All virus infections are perpetuated by a cycle of virus particles entering host cells, hijacking cellular machinery to make new particles, which are released to continue infection in other cells. The process of assembling new virus particles requires fitting together a pattern of puzzle pieces made up of proteins and lipids that in final form will encapsulate all the virus components necessary for later infections. Disrupting this process brings infection to halt— the desired outcome for treating viruses of major public health concern, such as the Zika virus.
Mostly recently, Zika virus caused large outbreaks in the Americas between 2015-2016. This virus and other members of the Flaviviridae family (i.e. West Nile Virus, Dengue virus) cause millions of yearly cases of hemorrhagic fever in humans. In some cases, Zika virus can lead to neurological deficits via Guillain-Barré syndrome in adults, and congenital malformations in newborns. Fortunately, the structural patterns of Flavivirus particles are generally shared, giving researchers an important opportunity for drug discovery.
A collaboration led by M.D.-Ph.D. student Nadia DiNunno and bridged by the labs of Joyce Jose and Susan Hafenstein (CIDD, Penn State Biochemistry and Molecular Biology Dept., and Hershey College of Medicine) investigated previously unidentified structural features of Zika virus capsids and their role in virus particle assembly. DiNunno employed a novel approach to reconstruct images of the Zika virus taken by Cryo-EM to achieve 3.4 Å resolution. This level of detail was possible by using an approach known as subvolume reconstruction. This technique uses smaller groups of images to define localized features of molecules, which can then be combined into a full image.
The improved virus structure was crucial in defining spatial features of the three capsid proteins as they exist in the mature Zika virus particle. Doing so revealed two pockets unfilled by proteins, but rather were occupied by lipids. In one example, the structural analysis suggested such lipids were important for stabilizing the capsid structural proteins. The role of these lipids was further explored in the Joyce lab through mutagenesis experiments meant to test the function of specific features using intentionally altered virus mutants. When the Joyce lab tested virus mutants that had their lipid pockets removed, they discovered the virus was unable to complete its assembly process.
In a time when infectious diseases are at the forefront of our global, public attention it is important to recognize that such virus outbreaks are an expanding threat to public health. The fundamental research conducted in this study provides technological and biological insight on previously unknown virus features that can also be targeted for medical applications. Development of antiviral drugs that make use of conserved features within virus families, such as the assembly mechanisms of Flaviviruses, will be vital to treating and preventing infections worldwide.
NA DiNunnio, DJ Goetschius, A Narayanan, SA Majowicz, I Moustafa, CM Bator, SL Hafenstein, J Joyce
Identification of a pocket factor critical to Zika virus assembly
Journal: Nature Communications