Ubiquitin-mediated proteolysis regulates the activity of diverse receptor systems. Here, we identify Smurf2, a C2-WW-HECT domain ubiquitin ligase and show that Smurf2 associates constitutively with Smad7. Smurf2 is nuclear, but binding to Smad7 induces export and recruitment to the activated TGF beta receptor, where it causes degradation of receptors and Smad7 via proteasomal and lysosomal pathways. IFN gamma, which stimulates expression of Smad7, induces Smad7-Smurf2 complex formation and increases TGF beta receptor turnover, which is stabilized by blocking Smad7 or Smurf2 expression. Furthermore, Smad7 mutants that interfere with recruitment of Smurf2 to the receptors are compromised in their inhibitory activity. These studies thus define Smad7 as an adaptor in an E3 ubiquitin-ligase complex that targets the TGF beta receptor for degradation.

The anaphase-promoting complex (APC) is highly expressed in postmitotic neurons, but its function in the nervous system was previously unknown. We report that the inhibition of Cdh1-APC in primary neurons specifically enhanced axonal growth. Cdh1 knockdown in cerebellar slice overlay assays and in the developing rat cerebellum in vivo revealed cell-autonomous abnormalities in layer-specific growth of granule neuron axons and parallel fiber patterning. Cdh1 RNA interference in neurons was also found to override the inhibitory influence of myelin on axonal growth. Thus, Cdh1-APC appears to play a role in regulating axonal growth and patterning in the developing brain that may also limit the growth of injured axons in the adult brain.

The receptor-regulated Smad proteins are essential intracellular mediators of signal transduction by the transforming growth factor-beta (TGF-beta) superfamily of growth factors and are also important as regulators of gene transcription. Here we describe a new role for TGF-beta-regulated Smad2 and Smad3 as components of a ubiquitin ligase complex. We show that in the presence of TGF-beta signalling, Smad2 interacts through its proline-rich PPXY motif with the tryptophan-rich WW domains of Smurf2, a recently identified E3 ubiquitin ligases. TGF-beta also induces the association of Smurf2 with the transcriptional co-repressor SnoN and we show that Smad2 can function to mediate this interaction. This allows Smurf2 HECT domain to target SnoN for ubiquitin-mediated degradation by the proteasome. Thus, stimulation by TGF-beta can induce the assembly of a Smad2-Smurf2 ubiquitin ligase complex that functions to target substrates for degradation.