Extracellular domain of the transforming growth factor-beta receptor negatively regulates ligand-independent receptor activation

Cholesterol modulates type I/II TGF-β receptor complexes and alters the balance between Smad and Akt signaling in hepatocytes

Cholesterol mediates membrane compartmentalization, affecting signaling via differential distribution of receptors and signaling mediators. While excessive cholesterol and aberrant transforming growth factor-β (TGF-β) signaling characterize multiple liver diseases, their linkage to canonical vs. non-canonical TGF-β signaling remained unclear. Here, we subjected murine hepatocytes to cholesterol depletion (CD) or enrichment (CE), followed by biophysical studies on TGF-β receptor heterocomplex formation, and output to Smad2/3 vs. Akt pathways. Prior to ligand addition, raft-dependent preformed heteromeric receptor complexes were observed. Smad2/3 phosphorylation persisted following CD or CE. CD enhanced phospho-Akt (pAkt) formation by TGF-β or epidermal growth factor (EGF) at 5 min, while reducing it at later time points. Conversely, pAkt formation by TGF-β or EGF was inhibited by CE, suggesting a direct effect on the Akt pathway. The modulation of the balance between TGF-β signaling to Smad2/3 vs. pAkt (by TGF-β or EGF) has potential implications for hepatic diseases and malignancies.

Dynamics of transforming growth factor β signaling and therapeutic efficacy

Transforming growth factor β (TGFβ) is a multifunctional cytokine, and its signalling responses are exerted via integrated intracellular pathways and complex regulatory mechanisms. Due to its high potency, TGFβ signalling is tightly controlled under normal circumstances, while its dysregulation in cancer favours metastasis. The recognised potential of TGFβ as a therapeutic target led to emerging development of anti-TGFβ reagents with preclinical success, yet these therapeutics failed to recapitulate their efficacy in experimental settings. In this review, possible reasons for this inconsistency are discussed, addressing the knowledge gap between theoretical and actual behaviours of TGFβ signalling. Previous studies on oncogenic cells have demonstrated the spatiotemporal heterogeneity of TGFβ signalling intensity. Under feedback mechanisms and exosomal ligand recycling, cancer cells may achieve cyclic TGFβ signalling to facilitate dissemination and colonisation. This challenges the current presumption of persistently high TGFβ signalling in cancer, pointing to a new direction of research on TGFβ-targeted therapeutics.

TGFβRIIb mutations trigger aortic aneurysm pathogenesis by altering transforming growth factor β2 signal transduction

BACKGROUND

Thoracic aortic aneurysm (TAA) is a common progressive disorder involving gradual dilation of the ascending and/or descending thoracic aorta that eventually leads to dissection or rupture. Nonsydromic TAA can occur as a genetically triggered, familial disorder that is usually transmitted in a monogenic autosomal dominant fashion and is known as familial TAA. Genetic analyses of families affected with TAA have identified several chromosomal loci, and further mapping of familial TAA genes has highlighted disease-causing mutations in at least 4 genes: myosin heavy chain 11 (MYH11), α-smooth muscle actin (ACTA2), and transforming growth factor β receptors I and II (TGFβRI and TGFβRII).

METHODS AND RESULTS

We evaluated 100 probands to determine the mutation frequency in MYH11, ACTA2, TGFβRI, and TGFβRII in an unbiased population of individuals with genetically mediated TAA. In this study, 9% of patients had a mutation in one of the genes analyzed, 3% of patients had mutations in ACTA2, 3% in MYH11, 1% in TGFβRII, and no mutations were found in TGFβRI. Additionally, we identified mutations in a 75 base pair alternatively spliced TGFβRII exon, exon 1a that produces the TGFβRIIb isoform and accounted for 2% of patients with mutations. Our in vitro analyses indicate that the TGFβRIIb activating mutations alter receptor function on TGFβ2 signaling.

CONCLUSIONS

We propose that TGFβRIIb expression is a regulatory mechanism for TGFβ2 signal transduction. Dysregulation of the TGFβ2 signaling pathway, as a consequence of TGFβRIIb mutations, results in aortic aneurysm pathogenesis.

