Cryo‐EM structure of the chain‐elongating E3 ubiquitin ligase UBR5.

UBR5 is a nuclear E3 ligase that ubiquitinates a vast range of substrates for proteasomal degradation. This HECT domain‐containing ubiquitin 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 human UBR5, revealing an α‐solenoid scaffold with numerous protein–protein interacting motifs, assembled into an antiparallel dimer that adopts further oligomeric states. Using cryo‐EM processing tools, we observe the dynamic nature of the UBR5 catalytic domain, which we postulate is important for its enzymatic activity. 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 and several distinct domains for protein–protein interactions may explain how UBR5 is linked to several different signalling pathways and cancers. Together, our data expand on the limited knowledge of the structure and function of HECT E3 ligases. Synopsis: Structure and mechanism of the oncogene‐regulating HECT E3 ubiquitin ligase UBR5 have remained elusive. Here, cryo‐EM data show that it assembles into dimers and tetramers, and efficiently extends ubiquitin chains on primed substrates, such as AKIRIN2.UBR5 dimers and tetramers assemble through distinct interfaces.Proteasome nuclear import factor AKIRIN2 binds UBR5.UBR5 is a potent chain‐elongating E3 ligase when presented with a primed substrate.The HECT domain of UBR5 displays conformational flexibility. [ABSTRACT FROM AUTHOR]