SARS-CoV-2 spike protein from 6VSB. The protomer that has its Receptor Binding Domain (RBD) flipped into the up (active) conformation has its domains colored as described in Figure 1 of: Wrapp, D., Wang, N., Corbett, K.S., Goldsmith, J.A., Hsieh, C.L., Abiona, O., Graham, B.S., McLellan, J.S. (2020) Science 367: 1260-1263
Blue: N-Terminal Domain (NTD)
Green: Receptor Binding Domain (RBD)
Tan: Remainder of S1
Cyan: Fusion Peptide
Yellow: Heptad Repeat 1
Orange: Central Helix
Purple: Connector Domain
Red: Remainder of S2
See also 3dpx-012877 for a related structure from SARS-CoV with all RBD down in the inactive conformation.