SMAD2, SMAD3 and SMAD4 mutations in colorectal cancer

Activation of the canonical TGF-β signaling pathway provides growth inhibitory signals in the normal intestinal epithelium. Colorectal cancers (CRCs) frequently harbor somatic mutations in the pathway members TGFBR2 and SMAD4, but to what extent mutations in SMAD2 or SMAD3 contribute to tumorigenesis is unclear. A cohort of 744 primary CRCs and 36 CRC cell lines were sequenced for SMAD4, SMAD2, and SMAD3 and analyzed for allelic loss by single-nucleotide polymorphism (SNP) microarray analysis. Mutation spectra were compared between the genes, the pathogenicity of mutations was assessed, and relationships with clinicopathologic features were examined. The prevalence of SMAD4, SMAD2, and SMAD3 mutations in sporadic CRCs was 8.6% (64 of 744), 3.4% (25 of 744), and 4.3% (32 of 744), respectively. A significant overrepresentation of two genetic hits was detected for SMAD4 and SMAD3, consistent with these genes acting as tumor suppressors. SMAD4 mutations were associated with mucinous histology. The mutation spectra of SMAD2 and SMAD3 were highly similar to that of SMAD4, both in mutation type and location within the encoded proteins. In silico analyses suggested the majority of the mutations were pathogenic, with most missense changes predicted to reduce protein stability or hinder SMAD complex formation. The latter altered interface residues or disrupted the phosphorylation-regulated Ser-Ser-X-Ser motifs within SMAD2 and SMAD3. Functional analyses of selected mutations showed reductions in SMAD3 transcriptional activity and SMAD2-SMAD4 complex formation. Joint biallelic hits in SMAD2 and SMAD3 were overrepresented and mutually exclusive to SMAD4 mutation, underlining the critical roles of these three proteins within the TGF-β signaling pathway.

Oligomeric interactions of TGF-β and BMP receptors

Transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) cytokines participate in a multiplicity of ways in the regulation of numerous physiological and pathological processes. Their wide-ranging biological functions are controlled by several mechanisms, including regulation of transcription, complex formation among the signaling receptors (oligomerization) and with co-receptors, binding of the receptors to scaffolding proteins or their targeting to specific membrane domains. Here, we address the generation of TGF-β and BMP receptor homo- and hetero-oligomers and its roles as a mechanism capable of fast regulation of signaling by these crucial cytokines. We examine the available biochemical, biophysical and structural evidence for the ternary structure of these complexes, and the possible roles of homomeric and heteromeric receptor oligomers in signaling.

Homomeric and heteromeric complexes among TGF-β and BMP receptors and their roles in signaling

Transforming growth factor-β (TGF-β) ligands and bone morphogenetic proteins (BMPs) play myriad roles in many biological processes and diseases. Their pluripotent activities are subject to multiple levels of regulation, including receptor oligomerization, endocytosis, association with co-receptors, cellular scaffolds or membrane domains, as well as transcriptional control. In this review, we focus on TGF-β and BMP receptor homomeric and heteromeric complex formation and their modulation by ligand binding, which regulate signaling on a near-immediate timescale. We discuss the current structural, biochemical and biophysical evidence for the oligomerization of these receptors, and the potential roles of distinct oligomeric interactions in signaling.

A novel glycosylation signal regulates transforming growth factor beta receptors as evidenced by endo-beta-galactosidase C expression in rodent cells

The αGal (Galα1-3Gal) epitope is a xenoantigen that is responsible for hyperacute rejection in xenotransplantation. This epitope is expressed on the cell surface in the cells of all mammals except humans and Old World monkeys. It can be digested by the enzyme endo-β-galactosidase C (EndoGalC), which is derived from Clostridium perfringens. Previously, we produced EndoGalC transgenic mice to identify the phenotypes that would be induced following EndoGalC overexpression. The mice lacked the αGal epitope in all tissues and exhibited abnormal phenotypes such as postnatal death, growth retardation, skin lesion and abnormal behavior. Interestingly, skin lesions caused by increased proliferation of keratinocytes suggest the role of a glycan structure [in which the αGal epitope has been removed or the N-acetylglucosamine (GlcNAc) residue is newly exposed] as a regulator of signal transduction. To verify this hypothesis, we introduced an EndoGalC expression vector into cultured mouse NIH3T3 cells and obtained several EndoGalC-expressing transfectants. These cells lacked αGal epitope expression and exhibited 1.8-fold higher proliferation than untransfected parental cells. We then used several cytokine receptor inhibitors to assess the signal transduction cascades that were affected. Only SB431542 and LY364947, both of which are transforming growth factor β (TGFβ) receptor type-I (TβR-I) inhibitors, were found to successfully reverse the enhanced cell proliferation rate of EndoGalC transfectants, indicating that the glycan structure is a regulator of TβRs. Biochemical analysis demonstrated that the glycan altered association between TβR-I and TβR-II in the absence of ligands.

