Conformational Dynamics Related to Membrane Fusion Observed in Single Viral Envelope Glycoproteins

All enveloped viruses enter cells by fusing the viral membrane with a cellular membrane. This fusion event in promoted by the viral envelope glycoprotein. The current model of class-I viral membrane fusion describes a static “spring-loaded” fusion domain. Upon triggering by acidic pH or interaction with a receptor, the fusion domain undergoes a singular, irreversible conformational rearrangement. Atomic-resolution structural models of several envelope glycoproteins describe the endpoints of this dynamic event. Yet no direct visualization of these conformational dynamics has ever been made, leaving the current model of class-I-mediated membrane fusion unverified. Here, using single-molecule Förster resonance energy transfer (smFRET) imaging we directly visualized the conformational changes of individual envelope glycoprotein trimers on the surface of viral particles. Thus far, we have visualized the dynamics of influenza hemagglutinin (HA), the canonical class-I viral glycoprotein, and Ebola GP, which shares structural similarities with HA. This novel approach was facilitated by introduction of non-canonical amino acids into HA, which were subsequently labeled with fluorophores. In contrast to the current model of class-I-mediated membrane fusion, we observe spontaneous and reversible exchange between pre-fusion and intermediate conformations. Acidification of pH and interaction with receptors shift the dynamic equilibrium, favoring progression toward membrane fusion. These observations are leading to a new understanding of viral membrane fusion, and conserved and divergent features of distinct class-I viral envelope glycoproteins.