Atomic Structure of a Non-Enveloped dsRNA Virus Reveals pH Sensing for Cell Entry

Abstract

Mechanism to monitor and response to environmental changes by non-enveloped dsRNA viruses during cell entry is largely unclear, which is in contrast to that by envelope viruses. In particular, how these viruses sense pH change inside endosomes and disrupt host membrane to gain entry into cytoplasm are not understood. To gain insight into cell entry by non-enveloped dsRNA viruses, we determined the atomic structure of the bluetongue virus (BTV) at pH 8.5, a non-enveloped dsRNA virus that undergoes a two-stage endosomal process of cell entry. Our structure reveals that the receptor-binding protein VP2 possesses a conserved zinc-finger motif which may be sensitive to low pH, and that the membrane penetration protein VP5 has three domains: dagger, unfurling and anchoring. The core of the anchoring domain not only contains a β-meander motif with a histidine cluster for low-pH sensing, but also has features potentially for membrane interactions: aromatic clusters, a hydrophobic surface and a WHXL motif. The cryoEM structure of BTV at low pH (5.5 & 3.4) reveals that the VP2 detaches from virus particles, and that the VP5 undergoes dramatic conformational change with the dagger and unfurling domains of VP5 trimer project outwards to form a long filament structure while the anchoring domain remains attached on virus inner capsid. Consistently, biochemical and structure-based mutagenesis studies support these mechanistic interpretations.