STRAP (serine-threonine kinase receptor-associated protein) binds to the activated TGF-beta receptor complex. In in vitro studies, STRAP is able to bind both TGFBR1 and TGFBR2 (Datta et al. 1998). This was deduced from experiments in which a recombinant mouse Strap and recombinant human TGFBR1 and TGFBR2 were expressed in COS1 cells. STRAP is also able to bind unphosphorylated TGF-beta receptor complex, but the physiological significance, if any, of this interaction is not known.