deteπnining binding affinity is provided below. Briefly, cells expressing a receptor on their ceU surface are plated into 35 mM dishes and incubated for 48 hours in DMEM (Dulbecco's modified Eagle medium) plus 10% fetal calf semm. Purified morphogen, here, e.g., OP-1, or an OP- 1 -analog is iodinated with Na ,25 I 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. Labeled 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 22°C in the presence of BSA, buffer and labeled morphogen (1 ng) and various concentrations (e.g., 0-lOmg/ml) of unlabelled competitor, e.g., unlabeUed morphogen or candidate Ugand analogs. FoUowing binding, ceUs 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 scintiUation 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 labeled 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 labeled 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 wiU flow, allowing the Ugand 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 an analog screening assay, the currently preferred protocol is a standard competition or radioimmunoassay (RIA). Here the OP-1 is labeled and the relative binding affinity of a candidate OP-1 analog ligand in a sample is measured by quantitating the ability ofthe candidate (unlabelled ligand analog) to inhibit binding ofthe labeled ligand (competitor morphogen) by the receptor. In performing the assay, fixed concentrations of receptor and labeled morphogen are incubated in the absence and presence of unknown samples containing candidate Ugands. Sensitivity can be increased by preincubating the receptor with candidate Ugand before adding the labeled morphogen. After the labeled competitor has been added, sufficient time is allowed for adequate competitor binding, and then free and bound labeled moφhogen 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 chemUuminescent or fluorogpαic substrates. In the same manner, OP- 1 -specific receptor analogs can be assayed for their affinity for OP-1 in competition assays with labeled OP-1 specific receptors. 4.2 Diagnostic Assay Considerations The ability to detect morphogens in solution provides a valuable tool for diagnostic assays, allowing one to monitor the level of moφhogen free in the body, e.g., in semm, urine, spinal or peritoneal fluid, breast exudate, and other body fluids. For example, OP-1 is an intimate participant in normal bone growth and resoφtion. Thus, soluble OP-1 is expected to be detected at higher concentrations in individuals experiencing high bone formation, such as children, and at substantiaUy lower levels in individuals with abnormaUy low rates of bone formation, such as patients with osteoporosis, aplastic bone disease, or osteopenia. Monitoring the level of OP-1 in semm thus provides a means for evaluating the status of bone tissue and bone homeostasis in an individual, as well as a means for monitoring the efficacy of a treatment to regenerate damaged or lost bone tissue. For semm assays, the semm preferably first is partially purified to remove some ofthe excess, contaminating semm proteins, such as semm albumin. Preferably the semm is 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 ofthe differential solubUity of soluble moφhogen complex or mature moφhogens relative to that ofthe other proteins present in semm. Further purification also can be achieved by chromatographic techniques well known in the art. The sample fluid then can be assayed for OP-1 using the OP-1- specific receptor(s) and binding assays as described herein. Moφhogens useful in the binding/screening assays contemplated herein include the soluble forms ofthe protein, e.g., the mature dimeric species complexed with one or two copies ofthe pro domain, the mature dimeric species alone, and tmncated forms comprising essentiaUy just the C-terminal active domain. Example 5. TRANSMEMBRANE SIGNAL INDUCTION ASSAYS/OP- 1 MIMETICS The kinase activity ofthe intracellular domains ofthe ALK-1 receptors can be tested in an autophophorylation assay as described by Mathews et al. (PCT/US92/03825, pubhshed November 26, 1992) . Briefly, the DNA fragment encoding at least the intracellular kinase domain of an OP-1-specific receptor is subcloned into pGEX-2T (Smith et α/.(1988) Gene 67: 31-40) to create a fusion protein between the putative kinase domain and giutathione S-transferase (GST). The plasmid is introduced into E. coli and the expressed fusion protein purified using giutathione affinity chromatography. About 100-200 ng of fusion protein or purified GST then are incubated with 25 mCi (gp 32 P) ATP in 50 mM Tris, 10 mM 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 phosphorylated, indicating that the kinase domain is functional. Phosphoamino acid analysis then can be performed to determine the predominant amino acid being phosphorylated. Similar assays can be performed using similar fusion constmcts expressed in mammaUan cells. Various signaling transduction assays are provided in Example 9, below. An assay can also be developed for testing kinase activity transduction upon ligand binding using 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 described by Accili et al. (1991) J. Biol. Chem. 266: 434-439 and Nakamura et al. (1992) J. Biol. Chem. 267: 18924-18928. For example, Ugand-induced kinase activity (e.g., receptor autophosphorylation) can be measured in vitro by incubating purified receptor in the presence and absence of ligand (here, OP-1 or OP-1 analog, e.g., IO "7 M) under conditions sufficient to allow binding ofthe ligand to the receptor, followed by exposure to 32 P-ATP (e.g., 100 mCi in the presence of 10 mM Tris-HCl (pH 7.6), 10 mM MgCl 2 , 10 mM MnCl 2 , 1 mM dithiothreitol, 0.15 M NaCl, 0.1% Triton X-100 and 3% glycerol) and the amount of phosphorylation measured, e.g., by SDS polyacrylamide gel electrophoresis and autoradiography following immunoprecipitation with antiphosphoserine, antiphosphothreonine 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 is anticipated to significantly increase (e.g., 5-20 fold increase) the amount of phosphorylation detected. In another assay 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 expressing the receptor on their cell surface. On the day ofthe experiment, cells are incubated with or without ligand (e.g., OP-1 or OP-1 analog, e.g., IO "7 M) at 37°C, and then quickly washed with a "stopping solution" containing ATP (e.g., 0.1 M NaF, 4 mM EDTA, 10 mM ATP, 10 mM sodium orthovanadate, 4 mM sodium pyrophosphate). The ceUs 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 electrophoresis using standard procedures and transferred to a membrane for Western blot analysis using standard procedures. Phosphorylation ofthe 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 125 I labeled protein A, followed by autoradiography. The amount of phosphorylated receptor detected is anticipated to be significantly greater (5-20 fold increase) in receptors incubated withjigand than receptors exposed to ATP in the absence of ligand. Ligand-induced receptor phosphorylation of exogenous substrates similarly can be assayed essentiaUy using the methodology described herein. Here, a suitable substrate (e.g., a synthetic polypeptide containing serine, threonine or tyrosine amino acids) is provided to the receptor foUowing Ugand exposure and prior to incubation with ATP. The substrate subsequently can be segregated by immunoprecipitation with an antibody specific for the substrate, and phosphorylation detected as described above. As for autophosphorylation, the amount of phosphorylated substrate detected following Ugand incubation is anticipated to be greater than that detected for substrates exposed to receptors in the absence of Ugand. Example 6. CHIMERIC RECEPTOR MOLECULES Chimeric receptor molecules, e.g., comprising an ALK or ALK analog extracellular and transmembrane region and, for example, part or aU of an intracellular domain from another, different receptor or an intraceUular domain from a different cell surface molecule, may be constmcted using standard recombinant DNA technology and/or an automated DNA synthesizer to constmct 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 intraceUular or the extracellular domains. Also envisioned are chimers where the extracellular domain or the intracellular domains themselves are chimeric sequences. Chimeric sequences are envisioned to be particularly useful in screening assays to determine candidate binding ligands (e.g., OP-1 analogs, see below), where the non-receptor