Specific down-regulator of the NEDD8 conjugation system. Recruits NEDD8, UBD, and their conjugates to the proteasome for degradation. Isoform 1 promotes the degradation of NEDD8 more efficiently than isoform 2. Directly interacts with NEDD8 and PSMD4/S5a, a member of the regulatory subunit of the 26S proteasome. Isoform 1 binds to NEDD8 more efficiently than isoform 2. 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. Predominantly nuclear. Widely expressed with lowest expression in the pancreas for isoform 1 and in leukocytes, liver, prostate and skeletal muscle for isoform 2. By TNF, IFNG/IFN-gamma and IFNB1/IFN-beta.