Transforming growth factor-β1 (TGF-β1) is the prototype of a large family of proteins that regulate a variety of biological processes. The pleiotropic responses to TGF-β are mediated via ligand-induced heteromeric complex formation by type I (TβR-I) and type II (TβR-II) serine-threonine kinase receptors. Several studies have shown that TβR-II acts as a primary receptor, binding TGF-β and phosphorylating TβR-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-β signaling. We have identified a novel WD domain-containing protein, designated STRAP (serine-threonine kinase receptor-associated protein), which interacts with TβR-I in a yeast twohybrid system. STRAP associates with both functional TβR-I and TβR-II in vivo. Overexpression of STRAP leads to inhibition of TGF-β-mediated transcriptional activation. It also shows synergistic inhibition of TGF-β signaling in concert with Smad7, but not with Smad6, as measured by TGF-β-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- β signal transduction.