125. Identification of a functional binding site on betaglycan for inhibin and TGFβ

Abstract

Betaglycan is a co-receptor that modulates signaling by TGFβ superfamily members including TGFβs and inhibins. Loss of betaglycan expression, or blocking of betaglycan function, has been implicated in several human diseases and in animal disease models. However, characterization of the superfamily ligands and receptors involved in these disease states is complicated because of the pleiotropic nature of betaglycan co-receptor action. Here we report the identification and characterization of a domain within the extracellular region of betaglycan that binds inhibin-A and TGFβ-1. We show that both ligands bind to the membrane proximal region (amino acids 591–760) of the betaglycan extracellular domain. This inhibin/TGFβ-binding region is within the ZP-domain of betaglycan, but is not integral to the conserved ZP motif. Using deletion studies and site-directed mutagenesis, we show that the inhibin and TGFβ binding sites on betaglycan overlap and identify individual amino acids essential for the binding of both ligands. In particular, point mutant V614Y abolishes inhibin and TGFβ binding to the membrane proximal domain of the betaglycan ECD. A full-length betaglycan construct containing this point mutation can still bind TGFβ-1 via a separate N-terminal binding site, but is unable to bind inhibin-A. This mutant is incapable of mediating inhibin’s antagonism of activin or BMP signalling. Mutation of betaglycan V614Y thus separates the co-receptor actions of betaglycan for inhibin and TGFβ. This will allow the clarification of the role of betaglycan in human disease states such as renal cell carcinoma and endometrial adenocarcinoma.