Wang, C.-Y. J. | bioRxiv Science

bioRxiv2021

Mutations at the Alphavirus E2/E1 inter-dimer interface have host-specific phenotypes

Alphaviruses are enveloped viruses transmitted by arthropod vectors to vertebrate hosts. The surface of the virion contains 80 glycoprotein spikes embedded in the membrane and these spikes mediate attachment to the host cell and initiate viral fusion. Each spike consists of a trimer of E2-E1 heterodimers. These heterodimers interact at two interfaces: (1) the intra-dimer interactions between E2 and E1 of the same heterodimer, and (2) the inter-dimer interactions between E2 of one heterodimer and E1 of the adjacent heterodimer (E1). We hypothesized that the inter-dimer interactions are essential for trimerization of the E2-E1 heterodimers into a functional spike. In this work, we made a mutant virus (CPB) where we replaced six inter-dimeric residues in the E2 protein of Sindbis virus (WT SINV) with those from the E2 protein from chikungunya virus, and studied its effect in both mammalian and mosquito cell lines. CPB produced fewer infectious particles in mammalian cells than in mosquito cells, relative to WT SINV. When CPB virus was purified from mammalian cells, particles showed reduced amounts of glycoproteins relative to capsid protein, and contained defects in particle morphology compared to virus derived from mosquito cells. Using cryo-EM, we determined that the spikes of CPB had a different conformation than WT SINV. Last, we identified two revertants, E2-H333N and E1-S247L, that restored particle growth and assembly to different degrees. We conclude the inter-dimer interface is critical for spike trimerization and is a novel target for potential antiviral drug design. IMPORTANCEAlphaviruses, which can cause disease when spread to humans by mosquitoes, have been classified as an emerging pathogen, with infections occurring worldwide. The spikes on the surface of the alphavirus particle are absolutely required for the virus to enter a new host cell and initiate an infection. Using a structure-guided approach, we made a mutant virus that alters spike assembly in mammalian cells but not mosquito cells. This is important because it identifies a region in the spike that could be a target for antiviral drug design.

microbiology↗

bioRxiv2021

Core Protein-Directed Antivirals and Importinβ Can Synergistically Disrupt HBV Capsids

Viral structural proteins can have multiple activities. Antivirals that target structural proteins have potential to exhibit multiple antiviral mechanisms. Hepatitis B Virus (HBV) core protein (Cp) is involved in most stages of the viral lifecycle: it assembles into capsids, packages viral RNA, is a metabolic compartment for reverse transcription, interacts with nuclear trafficking machinery, and disassembles to release the viral genome into the nucleus. During nuclear localization, HBV capsids bind to host importins (e.g. Imp{beta}) via Cps C-terminal domain (CTD); the CTD is localized to the interior of the capsid and is transiently exposed on the exterior. We used HAP12 as a representative Cp Allosteric Modulators (CpAMs), a class of antivirals that inappropriately stimulates and misdirects HBV assembly and deforms capsids. CpAM impact on other aspects of the HBV lifecycle is poorly understood. We investigated how HAP12 influenced the interactions between empty or RNA-filled capsids with Imp{beta} and trypsin in vitro. We showed that HAP12 can modulate CTD accessibility and capsid stability, depending on the saturation of HAP12-binding sites. We demonstrated that Imp{beta} synergistically contributes to capsid disruption at high levels of HAP12 saturation, using electron microscopy to visualize disruption and rearrangement of Cp dimers into aberrant complexes. However, RNA-filled capsids resisted the destabilizing effects of HAP12 and Imp{beta}. In summary, we show host protein-induced catalysis of capsid disruption, an unexpected additional mechanism of action for CpAMs. Potentially, untimely capsid disassembly can hamper the HBV lifecycle and also cause the virus to become vulnerable to host innate immune responses. IMPORTANCEThe HBV core, an icosahedral complex of 120 copies of the homodimeric core (capsid) protein with or without packaged nucleic acid, is transported to the host nucleus by its interaction with host importin proteins. Importin-core interaction requires the core protein C-terminal domain, which is inside the capsid, to "flip" to the capsid exterior. Core-protein directed drugs that affect capsid assembly and stability have been developed recently. We show that these molecules can, synergistically with importins, disrupt capsids. This mechanism of action, synergism with host protein, has potential to disrupt the virus lifecycle and activate the innate immune system.

microbiology↗