The TGF-beta family mediator Smad1 is phosphorylated directly and activated functionally by the BMP receptor kinase.
Bone morphogenetic proteins (BMPs) are members of the TGF-beta family that regulate cell proliferation, apoptosis, and differentiation, and participate in the development of most tissues and organs in vertebrates. Smad proteins function downstream of TGF-beta receptor serine/threonine kinases and undergo serine phosphorylation in response to receptor activation. Smad1 is regulated in this fashion by BMP receptors, and Smad2 and Smad3 by TGF-beta and activin receptors. Here, we report that BMP receptors phosphorylate and activate Smad1 directly. Phosphorylation of Smad1 in vivo involves serines in the carboxy-terminal motif SSXS. These residues are phosphorylated directly by a BMP type I receptor in vitro. Mutation of these carboxy-terminal serines prevents several Smad1 activation events, namely, Smad1 association with the related protein DPC4, accumulation in the nucleus, and gain of transcriptional activity. Similar carboxy-terminal serines in Smad2 are required for its phosphorylation and association with DPC4 in response to TGF-beta, indicating the generality of this process of Smad activation. As a direct physiological substrate of BMP receptors, Smad1 provides a link between receptor serine/threonine kinases and the nucleus.
Pubmed ID: 9136927 RIS Download
Activin Receptors | Activin Receptors, Type I | Amino Acid Sequence | Animals | Bone Morphogenetic Protein Receptors, Type I | Bone Morphogenetic Protein Receptors, Type II | COS Cells | Cell Line | Cell Nucleus | DNA-Binding Proteins | Epithelial Cells | Lung | Mink | Molecular Sequence Data | Mutation | Phosphopeptides | Phosphorylation | Protein-Serine-Threonine Kinases | Receptors, Growth Factor | Serine | Signal Transduction | Smad Proteins | Smad2 Protein | Trans-Activators | Transforming Growth Factor beta