Control of viral envelope glycoprotein function revealed by single-molecule imaging.

Viral envelope glycoproteins catalyze membrane fusion during entry into cells. Envelope glycoprotein function has traditionally been viewed through the lens of kinetic control, where environmental cues like pH trigger irreversible refolding from the pre-fusion conformation to the post-fusion conformation. Single-molecule Förster resonance energy transfer (smFRET) imaging has revealed an additional layer of thermodynamic control that governs the conformational dynamics of envelope glycoproteins in their pre-fusion form. smFRET studies of the envelope glycoproteins from HIV-1, SARS-CoV-2, MERS-CoV, Ebola virus, and influenza A virus demonstrate that these glycoproteins dynamically sample an ensemble of pre-fusion conformations whose relative stabilities respond to pH, receptor binding, ions, and host proteases. This thermodynamic tuning precedes the kinetically controlled transition that promotes membrane fusion. Collectively, smFRET imaging has transformed our understanding of viral entry, illustrating how the pre-fusion energy landscape of envelope glycoproteins is tuned by the host environment to maintain viral fitness.