The acylation of the haemagglutinin (HA) of different influenza viruses and of the envelope glycoproteins of Semliki Forest virus (SFV) were analysed. The fatty acid linkage in these acylproteins was found to be resistant to a variety of organic solvents and combinations of these, even after pretreatment with various detergents. Fatty acids are released from influenza virus HA at a pH value between 11.8 and 12.1 at room temperature. Although this mild alkaline cleavage occurs rapidly, the release of fatty acids by treatment with hydroxylamine is time-, temperature- and concentration-dependent. By comparison with model esters the linkage in HA is suggested to be of the oxygenester type rather than a thioester linkage. To assay for possible functions of protein-bound fatty acids the biological activities of influenza virus (A/FPV/Rostock/34) and its solubilized spike glycoproteins were measured after deacylation. While viral haemagglutination activity was not hampered at all, its ability to haemolyse erythrocytes and infectivity were drastically reduced. Likewise, viral spike glycoproteins solubilized with detergents failed to induce haemolysis at low pH when fatty acids had been cleaved off. These results indicate the possible involvement of protein-bound fatty acids in fusion inducation through the acylated fusogenic spike glycoproteins.


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