Structural changes in Influenza virus at low pH characterized by cryo-electron tomography

Area of Science:

• Virology
• Structural Biology
• Cell Biology

Background:

• Influenza virus entry into host cells relies on endocytosis.
• Low endosomal pH induces conformational changes in hemagglutinin (HA), mediating viral and endosomal membrane fusion.

Purpose of the Study:

• To visualize the structural changes of influenza A virus upon exposure to low pH (pH 4.9) using cryo-electron tomography.
• To identify intermediate states of HA and M1 protein layer during the fusion process.

Main Methods:

• Cryo-electron tomography (cryo-ET) was used to image influenza A virus at neutral and low pH (4.9).
• Subtomogram averaging was employed to analyze the structural conformations of HA.
• Virions were classified based on glycoprotein arrays and M1 layer integrity at different time points post-incubation at low pH.

Main Results:

• Low pH induced significant morphological changes, including loss of elongated particles, appearance of larger fused virions, HA spike disorganization, and M1 layer dissolution.
• Two intermediate HA conformations were identified, suggesting sequential events involving fusion peptide movement and HA1 subunit rearrangement.
• pH-induced changes in the M1 layer increased envelope pliability, facilitating fusion.

Conclusions:

• The study provides a high-resolution structural view of influenza virus fusion.
• Reversible conformational changes in HA are crucial for membrane fusion.
• Efficient viral infection depends on the timing of fusion relative to M1 protein coagulation and RNP aggregation.