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