Dynamics and self-assembly of the SARS-CoV-2 spike transmembrane domain

Abstract

AbstractThe spike (S) protein is a trimeric, membrane-anchored fusion protein that enables coronaviruses, such as the SARS-CoV-2, to recognize and fuse with their hosts’ cells. While the prefusion and postfusion structures of the ectomembrane domain of the spike protein are available, the corresponding organization of its transmembrane domain is obscure. Since the transmembrane and ectomembrane domains of fusion proteins are conformationally linked, an understanding of trimerization and transmembrane conformations in the viral envelope is a prerequisite to completely understand viral fusion by the spike protein. To address this, we computationally explored the self-assembly of the SARS-CoV-2 spike transmembrane domain, starting first by determining the membrane boundaries of the spike transmembrane helix. Using atomistic molecular dynamics simulations, we found the spike protein transmembrane domain to be plastic, and the transmembrane helix to be very dynamic. The observed movements of the helix changed the membrane embedded sequence, and thereby affected the conformational ensemble of the transmembrane assembly in Martini coarse grained simulations, even flipping the super-helical handedness. Analysis of the transmembrane organization of the spike transmembrane helix provided rich insights into the interfaces utilized to self-associate. Moreover, we identified two distinct cholesterol binding regions on the transmembrane helix with different affinities for the sterol. The cholesterol binding pockets overlapped with regions involved in the initiation of transmembrane protein-protein interaction. Together, the results from our multiscale simulations not only provide insight into understudied trimeric helical interfaces in biomembranes, but also enhance our understanding of the elusive transmembrane conformational dynamics of SARS-CoV-2 spike and more generally of viral fusion proteins. These insights should enable the inclusion of the conformations of the spike protein transmembrane domain into the prevalent models of virus fusion.SignificanceEnveloped viruses rely on fusion proteins, called spike proteins in coronaviruses, to infect cells by fusing the virus envelope with the host cell membrane. The transmembrane domain (TMD) of the coronavirus spike protein is critically involved in successful viral fusion and other aspects of the virus lifecycle, but is poorly studied. Using multiscale molecular dynamics simulations of the SARS-CoV-2 spike TMD, we explore its conformational dynamics and self-assembly in different lipid environments. The results provided here improve our understanding of transmembrane stabilization of spike trimers, which are indispensable for viral infection. Exploiting this knowledge to destabilize spike trimers should facilitate design of transmembrane domain targeted viral fusion inhibitors.