Novel insights into the structure and function of dengue virus NS1 protein

The dengue flavivirus is responsible for dengue fever. While generally mild, a proportion of cases develop into life-threatening dengue haemorrhagic fever. Flaviviruses produce a non-structural protein, NS1, that is essential to flavivirus genome replication, though it has no known catalytic function. Early in infection, dimeric NS1 localises on the endoplasmic reticulum membrane at the site of viral RNA replication and may be involved in the replication complexs formation. Later on NS1 is secreted as a hexameric lipoprotein that interacts with components of the complement-mediated immune system. In 2014, crystal structures of full-length, glycosylated, and truncated West Nile Virus and Dengue Type 2 Virus (DENV2) NS1 dimers were obtained, which have aided interpretation of low-resolution cryo-electron microscopy reconstructions of the hexameric structure of secreted NS1. This along with biochemical and NMR analyses of the NS1 lipid cargo was used to build and simulate models of the DENV2 NS1 hexamer with and without a lipid cargo. Coarse-grained molecular dynamics simulations showed that the glycosylation of NS1 is essential for hexamer stability, with the glycans compensating for the paucity of protein-protein contacts between dimers. Lipid loaded NS1 hexamers can pick up lipids from their environment and deposit lipids into membranes but are not able to pick up lipids from membranes, whereas NS1 hexamers without a lipid cargo are not effective at picking up environmental lipids. The lipid cargo forms a dumbbell shape, with polar lipids towards the outside of the bulbs and non-polar lipids in the centre of the bulbs and bar of the dumbbell. These results add to our somewhat scarce understanding of the otherwise enigmatic function of a protein whose presence is an important biomarker for dengue fever, but whose role in infection has been controversial and not well understood.