Different domains regulate homomeric and heteromeric complex formation among type I and type II transforming growth factor-beta receptors

Transforming growth factor-beta (TGF-beta) binds to and signals via two serine-threonine kinase receptors, type I (TbetaRI) and type II (TbetaRII). The oligomerization of TGF-beta receptors modulates ligand binding and receptor trafficking and may contribute to signal diversification. However, numerous features of the molecular domains that determine the homo- and hetero-oligomerization of full-length receptors at the cell surface and the mode of these interactions remain unclear. Here, we address these questions through computerized immunofluorescence co-patching and patch/fluorescence recovery after photobleaching measurements of different combinations of epitope-tagged receptors and their mutants in live cells. We show that TbetaRI and TbetaRII are present on the plasma membrane both as monomers and homo- and hetero-oligomers. The homodimerization of TbetaRII depends on a cytoplasmic juxtamembrane region (amino acid residues 200-220). In contrast, the cytoplasmic domain of TbetaRI is dispensable for its homodimerization. TbetaRI.TbetaRII hetero-oligomerization depends on the cytoplasmic domain of TbetaRI and on a C-terminal region of TbetaRII (residues 419-565). TGF-beta1 elevates TbetaRII homodimerization to some degree and strongly enhances TbetaRI.TbetaRII heteromeric complex formation. Both ligand-induced effects depend on the region encompassed between residues 200-242 of TbetaRII. Furthermore, the kinase activity of TbetaRI is also necessary for the latter effect. All forms of the homo- and hetero-oligomers, whether constitutively present on the membrane or formed upon TGF-beta1 stimulation, were stable in the time-scale of our patch/FRAP measurements. We suggest that the different forms of receptor oligomerization may serve as a basis for the heterogeneity of TGF-beta signaling responses.

Constitutive homo- and hetero-oligomerization of TbetaRII-B, an alternatively spliced variant of the mouse TGF-beta type II receptor

Transforming growth factor (TGF)-beta ligands signal through transmembrane type I and type II serine/threonine kinase receptors, which form heteromeric signalling complexes upon ligand binding. Type II TGF-beta receptors (TbetaRII) are reported to exist as homodimers at the cell surface, but the oligomerization pattern and dynamics of TbetaRII splice variants in live cells has not been demonstrated thus far. Using co-immunoprecipitation and bioluminescence resonance energy transfer (BRET), we demonstrate that the mouse TbetaRII receptor splice variant TbetaRII-B is capable of forming ligand-independent homodimers and heterodimers with TbetaRII. The homomeric interaction of mouse (m)TbetaRII-B isoforms, however, is less robust than the heteromeric interactions of mTbetaRII-B with wild-type TbetaRII, which indicates that these receptors may be more likely to heterodimerize when both receptors are expressed. Moreover, we demonstrate that mTbetaRII-B is a signalling receptor with ubiquitous tissue expression. Our study thus demonstrates previously unappreciated complex formation of TGF-beta type II receptors, and suggests that mTbetaRII-B can direct TGF-beta-induced signalling in vitro and in vivo.

Expression of the human activin type I and II receptor extracellular domains in Pichia pastoris

