# Glutamic Acid at Position 343 in PB2 Contributes to the Virulence of H1N1 Swine Influenza Virus in Mice

## TL;DR

A specific amino acid in a flu virus protein affects how sick mice get, showing how viruses adapt to hosts.

## Contribution

Identifies glutamic acid at position 343 in PB2 as a key factor in H1N1 swine influenza virulence in mice.

## Key findings

- Glutamic acid at PB2 position 343 enhances viral replication and pathogenicity in mice.
- Substituting glutamic acid with lysine at PB2-343 reduces virulence and polymerase activity.
- PB2-343E is highly conserved in human and swine influenza viruses, suggesting adaptive importance.

## Abstract

The H1N1 swine influenza viruses CQ91 and CQ445, isolated from pigs in China, exhibited distinct virulence in mice despite sharing similar genomic constellations. CQ91 demonstrated higher pathogenicity (MLD50: 5.4 log10 EID50) and replication efficiency in mice compared to CQ445 (MLD50: 6.6 log10 EID50). Through reverse genetics, we found that the attenuation of CQ445 was due to a single substitution of glutamic acid (E) with lysine (K) at position 343 in the PB2 protein. Introducing the CQ445-PB2 (343K) into CQ91 significantly reduced viral replication and pathogenicity in mice, while replacing CQ445-PB2 with CQ91-PB2 (343E) restored virulence. In vitro studies showed that the K343E mutation impaired viral replication in MDCK and A549 cells and reduced polymerase activity in minigenome assays. Mechanistically, the amino acid at position 343 in the PB2 affects the transcription stage of the viral replication process. Structural modeling indicated that the charge reversal caused by E343K altered local electrostatic interactions without major conformational changes. Phylogenetic analysis revealed that PB2-343E is highly conserved (>99.9%) in human and swine H1/H3 influenza viruses, suggesting that PB2-343E confers an adaptive advantage. This study identifies PB2-343E as a critical determinant of influenza virus pathogenicity in mammals, highlighting its role in host adaptation.