Transcriptional modulator activated by BMP (bone morphogenetic proteins) type 1 receptor kinase. SMAD5 is a receptor-regulated SMAD (R-SMAD). May form trimers with the co-SMAD SMAD4. Interacts with PEBP2-alpha subunit and SMURF1. Interacts with SUV39H1 and SUV39H2. Interacts (via MH2 domain) with LEMD3. Interacts with WWP1. Interacts with TMEM119 (By similarity). Interacts with ZNF8 (PubMed:12370310). Interacts with RANBP3L (PubMed:25755279). Cytoplasmic in the absence of ligand. Migrates to the nucleus when complexed with SMAD4. Ubiquitous. Phosphorylated on serine by BMP (bone morphogenetic proteins) type 1 receptor kinase. Ubiquitin-mediated proteolysis by SMAD-specific E3 ubiquitin ligase SMURF1. Belongs to the dwarfin/SMAD family.