Endosomal ionic balance and its role in bunyavirus entry

The Bunyavirales are the largest group of negative-sense RNA viruses with new members emerging due to changes in virus/host relationships and segment reassortment. The Bunyavirales are of interest as they require host-cell potassium (K+) channels during the early stages of their infectious lifecycle. The mechanism(s) governing this dependence had not been previously defined. In Chapter 1, using the prototype member of the family, Bunyamwera virus (BUNV), the role of K+ channels during virus entry was investigated. It was shown that BUNV encounters high-K+ containing endosomes during virus entry which is controlled by endosome-resident K+ channels. The inhibition of these channels disrupted endosomal K+ uptake and prevented viruses escaping the endosomal system. Mimicking the ionic environment of late endosomes in vitro (pH 6.3/K+) expedited BUNV entry and reduced its susceptibility to K+ channel inhibition. This highlighted endosomal K+ as a biochemical cue for BUNV, explaining its requirement for host-cell K+ channels. In Chapter 2, the effects of pH/K+ priming on virion architecture were investigated using cryo-electron tomography and sub-tomogram averaging (STA). BUNV glycoprotein (GP) spike averages identified key definitions in the GP trimer that became disordered in response to pH/K+ priming. STA indicated uncoupling of the GP trimers in response to K+, likely facilitating a pre-fusion intermediate that exposes the fusion loop. This begins to explain the changes triggered by endocytic pH/K+ to expedite infection. In Chapter 3, through K+ channel silencing it was identified that the two-pore K+ channel TWIK2 was necessary for BUNV infection. TWIK2 localized to endo/lysosomal compartments through which BUNV traversed during infection. This inferred a role for TWIK2 during BUNV entry and revealed this channel as a new anti-BUNV target. The culmination of these findings reveal for the first time, the basis for why inhibiting K+ channels impedes BUNV.