Required for the function of decapentaplegic. May play an important role in mediating Dpp signaling. Involved in the BMP signaling pathway. Homotrimer (PubMed:19557331). Interacts with MAN1 (PubMed:20036230). Interacts with Sec13 and Nup93-1 (PubMed:20547758). Stage 13 embryos display expression in the dorsal epidermis (at protein level). Is detected in all developmental stages, though it appears most abundant in pupae, adult stages and early embryos. Its abundance decreases throughout embryonic and larval development and then returns to high levels in pupae and adult females. Phosphorylation on Ser-453 and/or Ser-455 is required for interaction with Smurf (PubMed:12754252, PubMed:18327897). Phosphorylation on Ser-25 by key/Nemo promotes export from nucleus and antagonizes BMP signaling (PubMed:17507407). Ubiquitinated by Smurf upon phosphorylation; which promotes proteasomal degradation. Mutants exhibit defects in midgut morphogenesis, imaginal disk development and embryonic dorsal-ventral patterning that are very reminiscent of dpp mutant phenotypes. Belongs to the dwarfin/SMAD family.