Specific down-regulator of the NEDD8 conjugation system. Recruits NEDD8, UBD, and their conjugates to the proteasome for degradation (By similarity). Directly interacts with NEDD8 and PSMD4/S5a, a member of the regulatory subunit of the 26S proteasome. Interacts with AIPL1. The interaction with UBD via UBA domains facilitates the linking of UBD-conjugated target protein to the proteasome complex and accelerates UBD degradation and that of its conjugates (By similarity). Predominantly nuclear. Strongest expression at 7 dpc. Marked decrease 11, 15 and 17 dpc. By interferon.