The homotrimeric SARS-CoV-2 spike protein enables viral infection by undergoing a large conformational transition, which facilitates the fusion of the viral envelope with the host cell membrane. The spike protein is anchored to the SARS-CoV-2 envelope by its transmembrane domain (TMD), composed of three TM helices, each contributed by one of the protomers of spike. Although the TMD is known to be important for viral fusion, whether it is a passive anchor of the spike or actively promotes fusion remains unknown. Specifically, it is unclear if the TMD and its dynamics facilitate the prefusion to postfusion conformational transition of the spike. Here, we computationally study the dynamics and self-assembly of the SARS-CoV-2 spike TMD in homogeneous POPC and cholesterol containing membranes. Atomistic simulations of a long TM helix-containing protomer segment show that the membrane-embedded segment bobs, tilts and gains and loses helicity, locally thinning the membrane. Coarse-grained multimerization simulations using representative TM helix structures from the atomistic simulations exhibit diverse trimer populations whose architecture depends on the structure of the TM helix protomer. While a symmetric conformation reflects the symmetry of the resting spike, an asymmetric TMD conformation could promote membrane fusion through the stabilization of a fusion intermediate. Together, our simulations demonstrate that the sequence and length of the SARS-CoV-2 spike TM segment make it inherently dynamic, that trimerization does not abrogate these dynamics and that the various observed TMD conformations may enable viral fusion.

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