Abstract
The S protein of the SARS-CoV-2 is a Type I membrane protein that mediates membrane fusion and viral entry. A vast amount of structural information is available for the ectodomain of S, a primary target by the host immune system, but much less is known regarding its transmembrane domain (TMD) and its membrane-proximal regions. Here, we determined the nuclear magnetic resonance (NMR) structure of the S protein TMD in bicelles that closely mimic a lipid bilayer. The TMD structure is a transmembrane α-helix (TMH) trimer that assembles spontaneously in membrane. The trimer structure shows an extensive hydrophobic core along the 3-fold axis that resembles that of a trimeric leucine/isoleucine zipper, but with tetrad, not heptad, repeat. The trimeric core is strong in bicelles, resisting hydrogen-deuterium exchange for weeks. Although highly stable, structural guided mutagenesis identified single mutations that can completely dissociate the TMD trimer. Multiple studies have shown that the membrane anchor of viral fusion protein can form highly specific oligomers, but the exact function of these oligomers remain unclear. Our findings should guide future experiments to address the above question for SARS coronaviruses.