MEMBRANE-FUSION ACTIVITY OF THE INFLUENZA-VIRUS HEMAGGLUTININ - THE LOW PH-INDUCED CONFORMATIONAL CHANGE

  • 1 January 1985
    • journal article
    • research article
    • Vol. 260  (5) , 2973-2981
Abstract
The hemagglutinin (HA) spike glycoprotein of influenza virus catalyzes a low pH-induced membrane fusion event which releases the viral genome into the host cell cytoplasm. To study the fusion mechanism in more detail, the ectodomain of HA was prepared in water-soluble form by treating virus particles with bromelain. Under mildly acidic conditions (pH .ltoreq. 5.8), the ectodomain undergoes a conformational change which was found to be biochemically and immunologically equivalent to that in native viral HA. It became sensitive to proteinase K, it exposed new antigenic epitopes in its HA1 chain, and it acquired amphiphilic properties, notably the ability to bind to liposomes. The attachment to liposomes exhibited the same pH dependence to liposomes exhibited the same pH dependence and rapid kinetics as the conformational change and was mediately by HA2. The nature of the attachment resembled that of an integral membrane protein except that the bound HA was partially removed by base. As observed for virus fusion, attachment is independent of divalent cations and lipid composition. Temperature was found to be a critical parameter only with dimyristoylphosphatidycholine vesicles where attachment was partially blocked below the major phase transition. Evidently, the low pH-induced conformational change in the isolated ectodomain is equivalent to that occurring in intact viral HA, its attachment to liposomes can serve as a model for the initial stages in the HA induced membrane fusion reaction.