Neddylated species are NEDD8 E1 dependent. WT and NEDP1 KO HEK 293 cells were treated with NAE inhibitor MLN4924 at 3 μM for the indicated time. Lysed cells were then processed for Western blot analysis. NEDD8 E1 inhibition results in a time-dependent decrease in the amount of Cullin and non-Cullin NEDD8 reactive bands. 