Figure 1.

Example network of BMP signaling. BMP signaling is mediated by
    B
MP:Type I
    R
eceptor (C) forms either by direct interaction between BMP (A) and Type I (B) or via an intermediate state with BMP:Type I:Cv-2 (Z). Initially B interacts with A (Type I receptor), D (Cv-2) and forms C (BMP:Type I receptor) and E (BMP:Cv-2) complexes respectively. Then the complexes C, E can generate the intermediate state BMP:Cv-2:Type I complex (node Z) by interacting with D and B respectively. In the network, only BR(node C) has the ability to turn on downstream signaling. Upon BR formation, the complex recruits type II receptor, and later initiates the phosphorylation of intracellular Smad protein. Signaling leads to pSmad accumulation within the nucleus and gene expressions of BMP targets.

Karim et al. BMC Genomics 2012 13(Suppl 6):S10   doi:10.1186/1471-2164-13-S6-S10