Transforming Growth Factor (TGF-β)-specific Signaling by Chimeric TGF-β Type II Receptor with Intracellular Domain of Activin Type IIB Receptor

Members of the transforming growth factor-beta (TGF-beta) superfamily signal via different heteromeric complexes of two sequentially acting serine/threonine kinase receptors, i.e. type I and type II receptors. We generated two different chimeric TGF-beta superfamily receptors, i.e. TbetaR-I/BMPR-IB, containing the extracellular domain of TGF-beta type I receptor (TbetaR-I) and the intracellular domain of bone morphogenetic protein type IB receptor (BMPR-IB), and TbetaR-II/ActR-IIB, containing the extracellular domain of TGF-beta type II receptor (TbetaR-II) and the intracellular domain of activin type IIB receptor (ActR-IIB). In the presence of TGF-beta1, TbetaR-I/BMPR-IB and TbetaR-II/ActR-IIB formed heteromeric complexes with wild-type TbetaR-II and TbetaR-I, respectively, upon stable transfection in mink lung epithelial cell lines. We show that TbetaR-II/ActR-IIB restored the responsiveness upon transfection in mutant cell lines lacking functional TbetaR-II with respect to TGF-beta-mediated activation of a transcriptional signal, extracellular matrix formation, growth inhibition, and Smad phosphorylation. Moreover, TbetaR-I/BMPR-IB and TbetaR-II/ActR-IIB formed a functional complex in response to TGF-beta and induced phosphorylation of Smad1. However, complex formation is not enough for signal propagation, which is shown by the inability of TbetaR-I/BMPR-IB to restore responsiveness to TGF-beta in cell lines deficient in functional TbetaR-I. The fact that the TGF-beta1-induced complex between TbetaR-II/ActR-IIB and TbetaR-I stimulated endogenous Smad2 phosphorylation, a TGF-beta-like response, is in agreement with the current model for receptor activation in which the type I receptor determines signal specificity.