Acts as an E3 ubiquitin-protein ligase. Promotes SMAD4 ubiquitination, nuclear exclusion and degradation via the ubiquitin proteasome pathway. According to PubMed:16751102, does not promote a decrease in the level of endogenous SMAD4. May act as a transcriptional repressor. Inhibits the transcriptional response to TGF-beta/BMP signaling cascade. Plays a role in the control of cell proliferation. Its association with SMAD2 and SMAD3 stimulates erythroid differentiation of hematopoietic stem/progenitor (By similarity). Monoubiquitinates SMAD4 and acts as an inhibitor of SMAD4-dependent TGF-beta/BMP signaling cascade (Monoubiquitination of SMAD4 hampers its ability to form a stable complex with activated SMAD2/3 resulting in inhibition of TGF-beta/BMP signaling cascade). S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Homooligomer and heterooligomer with TRIM24 and TRIM28 family members. Interacts with SMAD4 in unstimulated cells. Found in a complex with SMAD2 and SMAD3 upon addition of TGF-beta. Interacts with SMAD2 and SMAD3. Interacts with SMAD4 under basal and induced conditions and, upon TGF-beta signaling, with activated SMAD2. Forms a ternary complex with SMAD4 and SMAD2 upon TGF-beta signaling. In discrete nuclear dots resembling nuclear bodies. Expressed in stem cells at the bottom of the crypts of the colon (at protein level). Expressed in colon adenomas and adenocarcinomas (at protein level). Expressed in brain, lung, liver, spleen, thymus, prostate, kidney, testis, heart, placenta, pancreas, small intestine, ovary, colon, skeletal muscle and hematopoietic progenitors. Sumoylated with SUMO1. A chromosomal aberration involving TRIM33 is found in papillary thyroid carcinomas (PTCs). Translocation t(1;10)(p13;q11) with RET. The translocation generates the TRIM33/RET (PTC7) oncogene. Belongs to the TRIM/RBCC family. Extended N-terminus.