B-MYB Positively Regulates Serine-Threonine Kinase Receptor-associated Protein (STRAP) Activity through Direct Interaction

Serine-threonine kinase receptor-associated protein (STRAP) functions as a regulator of both TGF-beta and p53 signaling. However, the regulatory mechanism of STRAP activity is not understood. In this study, we report that B-MYB is a new STRAP-interacting protein, and that an amino-terminal DNA-binding domain and an area (amino acids 373-468) between the acidic and conserved regions of B-MYB mediate the B-MYB.STRAP interaction. Functionally, B-MYB enhances STRAP-mediated inhibition of TGF-beta signaling pathways, such as apoptosis and growth inhibition, by modulating complex formation between the TGF-beta receptor and SMAD3 or SMAD7. Furthermore, coexpression of B-MYB results in a dose-dependent increase in STRAP-mediated stimulation of p53-induced apoptosis and cell cycle arrest via direct interaction. Confocal microscopy showed that B-MYB prevents the normal translocation of SMAD3 in response to TGF-beta 1 and stimulates p53 nuclear translocation. These results suggest that B-MYB acts as a positive regulator of STRAP.