Structural Asymmetry of Flaviviruses
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Flaviviruses are enveloped, positive-strand RNA viruses that
are spread by mosquitoes and ticks and can cause serious disease in humans.
Flavivirus virions undergo extensive structural changes during their life
cycle, including during maturation and fusion. Flaviviruses are initially
assembled at the endoplasmic reticulum in a non-infectious, immature state, and
then traffic to the trans-Golgi network, where a pH drop triggers a structural
rearrangement of glycoproteins prM and E on the virus surface from 60 trimers
to 90 dimers. A host protease, furin, then cleaves prM which makes the
transition irreversible. Upon exiting the host cell, pr disassociates from the
virus and the infectious, mature virus is able to enter a new cell.
In Chapter 1, an overview of flaviviruses is presented, including a brief history of their discovery and interaction with humans, followed by what is known about their life cycle and the maturation process. The structure of a mature flavivirus is then described, including the symmetrical arrangement of glycoproteins on the virion surface, the lipid membrane, and the nucleocapsid core, followed by an introduction of the structural proteins that assemble into the virion. The structure of the immature flavivirus is then described. The chapter concludes with a description of the dynamics and heterogeneity observed for flaviviruses.
The conformational rearrangements that occur during flavivirus maturation remain unclear. The structures of immature and mature flaviviruses determined with cryo-electron microscopy (cryo-EM) demonstrated that flaviviruses are icosahedral particles with 180 copies of glycoproteins on their surface. Icosahedral viruses typically have a quasi-equivalent arrangement of glycoproteins, but flaviviruses lack quasi-equivalence and instead the three subunits within an asymmetric unit occupy different chemical environments. Although the subunits are the same proteins, the unique environment of each subunit can be exploited for tracking subunits during conformational rearrangements. For example, the unique labeling of a subunit can be used to identify it in the immature and mature virion.
In Chapter 2, the maturation process was studied by
developing tools to differentially label protein subunits and trap potential
intermediates of maturation. The tools included heavy-atom compounds and
antibody Fabs, which were used to probe Kunjin virus (KUNV), an Australian
subtype of West Nile virus (WNV). One heavy-atom compound, potassium
tetranitroplatinate(II), was found to derivatize immature KUNV, likely at sites
on both E and prM. Higher-resolution studies will be required to determine if
the compound differentially labeled the three subunits. The other tool
developed was the E16 Fab. E16 Fab, originally isolated from a mouse immunized
with WNV E and found to bind to two out of three subunits on mature WNV, was
used to differentially label subunits in immature KUNV. Based on poor epitope
accessibility on immature KUNV, E16 Fab was hypothesized to trap an
intermediate state of maturation. In the cryo-EM reconstruction of E16 Fab
bound to immature KUNV it was found that the virion had localized distorted density
and apparent non-uniform binding of the E16 Fab. Based on this result it was
proposed that flaviviruses had imperfect icosahedral symmetry.
The structural asymmetry of immature and mature flaviviruses was investigated in Chapter 3. Icosahedral symmetry has always been imposed during cryo-EM reconstructions of flaviviruses, as it led to stable convergence of orientations. When reconstructions of immature KUNV and ZIKV were performed without imposing symmetry, the reconstructions showed that the flaviviruses had an eccentric nucleocapsid core, which was positioned closer to the membrane at one pole. At the opposite pole, the glycoprotein and inner leaflet densities were weak and distorted. Furthermore, there were protrusions from the core that contacted the transmembrane helices of the glycoproteins. In the asymmetric reconstruction of mature KUNV, the core was positioned concentric with the glycoprotein shell, in contrast to the immature virion, indicating that maturation alters the interactions between the core and the glycoproteins. The asymmetric reconstructions suggested that there is variable contact between the core and glycoproteins during assembly, which may be due to membrane curvature restrictions in the budding process.
In Chapter 4, extracellular vesicles (EVs) that were released during dengue virus (DENV) infection were characterized by mass spectrometry. EVs may play a significant role in flavivirus infection, as they have been shown to transport both viral proteins and infectious RNA. EVs likely represent alternative modes of virus transmission and aid in immune evasion. However, previous studies on EVs are controversial because EVs are potential contaminated during assays by co-purifying virions and other particulates. The identification of EV biomarkers would greatly reduce contamination because biomarkers would enable isolation of pure EVs by affinity purification. Therefore, a strategy was developed to isolate EVs and profile them with proteomics. The four proteins cystatin-A, filamin B, fibrinogen beta chain, and endothelin converting enzyme 1 were found to be statistically enriched in the DENV sample and represent potential EV biomarkers.