Transcriptional modulator activated by BMP (bone morphogenetic proteins) type 1 receptor kinase. SMAD1 is a receptor-regulated SMAD (R-SMAD) (By similarity). Has been shown to be also activated by TGF-beta (transforming growth factor) type I receptor kinase in rat intestinal epithelial cells (IEC 4-1). SMAD1/OAZ1/PSMB4 complex mediates the degradation of the CREBBP/EP300 repressor SNIP1 (By similarity).