TGF-β2 uses the concave surface of its extended finger region to bind betaglycan’s ZP domain via three residues specific to TGF-β and inhibin-α

Betaglycan (BG) is a membrane-bound co-receptor of the TGF- family that selectively binds transforming growth factor- (TGF-) isoforms and inhibin A (InhA) to enable temporal-spatial patterns of signaling essential for their functions in vivo. Here, using NMR titrations of methyl-labeled TGF-2 with BG's C-terminal binding domain, BG(ZP-C), and surface plasmon resonance binding measurements with TGF-2 variants, we found that the BG(ZP-C)-binding site on TGF-2 is located on the inner surface of its extended finger region. Included in this binding site are Ile-92, Lys-97, and Glu-99, which are entirely or mostly specific to the TGF- isoforms and the InhA -subunit, but they are unconserved in other TGF- family growth factors (GFs). In accord with the proposed specificity-determining role of these residues, BG bound bone morphogenetic protein 2 (BMP-2) weakly or not at all, and TGF-2 variants with the corresponding residues from BMP-2 bound BG(ZP-C) more weakly than corresponding alanine variants. The BG(ZP-C)-binding site on InhA previously was reported to be located on the outside of the extended finger region, yet at the same time to include Ser-112 and Lys-119, homologous to TGF-2 Ile-92 and Lys-97, on the inside of the fingers. Therefore, it is likely that both TGF-2 and InhA bind BG(ZP-C) through a site on the inside of their extended finger regions. Overall, these results identify the BG(ZP-C)-binding site on TGF-2 and shed light on the specificity of BG for select TGF--type GFs and the mechanisms by which BG influences their signaling.