Methods for the expression in Pichia pastoris and purification of the human activin receptor type I and II extracellular domains (ARIa/ARIb-ECDs, ARIIA/ARIIB-ECDs) are described. Key experimental aspects are also documented of the vector transformation methodology and the binding characteristics of these ECDs with activin A and inhibin. The cDNA constructs for these ECDs contained a C-terminal His6-tag with either the native signal (N) or the yeast alpha mating factor (alphaMF) sequence and were introduced into the pPICZ expression vector either as a single-copy or as a four-copy expression cassette. Hyper-resistant transformants (zeo(R): 500 microg/mL) generated from the cassette containing a single copy of the expression vector gave the stronger signal intensity with a DNA dot-blot screening assay. These transformants also produced higher quantities of the corresponding recombinant protein compared to transformants using the four-copy cassette vector. All receptor-ECD proteins expressed were found to be heterogeneously glycosylated, whereby the ARIIA-ECD and ARIIB-ECD had undergone two Asn-linked glycosylation events and the ARIb-ECD a single event. By SDS-PAGE, the de-glycosylated proteins migrated larger than the expected core size, indicating that they may have undergone O-linked glycosylation. Biacore-based procedures with the glycosylated and de-glycosylated ARIIA-ECD were employed to determine the kinetic and equilibrium binding parameters for the interaction with activin A and inhibin. The glycosylated ARIIA-ECD bound to activin A with a KD of 11.9 nM and inhibin with a KD of 21.1 nM. Although glycosylation of ARIIA-ECD was not strictly required for high affinity interactions with activin A or inhibin, it markedly improved the overall stability of the ARIIA-ECD.

A molecular recognition paradigm: promiscuity associated with the ligand-receptor interactions of the activin members of the TGF-β superfamily

The structure-function properties of the pleiotropic activins and their relationship to other members of the transforming growth factor-beta superfamily of proteins are described. In order to highlight the molecular promiscuity of these growth factors, emphasis has been placed on molecular features associated with the recognition by activin A and the bone morphogenic proteins of the corresponding extracellular domains of the ActRI and ActRII receptors. The available evidence suggests that the homodimeric activin A in its various functional roles has the propensity to fulfill key tasks in the regulation of mammalian cell behaviour, through coordination of numerous transcriptional and translational processes. Because of these profound effects, under physiologically normal conditions, activin A levels are closely controlled by a variety of binding partners, such as follistatin-288 and follistatin-315, alpha(2)-macroglobulin and other proteins. Moreover, the subunits of other members of the activin subfamily, such as activin B or activin C, are able to form heterodimers with the activin A subunit, thus providing a further avenue to positively or negatively control the physiological concentrations of activin A that are available for interaction with specific receptors and induction of cell signaling events. Based on data from X-ray crystallographic studies and homology modeling experiments, the molecular architecture of the ternary receptor-activin ligand complexes has been dissected, permitting rationalization in structural terms of the pattern of interactions that are the hallmark of this protein family.

Heart and liver defects and reduced transforming growth factor beta2 sensitivity in transforming growth factor beta type III receptor-deficient embryos

The type III transforming growth factor beta (TGFbeta) receptor (TbetaRIII) binds both TGFbeta and inhibin with high affinity and modulates the association of these ligands with their signaling receptors. However, the significance of TbetaRIII signaling in vivo is not known. In this study, we have sought to determine the role of TbetaRIII during development. We identified the predominant expression sites of TbetaRIII mRNA as liver and heart during midgestation and have disrupted the murine TbetaRIII gene by homologous recombination. Beginning at embryonic day 13.5, mice with mutations in TbetaRIII developed lethal proliferative defects in heart and apoptosis in liver, indicating that TbetaRIII is required during murine somatic development. To assess the effects of the absence of TbetaRIII on the function of its ligands, primary fibroblasts were generated from TbetaRIII-null and wild-type embryos. Our results indicate that TbetaRIII deficiency differentially affects the activities of TGFbeta ligands. Notably, TbetaRIII-null cells exhibited significantly reduced sensitivity to TGFbeta2 in terms of growth inhibition, reporter gene activation, and Smad2 nuclear localization, effects not observed with other ligands. These data indicate that TbetaRIII is an important modulator of TGFbeta2 function in embryonic fibroblasts and that reduced sensitivity to TGFbeta2 may underlie aspects of the TbetaRIII mutant phenotype.

