A molecular basis for Smad specificity

Bone morphogenetic proteins (BMPs) and activins are members of the TGF beta superfamily of growth factors, a crucial group of regulators of induction and patterning of embryonic germ layers in metazoa. In early Xenopus embryo s, activin, Vg1, and nodal are potent inducers of dorsal mesoderm, whereas BMPs can ventralize mesoderm, repress neural fate, and induce blood differe ntiation. These characteristic responses rely on ligand-specific signaling pathways, encompassing transmembrane kinase receptors and signal transducer s belonging to the Smad family. The overexpression in Xenopus embryos of BM P-activated Smad1 and of activin/Vg1/nodal-activated Smad2 is sufficient to specifically recapitulate ligand responses. In a search for determinants o f a Smad specificity code, we have identified two small regions within the conserved carboxyl-domain that are necessary and sufficient for specific Sm ad action, Swapping both residue clusters (C1 and C2) between Smad1 and Sma d2 completely switches Smad effects in vivo. Thus, Smad1 with swapped Smad2 clusters responds specifically to BMP but elicits an activin response, whi le a Smad2 protein containing the Smad1 clusters is activated by activin an d elicits a BMP response. Furthermore, association between Smads and FAST-1 , a mediator of mesoderm induction by activin, is dependent upon the presen ce of the Smad2 C1 sequence. Finally, the Smad1-specific antagonist Smad6 c an inhibit a Smad2 molecule harboring Smad1 C1 and C2 sequences. Thus, the C1 and C2 regions of Smads specify the association between Smads and pathwa y-specific partners, such as FAST-1 and Smad6, and account for activin- and BMP- specific responses in vertebrate embryogenesis. Dev Dyn 1999;214:269- 277, (C) 1999 Wiley-Liss, Inc.