intraceUular domain provides a suitable second messenger response system that is 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 molecules have particular utility in gene therapy protocols. For example, a population of cells expressing a chimeric moφhogen receptor molecule on their surface and competent for expressing a desired phenotype can be implanted in a mammal at a particular tissue locus. By careful choice ofthe ligand binding domain used on these receptors a physician can administer to the individual a moφhogen agonist capable of: (1) binding to the chimeric receptor alone and (2) stimulating the proUferation and/or differentiation ofthe implanted cells without affecting endogenous cell populations. Example 7. CONSIDERATIONS FOR IDENTIFYING OTHER OP-1 SPECIFIC RECEPTORS IN NUCLEIC ACID LIBRARIES Identification of ALK- 1 allows one to identify other ALK-1 related moφhogen receptor sequences in different species as well as in different tissues. ALK-1 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 No. 1 can be synthesized on commercially available DNA synthesizers, e.g. Applied Biosystems model 381 A, using standard techniques, e.g. Gait, Oligonucleotide Synthesis: A Practical Approach, (IRL Press, 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 ways standard 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 /., (1986) Nucleic Acids Research 14: 6115-6128. Preferably, low stringency conditions are employed when screening a Ubrary for moφhogen receptor sequences using a probe derived from OP- 1 -binding receptor. Preferred ALK-specific probes are those corresponding to bases encoding the extraceUular 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. Altematively, probes derived from the serine/threonine kinase domain can be used to identify new members ofthe receptor kinase family which can be screened for OP-1 binding using the methods described in Example 8. For example, for a probe of about 20-40 bases 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 55°C in 5xDenhardt's solution, 6xSSC (20xSSC consists of 175g NaCl, 88.2 g sodium citrate in 800 ml H 2 O adjusted to pH. 7.0 with 10 N NaOH), 0.1% SDS, and 100 mg/ml denatured salmon sperm DNA, (2) hybridization: incubate filters in prehybridization solution plus probe at 42°C for 14-48 hours, (3) wash; three 15 minutes washes in 6xSSC and 0.1% SDS at room temperature, followed by a final 1-1.5 minute wash in 6xSSC and 0.1% SDS at 55°C Other equivalent procedures, e.g. employing organic solvents such as formamide, are well known in the art. Altematively, moφhogen receptor-specific DNA can be amplified using a standard PCR (polymerase chain reaction) methodology such as the one disclosed herein, to ampUfy approximately 500 base pair fragments. As for the hybridization screening probes described above, the primer sequences preferably are derived from conserved sequences in the serine/threonine kinase domain. Examples of useful PCR amphfications 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. 7.1 Tissue Distribution of Morphogen Receptors Deteπnining the tissue distribution of OP- 1 -specific receptors can be used to identify tissue and cell sources which express these receptors, to identify new, related OP- 1 -specific receptor molecules, as weU as to identify target tissues for OP- 1 -receptor interactions under naturally occurring conditions. The OP-1 specific receptor molecules (or their mRNA transcripts) readily are identified in different tissues using standard methodologies and minor modifications thereof in tissues where expression may be low. For example, protein distribution can be determined using standard Western blot analysis or immunohistological detection techmques, and antibodies specific to the moφhogen receptor molecules of interest. Similarly, the distribution of moφhogen receptor transcripts can be determined using standard Northern hybridization protocols and transcript-specific probes or by in situ hybridization. Any probe capable of hybridizing specificaUy to a transcript, and distinguishing the transcript of interest from other related transcripts can be used. Because the moφhogen receptors described herein likely share high sequence homology in their intracellular domains, the tissue distribution of a specific moφhogen receptor transcript may best be determined using a probe specific for the extracellular domain ofthe molecule. For example, a particularly useful ALK-specific probe sequence is one derived from a unique portion ofthe 5' coding sequence, the sequence corresponding to the juxtamembrane region, or the 5' or 3' noncoding sequences. The chosen fragment then is labeled 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 tissues of various organisms, using standard methodologies well known to those having ordinary skill in the art. A detailed description of a suitable hybridization protocol is described in Ozkaynak, et al, (1991) Biochem. Biophys. Res. Comm. 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 standard methodologies such as by the method of Chomczynski et al. ((1987) Anal. Biochem 162: 156- 159) and described 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 tissue is fractionated on a 1% agarose/formaldehyde gel and transferred onto a Nytran membrane (Schleicher & Schuell). Following the transfer, the membrane is baked at 80°C and the RNA is cross-linked under UV light (generally 30 seconds at 1 mW/cm^). 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 37°C using a hybridization mix of 40% formamide, 5xDenhardts, 5xSSPE, and 0.1% SDS. Following hybridization, the non-specific counts are washed off the filters in O.lxSSPE, 0.1% SDS at 50°C Example 8. DEMONSTRATION THAT ALK-1 IS AN OP-1 -BINDING RECEPTOR Experimental Procedures Cell Culture A chondrogenic cell line, ATDC5 (Atsumi et al, 1990 Cell Diff. Div. 30: 109-116) was cultured in a 1 : 1 mixture of Dulbecco's modified Eagle's medium (DMEM) and Ham's F12 containing 5% fetal bovine semm, 10 μg/mi bovine insulin (Sigma Co., Cincinnati) and 100 units/ml penicillin. CeUs from a chemically-mutagenized Mvl Lu cell Une (R mutant, clone 4-2 (R4-2)) (Laiho et al, 1990 J. Biol. Chem. 285: 18518-18524) also were used. R4-2 cells were cultured in DMEM containing 10% FBS and 100 units/ml penicillin in 5% C0 2 atmosphere at 37°C Preparation of Polyclonal Antibodies Antisera against ALK-1 to -6 were made against synthetic peptides corresponding to the intracellular juxtamembrane parts ofthe receptors as described herein. Stable Transfection ofcDNA Stable expression plasmids of human ALK-1, ALK-3 (also referred to in the art as BMPR-1A) and ALK-5 (also referred to in the art as TβR-1) were generated by subcloning the corresponding full length cDNA into pMEP4 (Wrana et al, 1992, Cell 71 : 1003-1014), a Zn 2+ - inducible mammaUan expression vector, using convenient restriction enzyme sites in the polylinker region ofthe vector. To generate stable transfectants, R4-2 cells were transfected with 10 μg each of plasmids by a calcium phosphate precipitation method using a mammaUan transfection kit (Stratagene, La JoUa) as described by the manufacturer. After 2 days, selection was initiated by adding 120 U/ml hygromycin B (Wako Chemicals) to the culture medium. Independent colonies were cloned, and after screening by immunoprecipitation following the metabolic labeling ofthe ceUs, positive clones were chosen and further analyzed. More than two positive clones for each ofthe transfectants were subjected to the following experiments, in which no significant difference between independent clones was observed in the magnitude of receptor expression and response to the Ugands. Recombinant Proteins and Radioiodination Recombinant human TGF-β 1 and recombinant human activin A were obtained from H. Ohashi (Kirin Brewery Company Ltd.) and Y. Eto (Ajinomoto Company, Inc.), respectively. Recombinant human OP-1 was obtained as described (Sampath et al, 1992). Iodination of OP-1 was performed according to the chloramine T method as described (ten Dijke et al, (1994) J. Biol. Chem. 269: 16985-16988). Metabolic Labeling and Immunoprecipitation Metabolic labeling of stably transfected R4-2 cells was performed for 5 h in methionine- and cysteine-free DMEM with 100 μCi/mi of [ 35 S]methionine and [ 35 S]cysteine (Pro-mix cell labeling mix; Amersham) in the presence or absence of 100 μM ZnCl 2 . After labeling, the ceUs were washed in phosphate-buffered saline and lysed in 1 ml of 20 mM Tris-HCl, pH 7.5, 150 mM NaCl, 