The opening of initial fusion pores by low-pH forms of HA mimics
fusion between the viral envelope and the membrane of an acidified
endosome during virus entry. HA structure, its changes at the pH of
fusion, and the HA-mediated pathway that culminates in a fusion pore
opening have been studied in considerable detail [19–24]. Here, we
utilize HA to initiate fusion, but we concentrate on the transition from
early fusion pores to expanding cytoplasmic cell–cell connections
readily detectable with fluorescence microscopy. While the opening of
fusion pores requires only the presence of functional HA, this transition
requires cell metabolic energy and is negatively regulated by protein
kinase C (PKC). Late and cell-dependent stages of fusion yielding
syncytium formation are not blocked by microtubule- and actin