The phase separation of biomolecules into stress granules (SGs) allows the cell to tightly regulate translation activity in response to different stimuli, such as oxidative stress, starvation, or the recognition of non-cellular RNA. Recent reports suggest SGs induced during viral infection may act as a crossroad between the cellular stress response and the activation of the innate immune response. Here, we aimed to dissect the role of SGs in the context of yellow fever virus (YFV) infection. We found that YFV infection with both 17D vaccine and Asibi pathogenic strains resulted in translational shut-off and the formation of SGs at 48 hours post infection (hpi), although viral infection was already apparent at 24 hpi. This delay may be coupled to the ability of the YFV capsid to inhibit SG formation through an interaction with the SG scaffolding protein G3BP1. To elucidate the role of YFV-induced SGs during infection, we inhibited SG assembly using a small-molecule inhibitor as well as siRNA-mediated knockdown of SGs and found that while SG formation does not impact viral replication, silencing of SG scaffolding proteins is associated with increased expression of interferon-stimulated genes (ISG). Uncovering the proteome of YFV-induced SGs, our compositional analysis revealed a unique composition, distinct from NaAs-induced SGs, which displayed enrichment in mitochondrial proteins. Further functional analysis revealed that YFV infection is associated with mitochondrial damage and dysfunction, which coincides with formation of virus-induced SGs. Together, we propose that YFV-induced SGs may be involved not only in the modulation of cellular homeostasis but also in influencing mitochondrial functions.
Phase variation is a well-established phenomenon in some bacteria, where it generates heritable phenotypic variants within a population. These states, while reversible, are stable across generations, giving rise to quasi-clonal populations even when starting from a single bacterium. Underlying mechanisms include variation in short homopolymeric or heteropolymeric sequence repeats, DNA inversions mediated by site-specific recombination, reversible insertions and deletions, and switching between alternative methylation states. Although phase variation occurs in diverse bacteria, its contribution to phenotypic heterogeneity remains overlooked and use of the term is often absent in describing this phenomenon.
Structural studies of diverse viral RNA-dependent RNA polymerases reveal that the broad-spectrum antiviral Suramin binds distinct sites while repeatedly exploiting conserved RNA-interacting and electrostatic features. This Pearl highlights common vulnerabilities within viral RNA polymerases and discusses how structural studies provide a framework for future antiviral design.
H9N2 avian influenza viruses are characterized by significant genetic diversity. Deletions within the hemagglutinin (HA) receptor-binding domain represent a notable, albeit rare, evolutionary trajectory. Here, we systematically characterized the multifaceted impact of a single-amino-acid deletion at residue 227 of HA (designated HA Δ227 ), a naturally occurring deletion within the 220-loop that overlaps a key antigenic epitope, using isogenic reverse genetics H9N2 viruses. Paradoxically, the HA Δ227 imposed a fitness cost and was associated with enhanced HA receptor-binding avidity, reduced apparent virion-associated NA activity, altered HA/NA functional balance, attenuated viral replication in vitro , and reduced respiratory shedding in vivo . Notably, despite this compromised replicative fitness, infection with the mutant virus was associated with an altered host immune response in chickens, characterized by elevated levels of interferon-γ (IFN-γ), interleukin-10 (IL-10), interleukin-1β (IL-1β), and monocyte chemoattractant protein-1 (MCP-1). These altered immune responses were associated with more severe pulmonary lesions, indicating a dissociation between disease severity and viral RNA load. Furthermore, the deletion increased the pH threshold for HA-mediated membrane fusion and substantially altered antigenic relationships. Collectively, our findings identify HA residue 227 as a functionally important site at which a single-residue deletion is associated with a phenotypic trade-off between replicative capacity and disease severity: reduced replicative fitness accompanied by altered inflammatory responses and greater tissue injury. This study highlights the potential phenotypic significance of a rare deletion in influenza virus HA and demonstrates that viral burden alone may not adequately predict disease severity.
