In response to BMP2 treatment, RUNX2 (presumably associated with CBFB) forms a complex with SMAD1 (Wang et al. 2006). Phosphorylation of SMAD1 in response to BMP signaling and formation of a heterotrimeric complex between phosphorylated SMAD1 and SMAD4 are prerequisites for SMAD1 retention in the nucleus (Qin et al. 2001, Xiao et al. 2001).
BMP2 signaling is implicated in promoting formation of a complex between RUNX2, SMAD1 and acetyltransferase EP300 (p300) and facilitating EP300-mediated acetylation of RUNX2, which activates RUNX2 transcriptional activity. This may involve ERK-mediated phosphorylation of RUNX2 and/or EP300 downstream of BMP2, but the exact mechanism has not been elucidated (Jun et al. 2010).