S1 region attaches the virion to the cell membrane by interacting with host ANPEP/aminopeptidase N, initiating the infection. Binding to the receptor probably induces conformational changes in the S glycoprotein unmasking the fusion peptide of S2 region and activating membranes fusion. S2 region belongs to the class I viral fusion protein. Under the current model, the protein has at least 3 conformational states: pre-fusion native state, pre-hairpin intermediate state, and post-fusion hairpin state. During viral and target cell membrane fusion, the coiled coil regions (heptad repeats) regions assume a trimer-of-hairpins structure, positioning the fusion peptide in close proximity to the C-terminal region of the ectodomain. The formation of this structure appears to drive apposition and subsequent fusion of viral and target cell membranes. Homotrimer. During virus morphogenesis, found in a complex with M and HE proteins. Interacts with host ANPEP. Accumulates in the endoplasmic reticulum-Golgi intermediate compartment, where it participates in virus particle assembly. The KxHxx motif seems to function as an ER retrieval signal. Belongs to the alphacoronaviruses spike protein family. In contrast to serogroups 2 and 3, S glycoprotein from serogroup 1 is not cleaved into S1 and S2.