Structural protein that makes short spikes at the surface of the virus. Contains receptor binding and receptor-destroying activities. Mediates de-O-acetylation of N-acetyl-4-O-acetylneuraminic acid, which is probably the receptor determinant recognized by the virus on the surface of erythrocytes and susceptible cells. This receptor-destroying activity is important for virus release as it probably helps preventing self-aggregation and ensures the efficient spread of the progeny virus from cell to cell. May serve as a secondary viral attachment protein for initiating infection, the spike protein being the major one. May become a target for both the humoral and the cellular branches of the immune system