Epidermal growth factor receptor: association of extracellular domain negatively regulates intracellular kinase activation in the absence of ligand

The epidermal growth factor receptor (EGFR) plays an important role in many types of human cancers. Receptor amplification, autocrine activation and/or deletion of exons 2-7 of EGFR gene have all been associated with tumor development. The traditional model of EGFR activation via ligand induced dimerization and consequential kinase activation does not provide full understanding of its tumorigenicity. The main function of the receptor extracellular domain (ECD) has been thought to be ligand recognition and binding. We report that the EGFR ECD, through its association also negatively regulates the activity of the intracellular kinase in the absence of ligand. Even in the absence of its ligands, the EGF receptor forms homodimers, however, the ECD prevents constitutive receptor kinase activation through its intrinsic ligand-independent interaction. The removal of this domain, either partial or total, results in constitutive activation of the receptor kinase as observed by its phosphorylation in intact cells. Furthermore, EGF receptors truncated in the ECD induce phosphorylation of the wild-type full-length receptor, indicating an inter-molecular inhibitory mechanism by the receptor ECD. The tumor associated delta2-7EGFR mutant also dimerizes with and phosphorylates the wild type EGFR in the absence of ligand. Thus, in addition to its role in ligand recognition, EGFR ECD interacts with each other, imposing an inhibitory effect on the activation of the intracellular kinase.

The BMP7/ActRII extracellular domain complex provides new insights into the cooperative nature of receptor assembly

Activins and bone morphogenetic proteins (BMPs) elicit diverse biological responses by signaling through two pairs of structurally related type I and type II receptors. Here we report the crystal structure of BMP7 in complex with the extracellular domain (ECD) of the activin type II receptor. Our structure produces a compelling four-receptor model, revealing that the types I and II receptor ECDs make no direct contacts. Nevertheless, we find that truncated receptors lacking their cytoplasmic domain retain the ability to cooperatively assemble in the cell membrane. Also, the affinity of BMP7 for its low-affinity type I receptor ECD increases 5-fold in the presence of its type II receptor ECD. Taken together, our results provide a view of the ligand-mediated cooperative assembly of BMP and activin receptors that does not rely on receptor-receptor contacts.

Mixed lineage kinase 2 interacts with clathrin and influences clathrin-coated vesicle trafficking

Mixed lineage kinase 2 (MLK2) is a protein kinase that signals in the stress-activated Jun N-terminal kinase signal transduction pathway. We used immunoprecipitation and mass spectrometric analysis to identify MLK2-binding proteins in cell lines with inducible expression of green fluorescent protein-tagged MLK2. Here we report the identification of clathrin as a binding partner for MLK2 in both cultured cells and mammalian brain. We demonstrate that clathrin binding requires a motif (LLDMD) located near the MLK2 C terminus, which is similar to "clathrin box" motifs important for binding of clathrin coat assembly and accessory proteins to the clathrin heavy chain. A C-terminal fragment of MLK2 containing this motif binds strongly to clathrin, and mutation of the LLDMD sequence to LAAAD completely abrogates clathrin binding. We isolated clathrin-coated vesicles from green fluorescent protein-MLK2-expressing cells and from mouse brain lysates and found that MLK2 is enriched along with clathrin in these vesicles. In addition, we demonstrated that endogenous MLK2 co-immunoprecipitates with clathrin heavy chain from the vesicle-enriched fraction of mouse brain lysate. Furthermore, overexpression of MLK2 in cultured cells inhibits accumulation of labeled transferrin in recycling endosomes during receptor-mediated endocytosis. These findings suggest a role for MLK2 and the stress-signaling pathway at sites of clathrin activity in vesicle formation or trafficking.

Thyroid-stimulating hormone and thyroid-stimulating hormone receptor structure-function relationships

This review focuses on recent advances in the structure-function relationships of thyroid-stimulating hormone (TSH) and its receptor. TSH is a member of the glycoprotein hormone family constituting a subset of the cystine-knot growth factor superfamily. TSH is produced by the pituitary thyrotrophs and released to the circulation in a pulsatile manner. It stimulates thyroid functions using specific membrane TSH receptor (TSHR) that belongs to the superfamily of G protein-coupled receptors (GPCRs). New insights into the structure-function relationships of TSH permitted better understanding of the role of specific protein and carbohydrate domains in the synthesis, bioactivity, and clearance of this hormone. Recent progress in studies on TSHR as well as studies on the other GPCRs provided new clues regarding the molecular mechanisms of receptor activation. Such advances are a result of extensive site-directed mutagenesis, peptide and antibody approaches, detailed sequence analyses, and molecular modeling as well as studies on naturally occurring gain- and loss-of-function mutations. This review integrates expanding information on TSH and TSHR structure-function relationships and summarizes current concepts on ligand-dependent and -independent TSHR activation. Special emphasis has been placed on TSH domains involved in receptor recognition, constitutive activity of TSHR, new insights into the evolution of TSH bioactivity, and the development of high-affinity TSH analogs. Such structural, physiological, pathophysiological, evolutionary, and therapeutic implications of TSH-TSHR structure-function studies are frequently discussed in relation to concomitant progress made in studies on gonadotropins and their receptors.

