NMR Reveals the Synergistic Roles of Bivalent Metal Ions in Norovirus Infections

Norovirus is the most common cause of acute gastroenteritis worldwide. Murine noroviruses (MNV) are often used as model systems for human noroviruses (HuNoV). Therefore, it is important to identify common and divisive properties. Here, we compare the interactions of human and murine norovirus P-domains with bivalent metal ions. Binding of bivalent metal ions and bile acids to MNV P-domains have been shown to stabilize a contracted ("resting") as opposed to an extended ("raised") capsid conformation. This conformational change has been linked to infectivity, diarrheagenic potential, and immune escape. Likewise, the interaction of bivalent metal ions with human norovirus capsids results in contraction, suggesting a similar underlying mechanism. We used methyl TROSY NMR experiments to study the thermodynamics and kinetics of metal ion binding to P-domains, revealing a highly synergistic interaction with the bile acid glycochenodeoxycholic acid (GCDCA) for MNV. Neutralization assays support this synergistic behavior. It turns out that bivalent metal ion binding to MNV and HuNoV P-domains differs significantly. Therefore, although the transition between "raised" and "resting" capsid conformations and consequential modulation of infectivity appears to be triggered by bivalent metal ions in murine and human noroviruses, the underlying mechanisms must be different.