Structural insights into Ubr1-mediated N-degron polyubiquitination

Area of Science:

• Biochemistry and Molecular Biology
• Proteasome-mediated protein degradation
• Ubiquitination pathways

Background:

• The N-degron pathway targets proteins with destabilizing N-terminal residues for proteasomal degradation.
• In yeast, the single-subunit E3 ligase Ubr1 governs the Arg/N-degron pathway.
• The precise mechanism of Ubr1 in initiating and elongating ubiquitin chains via Ubc2 remains elusive.

Purpose of the Study:

• To elucidate the mechanistic basis of Ubr1-mediated ubiquitination initiation and elongation.
• To characterize the structural determinants of Ubr1's function in the Arg/N-degron pathway.
• To provide insights into linkage-specific ubiquitination catalyzed by a single E3 ligase.

Main Methods:

• Development of chemical strategies to mimic ubiquitin transfer reaction intermediates.
• Cryo-electron microscopy (cryo-EM) to determine structures of Ubr1-Ubc2-ubiquitin complexes.
• Structural analysis of complexes representing ubiquitination initiation and elongation steps.

Main Results:

• Cryo-EM structures revealed Ubr1 in complex with Ubc2, ubiquitin, and N-degron peptides.
• Identification and characterization of key structural elements: a Ubc2-binding region and an acceptor ubiquitin-binding loop on Ubr1.
• These elements are crucial for mediating both the initiation and elongation phases of ubiquitination.

Conclusions:

• The study provides unprecedented structural insights into Ubr1's mechanism of action.
• Key Ubr1 structural features dictate the initiation and elongation of ubiquitin chains.
• These findings advance our understanding of ubiquitin chain linkage specificity in protein degradation.