10 mM EDTA, 1% Triton X-100, 1% sodium deoxycholate, 1.5% Trasylol (Bayer) and 1 mM phenylmethylsulfonyl fluoride (Sigma). The cell lysates were centrifuged, and supernatants were subjected to immunoprecipitation as previously described (Ichijo, et al, (1993) J. Biol. Chem. 268: 14505-14513). Briefly, the metabolically labeled lysates were incubated with the antisera against each of ALKs for 45 min at 4°C Immune complexes were bound to protein A-Sepharose (Kabi-Pharmacia) for 30 min at 4°C, washed four times with a buffer containing 20 mM Tris-HCl, pH 7.5, 500 mM NaCl, 1% Triton X-100, 1% deoxycholate, 0.2% sodium dodecyl sulfate (SDS), foUowed by one wash in distilled water. The immune complexes were eluted by boiling for 5 min in SDS-sample buffer (100 mM Tris-HCl, pH 8.8, 0.01% bromophenol blue, 36% glycerol, 4% SDS) containing 10 mM dithiothreitol and analyzed by 8.5% SDS-gel electrophoresis. The gel was fixed, dried and subjected to the analysis using a Fuji BAS 2000 Bio-Imaging Analyzer (Fuji Photo Film). Binding, Affinity Cross-Linking and Immunoprecipitation For receptor affinity labeling , ATDC5 cells were incubated on ice for 3 hrs with 300 pM of 125 I-OP-l in the presence or absence of 15 nM unlabeled OP-1 in the binding buffer (phosphate-buffered saUne containing 0.9 mM CaCl 2 , 0.49 mM MgCl 2 and 1 mg/ml bovine serum albumin). For the ALK-transfected R4-2 cells, binding was aUowed after overnight culture ofthe cells in the presence and absence of 100 μm ZnCl 2 . After binding, cells were washed 3 times with the binding buffer and once with phosphate-buffered saline, and crosslinking was performed in the phosphate-buffered saline containing 0.28 mM of disuccinimidyl suberate (DSS; Pierce Chemical Co.) and/or 0.5 mM of Bis (sulfosuccinimidyl) suberate (BS3; Pierce) for 15 min on ice. CeUs were washed once with the buffer containing 10 mM Tris-HCl, pH 7.4, 1 mM EDTA and 10% glycerol. Cell lysates were collected after incubation ofthe cells with 10 mM Tris-HCl pH 7.8, 1% NP-40, 0.15 M NaCl, 1 mM EDTA and 1.5% Trasylol for 20 min, and the supernatants were analyzed by SDS-gel electrophoresis under reducing conditions or further subjected to immunoprecipitation. Immunoprecipitation ofthe labeled complex was done as described above. The immune complexes were eluted by boiling for 5 min in the SDS-sample buffer with 10 mM dithiothreitol, and analyzed using 7 or 8.5% polyacrylamide, followed by analysis using Bio- Imaging Analyzer. PAI-1 Assay PAI-1 assay was performed as described previously (Ohtsuki and Massague, (1992) Mol. Cell. Biol. 12: 261-265) with minor modifications. In brief, cells were seeded in 6-well cell culture plates and incubated with 100 μM ZnCl 2 overnight. The cells were exposed to OP-1, TGF-β 1 or activin A in semm-free DMEM without methionine and cysteine for 2 hrs. Thereafter, cultures were labeled with 30 μCi/ml of [ 35 S]methionine and [ 35 S]cysteine mixture for 2 hrs. The cells were removed by washing on ice once in phosphate-buffered saline, four times in 10 mM Tris-HCl, pH 8.0, 0.5% sodium deoxycholate and 1 mM phenylmethylsulphonyl fluoride, two times in 2 mM Tris-HCl, pH 8.0, and once in phosphate-buffered saline. Extracellular-matrix proteins were scraped off and extracted into SDS-sample buffer containing 10 mM dithiothreitol and analyzed by 10% SDS polyacrylamide gel electrophoresis, followed by analysis using Bio- imaging Analyzer. PAI-1 was identified as a 44 to 46 kDa doublet. 3 H-Thymidine incorporation Assay Cells were seeded in 24-well ceU culture plates at a density of 104 ceUs per well in DMEM with 10% FBS, and incubated overnight. The medium was changed to DMEM containing 0.2% FBS and 100 μM ZnCl 2 , and the cells were incubated for 5 h. The medium was then changed to fresh DMEM containing 0.2% FBS, 100 μM ZnCl 2 and various concentrations of OP-1 or TGF-βl. After 16 h of incubation, 0.25 μCi of 3 H-thymidine (Amersham) was added and the ceUs were incubated for an additional 2 h. Thereafter, the ceUs were fixed in 10% trichloroacetic acid for more than 15 min on ice, and solubUized with 1 M NaOH. The cell extracts were neutralized with 1 M HCl and 3 H radioactivity was determined in a Uquid scintillation counter. Identification ofOP-1 Receptors in An OP-1 Responsive Cell Line. To identify the signaling receptor(s) for OP-1, we first attempted to find ceUs were identified which respond to OP-1. The mouse chondrogenic cell Une ATDC5 was found to respond weU to OP-1 and its proUferation and by alkaline phosphates activity were strongly stimulated. When the ATDC5 cells were analyzed for the binding of 125 I-OP-l foUowed by affinity cross-linking and SDS polyacrylamide gel electrophoresis under reducing conditions, 125 I-OP-l formed three major crosslinked complexes of 50-60 kDa, 70-85 kDa and 95-110 kDa which were effectively competed for binding in the presence of excess unlabeled OP-1. The size ofthe 70-85 and 95-110 complexes were similar to those ofthe type I and type II receptor complexes for TGF-β and activin (ten Dijke et al, (1994) Science 264: 101-104). The 50-60 kDa component may be a degradation product of a larger complex. In order to exclude the background and to investigate whether certain ALKs may serve as type I receptors for OP-1 in this particular cell Une, the cross-linked complexes were further analyzed by immunoprecipitating these complexes using antisera against ALK-1 to ALK-6. Cross-linked complexes of 70-90 kDa could be precipitated by antisera to ALK-1 and ALK-3/BMPR-IA but not by others (e.g., ALK-2, -4, -5, -6), indicating that ALK-1 as well as ALK-3/BMPR-IA form part ofthe isolated complexes and are competent to bind OP-1. ALK-1 may act as an endogenous Type I receptor for OP-1 or an OP- 1 -related protein. Expression and OP-1 Binding of ALKs in Stably Transfected R 4-2 Cells. Cells of a highly transfectable mink lung epithelial mutant cell Une, R4-2, which lack endogenous type I receptor for TGF-β. Were stably transfected with the ALK-1, ALK-3/BMPR-IA and ALK-5/TβR-I cDNAs under the control of Zn 2+ -inducible metallothionein promoter in an expression vector pMEP4. The R4-2 transfectants were metabolicaUy labeled with [ 35 S]methionine and [ 35 S]cysteine in the presence or absence of 100 μM ZnCl 2 , and the labeled proteins were immunoprecipitated by the antisera against each of ALKs. Labeled components of 50-65 kDa which fit with the sizes predicted from each of ALK cDNA sequences were specificaUy immunoprecipitated only in the presence of ZnCl 2 . A broad migration pattem of the components on the gel likely represents heterogenous glycosylation and/or phosphorylation. In order to investigate whether ALK-1 and ALK-3/BMPR-IA can bind OP-1 when they were expressed in R4-2 ceUs, binding of 125 I-OP-l was analyzed by affinity cross-Unking foUowed by immunoprecipitation to eliminate the background. When ALK-1 or ALK-3/BMPR-IA was induced by ZnCl 2 , 125 I-OP-l bound to ALK-3/BMPR-IA, and to a lesser extent ALK-1, and the cross-linked complexes of 70-95 kDa were precipitated by the respective antisera. Intensity of the band for ALK-1 was weak, however, no cross-Unked complex was detected in cells without Zn 2+ induction, or in ALK-5/TβR-l expressing ceUs. When TGF-β 1 was used as a radiolabeled ligand, only ALK-5/TβR-l but not ALK-1 or ALK-3/BMPR-IA formed cross-linked complex with 125 I-TGF-βl. These results indicate that OP-1 and/or OP-1 related proteins can bind ALK-1 and ALK-3 when they are expressed in R4-2 ceUs. The results are presented in Table I below. Binding was specific as determined by standard competition assays. The values represented by "+/-", "+", "++", "+++", and "-" are all quaUtative descriptors ofthe relative amount of radioactivity measured when the crossUnked molecules were gel electrophoresed and subjected to autoradiography. More radioactivity measured indicates a stronger binding interaction detected. In the Table the strength of binding interaction is as follows: +++ > ++ > + > +/- > -. TABLE I 125 I-OP-l 125 I-TGF-β ALK-1 + ALK-3 ++ ++ ALK-5 OP-1 Signals through ALK-1 and ALK-3/BMPR-IA To investigate whether ALK-1 and ALK-3/BMPR-IA transduce any signals upon binding of OP-1, their abilities were determined to mediate signals for extracellular matrix production and cell growth production of PAI-1 was tested using the stable transfectants (Fig. 1). Production of PAI-1 in response to the stimulation by OP-1 in ALK-5/TβR-l transfected ceUs was not detected. However, in ALK-1 and ALK-3/BMPR-IA -transfected ceUs, TGF-β 1 and activin had no detectable effects on PAI-1 production, whereas OP-1 dramatically induced the PAI-1 protein in those ceUs in a ZnCl 2 dependent manner. These results indicate that ALK-1 and ALK-3/BMPR-IA can signal an extraceUular matrix production response upon binding of OP-1. 