The actin cytoskeleton inhibits pore expansion during PIV5 fusion protein-promoted cell–cell fusion

Paramyxovirus fusion (F) proteins promote both virus–cell fusion, required for viral entry, and cell–cell fusion, resulting in syncytia formation. We used the F-actin stabilizing drug, jasplakinolide, and the G-actin sequestrant, latrunculin A, to examine the role of actin dynamics in cell–cell fusion mediated by the parainfluenza virus 5 (PIV5) F protein. Jasplakinolide treatment caused a dose-dependent increase in cell–cell fusion as measured by both syncytia and reporter gene assays, and latrunculin A treatment also resulted in fusion stimulation. Treatment with jasplakinolide or latrunculin A partially rescued a fusion pore opening defect caused by deletion of the PIV5 F protein cytoplasmic tail, but these drugs had no effect on fusion inhibited at earlier stages by either temperature arrest or by a PIV5 heptad repeat peptide. These data suggest that the cortical actin cytoskeleton is an important regulator of fusion pore enlargement, an energetically costly stage of viral fusion protein-mediated membrane merger.