The MAD-Related Protein Smad7 Associates with the TGFβ Receptor and Functions as an Antagonist of TGFβ Signaling

The MAD-related (MADR) family includes the sma-2, These proteins are highly Division of Gastroenterology conserved across species, but have no known structural The Hospital for Sick Children motifs and bear no resemblance to components of other Toronto, Ontario M5G 1X8 signaling pathways. All members of the family share Canada certain structural domains including the highly con-† Millennium Pharmaceuticals, Inc. served amino and carboxy-terminal regions termed the 640 Memorial Drive MH1 and MH2 domains, respectively. Smad6 is the ex-Cambridge, Massachusetts 02139 ception to this, and encodes a truncated protein that ‡ Division of Research Technologies and possesses only the MH2 domain. Smads are critical Proteins intracellular mediators of Ser/Thr kinase receptor signal-Lilly Research Labs ing and play a role in specifying biological responses Indianapolis Indiana 46285 to ligands. Thus, Smad1 and 5 function in BMP signaling Smads are regulated by receptor-dependent phos-phorylation that is highly specific. Smad2 and Smad3 interact with TGF␤ receptors (Macías-Silva et al., 1996; Summary Zhang et al., 1996), while Smad1 is targeted by BMP pathways (Hoodless et al., 1996; Kretzchmar et al., TGF␤ signaling is initiated when the type I receptor 1997). Phosphorylation of Smads is mediated directly phosphorylates the MAD-related protein, Smad2, on by the activated type I kinase domain and occurs on C-terminal serine residues. This leads to Smad2 asso-serine residues within a conserved SSXS motif at the C ciation with Smad4, translocation to the nucleus, and terminus of the protein (Macías-Silva et al., 1996; Kretzchmar et al., 1997). Phosphorylation of these re-regulation of transcriptional responses. Here we dem-ceptor-regulated Smads results in formation of a hetero-onstrate that Smad7 is an inhibitor of TGF␤ signal-meric complex with another MAD-related protein, ing. Smad7 prevents TGF␤-dependent formation of Smad2/Smad4 complexes and inhibits the nuclear 5 appear to play specific roles in these pathways, while accumulation of Smad2. Smad7 interacts stably with Smad4 fulfills a common function (Lagna et al., 1996; the activated TGF␤ type I receptor, thereby blocking Zhang et al., 1997). Phosphorylation of Smads is re-the association, phosphorylation, and activation of quired for their accumulation in the nucleus (Macías-Smad2. Furthermore, mutations in Smad7 that inter-Silva et al., 1996; Kretzchmar et al., 1997) where the fere with receptor binding disrupt its inhibitory activity. proteins can function as transcriptional activators These studies thus define a novel function for MAD-through their interaction with DNA-binding proteins. For related proteins as intracellular antagonists of the type instance, in Xenopus, Smad2 interacts with …