UBQLN2 is necessary for UBE3A-mediated proteasomal degradation of the domesticated retroelement PEG10

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 that ubiquilin 2 (UBQLN2) regulates the proteasomal degradation of PEG10, a unique virus-like protein that 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 further investigation of the UBQLN2-mediated degradation of PEG10. Gag-pol and gag proteins undergo distinct degradation processes; both forms bind to UBQLN2 independently of their ubiquitylation status, but only gag-pol protein is degraded in a UBQLN2-, ubiquitin- and proteasome-dependent fashion. Cellular gag-pol is ubiquitylated, and mutation of key lysine residues in the pol region rendered gag-pol insensitive to UBQLN2. Degradation of gag-pol was also dependent on the E3 ubiquitin ligase UBE3A, which requires UBQLN2 to regulate gag-pol levels. Together, these data clarify our understanding of UBQLN2-mediated degradation and highlight the importance of UBE3A in regulating PEG10.