Structural insights into Ubr1-mediated N-degron polyubiquitination

The N-degron pathway targets proteins that bear a destabilizing residue at the N terminus for proteasome-dependent degradation(1). In yeast, Ubr1-a single-subunit E3 ligase-is responsible for the Arg/N-degron pathway(2). How Ubr1 mediates the initiation of ubiquitination and the elongation of the ubiquitin chain in a linkage-specific manner through a single E2 ubiquitin-conjugating enzyme (Ubc2) remains unknown. Here we developed chemical strategies to mimic the reaction intermediates of the first and second ubiquitin transfer steps, and determined the cryo-electron microscopy structures of Ubr1 in complex with Ubc2, ubiquitin and two N-degron peptides, representing the initiation and elongation steps of ubiquitination. Key structural elements, including a Ubc2-binding region and an acceptor ubiquitin-binding loop on Ubr1, were identified and characterized. These structures provide mechanistic insights into the initiation and elongation of ubiquitination catalysed by Ubr1.

Automated summary

This study reveals the mechanism of the N-degron pathway mediated by Ubr1, including key structural elements involved in the initiation and elongation steps of ubiquitination.