Cryo-EM structure of the chain-elongating E3 ligase UBR5
UBR5 is a nuclear E3 ligase that ubiquitinates a vast range of substrates for proteasomal degradation. This HECT E3 ligase has recently been identified as an important regulator of oncogenes, e.g., MYC, but little is known about its structure or mechanisms of substrate engagement and ubiquitination. Here, we present the cryo-EM structure of the human UBR5, revealing a building block of an antiparallel dimer which can further assemble into larger oligomers. The large helical scaffold of the dimer is decorated with numerous protein-interacting motifs for substrate engagement. Using cryo-EM processing tools, we observe the dynamic nature of the domain movements of UBR5, which allows the catalytic HECT domain to reach engaged substrates. We characterise the proteasomal nuclear import factor AKIRIN2 as an interacting protein and propose UBR5 as an efficient ubiquitin chain elongator. This preference for ubiquitinated substrates permits UBR5 to function in several different signalling pathways and cancers. Together, our data expand on the limited knowledge of the structure and function of HECT E3s.