Fig. 1.

Overview of the Canonical BMP Signaling Pathway. BMP ligand dimers bind Type I receptors with the help of Co-receptors. The ligand-bound Type I receptors form a heterotetrameric receptor complex with Type II receptors. The Type II receptor phosphorylates and activates the Type I receptor, which then phosphorylates the R-SMAD and changes it to an active conformation that complexes with the Co-SMAD and translocates into the nucleus to regulate transcription. i-SMADs negatively regulate this pathway at several points