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.