The transmembrane domain regulates the kinetics of the SARS-CoV-2 spike conformational transition

The homotrimeric SARS-CoV-2 spike glycoprotein comprises two subunits: S1, which recognizes host-receptors through its receptor-binding domains (RBDs), and S2, which is anchored to the viral membrane through its transmembrane domain (TMD) and which facilitates the fusion of the viral envelope with the host cell membrane. Upon host receptor engagement and proteolytic activation, S1 dissociates and triggers a large conformational transition in S2, involving structural rearrangements in the S2 ectomembrane domains and the TMD. While studies have focused on the ectomembrane-domain dynamics, the TMD has typically been modeled as being in a trimeric state. Here, we use molecular dynamics simulations of a coarse-grained structure-based model (SBM) with an implicit membrane to investigate the role of TMD dynamics in modulating S2 conformational conversion. We first recapitulate previous results from an all-atom SBM with a trimeric TMD and re-emphasize that the extended pre-hairpin intermediate state of S2, which brings the two membranes into apposition, is a by-product of the prefusion-to-postfusion transition. Next, by introducing dynamics into the TMD, we find a late fusion intermediate structurally similar to a recent cryoelectron microscopy structure. A dynamic TMD also makes the conformational transition faster. Simulations including the S1/S2 complex reveal coupled RBD-TMD dynamics: when all three RBDs are in the closed state, they can stabilize the TMD in a trimeric configuration, whereas the opening of a single RBD can trigger a transition to a dynamic TMD. So, the dynamics and the conformational preferences of the TMD can be tuned by the presence and conformation of S1. There is some evidence that the TMDs of class I viral fusion proteins, such as spike, contribute to viral fusion by modulating membrane properties. Our simulations indicate an expanded role for the function of the TMD, where it can directly regulate the kinetics of S2 conformational transitions.