MORPHOLOGICAL-CHANGES AND FUSOGENIC ACTIVITY OF INFLUENZA-VIRUS HEMAGGLUTININ

The kinetics of low-ph induced fusion of influenza virus with liposome s have been compared to changes in the morphology of influenza hemaggl utinin (HA). At ph 4.9 and 30 degrees C, the fusion of influenza A/PR/ 8/34 virus with ganglioside-bearing liposomes was complete within 6 mi n. Virus preincubated at pH 4.9 and 30 degrees C in the absence of lip osomes for 2 or 10 min retained most of its fusion activity. However, fusion activity was dramatically reduced after 30 min, and virtually a bolished after a 60-min preincubation. Cryo-electron microscopy showed that the hemagglutinin spikes of virions exposed to pH 4.9 at 30 degr ees C for 10 min underwent no major morphological changes. After 30 mi n, however, the spike morphology changed dramatically, and further cha nges occurred for up to 60 min after exposure to low ph, Because the m orphological changes occur at a rate corresponding to the loss of fusi on activity, and because these changes are much slower than the rate a t which fusion occurs, we conclude that the morphologically altered HA is inactive with respect to fusion-promoting activity, Molecular mode ling studies indicate that the formation of an extended coiled coil wi thin the HA trimer, as proposed for HA at low pH, requires a major con formational change in HA, and that the morphological changes we observ e are consistent with the formation of an extended coiled coil. These results imply that the crystallographically determined low-ph form of HA does occur in the intact virus, but that this form is not a precurs or of viral fusion. It is speculated that the motion to the low-ph for m may be responsible for the membrane destabilization leading to fusio n.