NEDD8 is a ubiquitin-like protein that controls vital biological events through its conjugation to cullin family members. Recently, we identified a negative regulator of the NEDD8 conjugation system, NUB1, which interacts with NEDD8 and down-regulates NEDD8 expression post-transcriptionally (Kito, K., Yeh, E. T. H., and Kamitani, T. (2001) J. Biol. Chem. 276, 20603-20609). Here, we show that NUB1 possesses a ubiquitin-like domain at the N-terminal region and binds to S5a of the 19 S proteasome activator (PA700). A GST pull-down assay revealed that the overexpression of NUB1 leads to a greater precipitation of NEDD8 conjugates with GST-S5a, suggesting that NUB1 might have an adaptor function between S5a and NEDD8. Furthermore, proteasome inhibitors completely block NUB1-mediated down-regulation of NEDD8 expression. These results suggest that NUB1 recruits NEDD8 and its conjugates to the proteasome for degradation, providing a direct functional link between the NEDD8 conjugation system and the proteasomal degradation pathway.