Mutational analysis of human norovirus VP2 elucidates critical molecular interactions for virus assembly

Human noroviruses (HuNoVs) are the leading cause of epidemic and sporadic gastroenteritis in all age groups worldwide. Yet, we lack vaccines or therapeutics for HuNoVs. Knowledge of the fundamental mechanisms governing HuNoV particle assembly is limited. Modern structural techniques have not resolved the complete structure of pandemic GII.4 norovirus or the localization of interior capsid proteins VP2 and VPg. Furthermore, VP2's functional roles during infection remain obscure. Studies of feline and murine caliciviruses show that VP2 may help deliver the viral genome into cells, suggesting that VP2 synergizes with VP1 and VPg. We identified a motif within the N-terminal α-helical domain of VP2, adjacent to a unique insertion site, that is essential for interaction with the major capsid protein VP1. We show VP2 uniquely interacts with the translation initiation protein VPg via its disordered C-terminus. These findings reveal principles of HuNoV capsid protein interactions and highlight VP2 as a bridge facilitating capsid assembly.