| The cytokines of the Transforming Growth Factor beta (TGF-beta) family are key regulators of metazoan embryo development and adult tissue homeostasis. In a canonical pathway shared by both the TGF-beta and the BMP (Bone Morphogenetic Protein) branches of this family, these cytokines bind to heteromeric serine/threonine kinase receptor complexes, which in turn phosphorylate and activate Smad transcription factors at the C-terminal tail. This phosphorylation event induces Smads to accumulate in the nucleus and assemble transcriptional complexes that regulate hundreds of target genes. MAPKs catalyze inhibitory phosphorylation of the Smad proteins within their respective linker regions, however, the mechanistic event that control this antagonistic effects of the MAPK pathway, as well as the biology of the phosphorylation of the linker region of Smad as a key point of regulation, were not well defined.;We show that, for the case of the BMP pathway, such phosphorylation inhibits Smad1 activity by enabling recognition of Smad1 by the HECT-domain ubiquitin ligase Smurf1. This interaction results in Smad1 polyubiquitination, and leads to proteasome-mediated degradation as well as cytoplasmic retention of Smad1.;We also show that small C-terminal domain phosphatases (SCP1, 2 and 3) dephosphorylate the Smad1 C-terminal tail region and the linker regions of the Smads 1, 2 and 3. Thus, SCPs attenuate BMP signaling by resetting Smad1 to the basal unphosphorylated state; in contrast, they enhance TGF-beta signaling by dephosphorylating exclusively the linker region.;Finally, we have uncovered a subsequent agonist-dependent Smad phosphorylation step that plays a dual role in the TGF-beta and BMP pathways. Following receptor-mediated phosphorylation at the C-terminus, Smad proteins become phosphorylated at the linker region. This event is catalyzed by CDK8 and CDK9 kinases, which are components of transcriptional mediator and elongation complexes. On target promoters, Smad1 linker phosphorylation provides docking sites for the cofactor YAP1, which supports Smad1-dependent expression of BMP target genes. Linker-phosphorylated Smads are eventually recognized by the ubiquitin ligases Smurf1 in the BMP pathway and Nedd4L in the TGF-beta pathway, which prime Smads for proteasome-mediated turnover. Thus, nuclear CDK8/9 drives a cycle of Smad utilization and disposal that is integral to the canonical BMP and TGF-beta pathways. |
