The interplay between bone morphogenetic proteins (BMPs) and their antagonists governs developmental and cellular processes as diverse as establishment of the embryonic dorsal-ventral axis, induction of neural tissue, formation of joints in the skeletal system and neurogenesis in the adult brain. So far, the three-dimensional structures of BMP antagonists and the structural basis for inactivation have remained unknown. Here we report the crystal structure of the antagonist Noggin bound to BMP-7, which shows that Noggin inhibits BMP signalling by blocking the molecular interfaces of the binding epitopes for both type I and type II receptors. The BMP-7-binding affinity of site-specific variants of Noggin is correlated with alterations in bone formation and apoptosis in chick limb development, showing that Noggin functions by sequestering its ligand in an inactive complex. The scaffold of Noggin contains a cystine (the oxidized form of cysteine) knot topology similar to that of BMPs; thus, ligand and antagonist seem to have evolved from a common ancestral gene.

Activins and inhibins, structurally related members of the TGF-beta superfamily of growth and differentiation factors, are mutually antagonistic regulators of reproductive and other functions. Activins bind specific type II receptor serine kinases (ActRII or IIB) to promote the recruitment and phosphorylation of the type I receptor serine kinase, ALK4 (refs 7-9), which then regulates gene expression by activating Smad proteins. Inhibins also bind type II activin receptors but do not recruit ALK4, providing a competitive model for the antagonism of activin by inhibin. Inhibins fail to antagonize activin in some tissues and cells, however, suggesting that additional components are required for inhibin action. Here we show that the type III TGF-beta receptor, betaglycan, can function as an inhibin co-receptor with ActRII. Betaglycan binds inhibin with high affinity and enhances binding in cells co-expressing ActRII and betaglycan. Inhibin also forms crosslinked complexes with both recombinant and endogenously expressed betaglycan and ActRII. Finally, betaglycan confers inhibin sensitivity to cell lines that otherwise respond poorly to this hormone. The ability of betaglycan to facilitate inhibin antagonism of activin provides a variation on the emerging roles of proteoglycans as co-receptors modulating ligand-receptor sensitivity, selectivity and function.

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.

Activins, inhibins, and BMPs 1 are structurally related members of the TGF␤ superfamily. BMPs are a large family with extensive and extremely complex roles both in development and adult life (1-3). For example, BMP-7, 1 of more than 20 BMPs, has roles in kidney morphogenesis and bone formation during development (4, 5) and is involved in regulating gonadal function in the adult (6, 7). BMPs stimulate target cells by assembling a cell surface complex containing type II and type I receptors (8). In this receptor complex, type II receptors activate type I receptors by phosphorylating the GS domain of the type I receptor. The activated type I receptors then activate Smad proteins, such as Smad1, which transduce signals into the cell nucleus (9, 10). Although BMP type I receptors (including ALK-2, ALK-3, and ALK-6) are largely specific for the BMP family, this is not true for type II receptors. BMPRII binds only BMPs (11-13), but ActRII and ActRIIB can bind both BMPs and activins (14) and can mediate signaling by either family. Individual BMPs may bind preferentially to specific type I or type II receptors, as illustrated by the preferentially binding of BMP-7 to ActRII (14) and ALK-2 (15, 16). In general, however, the BMP family as a whole makes use of all of these type I and type II receptors.Activins and inhibins were first identified as regulators of reproduction that antagonistically modulate the endocrine interaction of the pituitary and gonadal systems. Activins are local regulators of pituitary FSH release, whereas inhibins are produced by the gonads in response to FSH and act at the pituitary to attenuate activin effects (17). Activins, like BMPs, stimulate target cells by assembling receptor complexes containing type I and type II receptors at the cell membrane. In these ligand-receptor complexes, distinct activin-specific type I receptors are activated and in turn activate activin-specific Smads (18). Recently, betaglycan was identified as a co-receptor that binds inhibin and increases the affinity of inhibin for the type II activin receptors. When inhibin is bound to betaglycan it also binds to ActRII and ActRIIB and thereby sequesters them, preventing formation of the type II/type I receptor complex in response to activin and, thus, blocking activin signaling. This mechanism elaborates a model of inhibin function, where inhibin, as a competitive antagonist, competes with activin for access to ActRII and ActRIIB (19).In most studies the effects of inhibin have been explained by this blockade of activin signaling (20, 21), but some inhibin effects have been reported that seem inconsistent with this mode of action. These include responses to inhibin in systems that do not respond to activin (22) and systems that, instead of exhibiting antagonistic interactions, show similar responses to inhibin and activin (23,24). These findings led to the suggestion that inhibin may have additional mechanisms of action, such as its own independent signaling pathway (25), although there is little direct functional or bioch...

Betaglycan is a co-receptor that mediates signaling by transforming growth factor ␤ (TGF␤) superfamily members, including the distinct and often opposed actions of TGF␤s and inhibins. Loss of betaglycan expression, or abrogation of betaglycan function, is implicated in several human and animal diseases, although both betaglycan actions and the ligands involved in these disease states remain unclear. Here we identify a domain spanning amino acids 591-700 of the betaglycan extracellular domain as the only inhibinbinding region in betaglycan. This binding site is within the betaglycan ZP domain, but inhibin binding is not integral to the ZP motif of other proteins. We show that the inhibin and TGF␤-binding residues of this domain overlap and identify individual amino acids essential for binding of each ligand. Mutation of Val 614 to Tyr abolishes both inhibin and TGF␤ binding to this domain. Full-length betaglycan V614Y, and other mutations, retain TGF␤ binding activity via a distinct site, but are unable to bind inhibin-A. These betaglycan mutants fail to mediate inhibin antagonism of activin signaling but can present TGF␤ to T␤RII. Separating the co-receptor actions of betaglycan toward inhibin and TGF␤ will allow the clarification of the role of betaglycan in disease states such as renal cell carcinoma and endometrial adenocarcinoma.Betaglycan is a co-receptor for members of the TGF␤ 3 superfamily, with vital roles in mediating and regulating signaling of diverse superfamily members. The importance of betaglycan is highlighted by loss of expression or mutation of betaglycan in various human diseases. For example, there is a strict correlation between a loss of betaglycan expression in human patients and development of both renal cell carcinoma and endometrial adenocarcinoma (1, 2). These results also highlight the difficulty in understanding the role of betaglycan in different cellular contexts, because betaglycan can impinge on signaling by so many TGF␤ superfamily members. In the studies cited above, loss of betaglycan in renal cell carcinoma has been proposed to impair TGF␤-2 signaling, because impaired TGF␤ signaling due to loss of T␤RII expression has also been detected in these carcinomas (1). In contrast, loss of betaglycan in endometrial adenocarcinoma is speculated to involve betaglycan regulation of inhibin action, due to the fact that the inhibin ␣-subunit is also disrupted in endometrial adenocarcinomas (2). However, these conclusions will remain primarily correlative until methods are developed to dissect the various co-receptor roles of betaglycan.Betaglycan directly binds TGF␤ isoforms, which is functionally vital in the case of TGF␤-2. Betaglycan facilitates TGF␤-2 binding to the TGF␤ type II receptor T␤RII, leading to the formation of a ternary complex containing TGF␤-2, T␤RII, and betaglycan (3). Once this complex has formed, T␤RI (ALK5) then binds to TGF␤-2 and T␤RII, displacing betaglycan. T␤RII then phosphorylates T␤RI, which, in turn, propagates TGF␤ signals into the cell via phosphorylatio...