Differences in the 3HR1 core between α-HCoVs and β-HCoVs. (a) A side-by-side comparison illustrates the differences in the heptad-repeat packing geometry of HCoV-229E, HCoV-NL63 (PDB entry 2ieq), SARS (PDB entry 1wyy) and MERS (PDB entry 4njl). The hydrophobic 3HR1 core is depicted as a grey ribbon, cavities as a yellow surface mesh and the side chains of residues at `d/a' positions as cyan stick models. The helical turns are numbered from the N-terminus to the C-terminus on one of the HR1 helices in the trimer. On the left of each ribbon representation, the packing residues lining the trimeric coiled-coil interface are indicated in black, with residues at the `a' and `d' positions (canonical `a' and `d' layer) boxed in beige, residues at `x' positions (`x' layer) in magenta and residues at `d/a' positions (`da' layer) in cyan. (b) From left to right: cross-sections of representative `a', `d', `x' and `da' layers in the 3HR1 core of HCoV-229E. The 2Fo − Fc electron-density maps are contoured at 2σ. (c) Electrostatic surfaces illustrating differences in the 3HR1 core from HCoV-229E, HCoV-NL63, SARS and MERS. The red dashed boxes depict the regions on different 3HR1 cores where the electrostatic surface potentials are opposite in α-HCoVs and β-HCoVs. The black dashed boxes highlight the only regions on SARS and MERS where the electrostatic surface potentials are different.