3 H-thymidine incoφoration assays were performed to investigate the abiUty ofthe three type I receptors ALK-1, ALK-3, ALK-5 to confer growth responsiveness to OP-1. When ALKs were induced in the presence of ZnCl 2 , R4-2 cells expressing ALK-1 were found to respond to OP-1, and their growth was inhibited in a dose dependent manner (Fig. 2A). In contrast, OP-1 had no detectable effect on the ceUs expressing ALK-3/BMPR-IA or ALK-5/TβR-l (Fig. 2A). In addition, the growth inhibitory effect of OP-1 on the ALK-1 transfected cells were dependent on the expression of ALK- 1 since no inhibition of 3 H-thymidine incoφoration was observed in the absence of ZnCl 2 . (Fig. 2B). Under these conditions TGF-β only inhibited cell growth in ALK-5 transfected ceUs. The experimental results indicate that OP-1 binds ALK-1. The relatively weaker labeling intensity of ALK- 1 by 125 I-OP-l as compared with ALK-3 in ATDC5 ceUs and transfected R4-2 cells could be ascribed to reduced cross-linking efficiency with OP-1 since ALK-1 has fewer lysine residues in its extracellular domain (2 or 3, in human and mouse, respectively) than ALK-3 (7 lysines). Altematively, the presence of a Type II receptor could be required for tight complex formation; and/or an OP-1 related protein could bind with higher affinity. OP-1 has been shown to suppress cell proliferation and stimulate the expression of matrix proteins in rat osteosarcoma cells (MaUakal et al, 1994, Growth Factors 11 : 227-234). The data presented herein demonstrates that ALK-1 is competent to mediate growth-related signals in response to OP- 1. Example 9. OP-1, OP-1 -SPECIFIC RECEPTOR ANALOG SCREENING ASSAYS The present invention is useful to determine whether a ligand, such as a known or putative dmg, is capable of binding to and/or activating an OP- 1 -specific cell surface receptor as described herein. Ligands capable of specific binding interaction with a given OP- 1 -specific receptor (e.g., ALK-2, ALK-3, ALK-6) are referred to herein as OP-1 analogs and can be used for therapeutic and diagnostic applications. Some analogs wUl have the ability to stimulate moφhogenic activity in the ceU, mimicking both the receptor binding and signal transducing activity of OP-1. These are referred to OP-1 agonists or mimetics. Others wiU have strong binding affinity but will not stimulate moφhogenesis, these are OP-1 antagonists. The analogs can be amino acid-based, or they can be composed of non-proteinaceous chemical stmctures. The methods and kits described below simUarly can be used to identify OP- 1 -specific receptor analogs, capable of mimicking the binding affinity of ALK-2, ALK-3 or ALK-6 for OP-1. The analogs can be provided to a mammal to interact with serum-soluble OP- 1, effectively sequestering the protein and modulating its availabiUty for cell surface interaction. Transfection of an isolated clone encoding a moφhogen receptor into the cell systems described above provides an assay system for the abihty of Ugands to bind to and/or to activate the receptor encoded by the isolated DNA molecule. Transfection systems, such as those described above, are useful as living ceU cultures for competitive binding assays between known or candidate dmgs and Ugands which bind to the receptor and compete with the binding ofknown moφhogens, which are labeled by radioactive, enzymatic, spectroscopic or other reagents. Membrane preparations containing the receptor and isolated from transfected cells are also useful in these competitive binding assays. Altematively, 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 assay, e.g., as a competition assay, such as an RIA, described above. Finally, as described above, solution assays, and using only the receptor extracellular domain, also may be used to advantage in these assays. Functional assays of second messenger systems or their sequelae in transfection systems act as assays 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 "dmg discovery system", useful for the identification of natural or synthetic compounds with potential for dmg development that can be further modified or used directly as therapeutic compounds to activate or inhibit the natural functions ofthe receptor encoded by the isolated DNA molecule. Once such candidate dmgs (e.g., OP-1 or receptor-binding analogs thereof) are identified, they can be produced in reasonable, useful quantities using standard methodologies known in the art. Amino acid-based molecules can be encoded by synthetic nucleic acid molecules, and expressed in a recombinant expression system as described herein above or in the art. Alternatively, such molecules can be chemically synthesized, e.g., by means of an automated peptide synthesizer, for example. Non-amino acid-based molecules can be produced by standard organic chemical synthesis procedures. Where the candidate molecule is of undetermined stmcture, or composition, its composition readUy can be determined by, for example, mass spectroscopy. Two approaches to identifying analogs typically are practiced in the art: high flux screens and rational design of Ugand mimetics. High flux screens typically screen naturally sourced materials or chemical banks for their abiUty to bind a protein of interest, here, e.g. , the receptor. TypicaUy, compounds are obtained from a range of sources, e.g., chemical banks, microbial broths, plant and animal extracts, and the like. In a high flux screen typicaUy, purified receptor, preferably the soluble, Ugand binding extracellular domain, is plated onto a microtiter weU surface and a standard volume of a sample solution to be tested then is added. Also added is a standard volume having a known quantity of a purified Ugand known to bind the receptor with specificity. Preferably the Ugand is labeled with a substance that is readily detectable by automated means (e.g., radiolabel, chromophoric, fluorometric, enzymatic or spectroscopic label). The wells 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 scores are identified by the ability ofthe test substance to prevent interaction ofthe labeled 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 assays in parallel. Thus, with only 10-11 reiterations ofthe screening assay 15,625 (56) compounds can be screened for their bmding affinity. Even allowing for a maximum incubation time of 2 hours, all 15,625 compounds reasonably can be assayed in a matter of days. High flux screens exploit both the high degree of specificity ofthe labeled Ugand for its receptor, as weU as high throughput capacity of computer driven robotics and computer handling of data. Candidate analogs identified in this manner, then can be analyzed structurally and this information used to design and to synthesize analogs having enhanced potency, increased duration of action, increased selectivity and reduced side effects. Candidates also can be used in a rational design program as described below. FinaUy, candidate analogs also can be tested to determine moφhogenic effect, if any, as described below. The second approach to the identification of analogs uses a rational design approach to create molecules capable of mimicking the binding effect of OP-1 with an OP- 1 -specific receptor. Here the relevant stmcture for receptor binding is analyzed to identify critical sequences and stmctures necessary for binding activity and this information can be used to design and synthesize minimal size moφhogen analogs. As for candidate compounds in the high flux assay, design candidates can be tested for receptor binding activity as described above. As described above, a candidate sequence can be further modified by, for example standard biological or chemical mutagenesis 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 OP-1 binding can also be used as an analog. Antibodies can be generated as described above. OP-1 analogs may be evaluated for their abihty to mimic OP-1 or to inhibit OP-1 binding (e.g., agonists or antagonists) by monitoring the effect ofthe analogs on cells bearing an OP-1- specific receptor (e.g., ALK-1). OP-1 agonists are anticipated to have utility in any appUcation where tissue moφhogenesis is desired, such as in the regeneration of damaged tissue resulting from mechanical or chemical trauma, degenerative diseases, tissue destmction resulting from chronic inflammation, cirrhosis, inflammatory diseases, cancer and the like, and in the regeneration of tissues, organs and limbs. OP-1 antagonists are envisioned to have utiUty in applications where tissue moφhogenesis is to be Umited as, for example, in the treatment of maUgnant transformations including, but not Umited to, osteosarcomas and Paget's disease. Several exemplary systems for assaying the abUity of a candidate analog transduce an OP- 1 -specific signal across the cellular membrane are described below. 