Resolving the molecular gymnastics of the elusive herpes simplex fusion protein

Herpes Simplex Virus (HSV) afflicts ∼90 % of the population, causing a range of diseases from oral cold sores to encephalitis. Despite the widespread effects of HSV, current drugs cannot cure infection and there are no preventative vaccines. HSV infection requires entry into the host cell, which is mediated by the fusion protein, gB. Despite various structures being available for post-fusion gB, a high-resolution model for the pre-fusion state, required to understand the fundamental mechanism of HSV fusion, is still lacking. To elucidate this structure, full length gB was previously expressed on lipid vesicles and was shown via cryo-electron microscopy techniques to adopt the elusive pre-fusion state. Strikingly, two recent studies, one characterising such vesicles, generated two different pre-fusion gB models. The discrepancy between the models can be summarised by the position of the fusion loops: are they positioned facing away from the viral membrane or towards it in the pre-fusion state? Using the gB studded vesicle production method combined with a state-of-the-art Titan Krios cryo-electron microscope equipped with a Volta Phase Plate, a direct electron detector camera and an energy filter, all to increase contrast and signal-to-noise ratio, we aim to settle the dispute as to the structure of pre-fusion gB. Revealing the pre-fusion structure could lead to possible therapeutics to a disease that can have serious complications in newborns and the immunocompromised, a growing cohort in the modern world.