Due to the complexity of developmental cell fusion reactions and a
large number of proteins involved in controlling both pre- and postfusion
stages, it is very hard to distinguish contributions of
cytoskeleton rearrangements during fusion pore expansion from
those at the preceding stages of cell–cell binding, plasma membrane
expression of yet unknown fusogens, their activation, and, finally,
fusion pore opening. In this work we address the fundamental
question whether cytoskeleton drives syncytium formation, using an
experimental system that is much simpler than cell fusion in
development. A robust and relatively fast fusion reaction between
mammalian cells is initiated here by a low pH-triggered restructuring
of one of the best-characterized protein fusogens influenza virus
hemagglutinin (HA).