X-Ray Crystallography Reveals the Two Faces of Flavivirus Nonstructural Protein

High-resolution X-ray crystallography and electron microscopy have revealed that the Flavivirus NS1 (nonstructural protein 1) has two distinct faces, one that interacts with the interior of the infected host cell and the other that is exposed to antiviral elements in the host's immune system.

Flaviviruses are responsible for several severe diseases of humans including dengue fever, West Nile fever, tick-borne encephalitis, and yellow fever. All the flaviviruses produce nonstructural protein 1 (NS1), which functions in genome replication as an intracellular dimer and in immune system evasion as a secreted hexamer. In general, nonstructural proteins are encoded by the viral genome and are produced in the organisms they infect, but are not packaged into the virus particles. Some of these proteins may play roles within the infected cell during virus replication or act in regulation of virus replication or virus assembly.

Investigators at the University of Michigan (Ann Arbor, USA) and colleagues at Purdue University (Lafayette, IN, USA) chose X-ray crystallography as an approach to developing a better understanding of how NS1 functions.

The investigators isolated and crystallized NS1 from Dengue virus and West Nile virus. They then created a three-dimensional atomic structure map of the protein crystals based on data obtained from X-ray crystallography carried out at the Advanced Photon Source at the Argonne National Laboratory (Illinois, USA). Subsequently, electron microscopy was used to elucidate how NS1 associated with membranes of infected cells.

The investigators reported the crystal structures for full-length, glycosylated NS1 from West Nile and dengue viruses in the February 6, 2014, online edition of the journal Science. Their results revealed that the NS1 hexamer in crystal structures was similar to a solution hexamer visualized by single-particle electron microscopy. Recombinant NS1 bound to lipid bilayers and remodeled large liposomes into lipoprotein nanoparticles. The NS1 structures revealed distinct domains for membrane association of the dimer and interactions with the immune system, and will form a basis for elucidating the molecular mechanism of NS1 function.

"Isolating the protein in order to study it has been a challenge for researchers," said senior author Dr. Janet Smith, professor of biological chemistry at the University of Michigan. "Once we discovered how to do that, it crystallized beautifully. Seeing the design of this key protein provides a target for a potential vaccine or even a therapeutic drug."

"The two faces of NS1 define the regions responsible for its two major functions," said Dr. Smith. "This understanding will guide future research into dissecting and targeting these regions in disease treatment or prevention. We are now collaborating with the Purdue virologists to understand exactly how the two faces of NS1 help the virus survive and thrive in patients. These studies are the next steps toward a vaccine or an antiviral drug."

Related Links:
University of Michigan
Purdue University
Argonne National Laboratory