9.1 Induction of Osteoblast Differentiation Markers For example, OP-1 is known to preferentially induce differentiation of progenitor ceUs, including embryonic mesenchymal ceUs and primary osteoblasts (see, for example, PCT US92/07432) As one example, OP-1 analogs can be tested for their abiUty 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 foUows. In this and aU examples involving osteoblast cultures, rat osteoblast-enriched primary cultures preferably are used. Although these cultures are heterogeneous in that the individual ceUs are at different stages of differentiation, these cultures are beUeved to more accurately reflect the metaboUsm and function of osteoblasts in vivo than osteoblast cultures obtained from estabhshed ceU Unes. Unless otherwise indicated, aU chemicals referenced are standard, commercially avaUable reagents, readily avaUable from a number of sources, including Sigma Chemical, Co., St. Louis; Calbiochem, Coφ., San Diego and Aldrich Chemical Co., MUwaukee. Rat osteoblast-enriched primary cultures are prepared by sequential coUagenase digestion of newborn suture-free rat calvaria (e.g., from 1-2 day-old animals, Long-Evans strain, Charles River Laboratories, Wilmington, MA), foUowing standard procedures, such as are described, for example, in Wong et al, (1975) PNAS 72: 3167-3171. Rat osteoblast single ceU suspensions then are plated onto a multi-weU plate (e.g., a 24 well plate) at a concentration of 50,000 osteoblasts per weU in alpha MEM (modified Eagle's medium, Gibco, Inc., Long Island) containing 10% FBS (fetal bovine semm), L-glutamine and peniciUin/streptomycin. The cells are incubated for 24 hours 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 hours so that the cells are in serum-deprived growth medium at the time ofthe experiment. Alkaline Phosphatase Induction of Osteoblasts The cultured cells in semm-free medium are incubated with OP-1, OP-1 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 used. 72 hours after the incubation period the cell layer is extracted with 0.5 ml of 1% Triton X-100. The resultant cell extract then, is centrifuged, and 100 ml ofthe extract is added to 90 ml of paranitrosophenylphospate (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 mn 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 Uberated/30 minutes at 37°C. OP-1 induces a 5-fold increase in the specific activity of alkaUne phosphate by this method. Agonists are expected to have simUar induction effects. Antagonists should inhibit or otherwise interfere with OP-1 binding, and diminished alkaline phophatase induction should result when the assay is performed with an antagonist in the presence of a Umiting amount of OP- 1. Induction of PTH-Mediated cAMP. The effect of a moφhogen analog on parathyroid hormone-mediated cAMP production in rat osteoblasts in vitro may be demonstrated as foUows. Rat osteoblasts are prepared and cultured in a multiweU plate as described above. The cultured cells then are divided into three groups: (1) weUs 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 hours. At the end ofthe 72 hours the cells are treated with medium containing 0.5% bovine semm albumin (BSA) and 1 mM 3 -isobutyl- 1-methylxanthine for 20 minutes foUowed by the addition into half ofthe wells of human recombinant parathyroid hoπnone ( PTH, Sigma, St. Louis) at a concentration of 200 ng/ml for 10 minutes. 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 radioimmunoassay kit (e.g., Amersham, Arlington Heights, Illinois). OP-1 doubles cAMP production in the presence of PTH. Agonists are expected to have similar induction effects. Antagonists are expected to inhibit or otherwise interfere with OP-1 binding, and diminished cAMP production should result when the assay is performed with an antagonist in the presence of Umiting the amount of OP-1. 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 semm are used as a marker for osteoblast activity and bone formation in vivo. Induction of osteocalcin synthesis in osteoblast-enriched cultures can be used to demonstrate moφhogenic efficacy in vitro. Rat osteoblasts are prepared and cultured in a multi-weU plate as above. In this experiment the medium is supplemented with 10% FBS, and on day 2, cells are fed with fresh medium supplemented with fresh 10 mM b-glycerophosphate (Sigma, Inc.). Beginning on day 5 and twice weekly thereafter, ceUs are fed with a complete mineralization medium containing aU of the above components plus fresh L(+)-ascorbate, at a final concentration of 50 mg/ml medium. OP-1 or OP-1 analog then is added to the wells directly, e.g., in 50% acetonitrile (or 50% ethanol) containing 0.1% trifluoroacetic acid (TFA), at no more than 5 ml OP-l/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 synthesis is measured by standard radioimmunoassay using a commerciaUy 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 OP-1 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. Agonists are expected to have similar induction effects; antagonists are expected to inhibit or otherwise interfere with OP-1 binding, thereby substantially interfering with osteocalcin induction in the presence of a Umiting amount of OP-1. Mineralization is 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 23 °C for 10 min, foUowing rinsing cold 0.9% NaCl. Fixed ceUs then are stained for endogenous alkaline phosphatase at pH 9.5 for 10 min, using a commercially available kit (Sigma, Inc.) Puφle stained cells then are dehydrated with methanol and air dried. After 30 min incubation in 3% AgNO 3 in the dark, H 2 O-rinsed samples are exposed for 30 sec to 254 nm UV light to develop the black silver-stained phosphate nodules. Individual mineralized foci (at least 20 mm in size) are counted under a dissecting microscope and expressed as nodules/culture. OP-1 induces a 20-fold increase in initial mineralization rate. Agonists are expected to have similar induction effects; antagonists are expected to inhibit or otherwise interfere with OP-1 binding, thereby inhibiting mineralization induction in the presence of a Umiting amount of OP-1. 9.2 Induction of a Constructed Reporter Gene Altematively, a reporter gene constmct can be used to determine the abUity of candidate molecule to induce signal transduction across a membrane following receptor binding. For example, PAI-1 protein, (Plasminogen Activator Inhibitor- 1) expression can be induced by OP-1 in MvlLu- cells (see above). Also, as demonstrated above, ALK-1, when overexpressed in a chemically mutagenized derivative of these ceUs, can mediate PAI-1 induction in the presence of OP-1. Accordingly, PAI-1 promoter elements can be fused to a reporter gene and induction of the reporter gene monitored following incubation ofthe transfected cell with a candidate analog. As one example, the luciferase reporter gene can be used, in, for example, the constmct p3TP-Lux described by Wrana et al. (1992) Cell 71 : 1003-1014 and Attisano et al. (1993) Cell 74: 671-680. This reporter gene constmct includes a region ofthe human PAI-1 gene promoter in combination with three sets of tetradecanoyl phorbol - acetate responsive elements upstream of the luciferase open reading frame. In a typical assay, transfected cells starved in DMEM containing 0.1% fetal bovine semm and antibiotics (e.g., 100 units/ml penicillin and 50 μg/ml streptomycin) for 6 hrs., and then exposed to Ugand for 24 hr. Luciferase activity in the ceU 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" ceUs co-transfected with ALK-2 and ActRII, OP- 1 -mediated induction of luciferase activity. 