Functionality of IAV packaging signals depends on site-specific charges within the viral nucleoprotein.

The coordinated packaging of the segmented genome of the influenza A virus (IAV) into virions is an essential step of the viral life cycle. This process is controlled by the interaction of packaging signals present in all eight viral RNA (vRNA) segments and the viral nucleoprotein (NP), which binds vRNA via a positively charged binding groove. However, mechanistic models of how the packaging signals and NP work together to coordinate genome packaging are missing. Here, we studied genome packaging in influenza A/SC35M virus mutants that carry mutated packaging signals as well as specific amino acid substitutions at the highly conserved lysine (K) residues 184 and 229 in the RNA-binding groove of NP. Because these lysines are acetylated and thus neutrally charged in infected host cells, we replaced them with glutamine to mimic the acetylated, neutrally charged state or arginine to mimic the non-acetylated, positively charged state. Our analysis shows that the coordinated packaging of eight vRNAs is influenced by (i) the charge state of the replacing amino acid and (ii) its location within the RNA-binding groove. Accordingly, we propose that lysine acetylation induces different charge states within the RNA-binding groove of NP, thereby supporting the activity of specific packaging signals during coordinated genome packaging. IMPORTANCE: Influenza A viruses (IAVs) have a segmented viral RNA (vRNA) genome encapsidated by multiple copies of the viral nucleoprotein (NP) and organized into eight distinct viral ribonucleoprotein complexes. Although genome segmentation contributes significantly to viral evolution and adaptation, it requires a highly sophisticated genome-packaging mechanism. How eight distinct genome complexes are incorporated into the virion is poorly understood, but previous research suggests an essential role for both vRNA packaging signals and highly conserved NP amino acids. By demonstrating that the packaging process is controlled by charge-dependent interactions of highly conserved lysine residues in NP and vRNA packaging signals, our study provides new insights into the sophisticated packaging mechanism of IAVs.

Complex N-glycans are important for interspecies transmission of H7 influenza A viruses.

Influenza A viruses (IAVs) can overcome species barriers by adaptation of the receptor-binding site of the hemagglutinin (HA). To initiate infection, HAs bind to glycan receptors with terminal sialic acids, which are either N-acetylneuraminic acid (NeuAc) or N-glycolylneuraminic acid (NeuGc); the latter is mainly found in horses and pigs but not in birds and humans. We investigated the influence of previously identified equine NeuGc-adapting mutations (S128T, I130V, A135E, T189A, and K193R) in avian H7 IAVs in vitro and in vivo. We observed that these mutations negatively affected viral replication in chicken cells but not in duck cells and positively affected replication in horse cells. In vivo, the mutations reduced virus virulence and mortality in chickens. Ducks excreted high viral loads longer than chickens, although they appeared clinically healthy. To elucidate why these viruses infected chickens and ducks despite the absence of NeuGc, we re-evaluated the receptor binding of H7 HAs using glycan microarray and flow cytometry studies. This re-evaluation demonstrated that mutated avian H7 HAs also bound to α2,3-linked NeuAc and sialyl-LewisX, which have an additional fucose moiety in their terminal epitope, explaining why infection of ducks and chickens was possible. Interestingly, the α2,3-linked NeuAc and sialyl-LewisX epitopes were only bound when presented on tri-antennary N-glycans, emphasizing the importance of investigating the fine receptor specificities of IAVs. In conclusion, the binding of NeuGc-adapted H7 IAV to tri-antennary N-glycans enables viral replication and shedding by chickens and ducks, potentially facilitating interspecies transmission of equine-adapted H7 IAVs.IMPORTANCEInfluenza A viruses (IAVs) cause millions of deaths and illnesses in birds and mammals each year. The viral surface protein hemagglutinin initiates infection by binding to host cell terminal sialic acids. Hemagglutinin adaptations affect the binding affinity to these sialic acids and the potential host species targeted. While avian and human IAVs tend to bind to N-acetylneuraminic acid (sialic acid), equine H7 viruses prefer binding to N-glycolylneuraminic acid (NeuGc). To better understand the function of NeuGc-specific adaptations in hemagglutinin and to elucidate interspecies transmission potential NeuGc-adapted viruses, we evaluated the effects of NeuGc-specific mutations in avian H7 viruses in chickens and ducks, important economic hosts and reservoir birds, respectively. We also examined the impact on viral replication and found a binding affinity to tri-antennary N-glycans containing different terminal epitopes. These findings are significant as they contribute to the understanding of the role of receptor binding in avian influenza infection.

