Transcriptional modulator activated by BMP (bone morphogenetic proteins) type 1 receptor kinase. SMAD1 is a receptor-regulated SMAD (R-SMAD) (By similarity). May play a role in the initiation and maintenance of spermatogenesis. SMAD1/OAZ1/PSMB4 complex mediates the degradation of the CREBBP/EP300 repressor SNIP1 (By similarity). May act synergistically with SMAD4 and YY1 in bone morphogenetic protein (BMP)-mediated cardiac-specific gene expression (PubMed:15329343). Upon C-terminus phosphorylation: forms trimers with another SMAD1 and the co-SMAD SMAD4. Interacts with PEBP2-alpha subunit, CREB-binding protein (CBP), p300, SMURF1, SMURF2, USP15 and HOXC8. Associates with ZNF423 or ZNF521 in response to BMP2 leading to activate transcription of BMP target genes. Interacts with SKOR1. Interacts (via MH2 domain) with LEMD3. Binding to LEMD3 results in at least a partial reduction of receptor-mediated phosphorylation (By similarity). Also interacts with HGS, NANOG and ZCCHC12. Forms a ternary complex with PSMB4 and OAZ1 before PSMB4 is incorporated into the 20S proteasome (By similarity). Found in a complex with SMAD4 and YY1. Interacts (via MH2 domain) with FAM83G (via MH2 domain); in a SMAD4-independent manner. Interacts with ZC3H3 (PubMed:16115198). Interacts with TMEM119 (PubMed:21239498). Interacts (via MH1 and MH2 domains) with ZNF8 (PubMed:12370310). Interacts with RANBP3L; the interaction increases when SMAD1 is not phosphorylated and mediates SMAD1 nuclear export (PubMed:25755279). In the cytoplasm in the absence of ligand. Migration to the nucleus when complexed with SMAD4. Colocalizes with LEMD3 at the nucleus inner membrane. Exported from the nucleus to the cytoplasm when dephosphorylated (PubMed:25755279). Ubiquitous. Ubiquitously expressed during embryogenesis. Expression starts in some seminiferous tubules at 2 weeks of age. After mid-puberty a stage-specific expression is established. During the cycling of the seminiferous epithelium, expression initiates in the pachytene spermatocytes of stage V seminiferous tubules, peaks at stage X, then decreases as pachytene spermatocytes differentiate into secondary spermatocytes and then round spermatids. The MH2 domain mediates phosphorylation-dependent trimerization through L3 loop binding of phosphoserines in the adjacent subunit. Phosphorylation of the C-terminal SVS motif by BMP type 1 receptor kinase activates SMAD1 by promoting dissociation from the receptor and trimerization with SMAD4 (By similarity). Dephosphorylation, probably by PPM1A, induces its export from the nucleus to the cytoplasm (PubMed:25755279). Ubiquitinated by SMAD-specific E3 ubiquitin ligase SMURF1, leading to its degradation. Monoubiquitinated, leading to prevent DNA-binding. Deubiquitination by USP15 alleviates inhibition and promotes activation of TGF-beta target genes (By similarity). Belongs to the dwarfin/SMAD family.