A combinatorial ubiquitin code degrades substrates protected by deubiquitylation
Reversibility of protein ubiquitylation play essential roles in cellular protein homeostasis. How substrates stabilized by deubiquitylation are directed for degradation remains largely elusive. Here, we show that the branched ubiquitin chains promote the degradation of the deubiquitylase (DUB) OTUD5. OTUD5 is sequentially modified by TRIP12 and UBR5, E3s specific for the K29 and K48 linkages, respectively, resulting in the conjugation of K29/K48 branched ubiquitin chains. The TRIP12-OTUD5 antagonism regulates TNF-?-induced NF-?B signaling. Mechanistically, while OTUD5 readily cleaves K48-linkages, K29-linkages are resistant against OTUD5 activity. Consequently, K29-linkages overcome OTUD5 DUB activity to facilitate UBR5-dependent K48-linked chain branching. Regarding generality, this mechanism adopts to other TRIP12 substrates associated with OTUD5. These results uncover a cellular unique strategy in which the sequential addition of DUB-resistant and proteasome-targeting ubiquitin linkages efficiently promote degradation of substrates protected by deubiquitylation, underscoring the role of branched ubiquitin chains in the quality control of hard-to-degrade substrates. Overall design: HT1080 cells were transfected with the siRNAs (scrambled, siTRIP12, siOTUD5, and siTRIP12+siOTUD5 (double KD), and total RNAs were isolated.