Phosphorylation regulation of the interaction between Smad7 and activin type I receptor

Signal transduction of activin, one of the members in the transforming growth factor-beta superfamily, is initiated by ligand binding with two distinct membrane receptors (type II and type I) followed by activation of Smad2 or Smad3. We report here that activin-induced signaling is negatively regulated by another Smad molecule, Smad7. When expressed in Chinese hamster ovary cells, Smad7 inhibited the transcriptional response induced by either activin treatment or a constitutively active activin type I receptor (ALK-4). In addition, Smad7 also inhibited mouse FAST-2-mediated transactivation of the Xenopus Mix.2 promoter stimulated by the constitutively active ALK-4. Smad7 was able to directly associate with ALK-4 and this association was dependent on the phosphorylation of the type I receptor in its GS domain by activin type II receptors. Expression of kinase defective activin type II receptors decreased the association of Smad7 with ALK-4. Correspondingly, Smad7 bound poorly to a mutant ALK-4 bearing serine to alanine substitutions in four putative phosphorylation sites in its GS domain. These studies not only illustrated the counter regulatory function of Smad7 on activin signaling, but also indicated the involvement of phosphorylation at activin type I receptor in the inhibitory action of Smad7.