Transforming growth factor-beta1 (TGF-beta1) is the prototype of a large family of proteins that regulate a variety of biological processes. The pleiotropic responses to TGF-beta are mediated via ligand-induced heteromeric complex formation by type I (TbetaR-I) and type II (TbetaR-II) serine-threonine kinase receptors. Several studies have shown that TbetaR-II acts as a primary receptor, binding TGF-beta and phosphorylating TbetaR-I, whose kinase activity then propagates the signals. Therefore, intracellular proteins that interact with type I receptors are likely to play important roles in TGF-beta signaling. We have identified a novel WD domain-containing protein, designated STRAP (serine-threonine kinase receptor-associated protein), which interacts with TbetaR-I in a yeast two-hybrid system. STRAP associates with both functional TbetaR-I and TbetaR-II in vivo. Overexpression of STRAP leads to inhibition of TGF-beta-mediated transcriptional activation. It also shows synergistic inhibition of TGF-beta signaling in concert with Smad7, but not with Smad6, as measured by TGF-beta-dependent transcriptional reporters. The existence of the STRAP gene from yeast to mammals indicates an evolutionarily conserved function in eukaryotes. The data suggest a potential role for STRAP in TGF-beta signal transduction.