Signaling by chimeric erythropoietin-TGF-beta receptors: Homodimerization of the cytoplasmic domain of the type I TGF-beta receptor and heterodimerization with the type II receptor are both required for intracellular signal transduction

Transforming growth factor-beta (TGF-beta) affects multiple cellular functions through the type I and type II receptor Ser/Thr kinases(T beta RI and T beta RII). Analysis of TGF-beta signaling pathways has been hampered by the lack of cell lines in which both T beta RI and T beta RII are deleted, and by the inability to study signal transduction by T beta RI independently of T beta RII since T beta RI does not bind TGF-beta directly, To overcome these problems, we constructed and expressed chimeric receptors with the extracellular domain of the erythropoietin receptor (EpoR) and the cytoplasmic domains of T beta RI or T beta RII, When expressed in Ba/F3 cells, which do not express EpoR, Epo induces the formation of a heteromeric complex between cell surface EpoR-T beta RI and EpoR-T beta RII chimeras, Neither the EpoR-T beta RI nor the EpoR-T beta RII chimera interacts with endogenous TGF-beta receptors, Ba/F3 cells expressing both EpoR-T beta RI and EpoR-T beta RII chimeras, but not EpoR-T beta RI or EpoR-T beta RII alone, undergo Epo-induced growth arrest, When expressed in Ba/F3 cells in the absence of the EpoR-T beta RII chimera, EpoR-T beta RI((TD)-D-204), a chimeric receptor with a point mutation in the GS domain of T beta RI that is autophosphorylated constitutively, triggers growth inhibition in response to Epo, Thus, both homo- and heterodimerization of the cytoplasmic domain of the type I TGF-beta receptor are required for intracellular signal transduction leading to inhibition of cell proliferation, These chimeric receptors provide a unique system to study the function and signal transduction of individual TGF-beta receptor subunits independently of endogenous TGF-beta receptors.