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There is limited information about the molecular triggers leading to the uncoating of enteroviruses in physiological conditions. Using real-time spectroscopy and sucrose gradients with radioactively-labeled virus we show at 37 °C, formation of a low amount of albumin-triggered, metastable, uncoating intermediate of echovirus 1 without receptor engagement. This conversion was blocked by saturating the albumin with fatty acids. High potassium but low sodium and calcium concentrations, mimicking the endosomal environment, also induced the formation of a metastable uncoating intermediate of echovirus 1. Together, these factors boosted the formation of the uncoating intermediate and infectivity of this intermediate was retained, as judged by end-point titration. Cryo-electron microscopy reconstruction of the virions treated with albumin and high potassium, low sodium and low calcium concentrations resulted in a 3.6 Å resolution model revealing a fenestrated capsid showing 4 % expansion and loss of the pocket factor, similarly to altered (A-) particles described for other enteroviruses. The dimer interface between VP2 molecules was opened, the VP1 N-termini disordered and most likely externalised. The RNA was clearly visible, anchored to the capsid. The results presented here suggest that extracellular albumin, partially saturated with fatty acids, likely leads to the formation of the infectious uncoating intermediate prior to the engagement with the cellular receptor. In addition, changes in mono- and divalent cations, likely occurring in endosomes, promote capsid opening and genome release.There is limited information about uncoating of enteroviruses in physiological conditions. Here, we focused on physiologically relevant factors that likely contribute to opening of echovirus 1 and other B-group enteroviruses. By combining biochemical and structural data, we show, that before entering cells, extracellular albumin is capable of priming the virus into a metastable, yet infectious intermediate state. The ionic changes that are suggested to occur in endosomes, can further contribute to uncoating and promote genome release, once the viral particle is endocytosed. Importantly, we provide a detailed high-resolution structure of a virion after treatment with albumin and a preset ion composition, showing pocket factor release, capsid expansion and fenestration, and the clearly visible genome still anchored to the capsid. This study provides valuable information about the physiological factors that contribute to the opening of B-group enteroviruses.

Extracellular Albumin and Endosomal Ions Prime Enterovirus Particles for Uncoating That Can Be Prevented by Fatty Acid Saturation