The homotrimeric spike glycoprotein (S) from SARS-CoV-2, particularly its S2 subunit, is a fusion protein extraordinaire. It can fuse viral particles to cells and also fuse cells to cells to create multifarious syncytia among different cell phenotypes. Depending on which exact versions are under consideration, the spike can perform these feats via multiple mechanisms acting at both the intracellular and extracellular sides of cell membranes.
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Source: Phys.org