Figure 4: BMP inhibition by the antagonist NOGGIN. BMPs bind to receptors I and II (BMPR-I and BMPR-II) at the cell surface leading to phosphorilation of Smads 1, 5 and 8. After binding to Smad 4 the complex is translocated to the nucleus, to promote gene transcriptional activation. There is also an alternative pathway through MAP kinases. NOGGIN acts upstream in this pathway, preventing binding of BMPs to the receptors.