Structures of asymmetric particles of tick-borne encephalitis virus provide insight into flavivirus assembly and maturation

Full-length premembrane protein is not required for mature Zika virus particle assembly.

Assembly of flaviviruses such as Zika virus (ZIKV), dengue virus, and West Nile virus in the host cell endoplasmic reticulum is driven by the structural envelope (E) and premembrane (prM) proteins. The formation of an infectious virion requires cleavage of prM by the host furin protease during a maturation step that is dependent on a conformational change in virion structure. Here, we demonstrate that the biogenesis of flavivirus particles does not require an intact prM protein or proteolytic activation. The expression of E protein preceded by a truncated version of prM (M-E) was sufficient for the formation of noninfectious ZIKV subviral particles and pseudo-infectious reporter virions. Subviral particles encoded by a ZIKV M-E DNA vaccine elicited a neutralizing antibody response in macaques that was insensitive to the virion maturation state, a feature of flavivirus humoral immunity shown to correlate with protection. M-E vaccines that uniformly present structural features shared with mature virions offer a higher-quality and more broadly applicable approach for vaccination against flaviviruses.

Structures of Hepatitis B Virus Subviral Particles and Identification of Potential Druggable Pockets for the Discovery of Antiviral Agents.

The identification of two possible non-symmetrical small-molecule binding pockets at the dimer interface, and one in the CYL zing finger region are reported, and the druggability of these pockets, as well as unresolved issues in SVP biology, are discussed.

Zika Virus NS3 Drives the Assembly of a Replication Compartment-like Structure That Exerts the Structural and Physiological Functions of the Viral Replication Compartment

Zika virus (ZIKV) is a mosquito-transmitted orthoflavivirus that caused an epidemic in 2015–2016 in the Americas and raised serious global health concerns due to its association with congenital brain anomalies when infections occur during pregnancy. Various viruses can form compartments within the cell to facilitate viral replication and assembly, referred to as viroplasms, replication organelles, or virus factories depending on the type of virus. ZIKV assembles virus particles in virus-generated compartments adjacent to the nucleus, referred to here as a replication compartment (RC), which is formed by remodeling the host cell endoplasmic reticulum (ER). How the viral proteins control RC assembly remains unknown. Here we show that the ZIKV non-structural protein 3 (NS3), a dual-function protease and RNA helicase, is sufficient to drive the assembly of a replication compartment-like structure (RCLS) in human cells. While sufficient to generate the RCLS, NS3 is less efficient in several aspects compared to ZIKV-induced RC assembly. Nonetheless, the RCLS is similar to the ZIKV RC in its assembly at the nuclear periphery, its recruitment of ER, association with the Golgi and centrosome, and the arrangement of microtubules at its surface. Moreover, NS3 expression results in activation of the unfolded protein response (UPR), but attenuates expression of the downstream transcription factor CHOP, mirroring the manipulation of the different aspects of the UPR by ZIKV infection. We further show that the helicase domain and not the protease domain is required for optimal RCLS formation and organelle recruitment, yet each domain affects different control over the UPR. Overall, these findings advance our understanding of the mechanism of RC assembly by ZIKV, its involvement in hijacking the UPR, and the central role of NS3 in the process.

Structural Overview of Herpesvirus Tegument Proteins

Cryo-electron tomography of Nipah virus structural protein complexes in virus-like particles

This work provides novel insights into paramyxoviral protein complexes, structures, and morphology in NiV by interrogating the protein:protein interactions of the main NiV structural proteins M/N/F/G.

Plaque variant analysis of Entebbe bat virus reveals an envelope residue governing virus release from vertebrate cells.

Entebbe bat virus (ENTV) is a bat-associated orthoflavivirus with no known arthropod vector. The ENTV isolate UGA125 produces plaques of distinct sizes, designated large (L) and small (S), suggesting underlying genetic heterogeneity. While such variation may influence viral phenotypes, biological differences beyond plaque morphology remain uncharacterized. In this study, we identified single-amino-acid substitutions in the envelope (E) responsible for each phenotype. Growth curve analysis using infectious-clone-derived viruses revealed that rL (H86N) had similar kinetics relative to rWT, whereas rS (D379N) was significantly attenuated in mammalian cells. Mechanistic investigation revealed impaired viral release in rS, a defect that can be rescued by neutralizing the pH of the secretory pathway. This finding suggests that D379N may elevate the pH threshold required for the E trimer to dimer conformational rearrangement, resulting in premature prM cleavage within early secretory compartments that are insufficiently acidic to stabilize pr-E association. Sequence analysis shows that the domain III FG loop, where D379N is located, is substantially longer in mosquito-borne flaviviruses and ENTV-clade no-known-vector flaviviruses than in tick-borne flaviviruses and other no-known-vector viruses. This structural difference may explain reported differences in pH-dependent conformational reversibility among immature particles across flavivirus groups. In experimental infections using the parental isolate, the D379N was lost in both mosquitoes (Aedes aegypti and Ae. albopictus) and Jamaican fruit bats (Artibeus jamaicensis). In contrast, the H86N was lost in mosquitoes but became dominant in a bat's brain following peripheral inoculation.