THE STRUCTURES OF MATURE AND IMMATURE ST. LOUIS ENCEPHALITIS VIRUS REVEAL CONSERVED HISTIDINE RESIDUES AFFECTING VIRUS FITNESS
Orthoflaviviruses undergo significant structural changes through maturation and infection, yet the molecular mechanisms remain incompletely understood. Using St. Louis encephalitis virus (SLEV) as a model, a reemerging mosquito-borne orthoflavivirus endemic in the Americas, we elucidated the structures of immature and mature SLEV particles at resolutions of 4.4 [A] and 3.3 [A], respectively, using cryo-EM. SLEV is characterized by glycosylated E and prM proteins, the presence of lipid pockets, and is stabilized by an intricate network of inter- and intra-protein interactions between E and (pr)M across maturation stages. Several interactions were mediated by histidines that play different roles depending on SLEV maturation and pH. Non-lethal single mutations of H285R and H443R in E protein affect SLEV replication in mammalian and mosquito cell lines, and delay death in mouse models of infection. These histidine residues are conserved across orthoflaviviruses, illustrating the complexity of orthoflavivirus particles and indicating a possible strategy for attenuation.