The SARS-CoV-2 coronavirus is an enveloped, positive-sense single-stranded RNA virus that is responsible for the COVID-19 pandemic. The spike is a class I viral fusion glycoprotein that extends from the viral surface and is responsible for viral entry into the host cell and is the primary target of neutralizing antibodies. The receptor binding domain (RBD) of the spike samples multiple conformations in a compromise between evading immune recognition and searching for the host-cell surface receptor. Using atomistic simulations of the glycosylated wild-type spike in the closed and 1-up RBD conformations, we map the free energy landscape for RBD opening and identify interactions in an allosteric pocket that influence RBD dynamics. The results provide an explanation for experimental observation of increased antibody binding for a clinical variant with a substitution in this pocket. Our results also suggest the possibility of allosteric targeting of the RBD equilibrium to favor open states via binding of small molecules to the hinge pocket. In addition to potential value as experimental probes to quantify RBD conformational heterogeneity, small molecules that modulate the RBD equilibrium could help explore the relationship between RBD opening and S1 shedding.

Molecular dynamics (MD) is a powerful tool for studying intrinsically disordered proteins, however, its reliability depends on the accuracy of the force field. We here assess Amber ff14SB, Amber ff14SB,...

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) an enveloped, positive-sense single-stranded RNA virus that is responsible for the COVID-19 pandemic. The viral spike is a class I viral fusion glycoprotein that extends from the viral surface and is responsible for viral entry into the host cell, and is the primary target of neutralizing antibodies. However, antibody recognition often involves variable surface epitopes on the spike, and the receptor binding domain (RBD) of the spike hides from immune recognition underneath a glycan shield aside from brief dynamic excursions to search for the host-cell surface receptor ACE2. Using an atomistic model of the glycosylated wild-type spike in the closed and 1-up RBD conformations, we identified specific interactions that stabilize the closed RBD, and mapped the free energy landscape for RBD opening. We characterized a transient pocket associated with a hinge motion during opening of the RBD, suggesting the possibility of allosteric control of the RBD via this region. Substitution of a conserved alanine to bulkier leucine in the pocket shifted the RBD equilibrium to favor the open, exposed state, as did removal of a conserved lysine that forms a critical salt-bridge in the closed, hidden state. Results from our virtual screening, MD simulations and free energy landscape calculations for wild-type spike suggest that small molecules can spontaneously bind to the highly conserved hinge pocket, and that such binding can shift the RBD equilibrium to favor the open state. Stabilizing the open state may facilitate antibody recognition by forcing the spike to expose critical RBD epitopes, and also could increase the likelihood of premature triggering of the spike fusion machinery via S1 shedding, neutralizing the infectious ability of the virus.

This report describes the application of dielectric spectroscopy as a simple and fast way to guide protein adsorption experiments. Specifically, the polarization behavior of a layer of adsorbed lysozyme was investigated using a triangular-wave signal with frequencies varying from 0.5 to 2 Hz. The basic experiment, which can be performed in less than 5 min and with a single sample, not only allowed confirming the susceptibility of the selected protein towards the electric signal but also identified that this protein would respond more efficiently to signals with lower frequencies. To verify the validity of these observations, the adsorption behavior of lysozyme onto optically transparent carbon electrodes was also investigated under the influence of an applied alternating potential. In these experiments, the applied signal was defined by a sinusoidal wave with an amplitude of 100 mV and superimposed to + 800 mV (applied as a working potential) and varying the frequency in the 0.1-10000 Hz range. The experimental data showed that the greatest adsorbed amounts of lysozyme were obtained at the lowest tested frequencies (0.1-1.0 Hz), results that are in line with the corresponding dielectric features of the protein.

No abstract