Cells expressing the influenza virus hemagglutinin (HA) fuse to planar bilayer membranes under acidic conditions. After an electrically quiescent perfusion stage (Q), a fusion pore forms that enlarges in three subsequent stages. A repetitively flickering pore stage (R) develops into a securely open stage (S) that exhibits conductances ranging from a few to tens of nS. The pore then expands to a terminal stage (T) with a large conductance on the order of one microSiemens. We have studied how virus strain, HA receptors in the target bilayer membrane, and cytoskeleton affect the time a fusion pore remains in each stage. These intervals are referred to as waiting times. In the quiescent stage, waiting times were very sensitive to the virus strain and presence of gangliosides (HA receptors) in the bilayer. When bilayers contained gangliosides, the waiting times in the Q stage for cells infected with the PR/8/34 strain of virus were exponentially distributed, whereas waiting times for cells infected with the Japan/305/57 strain were not so distributed. Without gangliosides, the waiting time distribution for PR/8/34 infected cells was complex. The waiting times in the R and S stages of pore growth were exponentially distributed under all tested conditions. Within the R stage, we analyzed the kinetics of the flickering pore by fitting the open and closed time distributions with a sum of two exponentials. Neither the open and closed time distributions nor the flickering pore conductance distributions were appreciably affected by virus strain or gangliosides. Colchicine and cytochalasin B increased the flicker rates, without affecting the waiting time in the R stage. We conclude that variations in amino acid sequences of the HAs and the presence of gangliosides as receptors within the target membrane critically affect the kinetics of fusion pore formation, but do not affect subsequent stages.

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