The WW1 Domain Enhances Autoinhibition in Smurf Ubiquitin Ligases.

Downregulation of ubiquitin (Ub) ligase activity prevents premature ubiquitination and is critical for cellular homeostasis. Nedd4 Ub ligases share a common domain architecture and yet are regulated in distinct ways through interactions of the catalytic HECT domain with the N-terminal C2 domain or the central WW domain region. Smurf1 and Smurf2 are two highly related Nedd4 ligases with ~70% overall sequence identity. Here, we show that the Smurf1 C2 domain interacts with the HECT domain and inhibits ligase activity in trans. However, in contrast to Smurf2, we find that full-length Smurf1 is a highly active Ub ligase, and we can attribute this striking difference in regulation to the lack of one WW domain (WW1) in Smurf1. Using NMR spectroscopy and biochemical assays, we identified the WW1 region as an additional inhibitory element in Smurf2 that cooperates with the C2 domain to enhance HECT domain binding and Smurf2 inhibition. Our work provides important insights into Smurf regulation and highlights that the activities of highly related proteins can be controlled in distinct ways.

Author Correction: The structure of the ubiquitin-like modifier FAT10 reveals an alternative targeting mechanism for proteasomal degradation.

The original version of the Supplementary Information associated with this Article inadvertently omitted Supplementary Table 3. The HTML version of the Article has been updated to include a corrected version of the Supplementary Information.

The structure of the ubiquitin-like modifier FAT10 reveals an alternative targeting mechanism for proteasomal degradation.

FAT10 is a ubiquitin-like modifier that directly targets proteins for proteasomal degradation. Here, we report the high-resolution structures of the two individual ubiquitin-like domains (UBD) of FAT10 that are joined by a flexible linker. While the UBDs of FAT10 show the typical ubiquitin-fold, their surfaces are entirely different from each other and from ubiquitin explaining their unique binding specificities. Deletion of the linker abrogates FAT10-conjugation while its mutation blocks auto-FAT10ylation of the FAT10-conjugating enzyme USE1 but not bulk conjugate formation. FAT10- but not ubiquitin-mediated degradation is independent of the segregase VCP/p97 in the presence but not the absence of FAT10's unstructured N-terminal heptapeptide. Stabilization of the FAT10 UBDs strongly decelerates degradation suggesting that the intrinsic instability of FAT10 together with its disordered N-terminus enables the rapid, joint degradation of FAT10 and its substrates without the need for FAT10 de-conjugation and partial substrate unfolding.