Using genetic engineering techniques, two strategies for brane fusion functions of HA on to two different molecules. changing the receptor-binding specificity of the influenzaThe feasibility of this strategy was tested by looking for A virus haemagglutinin (HA) protein whilst preserving its fusion between NIP-conjugated red blood cells which membrane fusion function, have been explored. approach using a different virus attachment/entry protein with well understood structure-to-function relationships. The haemagglutinin (HA) protein of influenza A virus fits this description since it has been exceptionally well characterised in terms of both three-dimensional structure and function. [17][18][19] The HA protein's role in influenza virus replication is to bind the virus to potential host cells by interacting with sialic acid residues on the cell surface, and then to mediate virus entry into the cell by fusing together the virus envelope and cell membrane in a low pH-dependent process. 20 It is initially synthesised as a single monomeric polypeptide, HA0, which is subsequently cleaved by a nonviral protease into two polypeptides, HA1 and HA2, linked together by a disulphide bond. This cleavage process is essential for its fusion activity. In terms of three-dimensional structure, HA consists of a membrane-distal globular head joined by a hinge to a stem region, the end of which is embedded in the virus envelope (or infected cell membrane). The globular head, formed from the HA1 polypeptide, carries the sialic acid receptor binding site. The stem, formed from HA2 and components of HA1, contains the region responsible for mediating fusion. After synthesis,