Membrane fusion by surrogate receptor-bound influenza haemagglutinin

In influenza infections, haemagglutinin (HA) mediates the fusion of virus and cellular membranes at endosomal pH, between pH 5 and 6. In vitro, when reconstituted into virosomes, efficient fusion requires target membranes to contain sialic acid receptors or receptor analogues. In the experiments reported, lipid-associated anti-HA monoclonal Fab' fragments were used as surrogate receptors to investigate the fusion capacity of receptor-bound HA compared with unbound HA. The conclusions are drawn, in contrast to those from previous studies, that bound HA can mediate fusion and that fusion mainly involves bound HA when the liposome targets are densely packed with surrogate receptors.