Structural studies reveal that endosomal cations promote formation of infectious CVA9 A particles, facilitating RNA and VP4 release.

Abstract

Coxsackievirus A9, an enterovirus, is a common cause of paediatric aseptic meningitis and neonatal sepsis. During cell entry, enterovirus capsids undergo conformational changes leading to expansion, formation of large pores, externalization of VP1 N-termini and loss of the lipid factor from VP1. Factors such as receptor binding, heat, and acidic pH can trigger capsid expansion in some enteroviruses. Here we show that fatty-acid free bovine serum albumin or neutral endosomal ionic conditions can independently prime CVA9 for expansion and genome release. Our results show that CVA9 treatment with albumin or endosomal ions generates a heterogeneous population of virions, which could be physically separated by asymmetric flow field flow fractionation and computationally by cryo-EM and image processing. We report cryo-EM structures of CVA9 A-particles obtained by albumin or endosomal ion treatment and a control non-expanded virion to 3.5, 3.3 and 2.9 [A] resolutions, respectively. Where albumin promotes stabile expanded virions, the endosomal ionic concentrations induce unstable CVA9 virions which easily disintegrate losing their genome. Loss of most of the VP4 molecules and exposure of negatively-charged amino acid residues in the capsids interior after expansion, create a repulsive viral RNA-capsid interface, aiding genome release. ImportanceCoxsackievirus A9 (CVA9) is a common cause of meningitis and neonatal sepsis. The triggers and mode of action of RNA release into the cell unusually do not require receptor interaction. Rather, a slow process in the endosome, independent of low pH is required. Here, we show by biophysical separation, cryogenic electron microscopy and image reconstruction that albumin and buffers mimicking the endosomal ion composition can separately and together expand and prime CVA9 for uncoating. Furthermore, we show in these expanded particles that VP4 is present at only ~10% of the occupancy found in the virion, VP1 is externalised and the genome is repelled by the negatively-charged, repulsive inner surface of the capsid that occurs due to the expansion. Thus, we can now link observations from cell biology of infection with the physical processes that occur in the capsid to promote genome uncoating.