High-resolution mapping of the actin fusion focus reveals myosin V–dependent formin transport for aster formation

Many processes such as polarized growth and secretion require specific actin networks. In fungi, cell–cell fusion requires cell wall digestion mediated by local secretion of lytic enzymes. In Schizosaccharomyces pombe, the myosin V Myo52 transports enzyme-containing secretory vesicles on the actin fusion focus, an aster-like actin network assembled by the condensate-forming formin Fus1. The fusion focus also concentrates proteins regulating cell polarity, communication, cytoskeleton, exocytosis, and membrane merging. Here, using centroid tracking and averaging, we present a spatiotemporal map of the fusion site with 8-nm precision. We show that a pool of vesicles remains at constant distance from the membrane as the actin structure condenses. Unexpectedly, Myo52 detaches from this pool and colocalizes with Fus1 closer to the membrane. We show that Myo52 binds Fus1 and transports it along actin filaments, and that Myo52 and Fus1 actin assembly activity contribute to focus compaction. Thus, myosin V–driven transport of formin Fus1 along Fus1-nucleated actin filaments underlies positive feedback for actin aster formation.