A high resolution view of an adolescent flavivirus

Abstract

Mosquito-transmitted flaviviruses, such as Dengue virus (DENV) or Zika virus (ZIKV), are responsible for significant economic damage and human misery. In infected cells, flaviviruses first assemble into an immature form within the endoplasmatic reticulum (ER), and then undergo further processing by furin protease in the trans-Golgi. Despite substantial efforts, previous cryogenic electron microscopy (cryo-EM) studies of immature flaviviruses were restricted to low to medium resolutions, limiting our understanding of maturation. To better grasp the process of maturation, we have carried out cryo-EM reconstructions of immature Spondweni virus (SPOV), an emerging human flavivirus belonging to the same serogroup as ZIKV (~75% amino acid identity). By combining localized reconstruction and focused refinement, we were able to improve the resolution to 3.8 Å, yielding unprecedented insight into the immature form. The structure elucidates how, at neutral pH, polar interactions conceal the furin recognition site within trimeric envelope (E) protein spikes. Furthermore, we identify how a strictly conserved pH sensor anchors the precursor membrane (prM) protein to immature E. We reconstructed mature forms of SPONV and DENV to 2.6Å and 3.1Å, respectively. Comparison with immature virus shows a conserved binding pocket for a lipid headgroup, which forms as a consequence of the rearrangement of amphipathic stem-helices of E. We propose a structural role for the pocket and suggest it stabilizes mature E. Taken together, our data suggest a compelling rationale for low-pH triggered conformational rearrangement in the Golgi, which occurs during flavivirus maturation. especially D. Clare, for assistance with data collection and advice on data processing. We also thank L. Mendonça, T. Frosio, and members of the P. Zhang group for scientific discussions.

Molecular basis of the pH-controlled maturation of the tick-borne encephalitis flavivirus

“…[19][20][21] Interestingly, the association of the sE dimer (sE2) with two smaller protomer units (pr) has been pointed out as the strategy developed by the virus to avoid premature fusion despite the low pH of the TGN. [22][23][24][25] In particular at acidic pH, the tetrameric (pr/sE)2 form remains stable, while sE2 dissociates into the two constituent monomers. [18] Analysis of the crystal structures have pinpointed the main interactions between pr and sE, which might drive the stabilization of the (pr/sE)2 tetramer.…”

Effect of immature tick-borne encephalitis virus particles on antiviral activity of 5-aminoisoxazole-3-carboxylic acid adamantylmethyl esters

“…Their structures were described only for mosquito-borne flaviviruses, such as West Nile virus (WNV), dengue virus (DENV), Zika virus (ZIKV), and yellow fever virus (YFV), mostly on the basis of low-resolution (9-25 Å) cryo-EM studies [16][17][18]. Structures closer to atomic resolution are avalable for immature DENV [16] and Spondweni virus [19]: 6 and 7.8 Å, respectively. However, for TBEV such data are not yet available.…”