9.3 Inhibition of Epithelial cell proliferation OP-1 is known to inhibit epitheUal cells. Thus, the ability of a candidate analog to inhibit cell proliferation, as measured by 3 H-thymidine uptake by an epithelial cell can be used in an assay to evaluate signal transduction activity ofthe candidate. Analogs competent to inhibit epitheUal ceU growth are contemplated to have particular utility in therapeutic apphcations where Umitation of a proliferating cell population is desired. Such applications include chemotherapies and radiation therapies where Umiting the growth of a normally proliferating population of cells can protect these cells from the cytotoxic effects of these cancer therapies, (see e.g., WO 94/06420). In addition, psoriasis and other tissue disorders resulting from uncontrolled ceU proUferation, including benign and malignant neoplasties, can be modulated by use of an OP-1 analog. As an example, mink lung epithelial cell growth is inhibited by OP-1. (see, PCT US93/08885; WO 94/06420.) As described above, derivatives of these ceUs [e.g., "R-4 mutants", clone 4-2, Laiho et al. (1990) J. Biol. Chem. 265: 18518-18524] can be transfected with DNA encoding OP- 1 -specific receptors and induced to express these receptors. The transfected cells, then can be assayed for a candidate analog's ability to block cell growth. As one example, when R-4 cells are transfected with ALK-1 under a Zn 2+ -inducible promoter, and induced to express the receptor foUowing induction with ZnCl 2 , ceU growth can be inhibited in the presence of OP-1 in a dose dependent manner. In a typical assay, cells are seeded in 24-well cell culture plates at a density of 104 ceUs per well in DMEM with 10% FBS, and incubated overnight. The medium is replaced with DMEM containing 0.2% FBS and 100 μM ZnCl 2 , and the cells are incubated for 5 hrs, after which the medium is replaced with fresh DMEM containing 0.2% FBS, 100 μM ZnCl 2 and various concentrations of OP-1 or an analog candidate. After 16 h of incubation, 0.25 Ci of 3 H-thymidine (Amersham) are added and the ceUs incubated for an additional 2 hrs. Thereafter, the ceUs are fixed in 10% trichloroacetic acid for more than 15 min on ice, and solubilized with 1 M NaOH. The cell extracts are neutralized with 1 M HCl and 3 H radioactivity determined in a Uquid scintiUation counter. Example 10. SCREENING ASSAY FOR COMPOUNDS WHICH ALTER ENDOGENOUS OP-1 RECEPTOR EXPRESSION LEVELS Candidate compound(s) which can be administered to affect the level of a given endogenous OP-1 receptor can be found using the following screening assay, in which the level of OP-1 receptor production by a cell type which produces measurable levels ofthe receptor is determined by incubating the cell in culture with and without the candidate compound, in order to assess the effects ofthe compound on the cell. This can also be accomplished by detection ofthe OP-1 receptor either at the protein level by Western blot or immunolocaUzation, or at the RNA level by Northern blot or in situ hybridization. The protocol is based on a procedure for identifying compounds which alter endogenous levels of OP-1 expression, a detaUed description also may be found in PCT US 92/07359, incoφorated herein by reference. Cell cultures of, for example, bone, brain, intestine, lung, heart, eye, breast, gonads, kidney, adrenals, urinary bladder, brain, or other organs, may be prepared as described widely in the literature. For example, kidneys may be explanted from neonatal or new bom or young or adult rodents (mouse or rat) and used in organ culture as whole or sliced (1-4 mm) tissues. Primary tissue cultures and established cell Unes, also derived from kidney, adrenals, urinary, bladder, brain, mammary, or other tissues may be estabhshed in multiwell plates (6 well or 24 weU) according to conventional ceU culture techniques, and are cultured in the absence or presence of semm 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 semm (e.g., fetal calf semm at 1%-10%, Gibco) or in serum-deprived medium, as desired, or in defined medium (e.g., containing insulin, transferring, glucose, albumin, or other growth factors). CeU samples for testing the level of OP-1 receptor production are collected periodically and evaluated for receptor production by immunoblot analysis (Sambrook et al, eds., 1989, Molecular Cloning, Cold Spring Harbor Press, Cold Spring Harbor, NY), or, alternatively, a portion ofthe cell culture itself can be collected periodically and used to prepare poly(A) + RNA for mRNA analysis by Northern blot analysis. To monitor de novo receptor synthesis, some cultures are labeled according to conventional procedures with an 35 S-methionine/ 35 S-cysteine mixture for 6-24 hours and then evaluated to quantitate receptor synthesis by conventional immunoassay methods. Altematively, anti-receptor antibodies may be labeled and incubated with the cells or ceU lysates, and the bound complexes detected and quantitated by conventional means, such as those described hereinabove. Northern blots may be performed using a portion ofthe OP-1 receptor coding sequence to create hybridization probes, and following the RNA hybridization protocol described herein. Example 11. GENERAL FORMULATION/ ADMINISTRATION CONSIDERATIONS The analogs and constmcts described herein can be provided to an individual as part of a therapy to enhance, inhibit, or otherwise modulate the in vivo binding interaction between OP-1 and one or more OP- 1 -specific cell surface receptors. The molecules then comprise part of a pharmaceutical composition as described herein below and can be administered by any suitable means, 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 parenteraUy, such as by intravenous, subcutaneous, intramuscular, intraorbital, ophthalmic, intraventricular, intracranial, intracapsular, intraspinal, intracisternal, intraperitoneal, buccal, rectal, vaginal, intranasal or by aerosol administration, the therapeutic preferably comprises part of an aqueous solution. The solution preferably is physiologically acceptable so that in addition to delivery ofthe desired moφhogen 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, 0.15 M), pH 7-7.4 or other pharmaceutically acceptable salts thereof. Useful solutions for oral or parenteral administration can be prepared by any ofthe methods well known in the pharmaceutical art, described, for example, in Remington's Pharmaceutical Sciences, (Gennaro, A., ed.), Mack Pub., 1990. Formulations may include, for example, polyalkylene glycols such as polyethylene glycol, oUs of vegetable origin, hydrogenated naphthalenes, and the like. Formulations for direct administration, in particular, can include glycerol and other compositions ofhigh viscosity. Biocompatible, preferably bioresorbable polymers, including, for example, hyaluronic acid, collagen, tricalcium phosphate, polybutyrate, polylactide, polyglycohde and lactide/glycolide copolymers, may be useful excipients to control the release ofthe moφhogen in vivo. Other potentially useful parenteral delivery systems for these therapeutic molecules include ethylene-vinyl acetate copolymer particles, osmotic pumps, implantable infusion systems, and Uposomes. Formulations 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 solutions for administration in the form of nasal drops, or as a gel to be applied intranasally. Altematively, the moφhogens described herein may be administered orally. The therapeutic molecules can also be associated with means for targeting the therapeutic to a desired tissue. 