Structure and dynamics of the proton-selective histidine and the gating tryptophan in an inward rectifying hybrid influenza B and A virus M2 proton channel.

The M2 proteins of influenza A and B viruses form acid-activated proton channels that are essential for the virus lifecycle. Proton selectivity is achieved by a transmembrane (TM) histidine whereas gating is achieved by a tryptophan residue. Although this functional apparatus is conserved between AM2 and BM2 channels, AM2 conducts protons exclusively inward whereas BM2 conducts protons in either direction depending on the pH gradient. Previous studies showed that in AM2, mutations of D44 abolished inward rectification of AM2, suggesting that the tryptophan gate is destabilized. To elucidate how charged residues C-terminal to the tryptophan regulates channel gating, here we investigate the structure and dynamics of H19 and W23 in a BM2 mutant, GDR-BM2, in which three BM2 residues are mutated to the corresponding AM2 residues, S16G, G26D and H27R. Whole-cell electrophysiological data show that GDR-BM2 conducts protons with inward rectification, identical to wild-type (WT) AM2 but different from WT-BM2. Solid-state NMR 15N and 13C spectra of H19 indicate that the mutant BM2 channel contains higher populations of cationic histidine and neutral τ tautomers compared to WT-BM2 at acidic pH. Moreover, 19F NMR spectra of 5-19F-labeled W23 resolve three peaks at acidic pH, suggesting three tryptophan sidechain conformations. Comparison of these spectra with the tryptophan spectra of other M2 peptides suggests that these indole sidechain conformations arise from interactions with the C-terminal charged residues and with the N-terminal cationic histidine. Taken together, these solid-state NMR data show that inward rectification in M2 proton channels is accomplished by tryptophan interactions with charged residues on both its C-terminal and N-terminal sides. Gating of these M2 proton channels is thus accomplished by a multi-residue complex with finely tuned electrostatic and aromatic interactions.

Isolation, sequencing and phylogenetic analysis of the hemagglutinin, neuraminidase and nucleoprotein genes of the Chilean equine influenza virus subtypes H7N7 and H3N8

We report here on the isolation and sequencing of the hemagglutinin, neuraminidase and nucleoprotein genes of the Chilean equine influenza virus subtypes H7N7 (A⁄equi-1⁄Santiago⁄77, Sa77) and H3N8 (A⁄equi-2⁄Santiago⁄85, Sa85) . The sequences obtained allowed a variability analysis, which indicated significant differences when compared with other isolates. We found that Chilean isolates are more similar to the North American variety than to European isolates. Isolate Sa77 is a good candidate for inclusion in a vaccine as it is the latest isolate of the subtype H7N7 and is probably better-adapted to the equine host. Isolate Sa85, of subtype H3N8, also appears to be a good candidate since it has no significant differences in the main antigenic sites with recent isolates.

Influência do pH na atividade hemolítica de uma amostra aviária de vírus influnza A / Influence of pH on hemolytic activity of an avian sample of influenza a virus

A atividade de fusäo, depende de pH, de uma amostra aviária de Vírus Influenza A com eritrócitos de galinhas, foi investigada usando oteste de hemólise (HL). Este teste é baseado na quantificaçäo de hemoglobina (Hb) liberada como consequência da fusäo vírus/eritrócito. A atividade hemolítica desta preparaçäo viral pode ser demonstrada na faixa de pH ácido (5.0-5.8), o que está relacionado à ocorrência de uma mudança conformacional nas moléculasda hemaglutinina (HA) sob estas condiçöes. A atividade fusogênica era rapidamente inativada após incubaçäo do vírus, em pH ácido, a 37 graus C, na ausência de eritrócitos, sugerindo que a mudança conformacional da HA do vírus pode ser responsável pela ativaçäo e/ou inativaçäo da capacidade fusogênica. O estudo da reaçäo de HL mostra que elevando-se a concentraçäo de eritrócitos, enquanto a concentraçäo de vírus permanece constante, pode-se observar um aumento na liberaçäo de hemoglobina (Hb), o qual sugere que, provavelmente, os vírus tiveram a chance de encontrar um maior número de células susceptíveis. Uma vez que a Hb em soluçäo muda de tonalidade em funçäo do pH em que se encontra, um fator de correçäo foi utilizado após o ensaio colorimétrico.