Conformational changes in the hemagglutinin of influenza virus which accompany heat-induced fusion of virus with liposomes.

Incubation of X-31 influenza virus at 62 degrees results in changes in hemagglutinin structure which cause the virus to fuse with liposomes at neutral pH. Mutants of X-31 which contain different hemagglutinins and as a consequence fuse with liposomes at higher pH than wild-type virus also fuse at a lower temperature. The heat-induced changes in hemagglutinin structure were monitored by CD, fluorescence spectroscopy, electron microscopy, and proteolytic digestion and compared with the characteristic changes which occur at low pH required for hemagglutinin-mediated membrane fusion at physiological temperature. The results indicate that the heat-induced changes in hemagglutinin which allow fusion activity result in partial denaturation of the molecule. By comparison the changes in structure required for fusion at low pH and normal temperature are specifically restricted.