Signal transduction: membrane receptors transduce the signal so that transcription factors are activated through phosphorylation. TGFR family members(30 different ligands in humans)activate cell surface receptors( Serine-Threonine kinases) Ligand Xnr (2 BMP4 (3 R-Smad Kinase domain Co-Smad Type I Type ll recepto recepto Plasma membrane DNA-binding ( partner Nucleus 41 arget gene 9Signal transduction: membrane receptors transduce the signal so that transcription factors are activated through phosphorylation. TGFβ family members (30 different ligands in humans) activate cell surface receptors (Serine-Threonine kinases). Xnr BMP4 Fig. 6a