New classes of E3 ligases illuminated by chemical probes

Specificity in the ubiquitin system depends on E3 ligases, largely belonging to a handful of families discovered more than a decade ago. However, the last two years brought a quantum leap in the identification and/or mechanistic characterization of eukaryotic ubiquitin ligases, in part through implementation of activity-based chemical probes and cryo-EM. Here, we survey recent discoveries of RING-Cys-Relay, RZ-finger, and neddylated cullin-RING-ARIH RBR E3-E3 ubiquitin ligase mechanisms. These ligases transfer ubiquitin through unprecedented mechanisms-via novel catalytic domains or domain combinations-and collectively modify unconventional amino acids, non-proteinaceous bacterial lipid targets, and structurally-diverse substrates recruited to numerous cullin-RING ligases. We anticipate major expansion of the types, features, and mechanisms of E3 ligases will emerge from such chemical and structural approaches in the coming years.