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, these molecules may be included as useful agents for targeting therapeutics to bone tissue. Altematively, an antibody or other binding protein that interacts specifically with a surface molecule on the desired target tissue cells can also be used. Such targeting molecules further can be covalently associated to the therapeutic molecule e.g., by chemical crosslinking, or by using standard genetic engineering means to create, for example, an acid labile bond such as an Asp-Pro linkage. Useful 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. FinaUy, therapeutic molecules can be administered alone or in combination with other molecules known to have a beneficial effect on tissue moφhogenesis, including molecules capable of tissue repair and regeneration and/or inhibiting inflammation. Examples of useful cofactors for stimulating bone tissue growth in osteoporotic individuals, for example, include but are not limited to, vitamin D3, 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 useful cofactors mclude symptom-alleviating cofactors, including antiseptics, antibiotics, antiviral and antifungal agents and analgesics and anesthetics. Therapeutic molecules further can be formulated into pharmaceutical compositions by admixture with pharmaceuticaUy acceptable nontoxic excipients and carriers. As noted above, such compositions can be prepared for parenteral administration, particularly in the form of liquid solutions or suspensions; for oral administration, particularly in the form of tablets or capsules; or intranasaUy, particularly in the form of powders, nasal drops or aerosols. Where adhesion to a tissue surface is desired the composition may include the moφhogen dispersed in a fibrinogen- thrombin composition or other bioadhesive such as is disclosed, for example in PCT US91/09275, the disclosure of which is incoφorated herein by reference. The composition then can be painted, sprayed or otherwise applied to the desired tissue surface. The compositions can be formulated for parenteral or oral administration to humans or other mammals in therapeutically effective amounts, e.g., amounts which provide appropriate concentrations ofthe analog to target tissue 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 tissue or organ to be transplanted prior to removal of tissue 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 administration, using any ofthe means described above. Altematively or, in addition, once removed from the donor, the organ or living tissue can be placed in a preservation solution containing the therapeutic molecule. In addition, the recipient also preferably is provided with the analog just prior to, or concomitant with, transplantation. In aU cases, the analog can be administered directly to the tissue at risk, as by injection to the tissue, or it may be provided systemically, either by oral or parenteral administration, using any ofthe methods and formulations described herein and/or known in the art. Where the therapeutic molecule comprises part of a tissue or organ preservation solution, any commerciaUy avaUable preservation solution can be used to advantage. For example, useful solutions known in the art include Collins solution, Wisconsin solution, Belzer solution, EurocolUns solution and lactated Ringer's solution. Generally, an organ preservation solution usuaUy possesses one or more ofthe following properties: (a) an osmotic pressure substantiaUy equal to that ofthe inside of a mammaUan cell, (solutions typically are hyperosmolar and have K + and/or Mg 2+ ions present in an amount sufficient to produce an osmotic pressure slightly higher than the inside of a mammaUan cell); (b) the solution typically is capable of maintaining substantially normal ATP levels in the ceUs; and (c) the solution usuaUy allows optimum maintenance of glucose metaboUsm in the ceUs. Organ preservation solutions also may contain anticoagulants, energy sources such as glucose, fructose and other sugars, metabohtes, heavy metal chelators, glycerol and other materials ofhigh viscosity to enhance survival at low temperatures, free oxygen radical inhibiting and/or scavenging agents and a pH indicator. A detaUed description of preservation solutions and useful components can be found, for example, in US Patent No. 5,002,965, the disclosure of which is incoφorated herein by reference. As will be appreciated by those skiUed in the art, the concentration ofthe compounds desciibed in a therapeutic composition will vary depending upon a number of factors, including the dosage ofthe dmg to be administered, the chemical characteristics (e.g., hydrophobicity) of the compounds employed, and the route of administration. The preferred dosage of dmg to be administered also is Ukely to depend on such variables as the type and extent of tissue loss or defect, the overall health status ofthe particular patient, the relative biological efficacy ofthe compound selected, the formulation ofthe compound, the presence and types of excipients in the formulation, and the route of administration. 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 moφhogen-induced pathological lesions are induced when mature moφhogen (e.g., OP-1, 20 mg) is administered daily to normal growing rats for 21 consecutive days. Moreover, 10 mg systemic injections of moφhogen (e.g., OP-1) injected daily for 10 days into normal newborn mice does not produce any gross abnormalities. Other Embodiments The invention may be embodied in other specific forms 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 ofthe 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 ofthe claims are therefore intended to be embraced therein. SEQUENCE LISTING (1) GENERAL INFORMATION: (i) APPLICANT: ICHIJO, HIDENORI NISHITOH, HIDEKI SAMPATH, KUBER T. (ii) TITLE OF INVENTION: NOVEL SIGNALLING RECEPTOR FOR MORPHOGENIC PROTEINS (iii) NUMBER OF SEQUENCES: 5 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: Testa, Hur itz & Thibeault (B) STREET: 125 High St. (C) CITY: Boston (D) STATE: MA (E) COUNTRY: USA (F) ZIP: 02110 (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 #1.30 (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: (B) FILING DATE: (C) CLASSIFICATION: (viii) ATTORNEY/AGENT INFORMATION: (A) NAME: KELLEY Esq., Robin D. (B) REGISTRATION NUMBER: 34,637 (C) REFERENCE/DOCKET NUMBER: CRP-117 (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 ALK-1" (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 2 0 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 Ile 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 Ile 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 Ile 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 Ile Tyr Asn Thr Val Leu Leu Arg His Asp Asn Ile 245 250 255 CTA GGC TTC ATC GCC TCA GAC ATG ACC TCC CGC AAC TCG AGC ACG CAG 816 Leu Gly Phe Ile 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 Ile Thr His Tyr His Glu His Gly Ser Leu Tyr 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 Ile 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 Ile 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 Ile 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 Ile 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 Ile Arg Thr Asp Cys Phe Glu Ser Tyr Lys Trp Thr Asp Ile 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 Ile Ala Arg Arg Thr Ile Val Asn Gly 405 410 415 ATC GTG GAG GAC TAT AGA CCA CCC TTC TAT GAT GTG GTG CCC AAT GAC 1296 Ile 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 Ile 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 Ile Lys Lys Thr Leu Gin Lys Ile Ser Asn Ser Pro 485 490 495 GAG AAG CCT AAA GTG ATT CAA 1509 Glu Lys Pro Lys Val Ile 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 Ile 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 Ile 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 Ile Phe Ser Ser Arg Asp Glu Gin Ser Trp Phe 225 230 235 240 Arg Glu Thr Glu Ile Tyr Asn Thr Val Leu Leu Arg His Asp Asn Ile 245 250 255 Leu Gly Phe Ile Ala Ser Asp Met Thr Ser Arg Asn Ser Ser Thr Gin 260 265 270 Leu Trp Leu Ile 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 Ile Phe Gly Thr 305 310 315 320 Gin Gly Lys Pro Ala Ile Ala His Arg Asp Phe Lys Ser Arg Asn Val 325 330 335 Leu Val Lys Ser Asn Leu Gin Cys Cys Ile Ala Asp Leu Gly Leu Ala 340 345 350 Val Met His Ser Gin Gly Ser Asp Tyr Leu Asp Ile 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 Ile Arg Thr Asp Cys Phe Glu Ser Tyr Lys Trp Thr Asp Ile Trp Ala 385 390 395 400 Phe Gly Leu Val Leu Trp Glu Ile Ala Arg Arg Thr Ile Val Asn Gly 405 410 415 Ile 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 Ile 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 Ile Lys Lys Thr Leu Gin Lys Ile Ser Asn Ser Pro 485 490 495 Glu Lys Pro Lys Val Ile Gin 500 (2) INFORMATION FOR SEQ ID NO:3: (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= "OP-1" /standard_name= "OP-1" (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3: 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 Ile 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 Ile Leu Ser Ile 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 Ile 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 Ile Tyr Lys Asp 165 170 175 TAC ATC CGG GAA CGC TTC GAC AAT GAG ACG TTC CGG ATC AGC GTT TAT 633 Tyr Ile Arg Glu Arg Phe Asp Asn Glu Thr Phe Arg Ile 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 Ile 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 Ile 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 Ile 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 Ile 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 Ile Ile 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 Ile Val Gin Thr Leu Val His