Flavivirus NS1: Structure and Function of an Enigmatic Virulence Factor

Abstract

Viruses employ a variety of molecular strategies to carve out an existence in their host and to thwart host defenses. Crystal structures of viral proteins and of the host proteins deployed as molecular weapons contribute enormously to our understanding of viral pathogenesis and our efforts to combat viral infection. Mosquito‐borne flaviviruses, including the dengue, Zika, yellow fever, Japanese encephalitis and West Nile viruses, cause serious human diseases in much of the world. A growing number of diverse functions has been discovered for the enigmatic virulence factor known as NS1: it is an essential cofactor in viral genome replication, a mediator of the host immune response, and a trigger of host vascular leakage. NS1 lacks precedent in the structure and sequence databases, so an accurate 3D structure was key to understanding the basis for its various functions. A high‐throughput survey of expression conditions in baculovirus‐infected insect cells was essential to identifying conditions for production of recombinant NS1 in its natural glycosylated, disulfide‐linked form. We solved a crystal structure of West Nile virus NS1 from the anomalous scattering of the native sulfur atoms using an 18‐crystal data set [1,2]. The structure revealed a previously unknown protein fold with a fundamentally dimeric architecture, and led to assigned functions for two of the three NS1 structural domains. Subsequent structures of NS1 from dengue virus and Zika viruses established the basis for its membrane association in cells and its lipid encapsidation when secreted [3]. Recently discovered functions of this remarkable virulence factor emerged in follow‐up studies that built upon the crystal structures. Prime among these is the discovery that the NS1 protein, in a mouse model of dengue disease and in absence of virus, can induce the vascular leak that is a hallmark of severe dengue disease [4,5]. Clues to the molecular mechanism of NS1‐induced endothelial dysfunction emerged from a study of the protection afforded by a monoclonal antibody and the structure of the Fab fragment bound to the NS1 epitope [6].[1] Akey DL et al. & Smith JL (2014) Science 343, 881‐885.[2] Akey DL et al. & Smith JL (2014) Acta Crystallogr D70, 2719‐2729.[3] Brown WC et al. & Smith JL (2016) Nature Struct Molec Biol 23, 856‐867.[4] Beatty PR et al. & Harris E (2015) Science Trans Med 7, 304ra141.[5] Modhiran N et al. & Young PR (2015) Science Trans Med 7, 304ra142.[6] Biering SB, Akey DL et al. Smith JL & Harris E (2021) Science 371, 194‐200.