Promoting bone morphogenetic protein signaling through negative regulationof inhibitory Smads

Inhibitory Smads, i.e. Smad6 and Smad7, are potent antagonists of the BMP-S mad pathway by interacting with activated bone morphogenetic protein (BMP) type I receptors and thereby preventing the activation of receptor-regulate d Smads, or by competing with activated R-Smads for heteromeric complex for mation with Smad4. The molecular mechanisms that underlie the regulation of I-Smad activity have remained elusive. Here we report the identification o f a cytoplasmic protein, previously termed (a) under bar ssociated (m) unde r bar olecule with the (SH) under bar3 domain of STAM (AMSH), as a direct b inding partner for Smad6. AMSH interacts with Smad6, but not with R- and Co -Smads, upon BMP receptor activation in cultured cells. Consistent with thi s finding, stimulation of cells with BMP induces a co-localization of Smad6 with AMSH in the cytoplasm. Ectopic expression of AMSH prolongs BMP-induce d Smad1 phosphorylation, and potentiates BMP-induced activation of transcri ptional reporter activity, growth arrest and apoptosis. The data strongly s uggest that the molecular mechanism by which AMSH exerts its action is by i nhibiting the binding of Smad6 to activated type I receptors or activated R -Smads.