Filoviruses—such as Ebola and Marburg viruses—can cause hemorrhagic fevers with up to 90% lethality. Not only that, but there are very few tools (vaccines or drugs) to combat them.
Now, scientists at La Jolla Institute for Immunology (LJI) are working to guide the development of new antivirals. In a new study, the researchers used cryo-electron tomography to examine Ebola virus’s molecular structure to uncover targets for broadly effective antivirals.
This work is published in Cell, in the paper, “Intracellular Ebola Virus nucleocapsid assembly revealed by in situ cryo-electron tomography.”
In a first, the study reports detailed, complete, images of the Ebola virus nucleocapsid inside infected cells. The group used cryo-electron tomography of cells transfected with viral proteins, and infected with model Ebola virus, to illuminate assembly intermediates. They obtained a 9 Å map of the complete intracellular assembly. The findings reveal that the nucleocapsid is assembled by polymerization of the nucleoprotein along the viral genome, in conjunction with the viral proteins VP24 and VP35.
The nucleocapsid’s cylindrical shape is made up of three layers; each layer plays a different role as the virus replicates in host cells. Before LJI’s imaging studies, the existence of the outer layer was unknown.
More specifically, the authors note that the findings reveal “a previously unresolved third and outer layer of NP complexed with VP35. The intrinsically disordered region, together with the C-terminal domain of this outer layer of NP, provides the constant width between intracellular nucleocapsid bundles and likely functions as a flexible tether to the viral matrix protein in the virion.”
“We found that the core protein adopts different forms in the distinct layers of the nucleocapsid to play different roles,” says Erica Ollmann Saphire, PhD, professor, president and CEO of LJI.
The investigations revealed how the proteins in these layers make contact with each other during assembly in host cells—and how the Ebola virus rearranges these proteins when nucleocapsids help form new viral particles. The authors note that, “a comparison of intracellular nucleocapsids with prior in-virion nucleocapsid structures reveals that the nucleocapsid further condenses vertically in the virion.”
All pathogenic filovirus species known so far, including Ebola and Marburg virus, share a conserved nucleocapsid structure. The group is leading further research to study nucleocapsid assembly more closely in Marburg virus.
“A universal antiviral is the dream for stopping any kind of viral disease,” Reika Watanabe, PhD, LJI staff scientist. “This study brings us a step closer to finding a universal antiviral.”
