Degradation of TGF-β receptors. TGF-β receptors can be degraded through both ubiquitination-dependent and -independent ways. After ubiquitination, both TGF-β type I (TβRI) and type II (TβRII) receptors can be degraded via proteasome or lysosome. Although it is unclear how TβRII ubiquitination/degradation is regulated, Smad7 and Dapper2 are important adaptor proteins for ubiquitin E3 ligases (indicated in the green box)-mediated TβRI ubiquitination and degradation. Smad7-mediated ubiquitination/degradation of TβRI is finely controlled by Smad7-binding proteins indicated in the pink box.