Ubiquitin-Like protein 5 interacts with the silencing suppressor p3 of rice stripe virus and mediates its degradation through the 26S proteasome pathway

Ubiquitin like protein 5 (UBL5) interacts with other proteins to regulate their function but differs from ubiquitin and other UBLs because it does not form covalent conjugates. Ubiquitin and most UBLs mediate the degradation of target proteins through the 26S proteasome but it is not known if UBL5 can also do that. Here we found that the UBL5s of rice andNicotiana benthamianainteracted with rice stripe virus (RSV) p3 protein. Silencing ofNbUBL5sinN.benthamianafacilitated RSV infection, whileUBL5overexpression conferred resistance to RSV in bothN.benthamianaand rice. Further analysis showed that NbUBL5.1 impaired the function of p3 as a suppressor of silencing by degrading it through the 26S proteasome. NbUBL5.1 and OsUBL5 interacted with RPN10 and RPN13, the receptors of ubiquitin in the 26S proteasome. Furthermore, silencing ofNbRPN10orNbRPN13compromised the degradation of p3 mediated by NbUBL5.1. Together, the results suggest that UBL5 mediates the degradation of RSV p3 protein through the 26S proteasome, a previously unreported plant defense strategy against RSV infection. Author summary The ubiquitin-mediated 26S proteasome has been reported to inhibit viral infection in plants. We here found that the 26S proteasome also degraded the p3 suppressor of RNA silencing of rice stripe virus to inhibit viral infection, and that the degradation was mediated by ubiquitin-like protein 5 (UBL5) but not by ubiquitin. UBL5 has been reported to modulate the proteins with which it interacts by being part of a functional complex, but not by directly mediating its degradation. Our results suggest that UBL5 may also play a role in mediating target protein degradation, a previously unreported function and one that it shares with ubiquitin. This is also the first report that UBL5 interacts with a pathogen protein and that UBL5-mediated regulation might also participate in plant defense against pathogens.