The Ubiquitin Proteasome System

Katherine H. Reiter, Rachel E. Klevit, Reiter, Katherine H., Klevit, Rachel E.

Protein ubiquitination is an essential posttranslational modification that regulates nearly all cellular processes. E3 ligases catalyze the final transfer of ubiquitin (Ub) onto substrates and thus are important temporal regulators of ubiquitin modifications in the cell. E3s are classified by their distinct transfer mechanisms. RING E3s act as scaffolds to facilitate the transfer of Ub from E2-conjugating enzymes directly onto substrates, while HECT E3s form an E3~Ub thioester intermediate prior to Ub transfer. A third class, RING-Between-RING (RBR) E3s, are classified as RING/HECT hybrids based on their ability to engage the E2~Ub conjugate via a RING1 domain while subsequently forming an obligate E3~Ub intermediate prior to substrate modification. RBRs comprise the smallest class of E3s, consisting of only 14 family members in humans, yet their dysfunction has been associated with neurodegenerative diseases, susceptibility to infection, inflammation, and cancer. Additionally, their activity is suppressed by auto-inhibitory domains that block their catalytic activity, suggesting their regulation has important cellular consequences. Here, we identify technical hurdles faced in studying RBR E3s and provide protocols and guidelines to overcome these challenges.