An avidity-driven mechanism of extracellular BMP regulation by Twisted gastrulation

During dorsoventral patterning of bilaterian embryos, the conserved regulator Twisted gastrulation (Tsg) modulates BMP signalling by binding Chordin/Short gastrulation (Sog). Here, we elucidate the mechanism by which Tsg interacts with Sog/Chordin to promote formation of the inhibitory Tsg-Sog/Chordin-BMP complex and regulate BMP signalling extracellularly. We identify and validate in vitro a hydrophobic interface in the Tsg C-terminal domain that binds Chordin. Mutation of this epitope in Drosophila Tsg (TsgL100A) results in an unexpectedly mild perturbation to embryonic BMP gradient formation. We show that a protostome-specific Tsg C-terminal extension also binds Sog, and the presence of this second binding site allows partial rescue of Sog interaction with TsgL100A in the presence of BMP. Consistent with this, a truncated Tsg protein lacking both Sog binding regions is unable to support BMP gradient formation in vivo. As our data show that disruption of either Sog binding site in Tsg, but not both, can be overcome by Tsg-BMP and Sog-BMP interactions, we present a new avidity-driven mechanism of BMP gradient formation that will be relevant to a broad range of developmental contexts.