Binds to the N-acetyl-9-O-acetylneuraminic acid residues on the cell surface, bringing about the attachment of the virus particle to the cell. Plays a major role in the determination of host range restriction and virulence. Class I viral fusion protein. Responsible for penetration of the virus into the cell cytoplasm by mediating the fusion of the membrane of the endocytosed virus particle with the endosomal membrane. Low pH in endosomes induce an irreversible conformational change in HEF2, releasing the fusion hydrophobic peptide. Several trimers are required to form a competent fusion pore. Displays a receptor-destroying activity which is a neuraminidate-O-acetyl esterase. This activity cleaves off any receptor on the cell surface, which would otherwise prevent virions release. These cleavages prevent self-aggregation and ensure the efficient spread of the progeny virus from cell to cell (By similarity). H2O + N-acetyl-9-O-acetylneuraminate = acetate + H(+) + N-acetylneuraminate H2O + N-acetyl-4-O-acetylneuraminate = acetate + H(+) + N-acetylneuraminate Homotrimer of disulfide-linked HEF1-HEF2. In natural infection, inactive HEF is matured into HEF1 and HEF2 outside the cell by one or more trypsin-like, arginine-specific endoprotease. Belongs to the influenza type C/coronaviruses hemagglutinin-esterase family.