Murine norovirus allosteric escape mutants mimic gut activation

Area of Science:

• Virology
• Structural Biology
• Immunology

Background:

• Murine norovirus (MNV) exhibits significant conformational changes in its capsid (VP1 protein) in response to environmental conditions.
• The capsid's protruding (P) domain shifts from a loosely tethered state in neutral conditions to an activated state in simulated gut conditions, enhancing receptor binding and blocking antibodies.

Purpose of the Study:

• To investigate the structural basis of norovirus immune evasion by determining the cryo-electron microscopy (cryo-EM) structures of two escape mutants (V339I and D348E).
• To understand the allosteric mechanisms underlying capsid conformational changes and their impact on antibody neutralization.

Main Methods:

• Determined cryo-EM structures of MNV escape mutants V339I and D348E at neutral pH, with and without metal ions and bile salts.
• Performed dynamic simulations of the MNV P domain to analyze conformational flexibility and transition dynamics.

Main Results:

• The V339I and D348E mutants are constitutively in the activated capsid state, even without environmental triggers like metal ions or bile salts.
• Dynamic simulations indicate that C D loop movement is crucial for the conformational transition, and the V339I mutation facilitates this transition by increasing loop motion.

Conclusions:

• Allosteric escape mutants of MNV are locked in an activated conformation, explaining their resistance to neutralization.
• Understanding these conformational dynamics is key for developing norovirus vaccines that elicit broadly protective immunity against wild-type strains.