Structural and catalytic diversity of coronavirus proofreading exoribonuclease.

Summary

Comparative cryo-EM and biochemical analyses show that MERS-CoV ExoN has markedly lower catalytic activity than SARS-CoV-2 ExoN. The first ExoN structures outside sarbecoviruses reveal conserved determinants that govern 3'-end nucleotide excision, informing RNA proofreading and immune evasion mechanisms across coronaviruses.

Key Findings

• MERS-CoV ExoN exhibits substantially lower catalytic activity than SARS-CoV-2 ExoN in biochemical assays.
• Cryo-EM structures of MERS-CoV ExoN–RNA complexes (first outside sarbecoviruses) reveal the molecular basis of catalytic divergence.
• Two conserved structural determinants dictate efficient 3' nucleotide excision, underpinning proofreading and immune evasion.

Clinical Implications

Guides rational antiviral design by targeting ExoN or optimizing nucleos(t)ide analogs to evade ExoN excision, with potential to improve treatment durability against diverse coronaviruses.

Limitations

• Findings derived from in vitro structural/biochemical systems without in vivo validation of fitness effects
• Comparisons limited to two representative human coronaviruses; broader lineage coverage pending

Future Directions

Test ExoN inhibitors and excision-resistant analogs across coronavirus lineages; evaluate in vivo impacts on mutation rates, pathogenesis, and antiviral resistance.