# Cryo-electron tomography reveals coupled flavivirus replication, budding and maturation

## TL;DR

This study uses cryo-electron tomography to show how tick-borne flaviviruses coordinate replication, budding, and maturation within infected human cells.

## Contribution

The study reveals the spatial coupling of flavivirus replication, budding, and maturation using cryo-electron tomography.

## Key findings

- The RO membrane bud is shaped by curvature-establishing modifications and intraluminal RNA pressure.
- Immature virus particles bud from a protein complex at the RO base.
- Furin site variants determine virus maturation near replication organelles.

## Abstract

Flaviviruses replicate their genomes in replication organelles (ROs) formed as bud-like invaginations on the endoplasmic reticulum membrane, which also functions as the site for virion assembly. While this localization is well established, it is not known to what extent viral membrane remodeling, genome replication, virion assembly, and maturation are coordinated. Here, we image tick-borne flavivirus replication in human cells using cryo-electron tomography. We find that the RO membrane bud is shaped by a combination of a curvature-establishing membrane modification and the pressure from intraluminal template RNA. A protein complex at the RO base extends to an adjacent membrane, where immature virus particles bud. Naturally occurring furin site variants determine whether virus particles mature in the immediate vicinity of ROs. We further visualize replication in mouse brain tissue by cryo-electron tomography. Taken together, these findings reveal a close spatial coupling of flavivirus genome replication, budding, and maturation.

In this study, Dahmane et al use a method called cryo-electron tomography to uncover new details of how tick-borne flaviviruses transform cells into virus factories.