UBQLN2 facilitates degradation of the retrotransposon protein PEG10 via UBE3A activity
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Ubiquilins are a family of extrinsic ubiquitin receptors that are thought to facilitate protein degradation by shuttling proteins to the proteasome. However, the defining characteristics of Ubiquilin clients, and the steps of Ubiquilin-mediated degradation, have been elusive. Previously, we showed Ubiquilin 2 (UBQLN2) regulates the proteasomal degradation of PEG10, a unique virus-like protein which comes in two forms: a gag protein which is not regulated by UBQLN2, and a gag-pol protein which is dependent on UBQLN2. Here, we refine the model of Ubiquilin activity through the UBQLN2-mediated degradation of PEG10. UBQLN2 binding did not ensure degradation, and was independent of client ubiquitination, though ubiquitination of key lysine residues was necessary for gag-pol proteolysis. Ubiquitination was dependent on the E3 ubiquitin ligase UBE3A, which was surprisingly unable to regulate gag-pol in the absence of UBQLN2. Together, we have established a stepwise model of UBQLN2-mediated degradation that represents a new perspective on Ubiquilin function.
Abstract Figure
Graphical abstract: Working model of UBQLN2 and UBE3A-mediated degradation of PEG10 gag-pol. In an apparent first step, UBQLN2 binds both PEG10 gag and gag-pol through STI1:gag-dominated interactions, independent of PEG10 ubiquitination. Either concurrently or immediately thereafter, UBQLN2 binds to UBE3A through interactions facilitated by UBA:AZUL domain binding, though other protein domains contribute to this interaction as well. UBE3A and other unknown E3 ligases contribute to the ubiquitination of gag-pol on lysine residues of both the gag and pol regions, which is necessary for proteasomal degradation. The proteasome also interacts with UBQLN2:UBE3A:PEG10, likely through UBL domain interactions with the regulatory cap. Inhibition of E1 ubiquitin activation through TAK243, UBE3A activity with siRNA, or proteasomal degradation with Bortezomib all interfere with UBQLN2-mediated PEG10 degradation.