Phe Ile 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 Ile Ser Val Leu Tyr Phe Asp Asp Ser Ser Asn Val Ile 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 GGGGTGGGCA CATTGGTGTC TGTGCGAAAG GAAAATTGAC CCGGAAGTTC 1771 CTGTAATAAA TGTCACAATA AAACGAATGA ATGAAAAAAA AAAAAAAAAA A 1822 (2) INFORMATION FOR SEQ ID NO:4: (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:4: 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 Ile His Arg Arg Leu Arg Ser 35 40 45 Gin Glu Arg Arg Glu Met Gin Arg Glu Ile Leu Ser Ile 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 Ile Pro Glu Gly Glu Ala Val Thr Ala Ala Glu Phe Arg Ile 165 170 175 Tyr Lys Asp Tyr Ile Arg Glu Arg Phe Asp Asn Glu Thr Phe Arg Ile 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 Ile 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 Ile Asn Pro Lys Leu Ala Gly Leu Ile 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 Ile 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 Ile Ile 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 Ile Val Gin Thr Leu Val His 370 375 380 Phe Ile Asn Pro Glu Thr Val Pro Lys Pro Cys Cys Ala Pro Thr Gin 385 390 395 400 Leu Asn Ala Ile Ser Val Leu Tyr Phe Asp Asp Ser Ser Asn Val Ile 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:5: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 102 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (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:5: Cys Xaa Xaa His Glu Leu Tyr Val Xaa Phe Xaa Asp Leu Gly Trp Xaa 1 5 10 15 Asp Trp Xaa Ile 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 Ile 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

Claims: What is claimed is: 1. An isolated analog of OP- 1 which binds to the ligand binding domain of a cell surface receptor defined by the amino acid sequence of Seq. ID No. 2 (ALK-1) or a sequence variant thereof, said analog having greater than 60% homology with the C-terminal 96 amino acids ofthe sequence defined by Seq. ID No. 4 (OP-1). 2. The analog of claim 1 wherein the analog shares greater than 60% identity with the C-terminal 96 amino acids ofthe sequence defined by Seq. ID No. 4 (OP-1). 3. The analog of claim 1 wherein said analog has specific binding affinity for a Type II cell surface receptor. 4. The analog of claim 3 wherein said Type II receptor has bmding affinity for activin or BMP-4. 5. An isolated analog of OP- 1 which binds to the ligand binding domain of a cell surface receptor defined by the amino acid sequence of Seq. ID No. 2 (ALK-1) or a sequence variant thereof, said analog defined by OPX, Seq. ID No. 5. 6. A method for identifying an OP-1 analog, said analog being characterized as having substantially the same binding affinity for a ceU surface receptor as OP-1, the method comprising the steps of: (a) providing a sample containing a protein consisting of a polypeptide chain comprising an amino acid sequence defined Seq. ID No. 2 (ALK-1), or an OP- 1 -specific receptor analog thereof; (b) contacting said sample with a candidate OP-1 analog; and (c) detecting specific binding between said candidate OP-1 analog and said protein of step (a). 7. A method for identifying an OP-1 analog, said analog being characterized as having substantially the same binding affinity for a cell surface receptor as OP-1, the method comprising the steps of: (a) providing a sample containing a protein consisting of a polypeptide chain comprising an amino acid sequence defined Seq. ID No. 2 (ALK-1), or an OP- 1 -specific receptor analog thereof; (b) contacting said sample with a candidate OP-1 analog; and (c) detecting induction of an OP- 1 -mediated cellular response. 8. The method of claim 6 or 7 wherein said sample further comprises part or all of a Type II serine/threonine kinase receptor protein having binding affinity for OP-1, activin or BMP-4. 9. A method for producing an OP-1 analog, the method comprising the steps of: (a) obtaining, by the method of claim 6 or 7, a candidate OP-1 analog; and (b) producing either said candidate analog or a second OP-1 analog derived from said candidate and having substantially the same OP-1 receptor-binding domain as said candidate. 10. A kit for identifying OP-1 or a candidate OP-1 analog in a sample, the kit comprising: (a) a receptacle adapted to receive a sample and containing a protein consisting of a polypeptide chain comprising an amino acid sequence defined Seq. ID No. 2 (ALK- 1 ), or an OP- 1 -specific receptor analog thereof; (b) means for detecting interaction of OP-1 or a candidate OP-1 analog with said protein of part (a), said OP-1 or candidate analog comprising part of said sample provided to said receptacle. 11. The kit of claim 10 wherein said means in part (b) comprises either (i) means for detecting specific binding interaction of OP-1 or said candidate analog with said protein; or (U) means for detecting induction of an OP- 1 -mediated cellular response. 12. The kit of claim 10 or 11 further comprising a serine/threonine Type II receptor having binding specificity for OP- 1 , activin or BMP-4. 13. An isolated ligand-receptor complex comprising two molecules, the first said molecule defining said Ugand and comprising at least the C-terminal 96 amino acids of OP-1 (residues 335-431 of Seq. ID No. 4) or an analog thereof, and the second said molecule defining said receptor and consisting of a polypeptide chain comprising an amino acid sequence defined Seq. ID No. 2 (ALK-1), or an OP- 1 -specific receptor analog thereof. 14. The complex of claim 13 further comprising part or aU of a Type II serine/threonine kinase receptor. 15. An isolated binding partner having specific binding affinity for an epitope on a Ugand- receptor complex, said complex being characterized as comprising an OP-1 protein or an analog thereof in specific binding interaction with the Ugand binding domain of a ceU surface receptor defined by Seq. ID No. 2 (ALK-1), or an OP-1-specific receptor analog thereof, said binding partner having substantially no binding affinity for the uncomplexed form of said OP-1 protein or OP-1 analog. 16. The isolated binding partner of claim 15 wherein said binding partner is further characterized as having substantially no binding affinity for the uncomplexed form of said cell surface receptor protein or said analog thereof. 17. A method for (i) antagonizing OP-1 binding to a cell surface receptor; or (U) antagonizing induction of an OP-1 mediated ceUular response, the method comprising the step of providing the OP-1 analog of claim 1 or 2 to an OP- 1 -responsive ceU. 18. A method for antagonizing (i) OP-1 binding to a cell surface receptor; or (U) induction of an OP-1 mediated cellular response, the method comprising the step of providing to an OP- 1 -responsive cell a protein consisting of a polypeptide chain comprising an amino acid sequence defined Seq. ID No. 2 (ALK-1), or an OP- 1 -specific receptor analog thereof. 19. A method for antagonizing activin bmding to a ceU surface receptor, the method comprising the step of: providing a protein having binding specificity for the amino acid sequence defined by Seq. ID No. 2 (ALK-1) or an OP- 1 -specific receptor variant thereof, said protein sharing at least 60% amino acid sequence identity with residue 335-431 ofthe sequence defined by Seq. ID No. 4 (OP-1), such that said protein, when provided to a cell expressing said ceU surface receptor, is competent to interact specificaUy with said receptor, thereby substantially inhibiting activin binding to said receptor. 20. A method for antagonizing BMP-4 binding to a cell surface receptor, the method comprising the step of: providing a protein having binding specificity for the ligand binding domain defined by Seq. ID No. 2 (ALK-1), or an OP-1-specific receptor variant thereof, said protein sharing at least 60% amino acid sequence identity with residues 335-431 ofthe sequence defined by Seq. ID No. 4 (OP-l), such that said protein, when provided to a cell expressing a said cell surface receptor with said cell, is competent to interact specifically with said receptor, thereby substantiaUy inhibiting BMP-4 binding to said receptor.

Inventors: ICHIJO HIDENORI, NISHITOH HIDEKI, SAMPATH KUBER T, ICHIJO, HIDENORI, NISHITOH, HIDEKI, SAMPATH, KUBER, T.
Applicants: CREATIVE BIOMOLECULES INC, LUDWIG INST CANCER RES, CREATIVE BIOMOLECULES, INC., THE LUDWIG INSTITUTE FOR CANCER RESEARCH
Application year: 1996
Main classification: C07K14
IPC codes: A61K38, A61K45, A61P43, C07K14, C07K16, C12N15, C12P21, C12Q1, G01N33