Bone morphogenetic proteins (BMPs) play important roles in bone formation as well as other developmental processes, including dorsal-ventral embryonic patterning and epidermal-neural cell fate decisions. BMPs are members of the large Transforming Growth Factor-Beta (TGF-β) superfamily of secreted proteins. Similar to other TGF-β superfamily members, BMPs initiate their signal transduction cascade by binding to a heteromeric receptor complex comprised of two serine/threonine kinase receptors. In response to BMP, the type II receptor transphosphorylates the type I receptor and thus activates the kinase domain of the type I receptor. The activated type I receptor can then phosphorylate downstream signaling molecules, the best studied of which are the Smad family of proteins. Different Smad proteins appear to have specificity for different TGF-β superfamily ligands. Smadl is activated by direct phosphorylation by the BMP type I receptor, and accumulates in the nucleus upon activation where it is believed to act as a transcriptional activator. Extracellular BMP antagonists have been identified in a variety of organisms, however, little is known about the intracellular regulation of Smadl in the BMP pathway.
Here we report a novel Smad1 binding protein, SANE (S[barbelow]mad1 An tagonistic E[barbelow]ffector), which was identified in a yeast two-hybrid screen. SANE is expressed in a manner similar to Smad1 throughout Xenopus embryogenesis. SANE is a membrane associated protein and interacts with two components of the BMP signaling pathway, Smad1 and the BMP type I receptor. SANE binds the BMP-specific Smads, Smad1 and Smad5, and not the activin and TGF-β specific Smad2, or the common pathway shared Smad4. Functionally, SANE antagonizes BMP signaling specifically in Xenopus embryos and in a mammalian model of bone formation. In summary, these studies define a new class of protein, distinct from the Smad family of proteins, which functions intracellularly to regulate BMP signaling.
