 The phosphate group does not act as a docking site for coSMAD, rather the phosphorylation opens up an amino acid stretch allowing interaction.The phosphorylated RSMAD/coSMAD complex enters the nucleus where it binds transcription promoters/cofactors and causes the transcription of DNA.Bone morphogenetic proteins cause the transcription of mRNAs involved in osteogenesis, neurogenesis, and ventral mesoderm specification.