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 (Tsg L100A ) results in an unexpectedly mild perturbation to embryonic BMP gradient formation. We show that a protosome-specific Tsg C-terminal extension also binds Sog, and the presence of this second binding site allows partial rescue of Sog interaction with Tsg L100A 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. Summary statement Identification and mutational analysis of the binding epitopes that mediate Twisted gastrulation interaction with Short gastrulation/Chordin reveals an avidity-based model of embryonic BMP gradient formation.