MERS-CoV ORF4b employs an unusual binding mechanism to target IMPα and block innate immunity

Area of Science:

• Virology
• Structural Biology
• Immunology

Background:

• Middle East Respiratory Syndrome Coronavirus (MERS-CoV) is a dangerous emerging virus.
• MERS-CoV accessory proteins antagonize host innate immunity.
• The MERS-CoV ORF4b protein inhibits NF-κB immune responses by binding IMPα3.

Purpose of the Study:

• To determine the high-resolution structures of MERS-CoV ORF4b bound to IMPα family members.
• To elucidate the binding mechanism and its role in immune evasion.
• To understand how ORF4b binding interferes with NF-κB signaling.

Main Methods:

• X-ray crystallography to obtain high-resolution structures of ORF4b-IMPα complexes.
• Site-directed mutagenesis to study the role of specific binding regions.
• Structural analysis of NF-κB component p50 bound to IMPα2 and IMPα3.

Main Results:

• High-resolution structures revealed similar binding mechanisms of ORF4b to IMPα2 and IMPα3, lacking a canonical P2 Lys.
• Mutations in the Nuclear Localization Signal (NLS) region altered the binding mechanism to the canonical P2 Lys.
• Mutational studies confirmed the novel binding mechanism's importance for nuclear import, IMPα interaction, and immune signaling inhibition.
• p50 binding sites on IMPα2 and IMPα3 overlap with ORF4b binding sites, suggesting a mechanism for NF-κB inhibition.

Conclusions:

• The study provides a detailed structural basis for how MERS-CoV ORF4b targets IMPα nuclear import adapters to impair host immunity.
• The findings illustrate how viral mutations can alter IMPα binding mechanisms, impacting immune evasion.
• This research offers insights into viral strategies for subverting innate immune responses.