Identification of a functional site on the type I TGF-beta receptor by mutational analysis of its ectodomain

Six charged amino acid residues located in the ectodomain of the full-length type I transforming growth factor (TGF)-beta receptor were individually mutated to alanine. Mutation of residues D47, D98, K102 and E104 resulted in functionally impaired receptors as demonstrated by a marked decrease in ligand-dependent signaling and ligand internalization relative to the wild-type receptor. The other two mutants (K39A and K87A) exhibited wild-type-like activity. Molecular modeling indicates that the four functionally important residues are located on the convex face of the ectodomain structure. Since mutation of these four residues affects signaling and ligand internalization but not ligand binding, we propose that this functional site is an interacting site between type I and II receptors.

Regulation of transforming growth factor-beta signaling

Members of transforming growth factor beta (TGF-beta) family are potent regulators of multiple cellular functions, including cell proliferation, differentiation, migration, organization, and death. Yet the signaling pathways underpinning a wide array of biological activities of TGF-beta appear to be deceptively simple. At every step from TGF-beta secretion to activation of its target genes, the activity of TGF-beta is regulated tightly, both positively and negatively. Biologically active TGF-beta is cleaved from a precursor protein (latent form) and multiple process factors control the levels of active TGF-beta. The efficient secretion, correct folding and deposition to the extracellular matrices require the cosecretion of latent TGF-beta binding proteins (LTBPs). Once activated, TGF-beta ligand signals through a heteromeric receptor complex of two distinct type I and type II serine/threonine kinase receptors TbetaRI and TbetaRII. Many factors appear to influence the formation of the active ligand-receptor complex. The relative orientation of TbetaRI and TbetaRII in the ligand-receptor complex is critical for activation: through TbetaRI, the activated ligand-receptor complex directly binds and phosphorylates downstream intracellular substrates, called Smads. Inhibitory Smads, Smad6 and 7, can antagonize this process. The phosphorylation of Smads leads to the formation of complexes which translocate to the nucleus. Other signaling systems can modulate the activity of the Smads: e.g., ras activity can prevent Smad complexes from entering the nucleus and specific ubiquitin ligases can target Smad for degradation. In the nucleus, the Smad complexes associate with other transcription activators or suppressors to regulate gene expression, either positively or negatively. The combined effects of the positive and/or negative TGF-beta controlled gene expression together with the endogenous protein set of the target cell are responsible for the multiplicity of biological functions.

Identification of specific inhibin A-binding proteins on mouse Leydig (TM3) and sertoli (TM4) cell lines

The binding of human inhibin A to cell surface binding proteins of mouse Leydig (TM3) and Sertoli (TM4) cell lines was investigated. Scatchard analysis identified two classes of inhibin A-binding sites on TM3 (K(d(1)) = 85 pM and 4,160 sites/cell; K(d(2)) = 520 pM and 12,500 sites/cell) and TM4 (K(d(1)) = 61 pM and 2,620 sites/cell; K(d(2)) = 520 pM and 10,400 sites/cell) cells. Compared with inhibin A, inhibin B only partially competed [(125)I]inhibin A binding (6-8%), whereas activin A competed weakly (<0.01%). Chemical cross-linking of [(125)I]inhibin A to both cell lines identified five [(125)I]inhibin A binding complexes with apparent molecular masses of 70, 95, 145, 155, and more than 200 kDa. Inhibin A displacement of [(125)I]inhibin A from each of these cross-linked species (ED(50) = 60-110 pM) closely resembled displacement from intact TM3 (ED(50) = 97 +/- 32 pM) and TM4 (ED(50) = 75 +/- 28 pM) cells, suggesting that all of these proteins are involved in the high affinity inhibin A binding complex. Immunoprecipitation of iodinated inhibin A complexed to TM3 and TM4 cells with an antibody against human betaglycan identified protein complexes of more than 200, 145, and 95 kDa. It is concluded that the high affinity binding complex for inhibin A found in these cell lines consists of betaglycan and several proteins of unknown identity and may represent the putative inhibin receptor complex.