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Updated: Feb 8, 2026

Visualizing the Conformational Dynamics of Membrane Receptors Using Single-Molecule FRET
Published on: August 17, 2022
Direct Visualization of the Conformational Dynamics of Single Influenza Hemagglutinin Trimers
Dibyendu Kumar Das1, Ramesh Govindan1, Ivana Nikić-Spiegel2
1Department of Molecular Biology and Microbiology, Tufts University School of Medicine and Sackler School of Graduate Biomedical Sciences, Boston, MA 02111, USA.
Abstract:
Influenza hemagglutinin (HA) is the canonical type I viral envelope glycoprotein and provides a template for the membrane-fusion mechanisms of numerous viruses. The current model of HA-mediated membrane fusion describes a static "spring-loaded" fusion domain (HA2) at neutral pH. Acidic pH triggers a singular irreversible conformational rearrangement in HA2 that fuses viral and cellular membranes. Here, using single-molecule Förster resonance energy transfer (smFRET)-imaging, we directly visualized pH-triggered conformational changes of HA trimers on the viral surface. Our analyses reveal reversible exchange between the pre-fusion and two intermediate conformations of HA2. Acidification of pH and receptor binding shifts the dynamic equilibrium of HA2 in favor of forward progression along the membrane-fusion reaction coordinate. Interaction with the target membrane promotes irreversible transition of HA2 to the post-fusion state. The reversibility of HA2 conformation may protect against transition to the post-fusion state prior to arrival at the target membrane.
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