BMP signaling begins with the binding of BMP ligands to a heterodimeric complex of type I and type II serine/threonine kinase receptors on the cell surface. Upon ligand binding, the type II receptor phosphorylates the type I receptor, which in turn phosphorylates receptor-regulated SMADs (R-SMADs). These phosphorylated R-SMADs then form a complex with SMAD4 and translocate into the nucleus to regulate the transcription of target genes.
