A novel Smad nuclear interacting protein, SNIP1, suppresses p300-dependentTGF-beta signal transduction

Members of the transforming growth factor-beta superfamily play critical ro les in controlling cell growth and differentiation. Effects of TGF-beta fam ily ligands are mediated by Smad proteins. To understand the mechanism of S mad function, we sought to identify novel interactors of Smads by use of a yeast two-hybrid system. A 396-amino acid nuclear protein termed SNIP1 was cloned and shown to harbor a nuclear localization signal (NLS) and a Forkhe ad-associated (PHA) domain. The carboxyl terminus of SNIP1 interacts with S mad1 and Smad2 in yeast two-hybrid as well as in mammalian overexpression s ystems. However, the amino terminus of SNIP1 harbors binding sites for both Smad4 and the coactivator CBP/p300. Interaction between endogenous levels of SNIP1 and Smad4 or CBP/p300 is detected in NMuMg cells as well as in vit ro. Overexpression of full-length SNIP1 or its amino terminus is sufficient to inhibit multiple gene responses to TGF-beta and CBP/p300, as well as th e formation of a Smad4/p300 complex. Studies in Xenopus laevis further sugg est that SNIP1 plays a role in regulating dorsomedial mesoderm formation by the TGF-beta family member nodal. Thus, SNIP1 is a nuclear inhibitor of CB P/p300 and its level of expression in specific cell types has important phy siological consequences by setting a threshold for TGF-beta-induced transcr iptional activation involving CBP/p300.