Respiratory syncytial virus (RSV) induces pathogenic syncytia via its fusion (F) protein, yet how it orchestrates fusion within the crowded plasma membrane remains unclear. By integrating live-cell and super-resolution microscopy, we visualized the spatiotemporal dynamics of RSV F from secretion to membrane assembly. We show RSV F efficiently traffics to the plasma membrane and triggers F-actin-enriched protrusions that facilitate fusion, dependent on branched actin remodeling. Superresolution imaging further reveals that RSV F reorganizes from nanoscale clusters into near-continuous ribbon-like nanodomains at cell-cell contacts, forming a stable fusion platform. This work directly visualizes the actin-driven nanoscale assembly of a viral fusogen, defining a key mechanism in RSV pathogenesis and revealing a nanoscale target for antiviral intervention, thereby underscoring the power of advanced nanoscopy to unravel complex host-pathogen interactions at the molecular level.
