Figure 1.

TGF-β1 signaling pathway. TGF-β receptors phosphorylate intracellular SMAD proteins following ligand-binding. SMAD2 is phosphorylated by TGF-β receptor I and subsequently forms a complex with SMAD3 and the co-SMAD SMAD4. This complex is then transported into the nucleus where it binds an SBE and induces the transcription of TGF-β-dependent genes. I-SMAD may bind TGF-β receptor I and attenuate pathway activation. TGF-β, transforming growth factor β; I-SMAD, inhibitory SMAD; SBE, SMAD binding element; P, phosphorylated.