Mechanistic Insights into the NEDD8 Conjugation Cascade

Post‐translational modification by ubiquitin‐like proteins (ublps) such as ubiquitin, NEDD8, ISG15, and SUMO has emerged as a predominant cellular regulatory mechanism, with important roles in the immune response, development, vesicular trafficking, cell division and cancer. How is a ublp coordinated with its particular targets? These ublps are attached to their targets by parallel but distinct enzymatic cascades that sequentially involve an E1 activating enzyme, an E2 conjugating enzyme, and an E3 ligase. The E1 enzyme sits at the top of the cascade and selects its ublp and activates the ublp C‐terminus by adenylation. The E1 then forms a thioester intermediate between the E1's catalytic cysteine and the ublp's C‐terminus and transfers the ublp to an E2's catalytic cysteine. Finally, an E3 facilitates transfer of the ublp from the E2 to the target. The ubiquitin pathway involves one E1, tens of E2s, hundreds of E3s, and thousands of protein targets with in diverse biological functions. By contrast, the NEDD8 involves one E1, one E2, and few E3s, ultimately directing the ublp NEDD8 to relatively few targets. We have exploited the minimalist nature of the NEDD8 pathway to simplify the identification of protein‐protein interactions specific for a particular ublp's conjugation cascade. Recent progress toward understanding selective conjugation of NEDD8 and other ublps will be discussed.