Coronavirus entry into host cells is mediated by transmembrane spike (S) glycoprotein that forms  homotrimers protuding fom the viral surface. S comprises two functional subunits responsible for binding to the host cell receptor (S1 subunit) and fusion of the viral and cellular membrane (S2 subunit). For many CoVs, S is cleaved  at the boundary between the S1 and S2 subunits, which remain on-covalently bound in the perfusion  conformation.